carbonyl carbon

Meaning: The carbonyl carbon is the carbon in a C=O group, found in aldehydes, ketones, carboxylic acids, esters, amides, anhydrides and acyl chlorides. It is sp² hybridized and trigonal planar, with bond angles near 120°. Oxygen is more electronegative, so the bond is polarized and carbon carries a partial positive charge (δ+). A nucleophile attacks this carbon, the π electrons move onto oxygen, and a tetrahedral sp³ intermediate forms.

On the MCAT: Aldehydes and ketones undergo nucleophilic addition (Grignard, hydride reduction, hemiacetal and imine formation). Carboxylic acid derivatives undergo nucleophilic acyl substitution: the tetrahedral intermediate collapses and expels a leaving group. You may be asked to rank electrophilicity. Aldehydes are more reactive than ketones, and acyl chlorides are more reactive than anhydrides, which are more reactive than esters and acids, with amides least reactive. Alkyl groups and resonance donation from the substituent reduce the δ+ on carbon. In spectra, look for a strong IR band near 1700 cm⁻¹.

Common confusion: Students treat the oxygen as the site nucleophiles attack. The carbonyl carbon is the electrophile, and the oxygen is the basic site that gets protonated in acid catalysis. The neighboring alpha carbon is different: as an enolate it acts as a nucleophile.