Structure of the Carbonyl Group

The carbonyl carbon is sp2 hybridised, and the carbon-oxygen bond is analogous to the carbon-carbon double bond in alkenes. The carbonyl carbon forms three sigma bonds from its three sp2 hybrid orbitals: one to oxygen, and the other two either to hydrogen and carbon (in an aldehyde) or to two carbons (in a ketone) -- all three sigma-bonded atoms lie in the same plane, with bond angles of 120 degrees around the carbonyl carbon (Fig 12.1). The carbon's fourth valence electron remains in an unhybridised 2p orbital perpendicular to that plane, and this overlaps sideways with a 2p orbital on oxygen to form the carbon-oxygen pi bond; oxygen's own two nonbonding electron pairs occupy its remaining two p-type orbitals. Because oxygen is the more electronegative atom, it pulls the shared C=O electron pair toward itself, so the sigma-plus-pi carbon-oxygen bond is strongly polar -- carbon carries a partial positive charge and oxygen a partial negative charge, expressible as a resonance hybrid between the neutral C=O form and a dipolar C+-O(-) form. This polarisation of the carbonyl carbon is the single fact that dri …