carbonyl group

Meaning: A carbonyl group is a carbon double-bonded to oxygen, C=O. The carbon is sp² hybridized and trigonal planar (bond angles near 120°). Oxygen is more electronegative, so the carbon is partially positive (electrophilic) and the oxygen partially negative. A nucleophile attacks the carbon, the π electrons shift onto oxygen, and a tetrahedral intermediate forms. Carbonyls occur in aldehydes, ketones, carboxylic acids, esters, amides, anhydrides and acyl chlorides. Hydrogens on the alpha carbon are fairly acidic because the enolate conjugate base is resonance stabilized.

On the MCAT: Aldehydes and ketones undergo nucleophilic addition (Grignard reagents, hydride reduction, hemiacetal formation). Carboxylic acid derivatives undergo nucleophilic acyl substitution. Aldehydes are generally more reactive than ketones because they have less steric hindrance and fewer electron-donating alkyl groups. In spectroscopy, the C=O stretch gives a strong IR peak near 1700 cm⁻¹, and an aldehyde proton appears near 9–10 ppm in ¹H NMR.

Common confusion: A carboxyl group is a carbonyl with an OH on the same carbon (–COOH), so the two terms are not interchangeable. Also keep aldehydes and ketones apart: an aldehyde carbonyl carbon is bonded to at least one hydrogen (usually at the chain end), while a ketone carbonyl carbon is bonded to two carbons.