I want to ask a question concerning Markovnikov's rule.

Consider the following question:

enter image description here

We can tell that there will be two types of stabilisation upon formation of the carbocation:

  • Resonance via the phenyl group
  • sigma-hyperconjugation via the methyl group

When attempting this question, I thought the more stable carbocation would be the one where the positive charge lies to the left of the alkene double bond, and can be stabilised by the phenyl group, since resonance stabilisation is stronger than sigma hyperconjugation to yield the major isomer.

However, the answers state the opposite:

Marking Scheme

They state that the more stable carbocation is the one that can be stabilised by sigma-conjugation, yielding the alternate product, due to more carbon (groups) being attached to the carbon on the double bond. I thought it would be less stable as it is destabilised by the EWG aldehyde group and stabilised by the weaker sigma-hyperconjugation.

What is the correct interpretation of the answer?

  • $\begingroup$ When I tried modeling $\ce{[Mol\bond{...}X]+}$, both halogens ended up beta to the carbonyl group, so I'd conclude it really depends on the orientation with which the $\ce{ICl}$ reacts with. $\endgroup$ Sep 11 '20 at 12:53
  • $\begingroup$ @Martin-マーチン by "modelling $\ce{[Mol...X]+}$, both halogens ended up beta" do you mean the halogen which forms the cyclic intermediate? $\endgroup$ Sep 11 '20 at 13:00
  • $\begingroup$ @Safdar In the model, there is no cyclic intermediate, or at least there are significantly different leg lengths. See for example image: Regioselectivity of bromination of alkenes, but it is much more pronounced. $\endgroup$ Sep 11 '20 at 13:11
  • $\begingroup$ Does Anchimeric Assistance have to do something with this? $\endgroup$ Sep 11 '20 at 15:47
  • $\begingroup$ To be honest, I don't understand and you don't make it clear how Markovnikov's rule would apply here. Could you clarify please. $\endgroup$ Sep 11 '20 at 21:13

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