Which one among the following resonating structures is not correct?

Solution

Chemistry ma’am · 4 steps

Nitro pulls the π electrons towards itself, so the carbon next to it can take a minus, never a plus.

1. Name the molecule. All four structures belong to CH₂=CH-CH=CH-NO₂. Number the chain so that C1 carries the NO₂ group and C4 is the terminal CH₂.
2. Direction of shift. NO₂ is a strong -M group. It pulls π electrons towards itself. So in any acceptable contributor, the π electrons move towards the nitro end, and the negative charge lands on the side nearer NO₂. The positive charge lands on the side farther away.

   Option	Charges	Electron shift	Verdict
   (a)	C2 -, C1 + (next to NO₂)	away from NO₂	not correct
   (b)	C2 +, C1 -	towards NO₂	correct
   (c)	C4 +, both O -, N +	all the way onto the O atoms	correct
   (d)	C4 +, C3 -	towards NO₂; the C3 anion is conjugated with C₁=C₂-NO₂	correct

3. Why (a) fails. In (a) the electrons have moved away from NO₂. That leaves a carbocation directly on the carbon bearing an electron-withdrawing group, which destabilises it further. The -M effect never pushes electrons that way.
4. So. The resonating structure that is not correct is (a).

The idea to keep. With a -M group such as NO₂, CN or CHO on a conjugated chain, the π electrons always flow towards that group. The minus goes near it and the plus goes away from it.

Answer from the NTA final answer key. Solution by PrepWiser.