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.

 

2R COOH + Zn ⟶ (R COO)₂ Zn + H₂

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

RCOOH + NaOH ⟶ RCOONa + H₂O

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

RCOOH + NaHCO₃ ⟶ R COONa + CO₂ + H₂O

Thus carboxylic acids give brisk effervescence with aqueous NaHCO₃solution. 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.