Krapcho decarboxylation

The Krapcho decarboxylation is the chemical reaction of esters with halide anions. The ester must contain an electron-withdrawing group in the beta position, such as β-ketoesters, malonic esters, α-cyanoesters, or α-sulfonylesters. It works best with methyl esters, since it is an S_N2-reaction at carbon. It is driven by the entropy of the overall reaction, as the byproducts chloromethane and CO₂ are lost as gases. The reaction is a useful synthetic procedure to hydrolyze and decarboxylate malonic esters because it only cleaves one of the ester groups. The alternative way (e.g. basic hydrolysis) destroys both of the ester groups and a subsequent reaction is usually used to regenerate the alkylated ester.