Isocyanate
Isocyanate is the functional group with the formula –N=C=O. Organic compounds that contains an isocyanate group are referred to as isocyanates. An isocyanate that has two isocyanate groups is known as a diisocyanate. Diisocyanates are manufactured for reactions with polyols in the production of polyurethanes. The largest use of isocyanates in industry involves the production of polymers. Polymers of common isocyanates are used in the manufacture of foams, paints, lacquers and in electrical insulation.
Not to be confused with isocyanates are cyanates and isocyanides. Cyanate is another functional group, which is arranged as –O–C≡N. Isocyanides have the connectivity R-N≡C. Their behavior is very different from isocyanates.
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[edit] Production
Isocyanates are produced by treating amines with phosgene:
- RNH2 + COCl2 → RNCO + 2 HCl
These reactions proceed via the intermediacy of a carbamoyl chloride (RNHC(O)Cl). Owing to the hazards associated with phosgene, the production of isocyanates requires special precautions.[1]
[edit] Reactivity
[edit] Reactions with nucleophiles
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:[1]
If a diisocyanate 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:
- RNCO + H2O → RNH2 + CO2
This reaction is exploited in tandem with the production of polyurethane to give polyurethane foams. The carbon dioxide functions as a blowing agent.[2]
Isocyanates also react with amines to give ureas:
- R2NH + R'NCO → R2NC(O)N(H)R'
The addition of an isocyanate to a urea gives a biuret:
- R2NC(O)N(H)R' + R"NCO → R2NC(O)NR'C(O)NHR"
Reaction between a di-isocyanate and a compound containing two or more amine groups, produces long polymer chains known as polyureas.
[edit] Cyclization
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.
[edit] Hoffmann rearrangement
Isocyanates are intermediates in the Hofmann rearrangement, in which a primary amide is treated with sodium hypobromite to form an isocyanate intermediate. The isocyanate is then hydrolyzed to yield the corresponding primary amine.[3]
[edit] Common isocyanates
Numbering of the ring atoms shown with blue numbers
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.[4] A monofunctional isocyanate of industrial significance is methyl isocyanate (MIC), which is used in the manufacture of pesticides.
[edit] Hazards
Methylisocyanate were the causative agent in the Bhopal Disaster that killed thousands due to high exposure. Isocyanates are potentially dangerous irritants to the eyes and respiratory tract, despite their relatively low acute toxicities. LD50's are typically several hundred milligrams per kilogram.
Information on handling, personal protective equipment, exposure monitoring, transport, storage, sampling and analysis of MDI and TDI, dealing with accidents, and health and environmental information has been published.[5] All major producers of MDI and TDI are members of the International Isocyanate Institute, whose aim is the promotion of the safe handling of MDI and TDI in the workplace, community and environment.
[edit] See also
[edit] References
- ^ a b Christian Six, Frank Richter "Isocyanates, Organic" in Ulmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a14_611
- ^ Paul Painter and Michael Coleman. Fundamentals to Polymer Science, An Introductory Text (Second ed.). pp. 39.
- ^ http://alpha.chem.umb.edu/chemistry/orgchem/CH20Handout.pdf, Ch20Handout, University of Massachusetts Boston
- ^ Randall, D (2002). The Polyurethanes Book. Wiley. ISBN 0-470-85041-8.
- ^ Allport DC, Gilbert, DS and Outterside SM (eds) (2003).MDI and TDI: safety, health & the environment: a source book and practical guide. Chichester, Wiley.
[edit] 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
- [1] International Isocyanate Institute
- http://www.actsafe.ca/wp-content/uploads/resources/pdf/Isocyanates.pdf
- Isocyanates – Measurement Methodology, Exposure and Effects Isocyanates in Working Life Workshop Article (1999)
- Health and Safety Executive, Guidance Note (EH16) Isocyanates: Toxic Hazards and Precautions (1984)
- The Society of the Plastics Industry – Technical Bulletin AX119 MDI-Based Polyurethane
- Foam Systems: Guidelines for Safe Handling and Disposal (1993)
- An occupational hygiene assessment of the use and control of isocyanates in the UK by Hilary A Cowie et al. HSE Research Report RR311/2005. Prepared by the Institute of Occupational Medicine for the Health and Safety Executive
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