Question

Explain why the meso stereoisomer of stilbene dibromide is formed

Answer 1

trans-Stilbene was brominated by pyridinium perbromide to give meso-dibromostilbene in 85% yield after recrystallization. The quantitative analysis established stoichiometric incorporation of two bromine atoms and excluded the possibility of substitution on the double bond or phenyl ring. Physical and spectroscopic data indicate the formation of meso-stilbene dibromide and no other stereoisomers.

meso-Stilbene dibromide is formed by stereospecific antiperiplanar addition of bromine across the stilbene double bond. The stereochemistry is consistent with initial formation of a bromonium ion intermediate followed by nucleophilic attack of bromide ion with inversion of configuration as shown. It is interesting that no evidence of bromo acetate formation was found, despite its excess proportion as bulk solvent. Evidently the nucleophilicity of the charged bromide anion greatly exceeds that of the neutral acetic acid molecule.