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Addition of $\ce{HBr}$ to $\ce{Ph-CH2-CH=CH-CH3}$ possibly yields two products:

  • $\ce{Ph-CH2-CH2-CHBr-CH3}$ or
  • $\ce{Ph-CH2-CHBr-CH2-CH3}$

Which one of them should possibly be the major product? Markovnikov's rule (incorrectly) predicts that both of them should be the major product because the double-bonded carbon atoms in the substrate contains equal number of hydrogen atoms.

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    $\begingroup$ That would depend on reaction parameters. Also you're missing a rearrangement product, quite possibly even more important than the two. $\endgroup$
    – Mithoron
    Oct 19, 2022 at 13:34
  • $\begingroup$ @Mithoron Could you please elaborate about which rearrangement product you're talking of? $\endgroup$
    – ArijitKD
    Oct 20, 2022 at 17:04
  • $\begingroup$ Normal carbocation rearrangement when you got aromatic ring in neighbourhood. If you still don't get it check out your org. chem. book. $\endgroup$
    – Mithoron
    Oct 20, 2022 at 18:33

1 Answer 1

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Phenyl groups are bulky and cause steric hindrance.

This makes the $\ce{Br^-}$ nucleophile have an easier time targeting the carbon atom in the double bond that's closer from the right side of the reactant molecule.

On the other hand, $\ce{H^+}$ is small enough to bypass said steric hindrance.

Therefore, the first product is more likely to form.

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