Isocyanate

The isocyanate functional group

Isocyanate is the functional group of atoms –N=C=O (1 nitrogen, 1 carbon, 1 oxygen), not to be confused with the cyanate functional group which is arranged as –O–C≡N. Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An isocyanate may have more than one isocyanate group. An isocyanate that has two isocyanate groups is known as a diisocyanate. Diisocyanates are manufactured for reaction with polyols in the production of polyurethanes.

Chemistry

The isocyanate group reacts with the hydroxyl functional group to form a urethane linkage. If a diisocyanate is reacted with a compound containing two or more hydroxyl groups (a polyol), long polymer chains are formed, known as polyurethanes.

The isocyanate group also reacts with the amine functional group. Reaction between a diisocyanate and a compound containing two or more amine groups, produces long polymer chains known as polyureas.

The isocyanate group can react with itself. Aliphatic diisocyanates can form trimers, known as biurets.

The isocyanate group reacts with water to form carbon dioxide (CO₂). Carbon dioxide is used as a blowing agent (in situ) in order to produce polyurethane foams.^[1]

Isocyanates are intermediates in the Hofmann rearrangement, in which a primary amide is reacted with a hypohalite such as sodium hypobromite to form an isocyanate intermediate. The isocyanate is then hydrolyzed with water to yield the corresponding primary amine.^[2]

Common compounds

Methylene diphenyl 4,4'-diisocyanate (MDI)
Numbering of the ring atoms shown with blue numbers

A monofunctional isocyanate of industrial significance is methyl isocyanate (MIC), which is used in the manufacture of pesticides.

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.^[3]

Hazards

The reactivity of isocyanates makes them harmful to living tissue. They are toxic and exposure is known to result in hypersensitivity pneumonitis as well as occupational asthma in humans, both through inhalation exposure and dermal contact. Exposure to isocyanates and their vapors should be avoided. For the safe handling of isocyanates, information is available from ISOPA^[4], the European Diisocyanate and Polyol Producers Association.

See also

• Isothiocyanate

References

1. ^ Paul Painter and Michael Coleman. Fundamentals to Polymer Science, An Introductory Text (Second ed.). pp. 39. 
2. ^ http://alpha.chem.umb.edu/chemistry/orgchem/CH20Handout.pdf, Ch20Handout, University of Massachusetts Boston
3. ^ Randall, D (2002). The Polyurethanes Book. Wiley. ISBN 0-470-85041-8. 
4. ^ http://www.isopa.org ISOPA

External links

• NIOSH Safety and Health Topic: Isocyanates, from the website of the National Institute for Occupational Safety and Health (NIOSH)
• Health and Safety Executive, website of the UK Health and Safety Executive, useful search terms on this site — isocyanates, MVR, asthma