(a) Design an alkyl halide that will give only 2,4-diphenylpent-2-ene upon treatment with potassium tert-butoxide (a bulky base that promotes E2 elimination).

(b) What stereochemistry is required in your alkyl halide so that only the following stereoisomer of the product is formed?

E2 elimination is a one-step mechanism, and carbon-hydrogen and carbon-halogen bonds mostly break off to form a new double bond. The base used in this reaction has a huge influence on the mechanism and the reaction rate depends on both the substrate and the base involved. The beta-hydrogen and leaving group must be anti-coplanar with each other.

The following structure will always give the product 2,4-diphenylpent-2-ene regardless of which adjacent hydrogen is removed by tert-butoxide base.

Structure of an alkyl halide that will always give the product 2,4-diphenylpent-2-ene

On both carbons 2 and 4, the hydrogen has to be anti-coplanar with the bromine while leaving the other groups to give the same product. The diastereomer formed will be meso. Shown below are Newman projection, three-dimensional representation, and Fischer projection of required diastereomer.

Diastereomer formed will be meso structure