Protonation of aniline nitrogen versus carbon

This process occurs at either the amine nitrogen or the ring carbon atoms. In solution, the nitrogen atom is the preferred ANI protonation site, due to a higher stabilization by solvent of the A-protonated form86. 

FIGURE 8. Schematic energy profiles showing the unimolecular rearrangements of aniline. Relative energies in kJ mol 1 were obtained from G3 calculations based on B3LYP/6-31G(d,p) optimized geometries85 

It is of interest to mention some detailed mass spectrometric results that were obtained by performing distinct experiments (a) ion-molecule reactions in mixtures of haloben-zenes and ammonia under chemical ionization conditions (b) ion-molecule reactions of mass-selected ionized halobenzenes toward ammonia in a quadrupole collision cell of a hybrid tandem mass spectrometer (c) ion-molecule reactions of phenyl diazonium cations with ammonia in the same quadrupole collision cell and, finally, (d) electrospray ionization of anilines in a hybrid quadrupole-time of flight mass spectrometer (QTof). Characterization of the product ions relies on collisional activation experiments in the low or high kinetic energy regime98. 

The chemistry of mass-selected ionized halobenzenes toward ammonia in a quadrupole collision cell pointed out that the CA spectra of the m/z 94 ions, i.e. protonated aniline, prepared in each of these conditions are qualitatively similar, since all the fragmentations are common. The major observed differences concern the relative intensities of the peaks associated with charge stripping and ammonia loss. In this context, substituted benzenes protonated on a ring carbon atom are more prone to undergo double ionization than substituted benzenes protonated on a substituent. 

Applied to experimental data presented in Table 4, these conclusions seem to indicate that (i) mixtures of isomers are produced in all cases, and (ii) ANI itself is mainly protonated on the ring atom under Cl conditions, whereas the interaction between ionized iodobenzene and ammonia mainly affords pure nitrogen-protonated aniline. Starting with 

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