In malonic ester synthesis, β-diesters bearing acidic α protons in the presence of a base and an alkyl halide generate substituted acetic acids via alkylated malonic ester intermediates. Initially, the base removes the α proton of the diester to generate a nucleophilic enolate ion that is highly stabilized by three resonance structures. Alkylating the anionic carbon through an SN2 mechanism produces an alkyl malonic ester intermediate. This undergoes hydrolysis followed by acidification to give the β-diacid. The β-diacid, at high temperatures, undergoes a concerted process that expels CO2 to produce an enol. This rapidly tautomerizes to the stable keto form of the substituted acetic acid. As the reaction comprises the elimination of a single proton, the resulting acid is monosubstituted. If, however, the monoalkylated intermediate is subjected to the second alkylation process, a disubstituted acid will result.