Electrophilic aromatic substitution. Part 27. Chemical selectivities disguised by mass diffusion. Part 6. The kinetics of nitration in aqueous sulphuric acid of durene (1,2,4,5-tetramethylbenzene), nitrodurene, and nitroprehnitene (nitro-1,2,3,4-tetramethylbenzene). A comparison of the rates of nitration of methylnitrobenzenes in aqueous sulphuric acid with the corresponding rates from mixing–disguised nitrations with nitronium hexafluorophosphate in nitromethane based on a theoretical mixing–reaction model

Abstract

Durene (1,2,4,5-tetramethylbenzene) is nitrated in sulphuric acid at the encounter rate. Nitrations of nitrodurene and nitroprehnitene (nitro-1,2,3,4-tetramethylbenzene) are complicated by the formation of nitrous acid, presumably as a consequence of ipso-attack, and subsequent unidentified reactions of this. When an efficient nitrous acid trap is present the complications are removed and the kinetics become straightforward. Although nitrobenzene is 10⁸ times less reactive than benzene in nitration, nitroprehnitene and nitrodurene are only 41 and 20 times less reactive than their respective parent hydrocarbons. These reduced relative reactivities are a consequence of the fact that prehnitene and durene react at the encounter rate. The low relative reactivity of durene and 3-nitrodurene leads to the formation of some 3,6-dinitrodurene in the nitration of durene, even under the most favourable circumstances, and if mixing is inefficient the dinitro-compound may be the main product. By measuring the yield of 3-nitrodurene, as it varies with acidity, it is possible to determine the amount formed by direct attack at C-3 as distinct from that formed by ipso-attack followed by rearrangement. As a consequence the ratio of positional selectivity between C-3 and C-1 is shown to be 1 : 3.6. Thus, positional selectivity does not disappear. The intrinsic rate constants for nitronium ion nitrations in sulphuric acid of a number of methylnitrobenzenes show an excellent linear correlation with those for nitrations with nitronium hexafluorophosphate in nitromethane deduced by application of a theoretical mixing–reaction model. Differences between the two systems are not large, but appear to be in the direction showing the electrophile in sulphuric acid to be rather more reactive and more selective than that in the organic solvent.