Abstract
The mechanism of aromatic nitration is critically reviewed with particular emphasis on the paradox of the high positional selectivity of substitution in spite of low substrate selectivity. Early quantum chemical computations in the gas phase have suggested that the retention of positional selectivity at encounter-limited rates could be ascribed to the formation of a radical pair via an electron transfer step occurring before the formation of the Wheland intermediate, but calculations which account for the effects of solvent polarization and the presence of counterion do not support that point of view. Here we report a brief survey of the available experimental and theoretical data, adding a few more computations for better clarifying the role of electron transfer for regioselectivity.
Highlights
Apart from considerations suggested by charge density computations and a few semiempirical calculations [40], to our knowledge, the first theoretical analysis of the mechanism of aromatic nitration was provided by Politzer and coworkers [41] who carried out ab initio SCF calculations, with a minimal basis set, of some intermediate species which could be formed in the reaction of benzene and toluene with a nitronium ion
The results presented by Olah et al were fully consistent with the mechanistic model proposed by Kochi [29,30,48], involving a metastable charge-transfer complex as the precursor to electrophilic aromatic substitution reactions
Population analysis predicts a large amount of electron transfer from benzene to the nitronium ion, whereas, in mixed acid solutions, when association with a counterion is considered, no appreciable charge transfer is predicted [54]
Summary
Electrophilic aromatic substitution has long been studied and, given the large amount of data reported in the literature, it represents a cornerstone in the field of mechanistic models of organic reactions, which excellent reviews and books have been dedicated to [1,2,3,4]. Nitration has played a central role in the development of a mechanistic theory of aromatic reactivity [5,6,7]. Without the pretension of covering all the immense work done in the last seventy years, we present a critical review of both experimental and theoretical works related to aromatic nitration which have accumulated in the last half-century. We pose particular emphasis to the possible chemico-physical factors which make it possible that a very fast reaction can exhibit regioselectivity and add a few more computations aimed at better understanding the role of solvent in liquidphase nitration
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