Question

Stilbene can be dibrominated using HBr and hydrogen peroxide in ethanol. Common sense would suggest that the major product should be the bromo-ethoxy product due to reaction with bromonium ion followed by the solvent since it outnumbers the strong nucleophile. But, in practice this is not seen, or only seen in very small amounts. Why is this?

Answer 1

The halogenation reaction of stilbene with HBr and hydrogen peroxide does not produce significant amounts of bromo-ethoxy product due to the stronger nucleophilicity of the bromide ion and the formation of a bromonium ion intermediate.

Step-by-step explanation:

The question relates to the regioselectivity of a halogenation reaction of stilbene with HBr and hydrogen peroxide. Despite ethanol being the solvent and a potential nucleophile, the expected bromo-ethoxy product is not the major one.

This discrepancy can be attributed to the nature of the reaction mechanism and the role of the solvent in such reactions. Ethanol, being a weaker nucleophile compared to Br⁻, does not efficiently compete with the bromide ion in the reaction.

Additionally, the conditions favor the formation of a bromonium ion intermediate, which further supports the bromide ion as the nucleophile that opens the intermediate, leading predominantly to dibrominated stilbene rather than substitution with the solvent.