The Fischer-Hepp rearrangement N-nitrosoamines

It is well known that N-nitroso aromatic amines (XLIII) rearrange in acid solution to form the C-nitroso isomer. In most cases that have been recorded the para isomer is formed (XLIV), viz. 

There is no reference to the formation of the ortho nitroso amine in the benzene series, indeed the formation and characterisation of ortho nitroso amines from any reaction has only been reported once or twice79. When the para position is blocked by a substituent, de-nitrosation so the secondary amine can occur80 a certain amount (which depends on the conditions) of denitrosation occurs also, concurrently with the rearrangement81, so that N-nitroso-N-methylaniline (XLVII) yields N-methylaniline (XLVIII) as well as the rearrangement product p-nitroso-N-methylaniline XLIX, viz. 

On the basis of the evidence of trans-nitrosation and on the apparent specific catalysis by hydrochloric acid, a mechanism first proposed by Houben85 has become generally accepted 8 86,87. This involves a reversible de-nitrosation forming the secondary amine and nitrosyl chloride followed by C-nitrosation in  

Actually the evidence by no means requires this mechanism since there is no reason why the nitrosamine itself should not act as a primary nitrosating agent (thus allowing cross-nitrosation) and there was no rate data available to support the idea of specific catalysis by hydrochloric acid. 

In another investigation81,185, the rates of formation of para-nitroso-N-methyl-aniline from N-nitroso-N-methylaniline in hydrochloric acid solution at 31 °C were measured by noting the appearance of the absorption at 340 mn due to the protonated from of the para-nitroso-N-methylaniline. A typical run is shown in Fig. 3. The decreasing absorption at about 260 m/i represents the disappearance 

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