The single stereoisomer shown undergoes anti E2 elimination with sodium ethoxide, removing the benzylic hydrogen. Which structure is the alkene formed?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: C

The compound is (2R,3R)-2-bromo-3-phenylbutane. In the conformer with H and Br anti-periplanar the two methyl groups end up on the same side, so the phenyl group and the C-2 methyl group are trans after elimination. This is the (E)-2-phenylbut-2-ene. E2 is stereospecific, so the (2R,3S) diastereomer would give the Z alkene.

Practice Organic Reaction Mechanisms IV: Elimination More on E2 anti-periplanar requirement on drawn ring systems

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