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Hückel's rule predicts that; a planar ring molecule will have aromatic properties if it has 4n+2 π electrons in conjugation. If 4n π electrons are in cyclic conjugation, then it is anti-aromatic.

So, I was looking at resonance structures of Chlorobenzene and; enter image description here

C—Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction. - My Textbook

But there are 8π electrons (6 from the ring and 2 from the chlorine) that are in conjugation, then this should be anti-aromatic. So, it seems like one of these should be wrong. Which is it?

Is chlorobenzene aromatic? If yes, does that mean the lone pair of chlorine is not in resonance? If no, does that make it unstable, since it will be anti-aromatic?

Bharat Karri
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To apply the $4n+2/4n$ rule at all is wrong unless the conjugated $\pi$ electrons are precisely in a single ring. In the first structure you drew that is true, and you can confidently count six $\pi$ electrons and call that contributing structure aromatic. But when you draw the additional structures that bring in the chlorine atom, then with the pendant $\pi$ bond to the chlorine you don't have a single conjugated ring anymore. So no more $4n+2/4n$ rule.

We still may have aromaticity in all the contributing structures, not just the first. But we can't tell from the $4n+2/4n$ rule except for that first structure.

Oscar Lanzi
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  • Does that mean the chlorine doesn't undergo resonance since it breaking the resonance of the benzene ring? I'm particularly interested in C-Cl bond length. I tried searching in Wiki but I got no info. – Bharat Karri Jan 11 '24 at 07:13
  • No. The chlorine could still conjugate, but we would not be able to evaluate aromaticity with thr 4n+2/4n rule if it does. The rule is really applicable only under narrow circumstances. – Oscar Lanzi Jan 11 '24 at 10:34
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For this question i will suggest you to read about the L.C.C.S rule, which says to take the largest conjugated cyclic system. Here the lone pair of chlorine is not in the single cyclic system. You can only apply Huckel's rule for a single closed ring.

Also i would like to add that anti-aromatic molecules do not show resonance even if they have conjugation due to the presence of unpaired electrons in ABMO(Anti Bonding Molecular Orbit). You can observe it by drawing the Frost Circle of the molecule.

Aritra
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