Effect of constituents on the acidic strength of carboxylic acid

Since carboxyl group consists of carbonyl and hydroxyl groups, it is expected that acids show reactions due to both these groups. However the reactions expected of these groups are modified as a result of interaction between these groups due to close proximity.

Carboxylic acids are resonance hybrid of the following structures.

It is clear that the carbonyl part of the carboxyl group does not have a double bond character but a reduced double bond character. Thus it does not give the reactions of the carbonyl group. The carbonyl carbon in carboxyl group is less electrophilic than the carbonyl carbon in aldehydes and ketones due to resonance. Therefore many nucleophilic addition reactions of aldehydes and ketones do not take place with carboxylic acids. The same effect makes the hydroxyl group of carboxylic acids more acidic than the hydroxyl group of alcohols and phenols justifying why they are called acids.

Acidity

The aqueous solution, carboxylic acids dissociate as follows:

Since they liberate hydrogen ions in solution, they are acidic. However they are weaker than mineral acids, but stronger acids than alcohols and phenols.

Carboxylic compounds are amongst the most acidic organic compounds we have studied this far. This can be explained as follows:

Carboxylic acids as well as carboxylate ion both are stabilized by resonance. However, carboxylate ion is more stabilized by resonance because its contributing structures are exactly identical. The contributing structures of carboxylic acid involve charge separation.

Since carboxylate ion is more stabilized by resonance than carboxylic acid therefore equilibrium in above reaction lies very much in forward direction i.e., in favor of ionized form. Hence carboxylic acid behave as strong acids.

Carboxylic acids are stronger acids than phenols. It can be understood by comparing the hybrid structures of carboxylate ion and phenoxide ions. In carboxylate ion, the negative charge is equally distributed over two electronegative atoms (oxygen atoms) while in phenoxide ion, it is present only on one oxygen. Thus, carboxylate ion is more stabilized as compared to phenoxide ion. Hence, carboxylic acids ionize to the greater extent than phenols furnishing higher concentration of H⁺ ions. Therefore carboxylic acids behave as stronger acids than phenols.

Accordingly, carboxylic acids evolve hydrogen with electropositive metals as alcohols do.

Carboxylic acids form salts and water with alkalis similar to phenols.

Carboxylic acids react with weaker bases such as carbonates and bicarbonates to evolve CO₂.

Thus carboxylic acids give brisk effervescence with aqueous NaHCO₃ solution. This reaction can be used to detect the presence of carboxyl group in a compound. Also this reaction is used to differentiate between carboxylic acids and phenols which do not give effervescence with aqueous solution of NaHCO₃.

Effect of substituents on the acidity of carboxylic acids

The factors, which increase the stability of carboxylate ion more than the carboxylic acids, increase the acidic strength of acid and the factors that decrease the stability of carboxylate ion decrease the acid strength.

Electron withdrawing groups such as halo group, -NO₂, -CN etc increase the acidity of carboxylic acids. These electron withdrawing groups stabilize the carboxylate anion by dispersal of the negative charge and increase the strength of the acid.

Electron releasing groups like alkyl groups cause concentration of negative charge, destebalize the carboxylate anion and decrease the strength of the acid.

The more powerful and the more the number of such substituents and the closer they are to the carboxyl group, the greater is the inductive effect.

The FCH₂COOH > ClCH₂COOH > Br CH₂COOH > ICH₂COOH > CH₂COOH.

The electronegativity and thus the electron withdrawing power of the halogen substituent and thus the strength of the acid decreases in the given order.

CH₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH

The number of electron withdrawing chloro group and thus the acid strength decreases in this order.

CH₃CH₂CH(Cl)COOH > CH₃CHCl CH₂ COOH > ClCH₂CH₂CH₂COOH > CH₃CH₂CH₂ COOH

The inductive effect decreases rapidly with increasing distance from the carboxylic group and so does the acid strength.