Malonic acid

Malonic acid is a dicarboxylic acid with structure CH₂(COOH)₂. The ionized form of malonic acid, as well as its esters and salts, are known as malonates. For example, diethyl malonate is malonic acid's diethyl ester. The name originates from the Greek word μᾶλον (malon) meaning 'apple'.

Malonic acid is a naturally occurring substance found in many fruits and vegetables. There is a suggestion that citrus fruits produced in organic farming contain higher levels of malonic acid than fruits produced in conventional agriculture.

Malonic acid was first prepared in 1858 by the French chemist Victor Dessaignes via the oxidation of malic acid.

Hermann Kolbe and Hugo Müller independently discovered how to synthesize malonic acid from propionic acid, and decided to publish their results back-to-back in the Chemical Society journal in 1864. This led to priority dispute with Hans Hübner and Maxwell Simpson who had independently published preliminary results on related reactions.

The structure has been determined by X-ray crystallography and extensive property data including for condensed phase thermochemistry are available from the National Institute of Standards and Technology. A classical preparation of malonic acid starts from chloroacetic acid:

Sodium carbonate generates the sodium salt, which is then reacted with sodium cyanide to provide the sodium salt of cyanoacetic acid via a nucleophilic substitution. The nitrile group can be hydrolyzed with sodium hydroxide to sodium malonate, and acidification affords malonic acid. Industrially, however, malonic acid is produced by hydrolysis of dimethyl malonate or diethyl malonate. It has also been produced through fermentation of glucose.

Malonic acid reacts as a typical carboxylic acid forming amide, ester, and chloride derivatives. Malonic anhydride can be used as an intermediate to mono-ester or amide derivatives, while malonyl chloride is most useful to obtain diesters or diamides. In a well-known reaction, malonic acid condenses with urea to form barbituric acid. Malonic acid may also be condensed with acetone to form Meldrum's acid, a versatile intermediate in further transformations. The esters of malonic acid are also used as a ⁻CH₂COOH synthon in the malonic ester synthesis.

Malonic acid is a key component in the Briggs–Rauscher reaction, the classic example of an oscillating chemical reaction.