α-hydrogen

Meaning: An α-hydrogen is a hydrogen on the α-carbon, the carbon directly bonded to a carbonyl carbon. It is weakly acidic, far more so than an ordinary alkane C–H, because losing H⁺ gives an enolate whose negative charge is delocalized onto the carbonyl oxygen by resonance. Typical pKa values are about 17–20 for aldehydes and ketones, higher for esters, and much lower when the carbon sits between two carbonyls. The enolate can be protonated on oxygen to give an enol (keto-enol tautomerism) or react as a carbon nucleophile. In biochemistry the term can also mean the hydrogen on the α-carbon of an amino acid.

On the MCAT: Expect aldol and Claisen-type reactions, enolate alkylation, and base- or acid-catalyzed racemization or deuterium exchange at an α-carbon. Questions ask which compound can form an enolate (it needs an α-H) or which α-H is most acidic (the one flanked by two carbonyls).

Common confusion: The aldehyde C–H is not an α-hydrogen, because it is on the carbonyl carbon itself. Formaldehyde and benzaldehyde have no α-H, so they cannot form enolates. A carboxylic acid O–H is also far more acidic than any α-H, so a base removes it first.