Regioselectivity in addition of hydrogen iodide to vinyl chloride 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.



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?
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.



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 $\ce{Cl-}$).