Question

The correct order for electron density at ortho-para positions for the following compounds is :

• A. $\left(i\right)>\left(ii\right)>\left(iii\right)$
• B. $\left(iii\right)>\left(ii\right)>\left(i\right)$
• C. $\left(i\right)>\left(iii\right)>\left(ii\right)$
• D. $\left(ii\right)>\left(i\right)>\left(iii\right)$

Answer 1

The correct option is D $\left(ii\right)>\left(i\right)>\left(iii\right)$

Groups attached to benzene ring in structures (i) and (iii) shows $-M$ effect, whereas in structure (ii) $(-N{H}_2)$ group shows $+M$ effect.
$-M$ groups decreases the electron density at ortho/para positions on the benzene ring, whereas $+M$ group increases the electron density at ortho/para positions. So, electron density (on benzene ring at ortho and para position) will be the most in (ii).

The order of $-M$ strength for the given groups are :
$-CN>-COR$
Correct order is :
$\left(ii\right)>\left(i\right)>\left(iii\right)$

Order of $-M$ effect :
$-N{O}_2>-CN>-CHO>-COR>-COOCOR>-COOR>-COOH>-CON{H}_2>-CO{O}^{-}$