Application of the Hammett relationship to a series of tetrasubstituted thiophens. Kinetics of piperidino-debromination of some 2-bromo-3-nitro-5-X-thiophens and 2-bromo-4-methyl-3-nitro-5-X-thiophens in methanol

Abstract

The rates of piperidino-debromination of some 2-bromo-3-nitro-5-X-thiophens and 2-bromo-4-methyl-3-nitro-5-X-thiophens in methanol have been measured. The results indicate that the Hammett relationship can be applied to the tetrasubstituted as well as to the trisubstituted compounds. The kinetic data for 2-bromo-4-methyl-3-nitro-5-X-thiophens represent the first good fit of the Hammett equation for a series of ortho-tetrasubstituted compounds. This is possible with fully substituted thiophens because the geometry of the 5-membered ring lowers steric interactions which in a 6-membered ring (e.g. benzene) are known to be very large.