Question

Which of the following halides undergo $E_2$ elimination?

• A.
• B.
• C.
• D.

Answer 1

Answer: (A), (B), (D)

$E_2$ elimination occurs through an anti mechanism.
That compound will undergo $E_2$ mechanism in which the leaving group $-X$ and $-H$ at $\beta$ position are anti-periplanar.
In A and B, hydrogens present at $\beta$ positions are anti-periplanar and hence the compounds can undergo $E_2$ mechanism

In D, the cyclohexane will undergo ring flipping in which the bulky group will be in equatorial position and the $Br$ will comes in axial position. Thus, hydrogen present at $\beta$ position will be in anti-periplanar position. So the compound (D) will also undergo E2 elimination.