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I encountered a question which was stated as:

The reaction of vinyl chloride with hydrogen iodide to give 1-chloro-1-iodoethane is an example of anti-Markovnikov addition. Is this true or false?

The way I could see it, vinyl chloride has a dipole moment as shown below, since chlorine is more electronegative than the carbons in the double bond.

Dipole moment in vinyl chloride

Therefore the carbon on the left would have a greater partial positive charge than the carbon on the right. I therefore concluded that in a Markovnikov addition, the iodine atom would add to the CH2 carbon.

Since the question states that the product is 1-chloro-1-iodoethane, with the iodine atom adding to the CHCl carbon, that would thus be an example of anti-Markovnikov addition, according to the argument above.

I was told, however, that the reaction shown above was indeed Markovnikov addition. How is this so?

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    $\begingroup$ "Therefore the carbon on the left would have a greater partial positive charge than the carbon on the right" That's not correct. Dipoles don't work that way. $\endgroup$ – Zhe Apr 14 '17 at 13:25
  • $\begingroup$ But, due to chlorine pulling the electrons towards itself, wouldn't they move towards the right, hence causing a partial depletion of electrons on the carbon? Please explain. $\endgroup$ – Ajay Subramanian Apr 14 '17 at 14:29
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    $\begingroup$ Why do you think that effect extends through the entire molecule? $\endgroup$ – Zhe Apr 14 '17 at 17:25
  • $\begingroup$ What does Markovnikov's rule state? Does it mention hyper conjugation or does it mention dipoles? $\endgroup$ – Ajay Subramanian Apr 14 '17 at 19:04
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    $\begingroup$ It mentions neither of those things. The product created is via the more stabilized intermediate. But this is just a generalization of the Hammond Postulate for a transition state that is endergonic. $\endgroup$ – Zhe Apr 14 '17 at 19:19
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As far as I can see, that reaction follows Markovnikov's rule. The hydrogen goes where there is more hydrogen and the halide goes where there is less hydrogen. If you are confused as to why the product of that reaction is 1-chloro-1-iodoethane, it's because the electromeric effect the chloride atom exhibits trumps the inductive effect. We need to take the electromeric effects into consideration because the chloride atom is attached to a $\mathrm{sp^2}$ carbon. The inductive effect makes it attract electrons to itself, but the electromeric effect makes it push them away, like a $\ce{CH3-}$ would.

Resonance forms of vinyl chloride

It's the reason why $\ce{CH3-}$ and $\ce{Cl-}$, when attached to an aromatic nucleus, orient new substituents in the same positions, ortho and para (though $\ce{CH3-}$ has a stronger effect than $\ce{Cl-}$).

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