Structure of Carboxyl Group

Geometry at the Carboxyl Carbon

The carboxyl carbon is planar: the three groups attached to it — the alkyl/aryl group, the doubly-bonded oxygen, and the $-\text{OH}$ oxygen — all lie in a single plane, arranged around the carbon at bond angles of roughly $120^\circ$. This is the geometry expected of an $sp^2$-hybridised carbon, the same hybridisation state seen at the carbonyl carbon of aldehydes and ketones.

Why the Carboxyl Carbon Is Less Electrophilic

Despite that structural resemblance to a simple carbonyl, the carboxyl carbon behaves quite differently — it is noticeably less electrophilic than the carbonyl carbon of an aldehyde or ketone. The reason is resonance delocalisation involving the extra oxygen of the $-\text{OH}$ group.

A lone pair on the hydroxyl oxygen can push into the carbon–oxygen framework, generating a set of resonance structures for the carboxyl group:

Reading across these contributing structures: the lone pair on the $-\text{OH}$ oxygen can shift in to form a second bond to carbon, which in turn pushes electron density out onto the carbonyl oxygen. The net effect is that the positive character that would otherwise sit squarely on the carbonyl carbon (making it attractive to nucleophiles) gets spread out over the group, with the carbonyl oxygen picking up a share of the negative character instead. …