Name: Acetaldehyde and Acetone
Uploaded: 2019-07-19
Duration: 3 min 15 s
Description: Do you know carbonyl compounds are used in medicines? In this video, you will be able to understand the important uses of the two most common compounds, acet...

Question

Explain why carboxylic acids behave as acids. Discuss briefly the effect of electron withdrawing and electron donating substituents on acid strength of carboxylic acids.

Accepted Answer

Carboxylic acids are weaker than mineral acids, but stronger than alcohols, water and other organic compounds. The strength of an acid is determined by its $p K_{a}$ value; a lesser value indicates a stronger acid. Delocalization of charge through resonance is a main reason for the acidity of carboxylic acids. The carboxylic group is considered to be the resonance hybrid of all structures. Both acid and carboxylate ion are resonance stabilized. Since carboxylate ion is more stable than carboxylic acid, it ionizes to form more stable carboxylate ions, thus possessing acidic character.

The nature of substituents influences the degree of dissociation of acids. Substituents can be both electron withdrawing as well as electron donating. In a carboxylic acid, the electron withdrawing groups disperse the negative charge on the carboxylate ion. Thus, the carboxylate ion is stabilized, and the acid becomes more acidic. Increased stability results in increased acid strength. The electron donating groups intensify the negative charge on the carboxylate ion. Hence, the ion becomes destabilized, and the acid becomes less acidic. Decreased stability results in decreased acidic strength.

The acid strength increases with increase in number of electron withdrawing substituents, due to increase in (-) inductive effect of the substituents. Acid strength also increases with increasing electronegativity of substituents. The acid strength decreases with increase in the distance between the carboxyl group and the electron withdrawing group, due to decrease in inductive effect with increasing distance. The acid strength also decreases with increase in the size of the alkyl group.

In aromatic acids, the electron withdrawing groups tend to increase the acid strength, while the electron donating groups tend to decrease the acid strength in substituted benzoic acids. The acid strengthening effect of the electron withdrawing groups, and the acid weakening effect of electron donating groups is more pronounced at the para and not at the meta position. o-substituted benzoic acids are stronger acids than benzoic acid irrespective of the nature of the substituent. This is known as the ortho effect. For electron releasing groups, the m-isomer is stronger than benzoic acid, while the p-isomer is weaker. Similarly, for electron donating groups, the p-isomer is stronger than the m-isomer.

Explain why carboxylic acids behave as acids. Discuss briefly the effect of electron withdrawing and electron donating substituents on acid strength of carboxylic acids.

Important Questions on Aldehydes, Ketones and Carboxylic Acids

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