Electrophilic aromatic substitution. Part 20. The solvolyses in aqueous sulphuric acid of 4-methyl-4-nitrocyclohexa-2,5-dienyl acetate and some of its homologues, and their relevance to the nitration of methylbenzenes

Abstract

In aqueous sulphuric acid 4-methyl-, 3,4-dimethyl-, 1,4-dimethyl-, and 1,3,4-trimethyl-4-nitrocyclohexa-2,5-dienyl acetate, and 5-acetoxy-7a-nitro-5,7-dihydroindan are solvolysed too rapidly for the rates of reaction to be followed by conventional means. The products of solvolysis over a range of acidities have been determined quantitatively. In each case they can be accounted for by the occurrence of two competing reactions, E1 loss of nitrite and A_AL1 loss of acetic acid. From the secondary acetates the carbocation formed by loss of nitrite aromatises to an aryl acetate, and the nitro-W_i^Me(Wheland intermediate) formed by loss of acetic acid can either be captured by water to give (after elimination of nitrous acid) a phenol or rearrange by 1,2-nitro-group migration to give a nitro-compound. The reactions of the corresponding intermediates from the tertiary acetates are more complicated; they include 1,2- and 1,3-acetate migration, and for the nitro-W_i^Me rearrangement, capture by water, or reaction at a side-chain. The behaviour of the nitro-W_i^MeS formed in these solvolyses confirms the account given earlier of the results of nitrating methylbenzenes in aqueous sulphuric acid. It is confirmed that the positional reactivities in nitration are medium-dependent.