Toluene diisocyanate

Toluene diisocyanate (TDI) is an organic compound with the formula CH3C6H3(NCO)2. Two of the six possible isomers are commercially important: 2,4-TDI (CAS: 584-84-9) and 2,6-TDI (CAS: 91-08-7). 2,4-TDI is produced in the pure state, but TDI is often marketed as 80/20 and 65/35 mixtures of the 2,4 and 2,6 isomers respectively. It is produced on a large scale, accounting for 34.1% of the global isocyanate market in 2000, second only to MDI.[3] Approximately 1.4 billion kilograms were produced in 2000.[4] All isomers of TDI are colorless, although commercial samples can appear yellow.

Toluene-2,4-diisocyanate
Toluene diisocyanate
Ball and stick model of toluene diisocyanate
Names
Preferred IUPAC name
2,4-Diisocyanato-1-methylbenzene
Other names
Toluene diisocyanate
Toluene-2,4-diisocyanate
Methyl phenylene diisocyanate
Benzylene 2,4-diisocyanate
2,4-Di(nitrogencarbonyl)toluene
Identifiers
3D model (JSmol)
744602
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.008.678
EC Number
  • 2,4: 209-544-5
  • 2,6: 202-039-0
RTECS number
  • 2,4: CZ6300000
  • 2,6: CZ6310000
UNII
UN number 2078
  • InChI=1S/C9H6N2O2/c1-7-2-3-8(10-5-12)4-9(7)11-6-13/h2-4H,1H3 checkY
    Key: DVKJHBMWWAPEIU-UHFFFAOYSA-N checkY
  • 2,4: InChI=1/C9H6N2O2/c1-7-2-3-8(10-5-12)4-9(7)11-6-13/h2-4H,1H3
    Key: DVKJHBMWWAPEIU-UHFFFAOYAL
  • 2,6: InChI=1S/C9H6N2O2/c1-7-8(10-5-12)3-2-4-9(7)11-6-13/h2-4H,1H3
    Key: RUELTTOHQODFPA-UHFFFAOYSA-N
  • 2,4: Cc1ccc(cc1\N=C=O)\N=C=O
  • 2,6: CC1=C(C=CC=C1N=C=O)N=C=O
Properties
C9H6N2O2
Molar mass 174.2 g/mol
Appearance Colorless liquid
Odor sharp, pungent[1]
Density 1.214 g/cm3, liquid
Melting point 21.8 °C (71.2 °F; 294.9 K)
Boiling point 251 °C (484 °F; 524 K)
Reacts
Vapor pressure 0.01 mmHg (25°C)[1]
Hazards
GHS labelling:
GHS05: CorrosiveGHS06: ToxicGHS07: Exclamation markGHS08: Health hazard
Danger
H315, H317, H318, H319, H330, H334, H335, H351, H412
P201, P202, P260, P261, P264, P271, P272, P273, P280, P281, P284, P285, P302+P352, P304+P340, P304+P341, P305+P351+P338, P308+P313, P310, P312, P320, P321, P332+P313, P333+P313, P337+P313, P342+P311, P362, P363, P403+P233, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamond
3
1
1
Flash point 127 °C (261 °F; 400 K)
Explosive limits 0.9–9.5%[1]
Lethal dose or concentration (LD, LC):
14 ppm (rat, 4 hr)
13.9 ppm (guinea pig, 4 hr)
9.7 ppm (mouse, 4 hr)
11 ppm (rabbit, 4 hr)[2]
NIOSH (US health exposure limits):
PEL (Permissible)
 C 0.02 ppm (0.14 mg/m3)[1]
REL (Recommended)
 Ca[1]
IDLH (Immediate danger)
 Ca [2.5 ppm][1]
Related compounds
Related isocyanates
 Methylene diphenyl diisocyanate
Naphthalene diisocyanate, 1,3-Diisocyanatobenzene
Related compounds
 Polyurethane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)
Infobox references

Synthesis

2,4-TDI is prepared in three steps from toluene via dinitrotoluene and 2,4-diaminotoluene (TDA). Finally, the TDA is subjected to phosgenation, i.e., treatment with phosgene to form TDI. This final step produces HCl as a byproduct and is a major source of industrial hydrochloric acid.[4]

Distillation of the raw TDI mixture produces an 80:20 mixture of 2,4-TDI and 2,6-TDI, known as TDI (80/20). Differentiation or separation of the TDI (80/20) can be used to produce pure 2,4-TDI and a 65:35 mixture of 2,4-TDI and 2,6-TDI, known as TDI (65/35).

Applications

The isocyanate functional groups in TDI react with hydroxyl groups to form carbamate (urethane) links. The two isocyanate groups in TDI react at different rates: The 4-position is approximately four times more reactive than the 2-position. 2,6-TDI is a symmetrical molecule and thus has two isocyanate groups of similar reactivity, similar to the 2-position on 2,4-TDI. However, since both isocyanate groups are attached to the same aromatic ring, reaction of one isocyanate group will cause a change in the reactivity of the second isocyanate group.[3] It is also sometimes used in rocket propellants.[5]

It is used in the production of rigid polyurethane foams with a high temperature stability.

Hazards

The LD50 for TDI is 5800 mg/kg for oral contact and LC50 of 610 mg/m3 for the vapour. Despite the indicated low toxicity, TDI is classified as "very toxic" by the European Community.[4]

In the United States, the Occupational Safety and Health Administration has set a permissible exposure limit with a ceiling at 0.02 ppm (0.14 mg/m3), while the National Institute for Occupational Safety and Health has not established a recommended exposure limit, due to the classification of toluene diisocyanate as a possible occupational carcinogen.[6] This chemical was one of many that were stored by the company whose chemical warehouse stationed in Tianjin, China was the site of massive explosions on August 12, 2015.[7]

Information is available on handling, personal protective equipment, exposure monitoring, transport, storage, sampling and analysis of TDI, dealing with accidents, and health and environmental themes.[8] All major producers of TDI are members of the International Isocyanate Institute, whose aim is the promotion of the safe handling of TDI in the workplace, community, and environment.

High-level exposure can result in reactive airways dysfunction syndrome.

See also

References
  1. NIOSH Pocket Guide to Chemical Hazards. "#0621". National Institute for Occupational Safety and Health (NIOSH).
  2. "Toluene-2,4-diisocyanate". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  3. Randall, D.; Lee, S. (2003). The Polyurethanes Book. New York: Wiley. ISBN 978-0-470-85041-1.
  4. Six, C.; Richter, F. "Isocyanates, Organic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a14_611.{{cite encyclopedia}}: CS1 maint: multiple names: authors list (link)
  5. "Ababil-100/Al Fat'h". GlobalSecurity.org. Archived from the original on 15 April 2019.
  6. National Institute for Occupational Safety and Health (May 1994). "Documentation for Immediately Dangerous To Life or Health Concentrations (IDLHs)". Centers for Disease Control and Prevention.
  7. Ripley, Will; Jiang, Steven; Mullen, Jethro (13 August 2023). "Tianjin explosion: Dozens dead, areas of Chinese port city devastated". CNN. Retrieved 1 October 2023.
  8. Allport, D. C.; Gilbert, D. S.; Outterside, S. M., eds. (2003). MDI and TDI: Safety, Health and the Environment: A Source Book and Practical Guide. Wiley. ISBN 978-0-471-95812-3.
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