Question

Choose the incorrect statement :

• A. In $-R$ effect , transfer of $\pi$ (pi) electron takes place away from the conjugated system to an atom or a group.
• B. In $Ph-CN,-CN$ shows $+R$ effect.
• C. Electron withdrawing group attached to the conjugated system shows $-R$ effect
• D. (a) and (c) above.

Answer 1

The correct option is B In $Ph-CN,-CN$ shows $+R$ effect.

$-R$ effect :
In $-R$ effect , transfer of $\pi$ (pi) electron takes place away from the conjugated system to an atom or a group.
Electron withdrawing group attached to the conjugated system shows $-R$ effect
In $Ph-CN,-CN$ shows $-R$ effect.
So, option (b) is incorrect .


Theory :

In general, directly attached atom or group to the conjugated system shall possess for $-R$ effect:
1. Vacant Orbital
2. $-X=Y$
3.$-X\equiv Y$
where , electronegativity of Y is greater than X
Order of $-M$ effect :
$-N{O}_2>-CN>-CHO>-COR>-COOCOR>-COOR>-COOH>-CON{H}_2>-CO{O}^{-}$