Isocyanates diazotization method

This method is considerably quicker than the diazotization method and has the added advantage that an atmosphere containing more than one isocyanate can be analyzed in one step. 

A level of 0-01 ppm of isocyanate in the atmosphere is the maximum allowable for health reasons. This level is very small and its accurate determination requires a lengthy analytical procedure where a sample of the atmosphere is obtained for determination of the isocyanate content. There are two methods available for this measurement, diazotization and gas chromatography. T able 11.1 gives the values of quality-control data for commonly used industrial diisocyanates. 

Diazotization involves drawing a known volume of atmospheric air through an absorbent solution, hydrolyzing the isocyanate to amine, diazotization and coupling this amine, and comparison of colour formation with a range of standards. The method varies in detail with the specific isocyanate being determined. 

The isocyanate vapours are absorbed in an acid solution which converts the vapour into an amine. The amine is diazotized and coupled with V-naphthylethylene diamine and the colour measured in a spectrophotometer. Details of this common method are now given. 

The thermal decomposition of acyl azides into isocyanate intermediates is known as Curtius rearrangement reaction (Scheme 5.2) [27, 28]. Conventionally used general methods to synthesize aroyl azides are limited to diazotization of hydrazides and reactions of NaNs with acid chlorides, mixed anhydrides, and Al-acyl benzotriazoles [35-38]. However, these procedures involve highly reactive chemicals which put significant limitations on functionalities of the substrate. The development of methodologically new, highly functional-group tolerant, catalytic routes to aroyl azides is particularly desirable. 

---