Toluene diisocyanate

Overview
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.

Synthesis
There are six steps to the synthesis of pure 2,4-TDI:
 * 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)

Chemistry
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.

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