Toluene diisocyanate (TDI) is an

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

with the formula CH₃C₆H₃(NCO)₂. Two of the six possible

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

s 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. Approximately 1.4 billion kilograms were produced in 2000. All isomers of TDI are colorless, although commercial samples can appear yellow.

Synthesis

2,4-TDI is prepared in three steps from

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water Water is an inorganic compound with the c ...

via dinitrotoluene and

2,4-diaminotoluene 2,4-Diaminotoluene is an aromatic organic compound with the formula . It is one isomer of six with this formula. It is a white solid, although commercial samples are often yellow-tan. Preparation It is prepared by hydrogenation of 2,4-Dinitrotol ...

(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. : TDI-Synthese

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).

Description and description of reactivity

The isocyanate functional groups in TDI react with hydroxyl groups to form carbamate (urethane) links. The two isocyanate groups in Toluene diisocyanate 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.

Applications

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

rocket propellants Rocket propellant is used as reaction mass ejected from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines. Overview ...

Hazards

TDI is classified as "very toxic" by the European Community. This is due to the high pulmonary risk of prolonged exposure at low doses. In the United States, the

Occupational Safety and Health Administration The Occupational Safety and Health Administration (OSHA; ) is a regulatory agency of the United States Department of Labor that originally had federal visitorial powers to inspect and examine workplaces. The United States Congress established ...

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. 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. 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. 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.

References

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External links