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Question:

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Attempt:

The first reaction is addition of hydrogen halide. The reaction follows markovnikov's rule. The two possible products of the first reaction are:

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For each product, the carbocations formed are:

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After the formation of carbocation, $\ce{Cl-}$ attacks the carbocation forming a dihalide.

The final product after second reaction depends on the above intermediates.

The second carbocation faces electron withdrawing inductive effect and electron donating resonance effect from chlorine. But for halogens, inductive effect is more than resonance effect. So the carbocation is destabilized. So I chose option (A) as the answer since the first carbocation is formed.

But the answer given is (C) which means the second carbocation is formed.

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  • $\begingroup$ What is the question? $\endgroup$ – Lighthart Feb 15 '16 at 15:36
  • $\begingroup$ @Lighthart the answer given is C. But I got A. $\endgroup$ – Aditya Dev Feb 15 '16 at 15:37
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    $\begingroup$ Clearly the resonance effects for pi-bond formation with chlorine are stronger than the inductive effects. Most of the time resonance is a stronger effect that induction, unless fluorine is involved. $\endgroup$ – Lighthart Feb 15 '16 at 15:46
  • $\begingroup$ @Lighthart but that's not the case in chlorobenzene is it? $\endgroup$ – Aditya Dev Feb 15 '16 at 15:56
  • $\begingroup$ Chlorobenzene is an electrophilic aromatic substitution (EAS), which is a different mechanism that the electrophilic addition you have presented here. That being said, in EAS, chlorine's pi-donation directs ortho and para, so yes, the resonance effects are dominating the inductive effects in this case too. $\endgroup$ – Lighthart Feb 15 '16 at 16:03
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Indeed the second carbocation is formed. It's favored by the chlorine atom being able to delocalize the charge through resonance, draw resonance hybrids for the second cation and you should see it. In a sense you might say that the carbon bonded to chlorine is more effectively substituted for stabilizing the cation and so the Markovnikov rule is "bent" accordingly.

The usual version of the Markovnikov rule is for hydrocarbon substrates.

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