Dicarboxylic acid

A dicarboxylic acid is an organic compound containing two carboxyl functional groups (−COOH). The general molecular formula for dicarboxylic acids can be written as HO2C−R−CO2H, where R can be aliphatic or aromatic. In general, dicarboxylic acids show similar chemical behavior and reactivity to monocarboxylic acids. Dicarboxylic acids are also used in the preparation of copolymers such as polyamides and polyesters.

The most widely used dicarboxylic acid in the industry is adipic acid, which is a precursor used in the production of nylon. Other examples of dicarboxylic acids include aspartic acid and glutamic acid, two amino acids in the human body. The name can be abbreviated to diacid.

Long-chain dicarboxylic acids containing vicinal dimethyl branching near the centre of the carbon chain have been discovered in the genus Butyrivibrio, bacteria which participate in the digestion of cellulose in the rumen.

These fatty acids, named diabolic acids, have a chain length depending on the fatty acid used in the culture medium. The most abundant diabolic acid in Butyrivibrio had a 32-carbon chain length. Diabolic acids were also detected in the core lipids of the genus Thermotoga of the order Thermotogales, bacteria living in solfatara springs, deep-sea marine hydrothermal systems and high-temperature marine and continental oil fields.

It was shown that about 10% of their lipid fraction were symmetrical C30 to C34 diabolic acids. The C30 (13,14-dimethyloctacosanedioic acid) and C32 (15,16-dimethyltriacontanedioic acid) diabolic acids have been described in Thermotoga maritima.[13]

Some parent C29 to C32 diacids but with methyl groups on the carbons C-13 and C-16 have been isolated and characterized from the lipids of thermophilic anaerobic eubacterium Themanaerobacter ethanolicus. The most abundant diacid was the C30 a,ω-13,16-dimethyloctacosanedioic acid.

Biphytanic diacids are present in geological sediments and are considered as tracers of past anaerobic oxidation of methane.[15] Several forms without or with one or two pentacyclic rings have been detected in Cenozoic seep limestones. These lipids may be unrecognized metabolites from Archaea.

Dicarboxylic acids are crystalline solids. Solubility in water and melting point of the α,ω- compounds progress in a series as the carbon chains become longer with alternating between odd and even numbers of carbon atoms, so that for even numbers of carbon atoms the melting point is higher than for the next in the series with an odd number.[26] These compounds are weak dibasic acids with pKa tending towards values of ca. 4.5 and 5.5 as the separation between the two carboxylate groups increases. Thus, in aqueous solution at pH about 7, typical of biological systems, the Henderson–Hasselbalch equation indicates they exist predominantly as dicarboxylate anions.

The dicarboxylic acids, especially the small and linear ones, can be used as crosslinking reagents. Dicarboxylic acids where the carboxylic groups are separated by none or one carbon atom decompose when they are heated to give off carbon dioxide and leave behind a monocarboxylic acid.

Blanc’s Rule says that heating a barium salt of a dicarboxylic acid, or dehydrating it with acetic anhydride will yield a cyclic acid anhydride if the carbon atoms bearing acid groups are in position 1 and (3,4 or 5). So succinic acid will yield succinic anhydride. For acids with carboxylic groups at position 1 and 6 this dehydration causes loss of carbon dioxide and water to form a cyclic ketone, for example adipic acid will form cyclopentanone.