Transmission of substituent effects in systems with bonds of different order. Kinetics of the reactions of 3-bromo-2-nitro-4-X-thiophens and 3-bromo-4-nitro-2-X-thiophens with sodium benzenethiolate in methanol

Abstract

The rates of the reactions of some 3-bromo-2-nitro-4-X-thiophens (I) and of some 3-bromo-4-nitro-2-X-thiophens (II) with sodium benzenethiolate in methanol have been measured with the aim of obtaining information on the transmission of substituent effects through the C(2)–C(3) and C(3)–C(4) bonds of the thiophen ring, which are known to have different bond orders. The correlation of the log k values of these reactions with σ_p constants in series (I) and with σ_p^– constants in series (II) together with the ρ-values obtained (ρ-_I=+3·95, ρ_II=+8·18) suggest that a low conjugative effect and an extra conjugative one are exerted from position 4 and position 2 respectively upon position 3 of the thiophen ring.

The ρ_I-value compared with those reported for the same reaction of 3-bromo-2-nitro-5-X-thiophens (series III) and 1-halogeno-2-nitro-5- and 6-X-benzenes indicate a ρ_ortho:ρ_meta ratio for thiophen derivatives (0·88) greater than that observed for similar reactions of benzene derivatives (0·67). These results suggest that ortho-substituents can exert their polar effect on the reaction centre more effectively in the thiophen than in the benzene series owing to the lower steric effects between neighbouring substituents in 5-membered heterocyclic rings. The different features of the u.v. spectra of the products obtained by benzenethiolate debromination of compounds of series (I) and (II) are consistent with the kinetic results.