Carbodiimides

Carbodiimides are a class of organic compounds characterized by the carbodiimide functional group, with the general structure RN=C=NR' where R and R' are organic substituents. The group is linear and contains a central carbon double bonded to two nitrogens. Common derivatives include N,N'-dialkylcarbodiimides and N,N'-dicycloalkylcarbodiimides such as DCC (dicyclohexylcarbodiimide), DIC (diisopropylcarbodiimide), and EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide).

Carbodiimides are widely used as coupling reagents in organic synthesis, particularly for activating carboxylic acids toward nucleophilic attack. Activation proceeds via formation of an O-acylisourea intermediate that can react with amines to form amide bonds or with alcohols to form esters. They are also used as dehydrating agents in peptide synthesis and in polymer chemistry to promote bond formation. In many cases the reaction requires a base or additive to suppress side reactions and racemization; additives such as hydroxybenzotriazole (HOBt) or HOAt are employed with some carbodiimides to improve yields and selectivity.

A common limitation is the formation of byproducts such as the corresponding ureas (for example, dicyclohexylurea from DCC), which can complicate purification. Water sensitivity and potential for side reactions necessitate careful control of stoichiometry and reaction conditions. Water-soluble carbodiimides like EDC are often preferred when aqueous compatibility is desired, as they can be more easily removed from products.

Safety considerations include irritation potential and sensitization; many carbodiimides are moisture-sensitive and should be handled with appropriate protective equipment.