Benzo[b]thiophen derivatives. Part IX. Nitration of benzo[b]thiophen and the isomeric nitrobenzo[b]thiophens

Abstract

A study of the nitration of 3-nitrobenzo[b]thiophen has confirmed its lack of reactivity towards fuming nitric acid in glacial acetic acid at elevated temperatures, but has shown that substitution occurs at all positions in the benzene ring when it is treated in the cold either with a one molar proportion of potassium nitrate in concentrated sulphuric acid, or with a mixture of nitric acid, acetic acid, and acetic anhydride. The observed preference of substitution in the order 6 > 5 > 4 > 7-positions is discussed in terms of electron-density calculations for 3-nitrobenzo[b]thiophen and in terms of the influence of peri interaction in the 3,4-dinitro-compound. The ¹H n.m.r. spectra of 3,4-, 3,5-, 3,6-, and 3,7-dinitrobenzo[b]thiophens, the structures of which are confirmed by chemical means, and the ¹H n.m.r. spectra of the six isomeric mononitrobenzo[b]thiophens are recorded. Nitration of benzo[b]-thiophen is shown to give some 7-nitrobenzo[b]thiophen and reassignment of structure as 3,4,6-trinitrobenzo[b]thiophen is made to a compound previously incorrectly characterised. G.l.c. retention times are recorded for the six isomeric mononitrobenzo[b]thiophens.