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Isocyanate

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Title: Isocyanate  
Author: World Heritage Encyclopedia
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Subject: Polyurethane, Functional group, Hexamethylene diisocyanate, Phosgene, Amide
Collection: Commodity Chemicals, Functional Groups, Isocyanates
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Isocyanate

The isocyanate functional group

Isocyanate is the polyols in the production of polyurethanes, a class of polymers.

Isocyanates should not be confused with cyanate esters and isocyanides, whose behaviors are very different. The cyanate (cyanate ester) functional group (R–O–C≡N) is arranged opposite of the isocyanate group (R–N=C=O). Isocyanides have the connectivity R-N≡C, lacking the oxygen of the cyanate groups.

Contents

  • Production 1
  • Reactivity 2
    • Reactions with nucleophiles 2.1
    • Cyclization 2.2
    • Rearrangement Reactions 2.3
  • Common isocyanates 3
  • Hazards 4
  • See also 5
  • References 6
  • External links 7

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]

Reactivity

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]

ROH + R'NCO → ROC(O)N(H)R' (R and R' are alkyl or aryl groups)

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:

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.

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.

Rearrangement Reactions

Isocyanates are common intermediates in the synthesis of primary amines via hydrolysis:

Common isocyanates

Methylene diphenyl 4,4'-diisocyanate (MDI)
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.[7] A monofunctional isocyanate of industrial significance is methyl isocyanate (MIC), which is used in the manufacture of pesticides.

Hazards

Methyl isocyanate was the causative agent in the Bhopal Disaster that killed thousands of people. 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.[8] Polyurethanes have variable curing times, and the presence of free isocyanates in foams vary accordingly.[9]

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.

Isocyanates are associated with increased risk of non-small cell lung cancer in particular squamous cell.

See also

References

  1. ^ 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
  2. ^ Paul Painter and Michael Coleman. Fundamentals to Polymer Science, An Introductory Text (Second ed.). p. 39. 
  3. ^ http://alpha.chem.umb.edu/chemistry/orgchem/CH20Handout.pdf Ch20Handout, University of Massachusetts Boston
  4. ^ Mann, F. G.; Saunders, B. C. (1960). Practical Organic Chemistry, 4th Ed.. London: Longman. p. 128.  
  5. ^ Cohen, Julius (1900). Practical Organic Chemistry 2nd Ed.. London: Macmillan and Co., Limited. p. 72. 
  6. ^ Baumgarten, Henry; Smith, Howard; and Staklis, Andris (1975). "Reactions of amines. XVIII. Oxidative rearrangement of amides with lead tetraacetate". The Journal of Organic Chemistry 40 (24): 3554–3561.  
  7. ^ Randall, D (2002). The Polyurethanes Book. Wiley.  
  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.
  9. ^ Riedlich, C (2010). "Risk of isocyanate exposure in the construction industry". CPWR - The Center for Construction Research and Training: 1–8. 

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
  • Air Sampling Isocyanates
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