I am preparing for my exam, and then I stuck up here. The answer to this question given by my teacher was

No, this doesn't show resonance because there are two bulky groups alongside chlorine, due to this chlorine will leave the plane and as we know resonance is a planar phenomenon, it won't show resonance

My question to you is:

Why did chlorine leave the plane on the first place? And what is the whole concept behind it? Please help me with it.

My guess to this is that since these two groups are really big and bulky, and are present near Chlorine, they take up almost all the space around which causes chlorine to leave the plane.

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    $\begingroup$ In one line, it's Steric Inhibition of Resonance. $\endgroup$ – Soumik Das May 21 '19 at 6:12

2-chloro-1,3-dinitrobenzene is shown below:

enter image description here

I believe your question is referring to the lack of conjugation between the nitro groups and the aromatic ring. this is best shown in an X-ray crystal structure such as this one:

enter image description here


Due to the sterics between the nitro groups and the chloro substituent, the nitro groups cannot adopt a conformation planar with the ring. This prevents the $\pi$ system of the nitro group from achieving substantial overlap with the $\pi$ system of the ring and prevents full conjugation of the electrons. The sterics are better seen in a spacefilling view:

enter image description here

So, it is not the chloro group that is out of the plane, but rather the nitro groups.

| improve this answer | |
  • $\begingroup$ That 3D rendition of the molecule's structure is not exactly an "X-ray crystal structure". $\endgroup$ – William R. Ebenezer May 20 '19 at 21:17
  • $\begingroup$ @William R. Ebenezer. I will take it down if I am mistaken. It came from the Cambridge crystallographic database. Check the link and I will remove if you could explain my mistake. $\endgroup$ – Withnail May 20 '19 at 21:53
  • $\begingroup$ @Withnail thankyou so much. I understand it now ! $\endgroup$ – Ishi May 21 '19 at 3:20
  • $\begingroup$ @Withnail But hey, doesn't it go against common sense? $\endgroup$ – William R. Ebenezer May 21 '19 at 3:42
  • $\begingroup$ @Withnail I can't find it, could you be more specific? $\endgroup$ – William R. Ebenezer May 21 '19 at 3:43

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