Question

Which of the following(s) is/are an examples of $-E$ effect?

• A.
• B.
• C.
• D.

Answer 1

Answer: (B), (D)

Electromeric effect is an temporary effect, it is observed only in presence of attacking reagent. This involves the shifting of $\pi$ in the mutiple bond.
When the $\pi$electrons of the multiple bond are transferred to the atom to which the reagent gets attached, it is called positive electromeric effect (+E).
When the $\pi$electrons of the multiple bond are transferred to the atom to which the reagent does not get attached, it is called negative electromeric effect (-E).
In (a) and (c), the $\pi$electrons of the double bond is transferred to $H^{+}\text{and}B{r}^{+}$. Hence, these reaction shows +E effect.
In (b) and (d), the $\pi$electrons of the double bond is transferred to oxygen atom which is not the attacking reagent here. So (b) and (d) show -E effect.
Therefore, answer is option (b) and (d).