In the title system, meso-stilbene dibromide gave trans-stilbene, while dl-stilbene dibromide gave both stilbenes, [trans] /[cis] 'v (83 f 2)/(17 f 2). The rate data at 59.4' were, for meso, AH* = 20.6 kcal/mol and AS* = -9 eu; for dl, AH* = 28.9 kcal/mol and AS* = 8 eu; k(meso)/k(dl) = 50. Stannous chloride proved to be an efficient scavenger in the dl reaction, for which rate data could not otherwise have been obtained. In methanol, lithium bromide is inert; in the aprotic solvent (DMF), it is an effective debrominating agent. A complete conformational analysis has been performed: the reactant free-energy difference in benzene at 80" has been measured, ( G~L -G,,,) = 0.78 kcal/mol; the transition state free-energy difference in DMF at 80" has been estimated, (G*dr -G*,eso) Our rates show the trend, anti debromination of meso >> sgn debromination of dl = anti debromination of dl >>>> syn debromination of meso, in accordance with the idea that "discrete" stereoelectronic and steric factors may make aligned or opposed contributions to the rates.4.6 kcal/mol.
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