UNIT 4: Aromatic Acids

Acidity of Carboxylic Acids

Carboxylic acid is an organic compound containing a 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.

The Carboxylic Acid naming occurs when a substance donates a

proton; usually hydrogen to other things. The carboxylic acids are

acidic in nature because hydrogen belongs in the -COOH group.

Carboxylic acids dissociate in water to form carboxylate ion and the

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. The acidity of a carboxylic acid is

higher than alcohols and even phenols.

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 destabilize 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 to increase the

acidity of carboxylic acids in spite of decreasing the acidity due to

the inductive effect.

Aromatic acids

Aromatic acids include compounds that contain a COOH group

bonded to an aromatic ring. The simplest aromatic acid is benzoic

acid.

Aromatic carboxylic acids show not only the acidity and other

reactions expected of carboxylic acids (as an acid, benzoic acid is

slightly stronger than acetic acid) but, similar to other aromatic

compounds, also undergo electrophilic substitution reactions.

The COOH group is deact ivat ing, meaning e lect rophi l ic

substitutions take place less readily than with benzene itself

(Friedel-Crafts reactions do not occur), and meta-directing,

meaning that the incoming entity will enter at a position meta to the

COOH group, rather than at an ortho or para position, as in, for

example, the nitration of benzoic acid.

Some other important aromatic acids include the following: