diisocyanate is generally manufactured in a continuou process involving three steps:
1. Nitration of toluene to dinitrotoluene
2. ation of dinitrotoluene to toluenediamine
3. Phosgenation to

To 1:

The continuous nitration of toluene can be done under milder conditions than are necessary for benzene due to the activating effect of the methyl group. For example, the H₂O content of the nitrating acid can be 23%, compared to 10% for the nitration of benzene. The mixture of mononitrated products of toluene consists of the three isomers o-, p- and rn-nitrotoluene, whose distribution is influenced only slightly by reaction conditions. A typical composition is 63% o-, 33-34% p- and 4% m-nitrotoluene. The mixture can be separated by distillation or crystallization.

Nitrotoluenes are intermediates for dyes, pharmaceuticals, and perfumes, and precursors for the explosive 2,4,6-trinitrotoluene (TNT). A mixture of o- and p-nitrotoluene can be nitrated to dinitrotoluenes, the feedstocks for the manufacture of diisocyanates. The isomeric 2,4- and 2,6-dinitrotoluenes are obtained in a ratio of roughly 80:20:



Nitration and workup are done in the usual manner, e.g. analogous to manufacture of nitrobenzene.

To 2:

The hydrogenation of the dinitrotoluene mixture to the two toluenediamines is once again a standard process in aromatic synthesis. This reaction can be carried out with iron and aqueous hydrochloric acid like the reduction of nitrobenzene, but catalytic hydrogenation - for example in methanol with a Raney nickel catalyst at about 100 °C and over 50 bar, or with palladium catalysts - is preferred.

The dinitrotoluenes are reduced quantitatively in a succession of pressure hydrogenations. The selectivity to the toluenediamines is 98-99%. In contrast to the manufacture of aniline from nitro-benzene, gas-phase hydrogenations are not used commercially due to the ready explosive decomposition of the dinitrotoluenes at the required reaction temperatures. Purification is done in a series of distillation columns.

To 3:

Phosgenation of the toluenediamines can be carried out in several ways. 'Rase phosgenation', i. e., the reaction of the free primary amine with phosgene, is the most important industrially. In the first step, the amine and phosgene are reacted at 0-50°C in a solvent such as o-dichlorobenzene to give a mixture of carbamyl chlorides and amine hydrochlorides.

The reaction product is fed into the hot phosgenation tower where, at 170-185 °C, it isreacted further with phosgene to form the diisocyanates:

The excess phosgene can be separated from HCl in a deep freezing unit and reeyclcd to the process.

The phosgenation of the toluenediamine hydrochlorides is the second possible manufacturing process for the diisocyanates. In the Mitsubishi Chemical process, for example, the toluenediamines are dissolved in o-diehlorobenzenc and converted into a salt suspension by injecting dry HCl. Phosgene is reacted with the hydrochlorides at elevated temperatures and with strong agitation to give the diisocyanates. The HCl which evolves is removed with an inert gas stream:

The workup and purification are done by fractional distillation. The selectivity to toluene diisocyanates is 97% (based on diamine). In the Mitsubishi process, the overall selectivity to diisocyanatc is 81% (based on toluene). In addition to pure 2,4-toluene diisocyanate, two isomeric mixtures are available commercially, with ratios of 2,4- to 2.6-isomer of 80:20 and 65:35.

Because of the increased commercial interest in diisocyanates, new manufacturing routes without the costly phosgenation step - that is, without total loss of chlorine as HCl - have recently been developed. Processes for the catalytic carbonylation of aromatic nitro compounds or amines are the most likely to become commercially important.