Electrophilic substitution on the thiophen ring. Part IV. Kinetics and mechanism of the nitration of thiophen

Abstract

The kinetics of the nitration of thiophen and some substituted thiophens in sulphuric and perchloric acid have been studied. There is a linear relationship between log k₂(obs) and –(H_R+ log a_W)(where w = H₂O) and the mechanism of reaction appears to be the same as that for benzene compounds. However, it is not a suitable preparative procedure owing to the high reactivity of thiophen. Nitration of thiophen, with a solution of nitric acid in acetic acid, proceeds with explosive violence owing to the easy nitrosation of thiophen in an autocatalytic reaction, leading to a variety of unidentified products. If nitrous acid is removed by addition of urea, nitration of thiophen at low concentration is of the first order in thiophen and nitrothiophen is obtained in high yield. However, at high thiophen concentration nitrosation again becomes the predominant reaction. The reason for this is discussed. As has been known for some time, the most successful reagent for the nitration of thiophen is nitric acid in acetic anhydride. Acetic anhydride does not greatly increase the rate of nitration but appears to remove complications due to nitrosation.