In nucleophilic addition and nucleophilic acyl substitution, the carbonyl group accepts electrons from an incoming nucleophile. The carbon next to the carbonyl group can also participate in chemical reactions. Substitutions on this carbon, the alpha carbon, involve replacing one of its hydrogens with another group. The replaced hydrogen is typically more acidic because of the stabilized resonance of the intermediate enolate ion. The ion intermediate is stabilized by the delocalization of the negative charge between the alpha carbon and the oxygen atom in the carbonyl group. Substitutions at the alpha carbon typically occur when the carbonyl converts to its enolate ion or its enolate tautomer. Both nucleophilic addition and nucleophilic acyl substitution result from the loss of a hydrogen at the alpha carbon, and both resulting carbanions are nucleophilic. Thus, when they attack an electrophile, the electrophile takes the place of the removed hydrogen.
At A Glance
- 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.
If a ketone or aldehyde containing an alpha proton is present with an acid or base, the ketone or aldehyde exists in equilibrium between the keto form and the enol form.
- In acidic conditions, the mechanism for keto-enol tautomerism involves protonation of the carbonyl oxygen followed by deprotonation of the alpha hydrogen. In basic conditions, the mechanism for keto-enol tautomerism involves deprotonation of an alpha hydrogen and formation of a negatively charged enolate intermediate that is protonated on oxygen to form the enol.