The Mills–Nixon effect and chemical reactivity—methyl cation affinity of some cycloalkabenzenes

Abstract

The problem of the methyl cation attack on carbon atoms belonging to the benzene moiety fused to small rings is explored by the ab initio models at the MP2 level of sophistication. It is shown that the β-position is more reactive than the α-site in kinetically controlled reactions, which is in accordance with the original Mills–Nixon postulate. On the other hand, it appears that in thermodynamically controlled electrophilic substitution reactions the α-site should be slightly preferred for three-, four- and five-membered annelated rings. The differences between the methyl cation affinities MCA_β and MCA_α are analyzed and resolved into angular strain and the cationic resonance contribution. The latter involves the hyperconjugation/conjugation and relaxation effects. It turns out that the angular strain contribution is inversely proportional to the size of the annelated ring, whereas the opposite is the case for the cationic resonance interaction. Their interplay determines the selectivity and its extent in the electrophilic substitution reactions. The same analysis is applicable to other electrophilic groups.