Alpha Carbon and 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 ($\rm\alpha$ 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 (${\rm{C{=}O}}$) bound to one alkyl (${-}{\rm {R}}$) fragment and one hydrogen atom, with ${\rm{RC({=}O)H}}$ or ${\rm{R{-}CHO}}$ stoichiometry. A ketone is an organic compound that contains a carbonyl group (${\rm{C{=}O}}$) bound to two alkyl (${-}{\rm {R}}$) fragments, which can be the same (R and R) or different (R and R′) with ${ \rm{RC({=}O)R'}}$ or RCOR′ stoichiometry. A carboxylic acid is a compound that contains an alkyl or aryl group (R) attached to a carboxyl group (${-}{\rm{COOH}}$). 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. 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. The reactions of alpha carbons depend upon the presence of an alpha proton. Alpha protons are fairly acidic, with a pK_a around 19. Removing the alpha proton from an alpha carbon will result in a carbanion, which is a very reactive anion.