Hydrolysis of functional derivatives of carboxylic acids. Diazo compounds: reactions of diazonium salts with the release of nitrogen, synthetic possibilities of reactions
CHAPTER 6. REACTIVITY OF CARBOXYLIC ACIDS AND THEIR FUNCTIONAL DERIVATIVESCHAPTER 6. REACTIVITY OF CARBOXYLIC ACIDS AND THEIR FUNCTIONAL DERIVATIVES
6.1. Carboxylic acids
6.1.1. General characteristics
Carboxylic acids are compounds that functional group in which the carboxyl group is -COOH.
Depending on the nature of the organic radical, carboxylic acids can be aliphatic(saturated or unsaturated) RCOOH and aromatic ArCOOH (Table 6.1). Based on the number of carboxylic groups, they are divided into monocarboxylic, dicarbonic and tricarboxylic. This chapter deals only with monocarboxylic acids.
The systematic nomenclature of acids is discussed above (see 1.2.1). For many acids, their trivial names are used (see Table 6.1), which are often preferable to systematic ones.
Carboxylic acids are polar due to the carboxyl group and can participate in the formation of intermolecular hydrogen bonds(see 2.2.3). Such bonds with water molecules explain the unlimited solubility of lower acids (C 1 -C 4). In carboxylic acid molecules, a hydrophilic part (carboxyl group COOH) and a hydrophobic part (organic radical R) can be distinguished. As the proportion of the hydrophobic part increases, solubility in water decreases. Higher carboxylic acids of the aliphatic series (starting from C 10) are practically insoluble in water. Carboxylic acids are characterized by intermolecular association. Thus, liquid carboxylic acids, such as acetic acid, exist in the form of dimers. In aqueous solutions, dimers break down into monomers.
Table 6.1.Monocarboxylic acids
An increase in the ability to associate when moving from aldehydes to alcohols and then to acids is reflected in changes in the boiling points of compounds of these classes with similar molecular weights.
6.1.2. Reaction centers in carboxylic acids
The chemical properties of carboxylic acids are determined primarily by the carboxyl group, which, unlike the previously studied functional groups (alcohol, carbonyl), has a more complex structure. Within the group itself there is a p,l conjugation as a result of the interaction of the p orbital of the oxygen atom of the OH group with the π bond of the C=O group (see also 2.3.1).
The carbonyl group in relation to the OH group acts as an electron acceptor, and the hydroxyl group, due to the +M effect, acts as an electron donor, supplying electron density to the carbonyl group. Features of the electronic structure of carboxylic acids determine the existence of several reaction centers (Scheme 6.1):
OH-acidic center due to strong polarization of the O-H bond;
The electrophilic center is the carbon atom of the carboxyl group;
N- basic center - oxygen atom of the carbonyl group with a lone pair of electrons;
A weak CH-acid center, which appears only in derivatives of acids, since the acids themselves have an incomparably stronger OH-acid center.
Scheme 6.1.Reaction centers in a carboxylic acid molecule
6.1.3. Acid properties
The acidic properties of carboxylic acids are manifested in their ability to abstract a proton. The increased mobility of hydrogen is due to the polarity of the O-H bond due to p,p- connections (see diagram 6.1). The strength of carboxylic acids depends on the stability of the carboxylate ion, RCOO, formed by proton abstraction. In turn, the stability of an anion is determined primarily by the degree of delocalization of the negative charge in it: the better the charge in the anion is delocalized, the more stable it is (see 4.2.1). In the carboxylate ion, the charge is delocalized along a p,π-conjugated system involving two oxygen atoms and is distributed equally between them
(see 2.3.1).
For carboxylic acids, the values of pl a lie in the range of 4.2-4.9. These acids are significantly more acidic than alcohols (pK a 16-18), phenols (pK a ~ 10) and thiols (pK a 11-12) (see Table 4.5).
The length and branching of the saturated alkyl radical does not have a significant effect on acid properties carboxylic acids. In general, aliphatic monocarboxylic acids have almost the same acidity (pK a 4.8-5.0), with the exception of formic acid, whose acidity is an order of magnitude higher.
The higher acidity of formic acid can be explained with the involvement of another factor affecting the stability of the anion, namelysolvation. IN aquatic environment the charge in the small HCOO formate ion is better delocalized with the participation of polar solvent molecules than in larger carboxylate ions.
It should be noted that aromatic acids are slightly higher than aliphatic acids in acidity (pK a of benzoic acid 4.2). In the delocalization of the charge in the benzoate ion, the benzene ring acts as a weak electron acceptor, without participating in conjugation with electrons that determine the negative charge.
The acidity of carboxylic acids is significantly influenced by the substituents introduced into the hydrocarbon radical. Regardless of the mechanism
transfer of the electronic influence of a substituent in a radical (inductive or mesomeric), electron-withdrawing substituents contribute to delocalization negative charge, stabilize anions and thereby increase acidity. Electron-donating substituents, on the contrary, reduce it.
In aqueous solutions, carboxylic acids are weakly dissociated.
Acidic properties are manifested by the interaction of carboxylic acids with alkalis, carbonates and bicarbonates. The salts formed in this process are hydrolyzed to a noticeable extent, so their solutions have an alkaline reaction.
6.1.4. Nucleophilic substitution
Nucleophilic substitution at the sp 2 -hybridized carbon atom of the carboxyl group represents the most important group of reactions of carboxylic acids.
The carbon atom of the carboxyl group carries a partial positive charge, i.e. it is an electrophilic center (see diagram 6.1). It can be attacked by nucleophilic reagents, resulting in the replacement of the OH group with another nucleophilic species.
The hydroxide ion is a poor leaving group, so nucleophilic substitution reactions at the carboxyl group are carried out in the presence of acid catalysts, especially when weak nucleophilic reagents such as alcohols are used.
The most important reactions of monocarboxylic acids are shown in Scheme 6.2.
Scheme 6.2.Some nucleophilic substitution reactions in carboxylic acids
The esterification reaction is catalyzed by strong acids.
Mechanism of esterification reaction. The catalytic effect of sulfuric acid is that it activates a carboxylic acid molecule, which is protonated at the main center - the oxygen atom of the carbonyl group (see Scheme 6.1). Protonation leads to an increase in the electrophilicity of the carbon atom. Mesomeric structures show delocalization of the positive charge in the resulting cation (I).
Next, the alcohol molecule, due to the lone pair of electrons of the oxygen atom, joins the activated acid molecule. Subsequent migration of the proton leads to the formation of a good leaving group - a water molecule. At the last stage, a water molecule is split off with the simultaneous release of a proton (catalyst return).
Esterification is a reversible reaction. Shifting the equilibrium to the right is possible by distilling off the resulting ether from the reaction mixture, distilling off or binding water, or using an excess of one of the reagents. The reverse reaction of esterification results in hydrolysis of the ester to form a carboxylic acid and an alcohol.
Formation of amides. When carboxylic acids are exposed to ammonia (gaseous or in solution), direct substitution of the OH group does not occur, but an ammonium salt is formed. Only with significant heating do dry ammonium salts lose water and turn into amides.
Formation of acid anhydrides. Heating carboxylic acids with phosphorus(V) oxide leads to the formation of acid anhydrides.
6.2. Functional derivatives of carboxylic acids
6.2.1. General characteristics
Functional derivatives of carboxylic acids contain a modified carboxyl group, and upon hydrolysis they form a carboxylic acid.
The most important functional derivatives of carboxylic acids are salts, esters, thioesters, amides, anhydrides (Table 6.2). Acid halides are the most reactive derivatives that are widely used in organic chemistry, however, they do not participate in biochemical transformations due to their extreme sensitivity to moisture, i.e., ease of hydrolysis.
Nomenclature.The names of derivatives of carboxylic acids are based on the relationship of their structures with the structure of the carboxylic acid itself, in which the common fragment is acyl radical RC(O)-. These radicals are called by substitution combination -oic acid on -oil. Trivial names of acyl radicals are given in table. 6.3.
Saltsacids are named by listing the names of the acid's anion and cation (in the genitive case), for example, potassium acetate. The names of acid anions are, in turn, formed by replacing the suffix -il in the name of the acyl radical on -at.
Estersare called similarly to salts, only instead of the name of the cation they use the name of the corresponding alkyl or aryl, which is placed before the name of the anion and written together
Table 6.2.Some functional derivatives of carboxylic acids
with him. The ester group COOR can also be expressed descriptively, for example, “R-ester of such and such acid.”
Table 6.3.Trivial names of acyl radicals and acid derivatives
Symmetrical anhydrides acids are named by replacing the word acid in the name acid on anhydride, for example benzoic anhydride.
Titles amides with an unsubstituted NH group 2 derived from the names of the corresponding acyl radicals by replacing the suffix -oil (or-il) on -amide. In N-substituted amides, the names of the radicals at the nitrogen atom are indicated before the name of the amide with the symbol N-(nitrogen).
6.2.2. Comparative characteristics reactivity
Derivatives of carboxylic acids, like the acids themselves, are capable of undergoing nucleophilic substitution reactions at the sp 2 -hybridized carbon atom to form other functional derivatives. The mechanism of such substitution differs from the mechanism of nucleophilic substitution at the sp 3 -hybridized carbon atom in haloalkanes and alcohols discussed above (see 4.3).
Tetrahedral mechanism of nucleophilic substitution. First, a nucleophile attaches to the carbon atom of the C=O group to form an unstable intermediate anion. The reaction mechanism is called tetrahedral, since the carbon atom is transferred from sp 2 - in sp 3 is a hybrid state and takes on a tetrahedral configuration.
At the second stage, the Z particle is split off from the intermediate and the carbon atom again becomes sp 2 hybridized. Thus, this substitution reaction involves the steps accession And splitting off.
According to this mechanism, the reaction proceeds in the presence of a sufficiently strong nucleophile and a good leaving group Z, for example, in the case of alkaline hydrolysis of esters and other functional derivatives of carboxylic acids. The ease of nucleophilic attack depends on the magnitude of the partial positive charge δ+ on the carbon atom of the carbonyl group. In functional derivatives of carboxylic acids, it increases with increasing -I-effect of the Z substituent and decreases with increasing its M-effect. As a result of these effects, the amount of charge and therefore the ability to undergo nucleophilic attack in the compounds in question decreases in the following sequence. An analysis of the stability of the leaving groups Z - , which are highlighted in color, leads to the same conclusion (see 4.2.1).
Carboxylic acid derivatives are less susceptible to nucleophilic attack than aldehydes and ketones, since the electrophilicity of the carbonyl carbon atom is usually reduced
due to the +M effect of the Z substituent. For this reason, in nucleophilic reactions of functional derivatives of carboxylic acids, acid catalysis is often necessary by protonation of the oxygen atom of the carbonyl group. An example of such activation is the esterification reaction already discussed (see 6.1.3).
As a result of the interaction of carboxylic acids and their functional derivatives with alcohols or amines, an acyl residue is introduced into the molecules of these compounds. In relation to such reactions, the general name is used - acylation reactions. From this position, the esterification reaction can be considered as the acylation of an alcohol molecule.
Functional acid derivatives have different reactivity in acylation reactions. The most active are acid chlorides and anhydrides; Almost any acid derivatives can be obtained from them. Acids and esters themselves (with aliphatic alcohol residues) are much less active acylating agents. Substitution reactions involving them are carried out in the presence of catalysts. Amides undergo acylation reactions even more difficult than acids and esters.
Salts of carboxylic acids do not have acylating ability, since the carboxylic acid anion cannot be attacked by a negatively charged nucleophile or molecule with a lone pair of electrons.
6.2.3. Esters
Esters are derivatives of acids widespread in nature. Many drugs contain ester groups in their structure.
In addition to the esterification reaction, esters are formed, much more easily, when alcohols or phenols are acylated with acid anhydrides.
Some ester reactions are shown in Scheme 6.3.
Scheme 6.3.Ester reactions
Esters can be hydrolyzed in both acidic and alkaline environment. As already mentioned (see 6.1.3), acid hydrolysis of esters is the reverse reaction of esterification. Although this reaction is reversible, acid hydrolysis can easily be made irreversible by using a large excess of water.
In the alkaline hydrolysis of esters, alkali acts as a reagent (1 mole of alkali is consumed per 1 mole of ester).
Alkaline hydrolysis of esters is an irreversible reaction, since the resulting carboxylate ion is not able to interact with the alkoxide ion (particles with charges of the same name). This hydrolysis is also called saponification esters. This term is due to the fact that salts of higher acids formed during alkaline hydrolysis of fats are called soaps.
6.2.4. Thioethers
Thioesters - sulfur analogues of esters - find very limited use in classical organic chemistry, but play an important role in the body. It is known that in order to exhibit catalytic activity, most enzymes of a protein nature require participation coenzymes, which are low-molecular organic compounds of non-protein nature, diverse in structure. One of the groups of coenzymes is
acyl coenzymes that act as acyl group carriers. Of these, the most common acetyl coenzyme A.
Despite the complexity of the structure of the acetyl coenzyme A molecule, from the standpoint of a chemical approach, it can be determined that this coenzyme functions as a thioester.
The thiol involved in its formation is coenzyme A(abbreviated CoASH), the molecule of which is built from the residues of three components - 2-aminoethanethiol, pantothenic acid and adenosine diphosphate (additionally phosphorylated at position 3 in the ribose fragment). Adenosine diphosphate (ADP) is discussed further as a representative of another important group of coenzymes - nucleoside polyphosphates (see 14.3.1). Pantothenic acid forms, on the one hand, an amide bond with 2-aminoethanethiol, and on the other, an ester bond with the ADP residue.
In terms of acylating ability, all acyl coenzymes A, including acetyl coenzyme A, being thioesters, occupy the “golden mean” between highly reactive anhydrides and low-active carboxylic acids and esters. Their rather high activity is due, in particular, to the increased stability of the leaving group - the CoA-S - anion - compared to the hydroxide and alkoxide ions of acids and esters, respectively.
Acetyl coenzyme A in vivo is a carrier of acetyl groups to nucleophilic substrates.
In this way, for example, acetylation of hydroxyl-containing compounds is carried out.
With the use of acetyl coenzyme A, choline is converted into acetylcholine, which is an intermediary in the transmission of nervous excitation in nerve tissues (neurotransmitter) (see 9.2 1).
In addition, we can note the important participation in metabolic processes of coenzyme A itself, functioning as a thiol. In the body, any carboxylic acids are activated by conversion into reactive derivatives - thioesters.
6.2.5. Amides and hydrazides
Along with esters, an important group of acid derivatives are amides of carboxylic acids, which are also widespread in nature. It is enough to mention peptides and proteins, the structure of which contains numerous amide groups.
Depending on the degree of substitution at the nitrogen atom, amides can be monosubstituted or disubstituted (see 6.2.1).
Amides are formed by the acylation of ammonia and amines with anhydrides or esters.
Amides have the lowest acylating ability and are much more difficult to hydrolyze than other acid derivatives. Hydrolysis of amides is carried out in the presence of acids or bases.
The high resistance of amides to hydrolysis is explained by the electronic structure of the amide group, which is in many ways similar to the structure of the carboxyl group. The amide group is a p,l-conjugated system in which the lone pair of electrons of the nitrogen atom is conjugated with the π-electrons of the C=O bond. Due to the strong +M effect of the amino group, the partial positive charge on the carbonyl carbon of amides is less than that of other functional acid derivatives. As a result, the carbon-nitrogen bond in amides has a partially double character.
A consequence of conjugation is also the extremely low basicity of the nitrogen atom of the amide group. On the contrary, amides develop weak acidic properties. Consequently, amides have amphoteric properties.
Amidas are related hydrazides- derivatives of carboxylic acids containing a hydrazine H residue 2 NNH 2. Quite a few medicinal
agents are hydrazides in nature, for example, the anti-tuberculosis drug isoniazid (see 13.4.1). Like amides, hydrazides undergo hydrolysis under fairly harsh conditions with cleavage of the C-N bond.
6.2.6. Anhydrides
Acid anhydrides are more common in vivo in the form mixed anhydrides, including acyl residues of various acids, one of the acids being inorganic (most often phosphoric).
Acyl phosphates are good acyl group carriers because phosphate groups are good leaving groups in nucleophilic substitution reactions.
Substituted acyl phosphates are metabolites with the participation of which the body transfers acyl residues to hydroxyl, thiol groups and amino groups of various compounds.
6.3. Sulfonic acids
and their functional derivatives
Sulfonic acids RSO 3 H can be considered as derivatives of hydrocarbons in which the hydrogen atom is replaced by a sulfo group SO 3 H. The best known are sulfonic acids of the aromatic series; their simplest representative is benzenesulfonic acid. Like sulfuric acid, sulfonic acids are highly acidic.
Sulfonic acids, like carboxylic acids, form functional derivatives - salts, esters, amides, etc.
Great value N-substituted amides have been acquired in medical practice sulfanyl(n-aminobenzenesulfonic) acid - sulfonamide agents (see 9.3).
Voltaren (ortofen, diclofenac sodium) can be considered the best of modern NSAIDs. It combines a pronounced anti-inflammatory effect with particularly good tolerability, which makes it possible long-term use drug.
In the digestive tract it is absorbed almost completely, the maximum concentration is reached after 1-2 hours. The drug is actively metabolized and is excreted in the urine and bile in the form of associated metabolic products (some of which also have anti-inflammatory properties). Plasma concentrations are proportional to the dose used. The drug accumulates in areas of inflammation, in particular in the synovial fluid during arthritis, where, unlike plasma, it remains for a long time (up to 7 hours) in an almost unchanged concentration (the concentration in the blood decreases significantly during this period). When prescribed to nursing women, it is practically undetectable in milk. With the simultaneous administration of acetylsalicylic acid and voltaren, the maximum concentration of the latter in plasma is reduced by approximately 30% compared to the administration of voltaren alone.
The inhibitory effect of voltaren on inflammation is apparently based on the active inhibition of prostaglandin synthesis. The drug is the most powerful prostaglandin synthetase inhibitor among modern NSAIDs. Since this inhibition is irreversible [Key E. et al., 1974], its anti-inflammatory effect lasts much longer than the high concentration of the drug in the body. Voltaren is also able to inhibit the action of a number of enzymes involved in the development inflammatory process, including lysosomal hydrolases. There is evidence of inhibition of neutral protease isolated from human granulocytes.
The peculiarity of the effect of voltaren can be considered to be such a quickly manifested analgesic effect that even assumptions are made about its partial independence from the anti-inflammatory effect itself. An important feature of the drug was established when studying the dynamics of spontaneous gonarthrosis in mice. It turned out that it reduces its frequency of development and severity of the process, while other NSAIDs (including indomethacin) aggravate this pathology.
This is possibly due to the fact that Voltaren, unlike other drugs, does not negatively affect cartilage metabolism; in particular, in the experiment it does not inhibit the incorporation of sulfur into cartilage proteoglycans.Voltaren is available in a variety of forms: enteric-coated tablets of 25 and 50 mg, slow-release tablets (Voltaren-retard) of 100 mg, suppositories of 50 and 100 mg, in ampoules for intramuscular administration of 75 mg. The drug is prescribed mainly orally in tablet form. The average therapeutic dose is 150 mg, less often - 100 mg; if necessary, the dose is increased to 200 mg. Maintenance doses can be 75-100 mg. Using the drug in suppositories (in the same doses) gives identical results. If you want to achieve a particularly quick effect during the first period of treatment, use intramuscular injections Voltaren (on its own or in addition to its oral administration or in suppositories) 75 mg 1-2 times a day. The pain decreases noticeably 10-45 minutes after the injection. During the period of maintenance therapy, Voltaren-retard is very convenient, which is used at a dose of 100 mg (i.e., 1 tablet) once a day; the drug gives the same effect as taking regular tablets of 25 mg 4 times a day.
Voltaren is most widely used for rheumatoid arthritis, in which case it can be used continuously for many months and years. In mild cases, significant improvement occurs with the use of this drug alone. In seriously ill patients, in accordance with the general principles of treatment of rheumatoid arthritis, treatment with voltaren can be successfully combined with any of the long-acting (basic) drugs. Very good results were also obtained in the treatment of patients with arthrosis. In patients with ankylosing spondylitis, Voltaren turned out to be as effective as previously thought the best drug indomethacin, and in terms of tolerability the advantage of voltaren is undeniable. In fairly high doses (150-200 mg/day), the drug is used to relieve an acute attack of gout; in this case, its intramuscular administration is especially justified.
For lately It has been established that in the treatment of acute rheumatism, Voltaren, like indomethacin, can have a therapeutic effect similar to that of prednisolone. This applies to all manifestations of the disease, including rheumatic carditis. It is important that the long-term results of prescribing these three drugs were the same. It was found, in particular, that during treatment with voltaren, complete reverse development of valvulitis can occur. The obvious therapeutic effect of voltaren was also noted in other variants of the course of rheumatism, including in a number of patients resistant to other drugs.Good results obtained from patients with so-called soft tissue rheumatism (scapulohumeral periarthritis, bursitis, tendonitis, tenosynovitis), as well as with radicular syndromes, including acute pain. In the latter cases, injections of the drug are indicated.
The drug is also successfully used for non-rheumatic diseases manifested by inflammation, pain and fever, in particular in patients with thrombophlebitis, adnexitis, infections (in combination with adequate anti-infective agents), in postoperative period, for bruises, etc. The therapeutic effect of voltaren is of great importance for chronic glomerulonephritis, established by G. Lagrue and G. Hirbe (1979), M. Sasdelli et al. (1980). These researchers believe that the drug improves the prognosis of the disease by slowing the rate of progression of kidney failure.
Voltaren is superior to all other NSAIDs in its tolerability. It essentially does not cause severe complications and, if necessary, is used almost constantly. The drug can be used with caution even for peptic ulcers, although, of course, it is advisable to prescribe it in suppositories.
Among the very rare side effects You should keep in mind mild headache, nausea, abdominal pain, urticaria, the appearance of red blood cells in the urine (apparently due to the weak anticoagulant effect characteristic of all NSAIDs). After reducing the dose or discontinuing the drug, these phenomena quickly disappear. There are no absolute contraindications to the use of Voltaren; can be considered a relative contraindication for oral administration peptic ulcer stomach and duodenum in the acute stage.Tolmetin (tolectin) is a fairly popular anti-rheumatic drug, which is 1-methyl-5p-toluoylpyrrole-2-acetic acid. In some details of the structural formula it resembles indomethacin. Tolmetin is completely absorbed in the digestive tract, the maximum concentration in the blood is observed after 30-40 minutes, the plasma half-purification period lasts about an hour. It is quickly excreted in the urine in the form of glucuronides and inactive metabolites. The mechanism of therapeutic action has not been sufficiently studied; the main importance is given to the inhibition of prostaglandin synthesis.
Available in 200 mg tablets. There are reports of a clear positive effect in patients with rheumatoid arthritis, ankylosing spondylitis, osteoarthritis, glenohumeral periarthritis, epicondylitis, etc. The possibility of long-term use of the drug has been proven, in particular for rheumatoid arthritis up to 2 ½ years. Good results were obtained in the treatment of patients with juvenile rheumatoid arthritis. As is known, new non-steroidal drugs are rarely studied in this disease. It turned out that in terms of the results achieved, tolmetin is not inferior acetylsalicylic acid, which is still considered the standard anti-inflammatory drug in the treatment of juvenile rheumatoid arthritis. However, despite the unanimously positive overall assessment of tolmetin, this drug belongs to drugs whose analgesic properties prevail over anti-inflammatory ones.
The drug is well tolerated both for short-term and long-term use. Uncommon side effects include epigastric pain, nausea, vomiting, headache, dizziness, tinnitus, skin rashes, fluid retention, and increased blood pressure.
The daily dose is 800-1600 mg (usually about 1200 mg). The drug is often taken 4 times a day due to its rapid elimination from the body.
1. Classification of carboxylic acids.
2. Nomenclature, receipt.
3. Isomerism, structure.
4. Monocarboxylic acids (saturated, unsaturated, aromatic).
5. Dicarboxylic acids.
6. Derivatives of carboxylic acids.
Hydrocarbon derivatives containing the carboxyl group -COOH are called carboxylic acids.
Carboxylic acids are classified according to two structural characteristics:
a) according to the nature of the radical, there are aliphatic R(COOH)n (saturated, unsaturated) and aromatic acids Ar(COOH)n;
b) according to the number of carboxyl groups, they distinguish between monocarboxylic (n = 1), di- and polycarboxylic (n ≥ 2) acids.
Nomenclature. According to the IUPAC nomenclature, the names of acids are formed from the name of the hydrocarbon, adding the ending - oic acid, for example, CH 3 COOH - ethanoic acid. Trivial names of acids are widespread: acetic, butyric, oleic, tartaric, oxalic, etc.
Receipt.
a) O oxidation of alkenes, alkynes, primary alcohols and aldehydes (see " Chemical properties» corresponding connection classes):
R-CH = CH-CH 3 + [O] → R-COOH + CH 3 -COOH
R-CH 2 -OH + [O] → R-CH=O + [O] → R-COOH
alcohol aldehyde acid
Oxidizing agents - KMnO 4, K 2 Cr 2 O 7 in an acidic environment.
b) Oxidation of alkanes: R-CH 2 -CH 2 -R" + [O] → R-COOH + R"-COOH + H 2 O Oxidation is carried out in the presence of catalysts - cobalt or manganese salts.
V) Oxidation of alkylbenzenes (see “Chemical properties of aromatic hydrocarbons”). G) Hydrolysis of nitriles, derivatives of carboxylic acids in an acidic or alkaline environment: R-C≡N + 2H 2 O + HCl → R-COOH + NH 4 Cl
R-C≡N + H 2 O + NaOH → R-COONa + NH 3
X: -OR, -Hal, -OCOR, -NH 2.
d ) Organometallic synthesis:
Structure. The carbon and oxygen atoms of the carboxyl group are in a state of sp 2 hybridization. σ- the C-O bond is formed by the overlap of sp 2 -sp 2 hybridized orbitals, σ- O-H bond - overlapping sp 2 - s-orbitals, π- C-O bond - by overlapping unhybridized p-p orbitals. The carboxyl group is planar p,π- coupled system:
As a result of pairing S-O connection becomes shorter compared to a similar bond in alcohols, the C=O bond becomes longer compared to a similar bond in carbonyl compounds, i.e. there is a noticeable alignment of bond lengths in the carboxyl group.
The intermolecular interaction of carboxylic acids is characterized by strong hydrogen bonds, resulting in the formation of linear associates and cyclic dimers:
And 
The hydrogen bond in carboxylic acids is stronger than in alcohols. This determines the higher solubility in water, boiling and melting points of carboxylic acids compared to alcohols of similar molecular weight.
The mutual influence of the carbonyl and hydroxyl groups in the carboxyl group determines chemical properties that differ from the properties of carbonyl compounds and alcohols. Reactions involving a carboxyl group proceed along the following main directions: acid-base interaction, nucleophilic substitution, decarboxylation.
The chemical properties of carboxylic acids are discussed below using the example of saturated monocarboxylic acids.
Monocarboxylic acids(saturated, unsaturated, aromatic acids).
General molecular formula saturated monocarboxylic acids
СnН2nО2.
Table 4.
Homologous series of saturated monocarboxylic acids
|
T pl., С |
T kip. , С |
Acyl residue - acid residue |
||
|
Ant (methane) |
formyl - formates |
|||
|
Vinegar (ethane) |
acetyl - acetates |
|||
|
propionic (propane) |
CH3-CH2-COOH |
propionyl - propionates |
||
|
oil (butane) |
CH3-(CH2)2-COOH |
butyryl - butyrates |
||
|
valerian |
CH3-(CH2)3-COOH |
valeryl - valerates |
||
|
nylon |
CH3-(CH2)4-COOH |
capronoyl |
||
|
lauric |
CH3-(CH2)10-COOH | |||
|
palmitic |
CH3-(CH2)14-COOH |
palmityl palmitates |
||
|
stearic |
CH3-(CH2)16-COOH |
stearyl - stearates |
The table shows the names of acyl (R-CO-) and acidic (R-COO-) residues of some monocarboxylic acids of the limiting series.
Isomerism. Saturated monocarboxylic acids are characterized by structural isomerism (different structure of the carbon chain and different arrangement of the functional group). For example, the molecular formula C 4 H 8 O 2 corresponds to the following isomers: CH 3 -CH 2 -CH 2 -COOH (butanoic acid), (CH 3) 2 CH-COOH (2-methylpropanoic or isobutanoic acid), CH 3 -CH 2 -COOCH 3 (methylpropanoate) (for details, see the “Isomerism” section).
Physical properties. Acids with the number of carbon atoms from 1 to 9 are colorless liquids with unpleasant odors; those with C≥ 10 are odorless solids. Acids with the number of carbon atoms from 1 to 3 are highly soluble in water, with C≥ 4 - substances insoluble in water, but highly soluble in organic solvents (alcohol, ether).
Chemical properties.
a) acidic properties
Aqueous solutions of carboxylic acids have an acidic reaction:
acid carboxylate ion
Delocalization of electron density ( p,π- conjugation) in the carboxylate ion leads to a complete alignment of the orders of length of both C-O bonds, increasing its stability compared to alcoholate and phenolate ions. Therefore, carboxylic acids are stronger than alcohols and phenols, carbonic acid, but inferior to such mineral acids as hydrochloric, sulfuric, nitric and phosphoric.
The strength of carboxylic acids is significantly influenced by the nature of the radical at the carboxyl group: electron-donating groups destabilize the carboxylate ion and, therefore, reduce acidic properties, electron-withdrawing groups stabilize the carboxylate ion and increase acidic properties.
In the homologous series of saturated monocarboxylic acids, with an increase in the number of carbon atoms in the acid composition, the acidic properties decrease. The strongest acid is formic acid.
Carboxylic acids form salts when interacting with active metals, metal oxides, bases, and salts. For example, CH 3 -COOH + Na 2 CO 3 → CH 3 -COONa + CO 2 + H 2 O
Salts of lower carboxylic acids are highly soluble in water, while salts of higher ones are soluble only in sodium and potassium salts. Salts of carboxylic acids and alkali metals undergo hydrolysis and their aqueous solutions have an alkaline environment:
R-COO - Na + + HOH ↔ R-COOH + NaOH
Salts of carboxylic acids are used to obtain derivatives of carboxylic acids, hydrocarbons, and surfactants.
Sodium and potassium salts of higher fatty acids- soap. Ordinary solid soap is a mixture of sodium salts of various acids, mainly palmitic and stearic: C 15 H 31 COONa (sodium palmitate) and C 17 H 35 COONa (sodium stearate). Potassium soaps are liquid.
In ancient times, soap was made from fat and beech ash. During the Renaissance, they returned to the forgotten craft, the recipes were kept secret. Nowadays soaps are produced mainly from vegetable and animal fats.
Soaps are surfactants, a chemical hybrid consisting of a hydrophilic (carboxylate ion) and a hydrophobic (fear) end (hydrocarbon radical). Soaps sharply reduce the surface tension of water, cause wetting of particles or surfaces that have a water-repellent effect, and promote the formation of stable foam.
In hard water, the washing ability of soap sharply decreases; soluble sodium or potassium salts of higher fatty acids enter into an exchange reaction with soluble acidic carbonates of alkaline earth metals, mainly calcium, present in hard water:
2C 15 H 31 COONa + Ca(HCO 3) 2 → (C 15 H 31 COO) 2 Ca + 2NaHCO 3
The resulting insoluble calcium salts of higher fatty acids form precipitates.
Huge quantities of soap are used in everyday life for hygienic purposes, for washing, etc., as well as in various industries, especially for washing wool, fabrics and other textile materials.
b) nucleophilic substitution- S N (formation of functional derivatives of carboxylic acids)
The main type of reactions of carboxylic acids is nucleophilic substitution at the sp 2 -hybridized carbon atom of the carboxyl group, as a result of which the hydroxyl group is replaced by another nucleophile. Due to r,π-s Since in the carboxyl group the mobility of the hydroxyl group is much lower compared to alcohols, therefore nucleophilic substitution reactions are carried out in the presence of a catalyst - a mineral acid or alkali.
The reactions are accompanied by the formation of functional derivatives of carboxylic acids - acid halides (1), anhydrides (2), esters (3), amides (4):
V)decarboxylation
Decarboxylation is the removal of a carboxyl group in the form of CO 2 . Depending on the reaction conditions, compounds of different classes are formed. Electron-withdrawing groups in the radical at the carboxyl group facilitate the occurrence of reactions of this type.
Examples of decarboxylation reactions:
1) thermal decomposition of sodium or potassium salts in the presence of soda lime
R-COONa + NaOH → R-H + Na 2 CO 3
2) thermal decomposition of calcium or barium salts
R-COO-Ca-OOS-R → R-CO-R + CaCO 3
3) electrolysis of sodium or potassium salts (Kolbe synthesis)
2R-COONa + 2НН → R-R + 2NaОН +2CO 2 + Н 2
d) replacement of hydrogen atomsα-carbon atom
Halogen atom in α -halogenated acids are easily replaced by nucleophilic reagents. Therefore, α-halogen-substituted acids are starting materials in the synthesis of a wide range of substituted acids, including α-amino and α-hydroxy acids:
propionic acid α-chloropropionic acid
As a result of the influence of the halogen atom on the carboxyl group, halogenated acids (for example, trichloroacetic acid) are many times stronger acids and approach strong inorganic acids in this regard.
e) specific properties of formic acid
In the composition of formic acid, along with the carboxyl group, a carbonyl group can be distinguished, therefore formic acid exhibits the properties of both carboxylic acids and aldehydes:
1. oxidation
HCOOH + [O]→ CO 2 + H 2 O
oxidizing agents: Cu(OH) 2, OH ("silver mirror" reaction)
2. dehydration
HCOOH + H 2 SO 4 (conc.) → CO + H 2 O
Occurrence in nature and use of acids:
a) formic acid- colorless liquid with a pungent odor, miscible with water. It was first isolated in the 17th century from red ants by steam distillation. In nature, free formic acid is found in the secretions of ants, in nettle juice, and in the sweat of animals. In industry, formic acid is produced by passing carbon monoxide through heated alkali:
NaOH + CO → H-COONa
H-COONa + H 2 SO 4 → H-COOH + NaHSO 4
Formic acid is used in dyeing fabrics, as a reducing agent, and in various organic syntheses.
b) acetic acid
Anhydrous acetic acid (glacial acetic acid) is a colorless liquid with a characteristic pungent odor and sour taste, freezes at a temperature of +16 0 C, forming a crystalline mass resembling ice. A 70-80% aqueous solution of acid is called acetic essence.
It is widespread in nature, found in animal excretions, in plant organisms, and is formed as a result of fermentation and putrefaction processes in sour milk, cheese, souring wine, cooking butter, etc. They are used in the food industry as a flavoring and preservative, widely in the production of artificial fibers, solvents, and in the production of medicines.
c) butyric acid- colorless liquid, acid solutions have bad smell old butter and sweat. Occurs in nature in the form of esters; esters of glycerol and butyric acid are part of fats and butter. Used in organic synthesis to obtain aromatic esters.
c) isovaleric acid - colorless liquid with a pungent odor, in diluted solutions it has the smell of valerian. Found in the roots of valerian, it is used to obtain medicinal substances and essences.
d) palmitic, stearic acids
These are solids with faint odors and are poorly soluble in water. Widely distributed in nature, they are found in fats in the form of esters with glycerol. Used to produce suppositories and surfactants.
Unsaturated acids
Unsaturated acids are carboxylic acids containing multiple bonds (double or triple) in the hydrocarbon radical. The most important are unsaturated mono- and dicarboxylic acids with double bonds.
Nomenclature and isomerism.
The names for unsaturated acids are compiled according to the IUPAC nomenclature, but most often trivial names are used:
CH 2 =CH-COOH - 2-propenoic or acrylic acid
CH 3 -CH=CH-COOH - 2-butenoic or crotonic acid
CH 2 =C(CH 3)-COOH - 2-methylpropenoic or methacrylic acid
CH 2 =CH-CH 2 -COOH - 3-butenoic or vinyl acetic acid
CH 3 -(CH 2) 7 -CH=CH-(CH 2) 7 -COOH - oleic acid
CH 3 -(CH 2) 4 -CH=CH-CH 2 -CH=CH-(CH 2) 7 -COOH - linoleic acid
CH 3 -CH 2 -CH=CH-CH 2 -CH=CH-CH 2 -CH=CH-(CH 2) 7 -COOH-linolenic acid.
The structural isomerism of unsaturated acids is due to the isomerism of the carbon skeleton (for example, crotonic and methacrylic acids) and the isomerism of the position of the double bond (for example, crotonic and vinyl acetic acids).
Unsaturated acids with a double bond, as well as ethylene hydrocarbons, are also characterized by geometric or cis-trans isomerism.
Chemical properties. In terms of chemical properties, unsaturated acids are similar to mono- and dicarboxylic acids, but have a number of distinctive features due to the presence of multiple bonds and a carboxyl group in the molecule and their mutual influence.
Unsaturated acids, especially those containing a multiple bond in the α-position to the carboxyl group, are stronger acids than saturated acids. Thus, unsaturated acrylic acid (K=5.6*10 -5) is four times stronger than propionic acid (K=1.34*10 -5).
Unsaturated acids enter into all reactions at the site of multiple bonds characteristic of unsaturated hydrocarbons.
A)Eelectrophilic addition:
1. halogenation
β CH 2 = α CH-COOH + Br 2 → CH 2 Br-CHBr-COOH
propenoic acid α,β-dibromopropionic acid
This is a qualitative reaction to unsaturated acids; by the amount of halogen (bromine or iodine) consumed, the number of multiple bonds can be determined .
2. hydrohalogenation
α CH 2 δ+ = β CH δ- →COOH+ H δ+ - Br δ- → CH 2 Br-CH 2 -COOH
For α,β-unsaturated acids, the addition reaction proceeds against Markovnikov's rule.
b)Ghydrogenation
In the presence of catalysts (Pt, Ni), hydrogen is added at the site of the double bond and unsaturated acids become saturated:
CH 2 =CH-COOH + H 2 → CH 3 -CH 2 -COOH
acrylic acid propionic acid
Hydrogenation process ( hydrogenation) has great practical importance, especially for the conversion of higher unsaturated fatty acids into saturated fatty acids; This is the basis for the transformation of liquid oils into solid fats.
V)ABOUTacidification
Under the conditions of the Wagner reaction (see “Alkenes”), unsaturated acids are oxidized to dihydroxy acids, and during vigorous oxidation, to carboxylic acids.
a) acrylic CH 2 =CH-COOH and methacrylic CH 2 =C(CH 3 )-COOH acid - colorless liquids with pungent odors. Acids and their methyl esters easily polymerize, which is the basis for their use in the polymer materials industry (organic glass).
Acrylic acid nitrile - acrylonitrile CH 2 =CH-C≡N is used in the production of synthetic rubber and high-molecular polyacrylonitrile (PAN) resin, from which synthetic fiber nitron (or orlon) is produced - one of the types of artificial wool.
b) higher unsaturated acids
-cis-oleic acid in the form of an ester with glycerin is part of almost all fats of animal and vegetable origin, the content of oleic acid in olive (“Provence”) oil is especially high - up to 80%, potassium and sodium salts of oleic acid are soaps;
-cis, cis-linoleic and cis, cis- Linolenic acid in the form of an ester with glycerin is part of many vegetable oils, for example soybean, hemp, and flaxseed oil. Linoleic and linolenic acids are called essential acids because they are not synthesized in the human body. It is these acids that have the greatest biological activity: they are involved in the transfer and metabolism of cholesterol, the synthesis of prostaglandins and other vital substances, maintain the structure of cell membranes, are necessary for the functioning of the visual apparatus and nervous system, and affect the immune system. The absence of these acids in food inhibits the growth of animals, inhibits their reproductive function, and causes various diseases.
Acid esters are used in the production of varnishes and paints (drying oils).
Aromatic monocarboxylic acids
TO 
Islots are colorless crystalline substances, some of them have a faint, pleasant odor. They are characterized by a conjugate (π, π) system:
The most important representatives:
benzoic acid
phenylacetic acid
trance-cinnamic acid 
Aromatic acids are stronger acids than saturated acids (except formic acid). Acids of this type are characterized by all reactions of saturated carboxylic acids in the carboxyl group and reactions of electrophilic substitution in the benzene ring (the carboxyl group is a substituent of the 2nd kind, m-orientator).
Occurrence in nature and use of acids:
Aromatic acids are used to produce dyes, fragrances and medicinal substances; acid esters are found in essential oils, resins and balms. Benzoic acid and its sodium salt are found in the fruits of viburnum, rowan, lingonberries, cranberries, give them a bitter taste, have bactericidal properties, and are widely used in food preservation.
O-sulfobenzoic acid amide is called saccharin, it is 400 times sweeter than sugar.
Derivatives of carboxylic acids.
General formula of carboxylic acid derivatives:
Where X: - Hal, -OOS-R, -OR, -NH 2.
For derivatives of carboxylic acids, nucleophilic substitution reactions (S N) are most characteristic. Since the products of these reactions contain an acyl group R-C=O, the reactions are called acylation, and carboxylic acids and their derivatives are called acylating reagents.
In general, the acylation process can be represented by the following scheme: 
According to their acylating ability, derivatives of carboxylic acids are arranged in the following series:
salt< амиды < сложные эфиры <ангидриды <галогенангидриды
In this series, previous members can be obtained from subsequent ones by acylation of the corresponding nucleophile (for example, alcohol, ammonia, etc.). All functional derivatives can be obtained directly from acids and are converted to them by hydrolysis.
Amides, unlike other derivatives of carboxylic acids, form intermolecular hydrogen bonds and are solids (formic acid amide HCONH 2 is a liquid).
Esters
Receipt methods. The main method for producing esters is through nucleophilic substitution reactions:
a) esterification reaction R-CO HE + RABOUT-H ↔ R-CO-O R + H 2 O
The reaction is carried out in the presence of a catalyst - mineral acid. Esterification reactions are reversible. To shift the equilibrium towards the formation of an ester, an excess of one of the reactants or the removal of products from the reaction sphere is used.
b) acylation of alcohols with acid halides and anhydrides
c) from salts of carboxylic acids and alkyl halides
R-COONa + RCl → RCOOR + NaCl Nomenclature. According to IUPAC nomenclature, the names of esters are as follows:
CH 3 -CH 2 -CH 2 -WITH O-O CH 3
hydrocarbon radical
radical + hydrocarbon + oate - methyl butanoate.
If trivial names of acyl residues are indicated, then the name of this ester - methyl butyrate. Esters can be called by radical functional nomenclature - butyric acid methyl ester.
Physical properties. Esters are colorless liquids, insoluble in water and have low boiling and melting points compared to parent acids and alcohols, which is due to the absence of intermolecular hydrogen bonds in esters. Many esters have a pleasant odor, often the smell of berries or fruits (fruit essences).
Chemical properties. For esters, the most characteristic reactions are nucleophilic substitution (S N), occurring in the presence of an acid or base catalyst. The most important S N reactions are hydrolysis, ammonolysis and transesterification.
Acid hydrolysis of esters is a reversible reaction, alkaline hydrolysis is irreversible.
RCOOR + H 2 O(H +) ↔ RCOOH + ROH
RCOOR + NaOH → RCOO - Na + + ROH
Fats
Fats (triglycerides) are esters formed by glycerol and higher saturated and unsaturated acids.
Several dozen different saturated and unsaturated acids have been isolated from fats; almost all of them contain unbranched chains of carbon atoms, the number of which is usually even and ranges from 4 to 26. However, it is the higher acids, mainly with 16 and 18 carbon atoms, that are the main component of all fats. Of the saturated higher fatty acids, the most important are palmitic C 15 H 31 COOH and stearic C 17 H 35 COOH; among unsaturated fatty acids - oleic C 17 H 33 COOH (with one double bond), linoleic C 17 H 31 COOH (with two double bonds) and linolenic C 17 H 29 COOH (with three double bonds). Unsaturated acids containing a fragment (-CH 2 -CH=CH-) in the radical are called essential.
Simple triglycerides contain residues of identical fatty acids and mixed residues of different fatty acids. The names are based on the names of the acyl residues included in their composition of fatty acids:
tripalmitin dioleostearin
The importance of fats is extremely high. First of all, they are the most important component of human and animal food, along with carbohydrates and proteins. Vegetable oils have the greatest nutritional value, which, along with essential fatty acids, contain phospholipids, vitamins, and beneficial phytosterols (precursors of vitamin D) necessary for the body. The daily requirement of an adult for fats is 80-100g.
Fats are practically insoluble in water, but are highly soluble in alcohol, ether and other organic solvents. The melting point of fats depends on what acids they contain. Fats containing predominantly residues of saturated acids (animal fats - beef, lamb or lard) have the highest T pl. and are solid or ointment-like substances. Fats containing predominantly residues of unsaturated acids (vegetable oils - sunflower, olive, flaxseed, etc.), liquids with lower melting points.
Chemical properties triglycerides are determined by the presence of an ester bond and unsaturation:
a) hydrogenation (hydrogenation) of fats
The addition of hydrogen at the site of double bonds in acid residues is carried out in the presence of a catalyst - finely crushed metallic nickel at 160-240 0 C and a pressure of up to 3 atm. In this case, liquid fats and oils are converted into solid saturated fats - lard, which is widely used in the production of margarine, soap, and glycerin.
b) hydrolysis of fats
Alkaline hydrolysis (saponification) of fats produces salts of fatty acids (soaps) and glycerol, while acid hydrolysis produces fatty acids and glycerol.
c) addition and oxidation
Trilglycerides containing unsaturated fatty acid residues undergo addition reactions at the double bond (bromination, iodination) and oxidation with potassium permanganate. Both reactions allow you to determine the degree of unsaturation of fats.
All fats are flammable substances. When they burn, a large amount of heat is released: 1 g of fat when burned gives 9300 cal.
Did you know that
In 1906, Russian scientist S.A. Fokin developed it, and in 1909. He also implemented the method of hydrogenation (hardening) of fats on an industrial scale.
Margarine (from Greek - “pearls”) was obtained in 1869. Its various varieties are obtained by mixing lard with milk, and in some cases with egg yolk. The resulting product is reminiscent of butter in appearance; the pleasant smell of margarine is achieved by introducing special flavors into its composition - complex compositions of various substances, an indispensable component of which is diacetyl (butanedione), a yellow liquid found in cow butter.
However, there are also animal fats that contain a significant amount of unsaturated acids and are liquid substances (blue, cod oil or fish oil).
Vegetable fats and oils are extracted from the seeds and pulp of fruits of various plants. They are distinguished by a high content of oleic and other unsaturated acids and contain only a small amount of stearic and palmitic acids (sunflower, olive, cottonseed, linseed and other oils). Only in some vegetable fats are saturated acids predominant, and they are solids (coconut oil, cocoa butter, etc.).
Esters of fruit essences have a pleasant smell of fruits and flowers, for example isoamyl acetate - the smell of pears, amyl formate - cherries, ethyl formate - rum, isoamyl butyrate - pineapples, etc. They are used in the confectionery industry, in the production of soft drinks, and in perfumery.
An extremely valuable synthetic material - organic glass (plexiglass) - is prepared from polymethyl methacrylate. The latter is superior to silicate glass in transparency and ability to transmit UV rays. It is used in mechanical and instrument making, in the manufacture of various household and sanitary items, dishes, jewelry, and watch glasses. Due to its physiological indifference, polymethyl methacrylate has found application in the manufacture of dentures, etc.
Vinyl acetate is an ester of vinyl alcohol and acetic acid. It is obtained, for example, by passing a mixture of acetic acid and acetylene vapors over cadmium and zinc acetates at 180-220 o C:
CH 3 -COOH + CH≡CH → CH 3 -CO-O-CH=CH 2
Vinyl acetate is a colorless liquid that easily polymerizes, forming a synthetic polymer - polyvinyl acetate (PVA), used for the manufacture of varnishes, adhesives, and artificial leather.
Dicarboxylic acids
Dicarboxylic acids contain two carboxyl groups. The best known are linear acids containing from 2 to 6 carbon atoms:
NOOS-COON - ethane diova (IUPAC nomenclature) or oxalic acid (trivial nomenclature)
NOOS-CH 2 -COOH - propanedioic or malonic acid
NOOS-CH 2 -CH 2 -COOH - butanedioic or succinic acid
NOOS-CH 2 -CH 2 -CH 2 -COOH - pentanedioic or glutaric acid
NOOS-CH 2 -CH 2 -CH 2 -COOH - adipinoic acid
Physical properties. Dibasic acids are crystalline substances with high melting points, and for acids with an even number of carbon atoms it is higher; lower acids are soluble in water.
Chemical properties. In terms of chemical properties, dibasic acids are similar to monocarboxylic acids, but have a number of distinctive features due to the presence of two carboxyl groups in the molecules and their mutual influence.
Dicarboxylic acids are stronger acids than monocarboxylic acids with the same number of carbon atoms: Kion. oxalic acid (H 2 C 2 O 4) - 5.9 10 -2, 6.4 10 -5, acetic acid - 1.76 10 -5. As the distance between carboxylic groups increases, the acidic properties of dicarboxylic acids decrease. Dicarboxylic acids can form two series of salts - acidic, for example HOOC-COONa, and average - NaOOC-COONa.
Dicarboxylic acids have a number of specific properties, which are determined by the presence of two carboxyl groups in the molecule. For example, the ratio of dicarboxylic acids to heat.
Transformations of dicarboxylic acids upon heating depend on the number of carbon atoms in their composition and are determined by the possibility of the formation of thermodynamically stable five- and six-membered cycles.
When oxalic and malonic acids are heated, decarboxylation occurs to form monocarboxylic acids:
HOOC-CH 2 -COOH → CH 3 -COOH + CO 2
When heated, succinic and glutaric acids easily split off water to form five- and six-membered cyclic anhydrides:
When heated, adipic acid decarboxylates to form a cyclic ketone - cyclopentanone:
Dicarboxylic acids react with diamines and diols to form polyamides and polyesters, respectively, which are used in the production of synthetic fibers.
Along with saturated dicarboxylic acids, unsaturated, aromatic dicarboxylic acids are known.
Occurrence in nature and use of acids:
Oxalic acid widespread in the plant world. It is found in the form of salts in the leaves of sorrel, rhubarb, and sorrel. In the human body it forms sparingly soluble salts (oxalates), for example calcium oxalate, which are deposited in the form of stones in the kidneys and bladder. Used as a bleaching agent: removing rust, paints, varnish, ink; in organic synthesis.
Malonic acid (esters and salts - malonoates) found in some plants, such as sugar beets. Widely used in organic synthesis for the production of carboxylic acids.
Succinic acid (salts and esters are called succinates) participates in metabolic processes occurring in the body. It is an intermediate compound in the tricarboxylic acid cycle. In 1556, the German alchemist Agricola first isolated amber from the products of dry distillation. The acid and its anhydride are widely used in organic synthesis.
Fumaric acid (HOOC-CH=CH-COOH - trans- butenedioic acid), unlike cis- maleic , widespread in nature, found in many plants, many in mushrooms, and participates in the metabolic process, in particular in the tricarboxylic acid cycle.
Maleic acid(cis- butenedioic acid) does not occur in nature. The acid and its anhydride are widely used in organic synthesis.
Ortho-phthalic acid, acid derivatives - phthalic anhydride, esters - phthalates (repellents) are widely used.
Terephthalic acid is a large-scale industrial product used to produce a number of polymers - for example, lavsan fiber, polyethylene terephthalate (PET), from which plastic dishes, bottles, etc. are made.
Organic compounds containing a carboxyl group –COUN, belong to the class of acids.
Biologically important carboxylic acids:
| Acids (trivial name) | Anion name | Acid formula |
| Monobase | ||
| ant | formate | HCOOH |
| vinegar | acetate | CH3COOH |
| oil | butyrate | CH3(CH2)2COOH |
| valerian | valerate | CH3(CH2)3COOH |
| Unsaturated acids | ||
| acrylic | acrylates | CH 2 = CH-COOH |
| croton | crotonate | CH 3 – CH = CH - COOH |
| Aromatic | ||
| benzoin | benzoate | C6H5COOH |
| Dicarboxylic acids | ||
| oxalic acid | oxalates | NOOS - SOON |
| malonova | malonates | NOOS-CH 2 - COOH |
| amber | succinates | NOOS-CH 2 – CH 2 -COOH |
| glutaric | glutarates | NOOS –(CH 2) 3 - COOH |
| Unsaturated dicarbonate | ||
| Fumaric (trans isomer) | fumarates | HOOC-CH=CH-COOH |
Acidic properties of carboxylic acids:
RCOOH RCOO - + H +
Upon dissociation, a carboxylate anion is formed, in which the negative charge is evenly distributed between the oxygen atoms, which increases the stability of this particle. The strength of carboxylic acids depends on the length of the radical (the larger the radical, the weaker the acid) and the substituents (electron-withdrawing substituents increase acidity). CI 3 COOH is much stronger than CH 3 COOH. Dicarboxylic acids are stronger than monobasic acids.
Functional derivatives of carboxylic acids:
Carboxylic acids exhibit high reactivity. They react with various substances and form functional derivatives, i.e. compounds obtained as a result of reactions at the carboxyl group.
1. Formation of salts. Carboxylic acids have all the properties of ordinary acids. They react with active metals, basic oxides, bases and salts of weak acids:
2RCOOH + Mg → (RCOO) 2 Mg + H 2,
2RCOOH + CaO → (RCOO) 2 Ca + H 2 O,
RCOOH + NaOH → RCOONa + H 2 O,
RCOOH + NaHCO 3 → RCOONa + H 2 O + CO 2.
Carboxylic acids are weak, so strong mineral acids displace them from the corresponding salts:
CH 3 COONa + HCl → CH 3 COOH + NaCl.
Salts of carboxylic acids in aqueous solutions are hydrolyzed:
CH 3 COOC + H 2 O CH 3 COOH + CON.
The difference between carboxylic acids and mineral acids is the possibility of forming a number of functional derivatives.
2. Formation of functional derivatives of carboxylic acids. When the OH group in carboxylic acids is replaced by various groups (X), functional derivatives of acids are formed with the general formula R-CO-X; here R means an alkyl or aryl group. Although nitriles have a different general formula (R-CN), they are usually also considered to be derivatives of carboxylic acids, since they can be prepared from these acids.
Acid chlorides obtained by the action of phosphorus chloride (V) on acids:
R-CO-OH + PCl 5 → R-CO-Cl + POCl 3 + HCl.
Anhydrides are formed from carboxylic acids under the action of water-removing agents:
2R-CO-OH + P 2 O 5 → (R-CO-) 2 O + 2HPO 3.
Esters are formed by heating an acid with an alcohol in the presence of sulfuric acid (reversible esterification reaction):
Esters can also be obtained by reacting acid chlorides and alkali metal alcoholates:
R-CO-Cl + Na-O-R" → R-CO-OR" + NaCl.
Amides are formed by the reaction of carboxylic acid chlorides with ammonia:
CH 3 -CO-Cl + NH 3 → CH 3 -CO-NH 2 + HCl.
In addition, amides can be prepared by heating ammonium salts of carboxylic acids: t o
CH 3 -COONH 4 → CH 3 -CO-NH 2 + H 2 O
When amides are heated in the presence of dewatering agents, they dehydrate to form nitriles:
CH 3 -CO-NH 2 → CH 3 -C≡N + H 2 O
3. Properties of carboxylic acids due to the presence of a hydrocarbon radical. Thus, when halogens act on acids in the presence of red phosphorus, halogen-substituted acids are formed, and the hydrogen atom at the carbon atom (α-atom) adjacent to the carboxyl group is replaced by halogen: p cr.
CH 3 -CH 2 -COOH + Br 2 → CH 3 -CHBr-COOH + HBr
4. Unsaturated carboxylic acids capable of addition reactions:
CH 2 = CH-COOH + H 2 → CH 3 -CH 2 -COOH,
CH 2 = CH-COOH + Cl 2 → CH 2 Cl-CHCl-COOH,
CH 2 = CH-COOH + HCl → CH 2 Cl-CH 2 -COOH,
CH 2 = CH-COOH + H 2 O → HO-CH 2 -CH 2 -COOH,
The last two reactions proceed against Markovnikov's rule.
Unsaturated carboxylic acids and their derivatives are capable of polymerization reactions.
5. Redox reactions of carboxylic acids:
Carboxylic acids, under the action of reducing agents in the presence of catalysts, can be converted into aldehydes, alcohols and even hydrocarbons.
Formic acid HCOOH differs in a number of features, since it contains an aldehyde group.
Formic acid is a strong reducing agent and is easily oxidized to CO 2 . She gives the "silver mirror" reaction:
HCOOH + 2OH → 2Ag + (NH 4) 2 CO 3 + 2NH 3 + H 2 O,
or in a simplified form in an ammonia solution when heated:
HCOOH + Ag 2 O → 2Ag + CO 2 + H 2 O.
Saturated carboxylic acids are resistant to concentrated sulfuric and nitric acids. The exception is formic acid:
H 2 SO 4 (conc)
HCOOH → CO + H 2 O
6. Decarboxylation reactions. Saturated unsubstituted monocarboxylic acids are difficult to decarboxylate when heated due to the high strength of the C-C bond. To do this, it is necessary to fuse the alkali metal salt of a carboxylic acid with an alkali:
CH 3 -CH 2 -COONa + NaOH → C 2 H 6 + Na 2 CO 3
Dibasic carboxylic acids easily split off CO 2 when heated:
HOOC-CH 2 -COOH → CH 3 COOH + CO 2
Functional derivatives of carboxylic acids. Dibasic carboxylic acids. a , b -Unsaturated acids
Carboxylic acid derivatives
1. Acid halides .
When exposed to phosphorus halides or thionyl chloride, the formation of halides occurs:
CH 3 COOH + PCl 5 ® CH 3 COCl + POCl 3 + HCl
The halogen in acid halides is highly reactive. A strong inductive effect determines the ease of substitution of halogen with other nucleophiles: - OH , - OR , - N.H. 2, - N 3, - CN etc.:
CH 3 COCl + CH 3 COOAg ® (CH3CO)2O acetic anhydride + AgCl
1. Anhydrides.
Anhydrides are formed by the reaction of acid salts with their acid halides:
CH 3 COONa + CH 3 COCl ® NaCl + ( CH 3 CO ) 2 O
Acid anhydrides are highly chemically active and, like acid halides, are good acylating agents.
2. Amides .
Amides are obtained via acid halides
CH 3 COCl +2 NH 3 ® CH 3 CONH 2 acetamide +NH4Cl
or from ammonium salts of acids, during dry distillation of which water is split off and an acid amide is formed. Also, acid amides are formed as a by-product during the hydrolysis of nitriles. Amidation processes are important industrially for the production of a number of valuable compounds ( N , N-dimethylformamide, dimethylacetamide, ethanolamides of higher acids).
4. Nitriles. The most important representatives of nitriles are acetonitrile CH 3 CN(used as a polar solvent) and acrylonitrile CH 2 = CHCN(monomer for the production of synthetic neuron fiber and for the production of divinylnitrile synthetic rubber, which is oil and gasoline resistant). The main method for producing nitriles is the dehydration of amides on acid catalysts:
CH 3 CONH 2 ® CH 3 C - CN + H 2 O
5. Esters. Esters of carboxylic acids are of important practical importance as solvents, hydraulic fluids, lubricating oils, plasticizers and monomers. They are obtained by esterification of alcohols with acids, anhydrides and acid halides or by the reaction of acids and alkenes:
CH 3 -CH=CH 2 + CH 3 COOH ® CH 3 COOCH(CH 3) 2
Many esters are used as aromatic substances:
| CH 3 COOCH 2 CH 3 |
pear essence |
| CH 3 CH 2 CH 2 COOCH 2 CH 2 CH 2 CH 2 CH 3 |
pineapple essence |
| HCOOCH 2 CH 3 |
rum essence |
Dibasic saturated acids
Dibasic saturated (saturated) acids have the general formula CnH 2 n ( COOH ) 2 . Of these, the most important are:
NOOS-SOUN- oxalic, ethanedicarboxylic acid;
NOOS-CH 2 -COOH- malonic, propanedicarboxylic acid;
NOOS-CH 2 -CH 2 -COOH- succinic, butanedicarboxylic acid;
NOOS-CH 2 -CH 2 -CH 2 -COOH- glutaric, pentanedicarboxylic acid.
Methods of obtaining
General methods for producing dibasic acids are similar to methods for producing monobasic acids (oxidation of glycols, hydrolysis of dinitriles, Kolbe synthesis - see Lecture No. 27).
1. Oxidation of hydroxy acids :
OH-CH2CH2COOH ® HOCCH 2 COOH ® HOOC-CH2-COOH
2. Oxidation of cycloalkanes .
This is an industrial method for producing adipic acid HOOC - CH 2 CH 2 CH 2 CH 2 - COOH from cyclohexane.
Succinic and oxalic acids are also formed as by-products. Adipic acid is used for fiber synthesis nylon 6.6 and plasticizers.
Chemical properties
Dibasic acids are stronger than monobasic acids. This is explained by the mutual influence of carboxyl groups that facilitate dissociation:
In general, the reactions of dicarboxylic acids and their monocarboxylic analogues are almost the same. The reaction mechanism for the formation of diamides, diesters, etc. from carboxylic acids is the same as for monocarboxylic acids. The exception is dicarboxylic acids, which contain fewer than four carbon atoms between the carboxyl groups. Such acids, whose two carboxyl groups are capable of reacting with the same functional group or with each other, exhibit unusual behavior in reactions that proceed to form five- or six-membered closed activated complexes or products.
An example of the unusual behavior of carboxylic acids is the reactions that occur when heated.
At 150 o C, oxalic acid decomposes into formic acid and CO 2 :
HOOC-COOH ® HCOOH + CO2
2. Cyclodehydration .
When heated g-dicarboxylic acids, in which the carboxyl groups are separated by carbon atoms, undergo cyclodehydration, resulting in the formation of cyclic anhydrides:
3. Syntheses based on malonic ester .
Dibasic acids with two carboxyl groups on one carbon atom, i.e. malonic acid and its mono- and disubstituted homologues, when heated slightly above their melting temperatures, decompose (are subjected to decarboxylation) with the elimination of one carboxyl group and the formation of acetic acid or its mono- and disubstituted homologues:
HOOCCH 2 COOH ® CH 3 COOH + CO 2
HOOCCH(CH3)COOH ® CH3CH2COOH + CO 2
HOOCC(CH 3) 2 COOH ® (CH3) 2 CHCOOH + CO 2
The hydrogen atoms of the methylene group located between the acyl groups of malonic acid diethyl ester ( malonic ester), have acidic properties and give the sodium salt with sodium ethoxide. This salt - sodium malonic ester– alkylate by the mechanism of nucleophilic substitution S N 2 . Based on sodium malonic ester, mono- and dibasic acids are obtained:
-Na++RBr ® RCH(COOCH 2 CH 3) 2 + 2 H 2 O ®
R-CH(COOH)2 alkylmalonic acid ® R-CH2COOH alkylacetic acid +CO2
4. Pyrolysis of calcium and barium salts .
During pyrolysis of calcium or barium salts adipic (C 6), pimeline (C 7) And cork (From 8) acids are eliminated CO 2 and cyclic ketones are formed:
Unsaturated monobasic carboxylic acids
Unsaturated monobasic acids of the ethylene series have the general formula CnH 2 n -1 COOH, acetylene and diethylene series - CnH 2 n -3 COOH. Examples of unsaturated monobasic acids:
Unsaturated monobasic acids differ from saturated ones by large dissociation constants. Unsaturated acids form all the usual derivatives of acids - salts, anhydrides, acid halides, amides, esters, etc. But due to multiple bonds they enter into addition, oxidation and polymerization reactions.
Due to the mutual influence of the carboxyl group and the multiple bond, the addition of hydrogen halides to a,b-unsaturated acids occurs in such a way that hydrogen is directed to the least hydrogenated carbon atom:
CH 2 = CHCOOH + HBr ® BrCH 2 CH 2 COOH b-bromopropionic acid
Ethylene acids such as acrylic acid and their esters undergo polymerization much more easily than the corresponding hydrocarbons.
individual representatives
Acrylic acid obtained from ethylene (via chlorohydrin or ethylene oxide), by hydrolysis of acrylonitrile or oxidation of propylene, which is more efficient. In technology, derivatives of acrylic acid are used - its esters, especially methyl ( methyl acrylate). Methyl acrylate easily polymerizes to form transparent glassy substances, so it is used in the production of organic glass and other valuable polymers.
Methacrylic acid and its esters are prepared on a large scale by methods similar to those for the synthesis of acrylic acid and its esters. The starting product is acetone, from which acetone cyanohydrin is obtained, subjected to dehydration and saponification to form methacrylic acid. By esterification with methyl alcohol, methyl methacrylate is obtained, which, upon polymerization or copolymerization, forms glassy polymers (organic glasses) with very valuable technical properties.
Dibasic unsaturated acids
The simplest unsaturated dibasic acids are fumaric And maleic - have the same structural formula HOOCCH = CHCOOH, but different spatial configuration: fumaric - trance-, maleic - cis-. Maleic acid (labile form) under the influence of bromine, iodine, nitrous acid easily transforms into a stable (stable) form - fumaric acid. The reverse transition is carried out under the influence of ultraviolet rays. Maleic acid on a technical scale is obtained by the catalytic oxidation of benzene and naphthalene with atmospheric oxygen.
Both acids are capable of forming salts, esters, amides and some other acid derivatives. However, maleic acid, unlike fumaric acid, easily forms a cyclic anhydride, since both carboxyl groups are located on the same side of the double bond ( cis-isomer). Maleic anhydride serves as a characteristic reagent for the detection of 1,3-diene compounds: it readily reacts in diene synthesis and in many cases gives valuable products. Maleic anhydride is widely used in the production of polyester resins and copolymers with styrene, acrylic and methacrylic esters. By hydrating maleic anhydride, malic acid is obtained, which is used in the food industry.
Aromatic monocarboxylic acids
Aromatic carboxylic acids are called benzene derivatives containing carboxyl groups directly bonded to the aromatic ring. Acids containing carboxyl groups in the side chain are considered as fatty-aromatic . Based on the number of carboxylic acid groups, aromatic acids are divided into mono-, dibasic, etc. The name of the acid is derived from the aromatic hydrocarbon (benzoic acid, n-toluic acid).
Methods of obtaining
1. Oxidation of aromatic hydrocarbons .
For the synthesis of aromatic acids, methyl homologues of benzene are most suitable, the radical chain oxidation of which proceeds through the stages of primary hydroperoxide and aldehyde:
ArCH 3 + O 2 ® ArCH2OOH ® ArCHO+ O2 ® ArCOOH
Mono- and dicarboxylic aromatic acids are produced industrially by the liquid-phase oxidation of methylbenzenes with atmospheric oxygen.
2. Oxidation of alcohols, aldehydes and ketones .
Aromatic alcohols, aldehydes and ketones oxidize more easily than hydrocarbons. Oxidation is usually carried out using hypochlorites according to the following scheme:
C 6 H 5 - CO - CH 3 + 4 NaOCl ® C 6 H 5 - COOH + NaCl + H 2 O + CO 2
3. Hydrolysis of halogen derivatives .
This method is widely used in technology.
C 6 H 5 CCl 3 + 2 H 2 O ® C 6 H 5 COOH + 3 HCl
When toluene is chlorinated, three types of halogen derivatives are obtained: benzyl chloride for the production of benzyl alcohol; benzylidene chloride - to obtain benzaldehyde; benzotrichloride is converted to benzoic acid.
4. Synthesis Grignard .
C6H5Li + CO2 ® C6H5COOLi + LiBr
Chemical properties
In aqueous solutions, monocarboxylic acids exhibit a greater degree of dissociation than aliphatic acids ( Benzoin acid=6.6×10 -5, Vinegar=1.8×10 -5). The high degree of dissociation of benzoic acid is due to the electrophilic nature of the benzene ring:
The acidity of aromatic acids is almost independent of resonance effects.
Aromatic acids undergo all the reactions that are characteristic of fatty acids. Due to the carboxyl group, various acid derivatives are formed: the action of acids on alkalis and carbonates produces salt , ethers- heating a mixture of acid and alcohol in the presence of mineral acid.
If the substituents in ortho-position is not present, then esterification of the carboxyl group occurs as easily as in the case of aliphatic acids. If one of ortho-positions are substituted, then the rate of esterification is greatly reduced, and if both are occupied ortho-position, then esterification does not occur.
Ethers ortho-substituted benzoic acids can be prepared by reacting silver salts with haloalkanes. They are difficult to hydrolyze. This phenomenon is called spatial (steric) difficulties. Groups larger than hydrogen fill the space around the carbon atom of the carboxyl group to such an extent that it makes it difficult to transition to an intermediate state during the formation or saponification of an ester.
Acid chlorides obtained by acting on acids with thionyl chloride or phosphorus pentachloride:
C 6 H 5 COOH + SOCl 2 ® C 6 H 5 COCl + HCl + SO 2
Anhydrides obtained by distillation of a mixture of acid and acetic anhydride or the action of acid chlorides on salts:
C 6 H 5 COCl + NaOOCC 6 H 5 ® ( C 6 H 5 CO ) 2 O + 2 NaCl
When a salt of an aromatic carboxylic acid is fused with an alkali, the carboxyl group is replaced by hydrogen:
C 6 H 5 COONa + NaOH ® ArH + Na 2 CO 3
The most important representatives
1. Benzoic acid . The main methods for producing benzoic acid are the oxidation of toluene and the decarboxylation of phthalic acid. It is used as a preservative in the food industry due to its strong antiseptic effect, as well as in the production of dyes and fragrances. A very important derivative of benzoic acid is its acid chloride - benzoyl chloride. It is a liquid with a characteristic odor and a strong lachrymatory effect.
2. n-tert -Butylbenzoic acid obtained on an industrial scale by oxidation rubs-butyltoluene in the presence of a soluble cobalt salt as a catalyst. Used in the production of polyester resins.
Dicarboxylic aromatic acids
There are three known benzenedicarboxylic acids: phthalic (O-isomer), isophthalic (m-isomer) and terephthalic (n-isomer). Terephthalic acid is a crystalline substance ( T sublime. 300 o C), compared to isomeric acids, it is least soluble in water and organic liquids. Terephthalic acid and its dimethyl ester play an important role in the production of synthetic fiber lavsan (terylene) - the product of their polycondensation with ethylene glycol. Terephthalic acid is produced by oxidation n-xylene.
Isophthalic acid is used for the production of polyesters. It is obtained similarly to terephthalic acid - by liquid-phase oxidation m-xylene.