Carboxylic acid is an organic compound containing carboxyl group (COOH) attached to an alkyl or aryl group. They react with metals and alkalis to generate carboxylate ions. These reactions of carboxylic acids indicate their acid nature. Acidity of carboxylic acids is higher in comparison to simple phenols as they react with weak bases like carbonates and bicarbonates to liberate carbon dioxide gas.
Carboxylic acids dissociatein water to form carboxylate ion and hydronium ion. The carboxylate ion formed is stabilized through resonance by effective delocalization of the negative charge.
Carboxylic acids are weaker than mineral acids but are strongest among the organic compounds. Acidity of carboxylic acid is higher than alcohols and even phenols. As discussed above, carboxylate ion, the conjugate base of carboxylic acid is stabilized by two equivalent resonance structures in which the negative charge is effectively delocalized between two more electronegative oxygen atoms. On the other hand in case of phenols,
On the other hand in case of phenols, negative charge is less effectively delocalized over one oxygen atom and less electronegative carbon atoms in phenoxide ion. Therefore, the carboxylate ion exhibits higher stability in comparison to phenoxide ion. Hence, the carboxylic acids are more acidic than phenols.
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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. Thus, electron withdrawing groups increase the stability of the conjugate base formed and hence the acidity of carboxylic acids. On the other hand, electron donating groups destabilizes the conjugate base formed and hence decreases the acidity of carboxylic acids. A general trend can be seen as:
CF3COOH > CCl3COOH > CHCl2COOH > NO2CH2COOH > NC-CH2COOH
Due to the resonance effect, phenyl or vinyl groups too, increase the acidity of carboxylic acids in spite of decreasing the acidity due to induction effect.
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