Answer: cis (Z)-2-phenyl-2-butene due to anti periplanar arrangement requirement.
- A trans (E)-2-phenyl-2-butene, formed from the corresponding diastereomeric bromide instead
- B An equal mixture of cis and trans alkenes regardless of stereochemistry
- C Only the terminal alkene 1-phenyl-2-butene via a different elimination pathway
- D cis (Z)-2-phenyl-2-butene due to anti periplanar arrangement requirement
Correct answer: D. cis (Z)-2-phenyl-2-butene due to anti periplanar arrangement requirement
Explanation: E2 requires anti-periplanar arrangement of H and Br. In (2R,3S) configuration, the anti periplanar conformer has H and Br on adjacent carbons arranged so the resulting alkene has the phenyl and methyl on the same side: Z (cis) alkene is the major product.
SN2 proceeds in a single step with backside attack and inversion of configuration, while SN1 forms a planar carbocation intermediate first, leading to racemisation.
Concept context
Study carbon-halogen compounds: how they are made, how they react via SN1/SN2 and elimination, their stereochemistry, and their environmental impact.