Carboxylic acid is an organic compound that contains carboxyl group (COOH) attached to an alkyl or aryl group. Itreacts with metals and alkalis and generates carboxylate ions. These reactions indicate the acidic nature of carboxylic acid. Their acidity is higher in comparison to simple phenols as they react with weak bases like carbonates and bicarbonates to liberate carbon dioxide gas. When a substance donates a proton; usually hydrogen to other things,the carboxylic acid naming occurs. The acidic nature of the carboxylic acids comes from the hydrogen in the -COOH group.
Disassociation of Carboxylic acid takes place in the water to form carboxylate ion and hydronium ion. The carboxylate ion formed stabilizes itself through resonance by effective delocalization of the negative charge.
Carboxylic acids but are strongest among the organic compounds but they are weaker than mineral acids. Compared to other compounds, the acidity of carboxylic acid is higher than alcohols and even phenols. A seen above, carboxylate ion which is the conjugate base of carboxylic acid is stabilized by two equivalent resonance structures. In the negative charge is effectively delocalized between two more electronegative oxygen atoms.
When it comes to phenols, negative charge is less effectively delocalized over one oxygen atom and less electronegative carbon atoms in phenoxide ion. Therefore, higher stability is exhibited by carboxylate ion in comparison to phenoxide ion. Hence, the acidity of carboxylic acids is more than phenols. When reacted with metals and alkalis carboxylic acids form carboxylate ions, which only get stabilized due to resonance. The easiest way to understand carboxyl groups is simply by understanding that withdrawal of electrons leads to the increased acidity of carboxyl groups. Same way the donation of an electron leads to the decrease of acidity in carboxyl groups.
The acidity of carboxylic acids further depends on the nature of substituent alkyl or aryl group attached to the carboxyl group. An electron withdrawing group ensures effective delocalization of negative charge through resonance or inductive effect. Therefore electron withdrawing groups increase the stability of the conjugate base formed and hence the acidity of carboxylic acids. In the same way, electron donating groups destabilize the conjugate base formed and hence decreasethe acidity of carboxylic acids. A general trend can be seen as:
CF3COOH > CCl3COOH > CHCl2COOH > NO2CH2COOH > NC-CH2COOH
Phenyl or vinyl groups increase the acidity of carboxylic acids in spite of decreasing the acidity due to induction effect. This is due to the resonance effect.