α-carbon

Meaning: The α-carbon is the carbon directly bonded to the carbonyl carbon; the next one out is β. Removing an α-hydrogen gives an enolate, whose negative charge is delocalized onto oxygen by resonance. This makes α-hydrogens far more acidic (pKa near 20 for a simple ketone) than ordinary alkane C–H hydrogens. The enolate usually reacts as a carbon nucleophile at the α-carbon, for example attacking a carbonyl in the aldol reaction. The carbonyl compound also equilibrates with its enol tautomer. A carbonyl with no α-hydrogens cannot form an enolate. In biochemistry, the α-carbon of an amino acid is the central carbon bearing the amino group, carboxyl group, H and side chain.

On the MCAT: Passages describe aldol reactions, enolate alkylation or keto-enol tautomerism. Questions ask which hydrogen is most acidic, which compound can serve as the enolate nucleophile, or why a chiral α-carbon racemizes in base (the planar enolate can be reprotonated from either face). A carbon between two carbonyls is especially acidic.

Common confusion: The carbonyl carbon itself is not the α-carbon, and the C–H of an aldehyde is not an α-hydrogen. The acidic hydrogens are on the carbon next to C=O.