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Toluene-2,4-diisocyanate
IUPAC name	2,4-diisocyanato-1-methyl-benzene
Other names	Tolylene diisocyanate Methyl phenylene diisocyanate
Identifiers
CAS number	86-91-9
RTECS number	CZ6300000
SMILES	c1(C)c(N=C=O)cc(N=C=O)cc1
Properties
Molecular formula	C9H6N2O2
Molar mass	174.2 g/mol
Appearance	Colorless to pale yellow, liquid
Density	1.214 g/cm3, liquid
Melting point	21.8 °C (295.0 K)
Boiling point	251 °C (524 K)
Solubility in water	Reacts
Hazards
MSDS	External MSDS
EU classification	Very toxic (T+) Carc. Cat. 3
NFPA 704	1 3 1
R-phrases	R26, R36/37/38, R40, R42/43, R52/53
S-phrases	(S1/2), S23, S36/37, S45, S61
Flash point	127 °C
Related Compounds
Related isocyanates	Methylene diphenyl diisocyanate Naphthalene diisocyanate
Related compounds	Polyurethane
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Toluene diisocyanate (TDI) is an aromatic diisocyanate. It is produced for reaction with polyols to form polyurethanes. It exists in two isomers, 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 a highly produced diisocyanate, accounting for 34.1% of the global isocyanate market in 2000, second only to MDI.^[1]

There are six steps to the synthesis of pure 2,4-TDI:^[1]

• Nitration: Reaction of toluene with nitric acid and a catalyst to form dinitrotoluene
• Hydrogenation: Reaction of the dinitrotoluene with hydrogen and a catalyst to form a mixture of isomers of toluene diamine (TDA)
• Purification: Distillation of the TDA mixture to produce meta-TDA
• Phosgenation: Reaction of the meta-TDA with phosgene to form a crude TDI mixture
• Purification: Distillation of the crude TDI mixture to produce an 80:20 mixture of 2,4-TDI and 2,6-TDI, known as TDI (80/20)
• Differentiation: Separation of the TDI (80/20) to produce pure 2,4-TDI and a 65:35 mixture of 2,4-TDI and 2,6-TDI, known as TDI (65/35)

Each of the isocyanate functional groups in TDI can react with a hydroxyl group to form a urethane linkage.

2,4-TDI is an asymmetrical molecule and thus has two isocyanate groups of different reactivity. 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.^[1]

Exposure to TDI and its vapors should be avoided. It is a well-documented causative agent of asthma.

• Methylene diphenyl diisocyanate
• Hexamethylene diisocyanate
• Isophorone diisocyanate

• International Chemical Safety Card 0339
• IARC Monograph: "Toluene Diisocyanates"
• NIOSH Pocket Guide to Chemical Hazards
• Hazards of TDI, MDI, and HDI
• NIOSH Safety and Health Topic: Isocyanates, from the website of the National Institute for Occupational Safety and Health (NIOSH)