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 isocyanate that has two isocyanate groups is known as a di-isocyanate. Di-isocyanates are manufactured for reactions with polyols in the production of polyurethanes, a class of polymers.
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.
Isocyanates are electrophiles, and as such they are reactive toward a variety of nucleophiles including alcohols, amines, and even water. Upon treatment with an alcohol, an isocyanate forms a urethane linkage:
If a di-isocyanate is treated with a compound containing two or more hydroxyl groups, such as a diol or a polyol, polymer chains are formed, which are known as polyurethanes. Isocyanates react with water to form carbon dioxide:
Isocyanates also react with amines to give ureas:
The addition of an isocyanate to a urea gives a biuret:
Reaction between a di-isocyanate and a compound containing two or more amine groups, produces long polymer chains known as polyureas.
Isocyanates also can react with themselves. Aliphatic di-isocyanates can form trimers, which are structurally related to cyanuric acid. Isocyanates participate in Diels-Alder reactions, functioning as dienophiles.
Isocyanates are common intermediates in the synthesis of primary amines via hydrolysis:
The global market for diisocyanates in the year 2000 was 4.4 million tonnes, of which 61.3% was methylene diphenyl diisocyanate (MDI), 34.1% was toluene diisocyanate (TDI), 3.4% was the total for hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI), and 1.2% was the total for various others. A monofunctional isocyanate of industrial significance is methyl isocyanate (MIC), which is used in the manufacture of pesticides.
LD50s are typically several hundred milligrams per kilogram. Despite this low acute toxicity, an extremely low short-term exposure limit (STEL) of 0.07 mg/m−3 is the legal limit for all isocyanates (except methyl isocyanate: 0.02 mg/m−3) in the United Kingdom. These limits are set to protect workers from chronic health effects such as occupational asthma, contact dermatitis or irritation of the respiratory tract.
All major producers of MDI and TDI are members of the International Isocyanate Institute, which promotes the safe handling of MDI and TDI.