Question

Briefly discuss the effect of substituents on the acidity of carboxylic acids.

Answer 1

Effect of substituents on the acidity of carboxylic acid.

(i) Electron releasing alkyl group decreases the acidity: The electron-releasing groups (+1 groups) increase the negative charge on the carboxylate ion and destabilize it and hence the loss of proton becomes difficult. For example, formic acid is more stronger than acetic acid,

eg:

(ii) Electron withdrawing substituents increases the acidity: The electron-withdrawing substituents decrease the negative charge on the carboxylate ion and stabilize it. In such cases, the loss of proton becomes relatively easy.

Acidity increases with increasing electronegativity of the substituents. For example, the acidity of various halo acetic acids follows the order F—CH₂—COOH > Cl—CH₂—COOH > Br—CH₂—COOH > I —CH₃—COOH

Acidity increases with increasing number of electron – withdrawing substituents on the a – carbon.

For example

Cl₃C—COOH > Cl₂CH—COOH > ClCH₂COOH > CH₃COOH

The effect of various, electron withdrawing groups on the acidity of a carboxylic acid follows the order,

—NO₂ > —CN >—F >—Cl >—Br >—I >Ph

The relative acidities of various organic compounds are

RCOOH > ArOH > H₂O > ROH >RC ≡ CH