What Is Adipic Acid?
Adipic acid is an important dicarboxylic acid in the industry, also known as hexanedioic acid or 1,4-butanedicarboxylic acid. It’s a white, odorless, crystalline powder with the chemical formula (CH2)4(COOH)2. Initially prepared from fats, its name originates from the Latin words ‘adeps’ and ‘adipis’, meaning ‘animal fat’.
Properties of Adipic Acid
Adipic acid is soluble in ethanol, hot water, and acetone, but slightly soluble in water. Its acidic aqueous solutions exhibit a two-step proton dissociation with pKa values of 4.4 and 5.4. Upon heating with barium hydroxide, it undergoes ketonization to form cyclopentanone.
Adipic Acid Manufacturing Process
Commercially, adipic acid is produced by oxidizing cyclohexane. The process begins with the formation of a mixture of cyclohexanone and cyclohexanol, oxidized with nitric acid. This multi-step pathway also produces by-products like glutaric acid and succinic acid. Alternative methods to avoid nitrous oxide emissions include hydrocarboxylation of butadiene and oxidative cleavage of cyclohexene with hydrogen peroxide.
Uses of Adipic Acid
Adipic acid is primarily used as a raw material in polymers like Nylon 66 and polyester resin. It’s also a key ingredient in alkyd resins, pharmaceuticals, food additives, and as a standard substance in acid-base reactions. Its diesters serve as plasticizers and synthetic lubricants.
Safety of Adipic Acid
While mildly toxic and a skin irritant, adipic acid’s major environmental concern is the nitrous oxide emitted during its production. This gas contributes significantly to ozone depletion and has a high greenhouse effect. Manufacturers have introduced processes to convert this gas into nitrogen and oxygen to mitigate environmental impact.