Diphenylethylene, from diphenylacetylene

The necessary vicinal dihalides are themselves readily available by addition of Br2 or Cl2 to alkenes. Thus, the overall halogenation/dehvdrohalogenation sequence makes it possible to go from an alkene to an alkyne. for example, diphenylethylene is converted into diphenylacetylene by reaction with Br2 and subsequent base treatment. 

Dehalogenation is not important for preparation of acetylenic compounds. A few vicinal dibromoalkenes have been dehalogenated by metals — usually finely divided metals in an appropriate solvent. For example, diphenylacetylene may be made by treating l,2-dibromo-l,2-diphenylethylene with zinc in acetone (yield 85%) 200 and about 80% of fluoroacetylene is formed on removal of bromine from 1,2-dibromo-l-fluoroethylene with magnesium in tetra-hydrofuran 201 M... 

If meso-stilbene dibromide is treated with KOH in ethanol, it is possible to isolate the l-bromo-l,2-diphenylethylene that is formed from the first dehydrobromination. The E2 elimination of the second molecule of hydrogen bromide from this intermediate alkene to give diphenylacetylene has a higher activation enthalpy than the first elimination and thus requires a higher reaction temperature. Explain. 

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