The single stereoisomer shown undergoes anti E2 elimination with sodium ethoxide toward the benzylic carbon. What is the geometry of the 2-phenylbut-2-ene formed, and what geometry would its (2R,3R) diastereomer give under the same conditions?

AE; Z
BE; E
CZ; E
DZ; Z

Answer: C

The compound is (2R,3S)-2-bromo-3-phenylbutane. Placing H and Br anti-periplanar leaves the C-2 methyl group and the phenyl group cis, giving the (Z)-alkene. Anti E2 is stereospecific, so the (2R,3R) diastereomer must give the opposite geometry, (E).

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