Reactions with anilinium ions

It has already been pointed out that nitrosation is probably the first step in diazotization. Ingold (1952) describes the reaction as N-nitrosation and classifies it as an electrophilic substitution, together with related processes such as the formation of 4-nitrosophenol, an example of a C-nitrosation. It was probably Adamson and Kenner (1934) who first applied these ideas to diazotization and realized that in aniline itself the electron density at the nitrogen atom is greater than in the anilinium ion, so that the base is more reactive. On the other hand, the nitrosoacidium ion (3.1), the addition product of nitrous acid and a proton, is a more powerful electrophilic reagent than the HN02 molecule. They therefore represented the first step of diazotization as in Scheme 3-5. 

In the modified Ridd mechanism for region B the deprotonation of the A-nitroso-anilinium ion Ar —NH2NO in Scheme 3-23 is rapid, and therefore does not influence the overall rate. However, the second-order term in the rate equation for region C (Scheme 3-25) is consistent with a mechanism in which the deprotonation of the A-nitrosoanilinium ion (Scheme 3-24) and of the C-nitrosoanilinium dication (Scheme 3-22) belongs to the rate-determining part of the reaction. 

A long series of studies of aromatic nucleophilic substitution included the kinetics of reactions of l-chloro-2,4-bis(trifluoromethylsulfonyl)benzene, 3-nitro-4-chlorophenyl trifluoromethyl sulfone and 2-chlorophenyl trifluoromethyl sulfone with sodium methox-ide or ammonia in methanol . The SO2CF3 group was found to have an enormous accelerating effect, in accord with the value of 1.65, based on the dissociation of anilinium ion. Further examples of the promotion of nucleophilic aromatic substitution by fluoro-substituted sulfonyl groups are given by Yagupol skii and coworkers . 

The presence of a fluorine or a chlorine atom on the aromatic ring clearly leads to two different behaviors, giving two different products—aniline-1- or anilinium ion with formation of HX or X. Apparently, two different reaction coordinates are involved. In disubstituted aromatic compounds, depending on the nature of the substitutents, these two processes are competitive. 

In the nitration of aniline in strongly acid medium one is concerned with the reaction of the anilinium ion, without resonance, with the N02+ ion. Electrons are withdrawn from the phenyl nucleus by the —NH3+ group and so meta and para nitraniline is produced but not ortho. If the aniline is first acetylated then mainly ortho and para nitraniline is produced through the R effect. 

The acetylation of aniline in aqueous solution to give acetanilide serves to protect the amine group from reaction with chlorosulfonic acid. The acetylation takes place readily in aqueous solution. Aniline reacts with acid to give the water-soluble anilinium ion ... 

An extensive study of the decomposition of the N,N-dimethylbenzyl-anilinium ion 22 was recently carried out by Lepley and coworkers 93.99) As expected, displacement reactions occur exclusively with hydroxide and alkoxide, an orf/io-rearrangement occurs with amide, and a variety of products are formed with various organolithium reagents. The products found when butyllithium was employed are listed below. 

Comparison of relative reactivities at constant steric effects in terms of a few selected Hammett s p values is shown in Table VII for ArS- and in Table VIII for anilines. Although some of the data in Table VII are based only on two points, clearly the p values are structure-dependent. Moreover, these values show no clear trend. The reactivity of P,P-dihalovinyl sulfones are similar to those of the P-halovinyl sulfones, but their p values are much lower. The addition reactions with ArS- show higher p values than for most of the substitutions, but for the structurally similar vinyl sulfones, the p for the substitution is higher. In general, the p values for the anilines are significantly higher, but this fact does not necessarily mean an earlier transition state for the anionic nucleophiles because the p values for the equilibrium acidities of the anilinium ions are higher than those for the thiols. 

MDA does not form in the absence of acid The aminals and the A/-methylolaniline decompose rapidly in acid solution to form an anilinium ion (intermediate B). This reactive intermediate combines with aniline to form A/-(p-anainoben2yl)aniline [17272-83-2] (C 2H 4N2) which reacts with intermediate B in the presence of acid to form oligomeric ben2ylamines (PBA) that exist in equiHbrium with the monomer. The formation of this equihbrium mixture completes the condensation phase of the synthesis. Almost aU of the side reactions take place during the condensation phase of the reaction. The typical side products formed are the N-methyl and quina2oline derivatives of aniline and MDA (13,15). Commercial processes have been successful in minimising these side reactions. 

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