Electronic Structure of the Carboxylic Group

In carboxylic group the carbon atom and the two oxygen atoms are sp2 hybridized. The two sp2 hybridized orbitals of the carboxylic carbon atom overlap with one sp2 with one sp2 hybridized orbital of each oxygen atom while the third sp2 hybridized orbital of carbon atom overlaps with a sp3 hybridized orbital of carbon atom of the alkyl group to form three bonds and the one unhybridized p-orbital is perpendicular to the bonding skeleton.

All the bonds to the carboxylic carbon atom lie on one plane and are separated by about 120o from each other. The molecular of carboxylic acid can be represented as a resonance hybrid of the following structure.

From these structures, it is clear that the carbonyl part of the carboxylic group is electrically neutral (compare it with aldehydes and ketones in which only one structure is electrically neutral) and  does not have a double bond character but a reduced double bond character. Thus, carboxylic acid does not five the reactions of the carbonyl group.

The stability of carboxylate ion can also be explained in terms of delocalization of ? electron density. As the carbonyl carbon involves sp2 hybridization leaves one unhybridized ? orbital, therefore, this ? orbitals of oxygen atom forming ? bond. The resulting ? orbital cloud is spearad over both oxygen atoms and carbon atom.