Toluene diisocyanate (TDI) is an

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...

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

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Is ...

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 In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...

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. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) ...

via

dinitrotoluene Dinitrotoluenes could refer to one of the following compounds: * 2,3-Dinitrotoluene * 2,4-Dinitrotoluene 2,4-Dinitrotoluene (DNT) or dinitro is an organic compound with the formula C7H6N2O4. This pale yellow crystalline solid is well known as a ...

and

2,4-diaminotoluene 2,4-Diaminotoluene is an organic compound with the formula C6H3(NH2)2CH3. 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-dinitr ...

(TDA). Finally, the TDA is subjected to ''phosgenation'', i.e., treatment with

phosgene Phosgene is the organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. Phosgene is a valued and important industrial building block, esp ...

to form TDI. This final step produces

HCl HCL may refer to: Science and medicine * Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia * Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development * Hollow-cathode lamp, a s ...

as a byproduct and is a major source of industrial hydrochloric acid. : 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

functional groups in TDI react with hydroxyl groups to form

carbamate In organic chemistry, a carbamate is a category of organic compounds with the general formula and structure , which are formally derived from carbamic acid (). The term includes organic compounds (e.g., the ester ethyl carbamate), formall ...

(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. It is also sometimes used in rocket propellants. It is used in the production of rigid polyurethane foams with a high temperature stability.

Hazards

The

LD50 In toxicology, the median lethal dose, LD50 (abbreviation for "lethal dose, 50%"), LC50 (lethal concentration, 50%) or LCt50 is a toxic unit that measures the lethal dose of a toxin, radiation, or pathogen. The value of LD50 for a substance is the ...

for TDI is 5800 mg/kg for oral contact and

LC50 In toxicology, the median lethal dose, LD50 (abbreviation for " lethal dose, 50%"), LC50 (lethal concentration, 50%) or LCt50 is a toxic unit that measures the lethal dose of a toxin, radiation, or pathogen. The value of LD50 for a substance is ...

of 610 mg/m³ for the vapour. Despite the indicated low toxicity, TDI is classified as “very toxic” by the European Community. In the United States, the

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

has set a

permissible exposure limit The permissible exposure limit (PEL or OSHA PEL) is a legal limit in the United States for exposure of an employee to a chemical substance or physical agent such as high level noise. Permissible exposure limits are established by the Occupationa ...

with a ceiling at 0.02 ppm (0.14 mg/m³), while the

National Institute for Occupational Safety and Health The National Institute for Occupational Safety and Health (NIOSH, ) is the United States federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness. NIOSH is part of the C ...

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 caused two massive explosions in a chemical warehouse stationed in Tianjin, China on August 13, 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 Reactive airway disease (RAD) is an informal label that physicians apply to patients with symptoms similar to those of asthma. An exact definition of the condition does not exist. Individuals who are typically labeled as having RAD generally have ...

References

{{reflist

External links

International Chemical Safety Card 0339

NIOSH Safety and Health Topic: Isocyanates
from the website of the National Institute for Occupational Safety and Health (NIOSH) * International Isocyanate Institute http:
www.diisocyanates.org
Isocyanates Monomers IARC Group 2B carcinogens Alkyl-substituted benzenes

Synthesis

Applications

Hazards

See also

References

External links