alphahydrogen

Alpha hydrogen (also called an α-hydrogen) refers to a hydrogen atom attached to the carbon atom that is adjacent to a functional group, most commonly a carbonyl group in organic molecules. The term is used to describe hydrogens on the α-carbon, the position next to the reactive center in many chemical transformations. Alpha hydrogens also appear on the alpha carbon of amino acids, sugars, and other structurally relevant motifs.

In carbonyl compounds such as ketones and aldehydes, the α-carbon bears one or more hydrogens that can be readily removed under basic conditions to give an enolate or enol equivalent. The resulting enolate is resonance-stabilized, with negative charge delocalized onto the carbon and oxygen atoms of the carbonyl group. This stabilization makes α-hydrogens disproportionately more acidic than ordinary alkyl hydrogens.

Typical acidity ranges: α-hydrogens of ketones and aldehydes have pKa values around 19–20, reflecting enolate formation. α-hydrogens of esters, nitriles, and amides are somewhat less acidic, typically in the mid-20s to low-30s. In contrast, ordinary alkane C–H bonds are far less acidic, with pKa values around 50. The relative acidity governs the site and outcome of many reactions, including enolate alkylation, aldol condensations, and Michael additions.

Applications of α-hydrogen chemistry include enolate formation for carbon–carbon bond formation, isotopic labeling studies (deuterium transfer), and mechanistic investigations in organic and bioorganic contexts.