Crystallization toluene diisocyanate

TDI is commonly sold as a mixture of isomers. For example 80/20 TDI is a mixture of 80% 2,4- and 20% 2,6-toluene diisocyanate, whilst 65/35 is a mixture of 65% 2,4- and 35% 2,6-isomer [2165]. The amine groups of the starting material are therefore in 1,3-positions relative to each other. However, a small quantity of impurity in which the amine groups are in 1,2- positions relative to each other can give a disproportionate loss in yield, as a result of the cyclization reaction illustrated in Equation (4.4) [1282] the cyclic urea is volatile and distills with TDI, crystallizing from it and contributing to a loss in quality. 

Hexamethylene diisocyanate (252 g) in toluene (660 ml) is treated with anhydrous hydrogen cyanide (120 ml). The mixture is thoroughly cooled, pyridine (10 ml) is added, and the whole is set aside. The temperature rises, slowly at first, but as soon as the product separates, then instantaneously to 45°. Next day the slurry that has been formed is washed from the flask in toluene and filtered. The crystals are washed free from hydrogen cyanide by toluene and, for purification, dissolved in warm ethyl acetate (750 ml), to which toluene (1500 ml) is added. This affords white needles (290 g), m.p. 102°. 

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