Conformational effect in the stannous chloride debromination of sym-tetrabromoethane

The initial ratios of cis- to trans-dibromoethene product in the stannous chloride reduction of sym-tetrabromoethane in dimethylformamide were 1.51 ± 0.04 at 25°, 1.36 ± 0.03 at 50°, and 1.20 ± 0.03 at 75°. Transition state energy differences in the trans–gauche tetrabromoethane rotamers are estimated as (Ht≠ – Hg≠) = 480 and (Ft≠ – Fg≠) = 244 cal/mole at 25°. A conformational energy analysis was performed. We have shown how a rate-equilibrium parallelism can be adapted to a conformational analysis for systems of this type, and how α, a constant between zero and unity, can be used to characterize the transition state relative to reactants and products (eq. [8]). In the present system, however, there is no rate-equilibrium parallelism and α cannot be defined.