Protonation halogenated anilines

TABLE 6. Calculated proton affinities (PA, kJ mol-1 at 0 K) of halogenated anilines... 

Amines tend to give more complex results. The compound obtained by treating a 5-halofuran-2-carboxaldehyde with aniline is now thought to have a structure resulting from anil formation, halogen displacement, protropy and protonation. The free base is too unstable to be studied easily but acetylation of the salt provides readily recognizable products.182 A related example is discussed later (Section X,A,2). 

The halogenation of phenols and aromatic amines in aqueous solution also provides evidence for diffusion control, but the interpretation is complicated by the fact that either the formation of the o-complex or the proton loss from the (7-complex can be rate-determining. The reaction path for the halogenation of aromatic amines in aqueous acids is believed to be that shown for N,N-dialkyl anilines in Scheme 9. Where the formation of the o-complex is rate-determining, the kinetic form for attack by the molecular halogen is given by (39). In this equation, the observed rate coefficient (k ) is related to the rate coefficient for the reaction of the amine molecule (k) by (40), where KSH+ is the... 

In summary, it is confirmed that while the nitrogen protonation in the unsubstituted ANI is favored marginally, by about 4 kJ mol-1, over the para-C4-protonation, the protonation sites in substituted anilines are essentially determined by the nature and position of the substituents. The halogen atoms consistently reduce the basicity whereas the alkyl groups enhance it. Substitution at meta positions induces a larger increment than that at ortho positions. The overall effects of substituents on PAs are essentially additive. 

There are of course many other kinetic studies of electrophilic aromatic substitution, particularly of the halogenation of activated aromatic species such as phenols and anilines. These are frequently complicated by the effect of pH and halide ion concentration upon equilibria between different halogenating species and between different forms of the aromatic substance. The role of bases in removing a proton has been rarely investigated directly, and much of the mechanistic information comes from a study of isotope effects, which will be considered in Chapter 12. 

Already, in the original study, it was demonstrated that 7(r) can predict regioselectivity for S Ar. It was shown for benzene and eight substituted benzenes that the lowest 7.. are found at the positions that are preferred for nitration and halogenation, for example, in toluene, 7 of similar magnitude are found over the ortho and para positions, whereas in nitrobenzene the lowest 7 is found at the meta position [82]. Even anomalous systems, such as protonated aniline (CjHjNHj ), which is a metalpara director, were correctly predicted. Furthermore, it was demonstrated that there is a linear correlation between the 7 at the para and/or meta positions for each system and the corresponding Hammett constant or Thus, the magnitude of the 7 reflects the relative reactivity of the different systems toward S Ar. Later studies showed that 7(r) reflects positional selectivity and relative reactivity also in heteroaromatic systems [75, 86, 87]. 

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