BENZO[B]THIOPHEN DERIVATIVES .X. NITRATION OF BENZO[B]THIOPHEN-3-AARBOXYLIC ACID

Nitration of benzo[b]thiophen-3-carboxylic acid affords substitution at all four available positions in the benzene ring, although the relative composition of the mixture of products varies with the conditions employed. Three nitration procedures are described. When nitration is carried out with 10% sulphuric acid in acetic acid solution at 60° with concentrated nitric acid, the 4-nitro-isomer is the predominant isomer, but when a potassium nitrate-sulphuric acid solution at 0° is employed the 6- and/or 5-nitra-isomers become the major nitro-products. Repetition of the nitration procedure employed by Van Zyl et at. revealed that the 6- and/or 5-nitro-isomers, and not the 4-isomer, were the main nitro-isomers in the product, in contrast to their results. A minor reaction path involves replacement of the carboxy-group by a nitro-group, but under none of the conditions employed in this work does substitution occur in the thiophen ring at the 2-position adjacent to the carboxylic function. Electron-density calculations for the parent acid and the 1H n.m.r. spectra of the methyl esters of its 4-, 5-, 6-, and 7-nitro-derivatives are recorded.