Diazotization with nitrosyl halides

The rate-limiting step in diazotizations with nitrosyl halides can in some cases be the formation of the nitrosyl halide (Scheme 3-26) this occurs with very reactive aniline derivatives (Hughes and Ridd, 1958). Alternatively it can be the deprotona-... 

Schmid s observation of the dependence of the reaction rate on the square of the concentration of nitrous acid was interpreted by Hammett (1940, p. 294) as due to the rate-limiting formation of dinitrogen trioxide, N203. The consequent attack of the amine by N203 was postulated to be faster therefore the concentration of the amine has no influence on the overall rate (zero order with respect to amine). Similarly, Hammett regards the second factor of Schmid s equation for diazotization in the presence of hydrochloric or hydrobromic acid as the result of the formation of nitrosyl halide. 

Diazotization is a complex reaction (Scheme 1). When performed in acidic media with sodium nitrite, NOx or nitrosyl halides, its kinetics are dependent upon the acidity of the medium in media with a Hammett acidity constant (-H0) from — 1 to 3 the reaction rate increases with acidity and the formation of the nitroso cation is the rate-limiting step, in more acidic media (-H0 > 4) the reaction rate decreases when acidity increases and the deprotonation of intermediate 6 is the rate-limiting step.6-9... 

Fig. 1. Diazotization of aromatic amines through the intermediate formation of nitrosyl halides (NOX). The variation of the rate coefficient k (eqn 16) with the pXa-value of the amine. The broken line indicates the expected limit for a diffusion-controlled reaction. +. .. X = Cl O X = Br...

The first evidence for the involvement of these nitrosyl halides in nitrosation came from kinetic studies of diazotization. The rate equation for diazotization in aqueous hydrochloric and hydrobromic acids includes the kinetic term given in (15) where S stands for the free amine (Schmid, 1937 Schmid and Muhr, 1937). It was later recognized that this can be interpreted as the attack of the nitrosyl halides on the free amine (Hammett, 1940) and the kinetic term was rewritten as (16).7 The corresponding rate coefficients and activation energies calculated from the equilibrium concentration of the nitrosyl halides are given in Tables 3 and 4 together with later results (Williams, 1977). 

Nitrosyl halides, particularly NOCl and NOBr, can also be used as diazotization reagents undo" anhydrous conditions, as these gases (NOCl mp —59.6°C, bp —6.4°C NOBr bp 0°C) are readily soluble in many organic solvents. They can also be generated in situ in chlorinated hydrocarbons by reaction of trimethylsilyl halides with alkyl nitrites (equation 13) as shown by Weiss and Wagner . The group of Weiss found also that NW-bissUylated anilines react in aprotic dichloromethane with generation of diazonium salts and formation of the nonnucleophilic hexamethyldisiloxane (equation 14). 

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