malonic ester synthesis

The malonic ester synthesis is a recipe for making substituted acetic acids — carboxylic acids of the form R-CH2-COOH or R2CH-COOH — by building up the chain on a borrowed scaffold. The scaffold is diethyl malonate, EtO2C-CH2-CO2Et, whose central CH2 sits between two ester carbonyls. Those two flanking esters make the central protons exceptionally acidic (pKa ~13), so a mild base like sodium ethoxide fully removes one to give a stabilized enolate.

From there it is a clean three-act play. Act one: deprotonate, then alkylate the central carbon with an alkyl halide (SN2), attaching an R group — you can alkylate once or, by repeating, twice. Act two: hydrolyze both esters back to carboxylic acids, giving a substituted malonic acid, R-CH(COOH)2. Act three: heat. A malonic acid (a beta-diacid, with a carboxyl beta to another carboxyl) loses CO2 on heating — decarboxylation — and you are left with a single substituted acetic acid, R-CH2-COOH. The second ester was just a temporary 'activating handle' that did its job and then left.

Why bother with this detour instead of alkylating acetic acid directly? Because you cannot easily make and alkylate the enolate of a plain carboxylic acid or simple ester — its alpha-proton is not acidic enough for mild bases, and the acid's own O-H gets in the way. Malonic ester's doubly-activated, easy-to-form enolate solves that, then decarboxylation discards the helper. The strategy is a model of 'synthon' thinking: use an easy-to-control building block that you later trim down to the target.