Isocyanate

In organic chemistry, isocyanate is the functional group with the formula R−N=C=O. Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyanates are manufactured for the production of polyurethanes, a class of polymers.^{[page needed][page needed]}

Isocyanates should not be confused with cyanate esters and isocyanides, very different families of compounds. The cyanate (cyanate ester) functional group (R−O−C≡N) is arranged differently from the isocyanate group (R−N=C=O). Isocyanides have the connectivity R−N≡C, lacking the oxygen of the cyanate groups.

In terms of bonding, isocyanates are closely related to carbon dioxide (CO₂) and carbodiimides (C(NR)₂). The C−N=C=O unit that defines isocyanates is planar, and the N=C=O linkage is nearly linear. In phenyl isocyanate, the C=N and C=O distances are respectively 1.195 and 1.173 Å. The C−N=C angle is 134.9° and the N=C=O angle is 173.1°.

Isocyanates are usually produced from amines by phosgenation, i.e. treating with phosgene:

These reactions proceed via the intermediacy of a carbamoyl chloride (RNHC(O)Cl). Owing to the hazardous nature of phosgene, the production of isocyanates requires special precautions. A laboratory-safe variation masks the phosgene as oxalyl chloride. Also, oxalyl chloride can be used to form acyl isocyanates from primary amides, which phosgene typically dehydrates to nitriles instead.

Another route to isocyanates entails addition of isocyanic acid to alkenes. Complementarily, alkyl isocyanates form by displacement reactions involving alkyl halides and alkali metal cyanates.

Aryl isocyanates can be synthesized from reductive carbonylation of nitro- and nitrosoarenes; a palladium catalyst is necessary to avoid side-reactions of the nitrene intermediate.

Three rearrangement reactions involving nitrenes give isocyanates: