Alpha Carbonyl Chemistry

Alpha Carbon and Alpha Protons

The carbon directly next to a carbonyl (ketone, aldehyde, or carboxylic acid derivative) is called the alpha carbon, and any hydrogens connected to an alpha carbon are called alpha protons.
The alpha carbon of ketones, aldehydes, and carboxylic acid derivatives can serve as nucleophiles (electron-rich species) when an alpha hydrogen atom is removed to form a carbanion, a negatively charged carbon. An alpha carbon (α\rm\alpha carbon) is a carbon that is directly attached to the carbonyl group (on either side). A carbonyl (or carbonyl group) is a functional group in which a carbon atom is double-bonded to an oxygen atom. An aldehyde is an organic compound that contains a carbonyl group (C=O{\rm{C{=}O}}) bound to one alkyl (R{-}{\rm {R}}) fragment and one hydrogen atom, with RC(=O)H{\rm{RC({=}O)H}} or RCHO{\rm{R{-}CHO}} stoichiometry. A ketone is an organic compound that contains a carbonyl group (C=O{\rm{C{=}O}}) bound to two alkyl (R{-}{\rm {R}}) fragments, which can be the same (R and R) or different (R and R′) with RC(=O)R{ \rm{RC({=}O)R'}} or RCOR′ stoichiometry. A carboxylic acid is a compound that contains an alkyl or aryl group (R) attached to a carboxyl group (COOH{-}{\rm{COOH}}).
Alpha carbons are those that are directly attached to the carbonyl group.
To describe the chemistry that takes place at alpha carbons, it is useful to define an alpha proton as any proton (H+) joined to an alpha carbon. The alpha proton can also be called an alpha hydrogen when bound to the alpha carbon.
An alpha proton is any hydrogen that is attached to an alpha carbon.
Not all alpha carbons will have alpha protons. For example, in benzaldehyde (C6H5CHO), there is not an alpha hydrogen. The hydrogen (H) attached to the aldehyde is not connected to an alpha carbon and thus is not an alpha proton. The alpha carbon is part of a benzene ring, but it is not attached to a proton.
Not all alpha carbons have alpha protons. Benzaldehyde has a hydrogen on one side of the carbonyl and a benzene ring on the other side. Therefore, there are no alpha protons on benzaldehyde.
The reactions of alpha carbons depend upon the presence of an alpha proton. Alpha protons are fairly acidic, with a pKa around 19. Removing the alpha proton from an alpha carbon will result in a carbanion, which is a very reactive anion.

Identifying Alpha Carbons and Alpha Protons

Substitutions at the alpha carbon typically involve the replacement of an alpha proton.