Question

Which of the following compound is most likely to follow E1cB mechanism when treated with $C_2{H}_{5}ONa$ in ethanol?

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

$E_{1}cB$ reaction is the process by which a substituted alkane form its corresponding alkene taking hydrogen from the beta carbon. It is a unimolecular reaction.
For $E_{1}cB$ to occur, the substituted alkane must have strong EWG at the $\beta$ carbon and poor leaving group. These conditions will favors the E1cB reaction.
It also must have a hydrogen on one of its beta carbon.
Compounds forming resonance stabilised carbanion after abstraction of $(\beta -H)$ will favours the most to E1cB mechanism.

In (a), we don't have electron withdrawing group at beta carbon.
In (b), reactant has fluorine which is a poor leaving group and also a electron withdrawing phenyl group so it undergo E1cB reaction.
In (c) and (d), compound lacks electron withdrawing group.
Thus, compound (b) will undergo E1cB easily.