Question

How can I illustrate Markovnikov's rule by the reaction of propene with hydrobromic acid?

Answer 1

Treat the acid as `D-Br`, i.e. `D` = deuterium. Consider the intemediate after the olefin has reacted with `D^+`.

Explanation:

Markownikow addition:

`H_2C=CHCH_3 + D-Br rarr H_2DC-C^+HCH_3 + Br^-`

Anti-Markownikow addition:

`H_2C=CHCH_3 + D-Br rarr H_2C^+(-CHDCH_3) + Br^-`

As you know, the secondary carbocation is more stable than the primary carbocation. Because the carbocation intermediate is then going to react with the electrophile, `Br^-`, we should see more `H_2(D)C-CHBrCH_3` than the anti-Markownikow product, `H_2(Br)C-CHDCH_3`.

For further background see: Hughes, P. J. Chem. Ed. 2006 , 83 , 1152. Was Markovnikov's Rule an Inspired Guess? I can't get to this article, but your librarian should supply it. J. Chem. Ed. articles are generally very good.