Structure of Carboxylic Acids

Carboxylic acids are compounds with a carboxylic functional group attached to either hydrogen or carbon of the side group. The carbonyl and the hydroxyl groups are attached directly to form the carboxyl group. The carbonyl carbon is sp²-hybridized, with the three σ bonds formed with the two oxygens and one hydrogen or α carbon of the side group, oriented in a trigonal planar geometry, while the unhybridized 2p orbital forms a π bond with the 2p orbital of the carbonyl oxygen. The two carbon–oxygen bond lengths are unequal, as shown below.

The C–O single bond in a carboxylic acid is an sp²–sp³ σ bond, while in alcohols and ethers, they are sp³–sp³ σ bonds. Due to this higher s-character, carboxylic acids have smaller bond lengths compared to the C–O bonds of alcohols and ethers.

Moreover, the lone pair of electrons on the carboxylate oxygen remain delocalized over the π system of the carbonyl group giving two resonance structures. This delocalization reduces the electrophilicity of the carbonyl carbon in carboxylic acids compared to aldehydes or ketones.