Question

Reactions of dideuterated bromocyclohexanes, 1 and 2, by the E2 mechanism give monodeuterated cyclohexene A and dideuterated cyclohexene B. Which of the statements (a)-(d) best indicates the most probable selectivities of two reactions.

• A. Product ia A from both 1 and 2
• B. Product ia B from both 1 and 2
• C. Product is A from 1 and B from 2
• D. Product is B from 1 and A from 2

Answer 1

Answer: (C)

Product is A from 1 and B from 2
Elimination reactions by the E2 mechanism are facilitated when the nucleofuge and H on adjacent C atoms can achieve an antiperiplanar relationship. In the conformation of 1 with the Br axial, the trans $C-D$ bonds are both axial and coplanar with the Br, and elimination of either D with the Br will yield by the E2 mechanism. Correspondingly, in the conformation of 2 with the Br axial, two adjacent trans $C-H$ bond are axial and coplanar with the Br, and elimination of either with the Br will yield by the E2 mechanism.