The rateof diazotisation of aniline in 3M-perchloric acid is accelerated b y m -and p-methyl and -methoxy-groups and retarded by chlorine atoms in the rneta-andpara-positions: the strongest acceleration is given by the m-methoxygroup. Similar results are reported for the effect of three substituents on the nitrosation of N-methylaniline under the same conditions. The substituent effects of p-methyl and p-chloro-substituents are not very sensitive to an increase in the acidity of the medium. The substituent effects are shown to be inconsistent with reaction through the free amine and are interpreted in terms of an initial attack of the nitrosating agent on the aromatic ring of the protonated amine followed by proton loss from the -NRH,+ group and migration of the nitrosating agent to this group.* Calculated from the data for o-chloroaniline and p-nitro-At low ionic strengths,S the limiting aniline in refs. 3 and 6. value of k,' is about 180 moles2 sec.-l l.2.
The rate and kinetic form of the N-nitrosation of N-methylaniline with nitrous acid in aqueous perchloric acid (0.002-6.5~) are similar to those observed for the diazotisation of aniline over the same range of acidity. At acidities below about 1 M. the nitrosation of the free amine by nitrous anhydride provides the most important reaction path but, as the acidity is increased, this is supplanted by a strongly acid-catalysed mechanism which w e attribute to the attack of a positive nitrosating agent on the protonated amine. The similarity of the nitrosation results with the diazotisation results shows that N-nitrosation is the rate-determining stage of diazotisation over the whole of this range of acidity.THE previous Papers in this Series have been mainly concerned with the kinetics and mechanism of diazotisation in media of progressively increasing acidity. The initial nitrosation stage has been considered throughout to be rate-determining and, at low acidities, this has been held to involve attack on the free amine [stage (a), Scheme 11. However, in the last Paper,l the kinetic form and substituent effects for the diazotisation of aniline in 3~-perchloric acid were considered to be inconsistent with a reaction of the free amine, and the direct nitrosation of the protonated amine was held to be rate-determining [stage (b), Scheme 11 through a transition state involving partial bonding of the electrophile to the aromatic ring. The work in the present two Papers was carried out to provide further evidence on this unusual reaction path; first, by a study of the related N-nitrosation of secondary amines and then (as described in the following Paper) by a more detailed study of substituent effects.
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