Structure of the Carboxyl Group

The carboxyl group represents a planar arrangement of atoms: in -COOH, the central carbon and BOTH oxygen atoms are sp2 hybridised. The carbon's three sp2 hybrid orbitals overlap as follows -- two of them bond to the two oxygens (one to the carbonyl oxygen, one to the hydroxyl oxygen), while the third bonds either to a hydrogen atom (in formic acid) or to an sp3-hybridised alkyl carbon (in every other carboxylic acid), forming three sigma bonds in total. Each of the two oxygen atoms, and the carbon itself, is additionally left with one UNHYBRIDISED p-orbital, perpendicular to this sigma-bonding plane. These three parallel p-orbitals all overlap together to form an extended pi system that is PARTLY DELOCALISED across all three atoms -- from the carbonyl carbon-oxygen pi bond on one side, and from one of the hydroxyl oxygen's lone pairs feeding into that same system on the other -- so that RCOOH is genuinely better represented as a resonance hybrid of two canonical structures (one with the negative charge/lone pair on the carbonyl oxygen, one with it delocalised toward the hydroxyl oxygen) rather than as a single fixed Lewis structure. A direct chemical consequence follows: the carboxylic carbon is LESS electrophilic than an ordinary aldehyde/ketone carbonyl carbon, because this resonance delocalises electron density (from the hydroxyl oxygen's lone pair) back onto the carbonyl carbon, partially neutralising the positive charact …