Propanedioate

Propanedioate is the dianion of propanedioic acid, commonly known as malonate. In neutral form, propanedioic acid is HOOC-CH2-COOH; deprotonation of both carboxyl groups yields the propanedioate dianion, [−OOC-CH2-COO−]2−. Salts and esters of this dianion are referred to as malonates.

Chemically, the two carboxylate groups withdraw electron density from the central methylene, making its hydrogens relatively acidic and enabling enolate formation. Malonic acid has pKa values around 2.83 for the first dissociation and 5.69 for the second, reflecting the increasing difficulty of removing a proton after the first deprotonation. The malonate framework is characterized by resonance-stabilized carboxylate groups that coordinate well in salts and esters.

Propanedioate and malonate esters are widely used in organic synthesis. Diesters such as diethyl malonate serve as substrates in the malonic ester synthesis, where alkylation at the methylene carbon followed by hydrolysis and decarboxylation yields substituted acetic acids. The malonate dianion also participates in various alkylation and condensation reactions and in Knoevenagel condensations, acting as a versatile nucleophile.

Commercially, malonate salts (for example, disodium malonate) and malonate esters find use as intermediates in chemical synthesis, chelating ligands in coordination chemistry, and, in some contexts, as stabilizers or additives. Safety considerations are similar to other carboxylate salts: they can be irritants to skin and eyes; appropriate handling and protective equipment are advised.