Question

In bromination of E-stilbene, we get meso-stilbene dibromide. In this reaction, why do we use H202?

Then from meso-stilbene dibromide, we carry out two elimination reactions to get diphenylacetylene. Why do we use KOH here?

Answer 1

Molecular bromine is very corrosive and fuming liquid.A direct addition of bromine with E-stilbene without solvent is a very exothermic reaction and not usually done by this process.Instead we generate bromine in situ from HBr and H2O2,it gives bromine and water and we use environment friendly solvent such as ethanol.So for in situ genaration of bromine we used H2O2.

To form diphenylacetylene from mesostilbene dibromide we must have to used a 2 equivalents strong base,such as KOH.Because by by abstracting one proton,dehydrobromination occurs and form alkenyl halide.Then another proton abstraction will be difficult from an alkenyl bromide if we not use strong base,So another equivalent of KOH successfully done 2nd dehydrobromination and therefore forms diphenyl acetylene