Substitution reactions of benzo[b]thiophen derivatives. Part III. Nitration of benzo[b]thiophen-2-carboxylic acid and its 3-methyl derivative

Abstract

The nitration of benzo[b]thiophen-2-carboxylic acid has been re-examined under two different sets of conditions (in sulphuric acid and acetic acid at 60° and in acetic acid and acetic anhydride at 0°). A mixture of the 3-, 4-, 6-, and 7-substitution products was obtained in each case, together with a small amount of 2-nitrobenzo[b]thiophen formed by electrophilic displacement of the carboxy-group. Under similar conditions, the nitration of 3-methyl-benzo[b]thiophen-2-carboxylic acid took place in all positions of the benzene ring apart from the 5-position, and the replacement of the carboxy-group by a nitro-group became a more significant process, particularly in the higher temperature reaction. Structures of new compounds were established by n.m.r. spectroscopy; that of 3-methyl-7-nitrobenzo[b]thiophen-2-carboxylic acid was confirmed by synthesis from 2′-chloro-3′-nitroacetophenone. 6-Acetamidobenzo[b]thiophen was nitrated in an attempt to obtain the 7-nitro-compound, but a large amount of the 2-nitro-isomer was also formed. Attention is drawn to unusual features in the i.r. spectra of some esters of 3-methyl-7-nitrobenzo[b]thiophen-2-carboxylic acid.