Debromination of meso- and DL-stilbene dibromides by lithium bromide in dimethylformamide

Abstract

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.

Kinetics and mechanisms of electrophilic addition. II. Thermochemical-kinetic approach to transition state structure

“…/ Corrected for the latent heat of fusion of the trans isomer, 7.19 kcal/mol.17 27°.19 The enthalpy difference between the solid trans isomer and the liquid cis isomer is -12.85 kcal/mol from hydrogenation measurements15 and -10. 29 kcal/mol from combustion results. 14 When corrected for the heat of fusion of the trans isomer (7.19 kcal/mol),17 the AH, values for the pure liquids listed in Table II are obtained.…”

Degradation of Naled and Dichlorvos Promoted by Reduced Sulfur Species in Well-Defined Anoxic Aqueous Solutions

“…The reductive transformation of halogenated organic compounds has been widely reported to occur in reducing environments ( − ). Dehalogenation of vicinal dihalides can be promoted by a great variety of nucleophiles or two-electron reductants, which include halide ions ( − ) except fluorides, sulfur nucleophiles ( − ), phosphorus nucleophiles ( − ), organometallic compounds (), and nitrogen and oxygen nucleophiles ( , ). As other vic-dihalides, naled may be debrominated by treating it with a reducing agent ( 14 , 41 , 42 ).…”