I came across this reaction where simple electrophilic addition mechanism will result in a vic-diiodide.

$$\ce{H2C=CH-CH2-I ->[HI, excess][CCl4]}$$

But the solution says that vicinal iodine groups are unstable and the following reaction occurs:

$$\ce{CH3-\underset{(unstable)}{\underset{\underset{\huge I}{|}}{C}H-\underset{\underset{\huge I}{|}}{C}H2} ->[][$-$I2] CH3-CH=CH2}$$

I am unable to figure out how the dihaloalkane is unstable. It might be due to steric reasons, but still the two iodine can arrange in a way that reduces the repulsions.

But even if it is unstable, how did $\ce{I2}$ leave the haloalkane to yield an alkene? This dehalogenation does happen when a metal is used though (Zn dust, Mg in acetone, I guess) but what would be the mechanism here?

  • $\begingroup$ As for the question, I doubt one can really comment on the stability of the compound without knowing what else is present besides the compound. Apparently, the source also omitted the presence of any reagent that could have been used to cause the dehalogenation, only showing the $\ce{(-I2)}$. $\endgroup$
    – user79161
    Mar 25, 2021 at 16:09

1 Answer 1


That 1,2-diodopropane is unstable does not indicate whether it is as the neat halide or in solution. Breaking (BDE) two $\ce{C-I}$ bonds costs $\pu{+114 kcal/mol}$ while forming the $\pi$-bond of propene is worth approximately $\pu{-62 kcal/mol}$ and the formation of iodine, $\pu{-36 kcal/mol}$. The net reaction is endothermic by $\pu{+16 kcal/mol}$. The reaction is entropically favored and the products are volatile. Therefore, the unfavorable equilibrium can be shifted to the right.

  • 1
    $\begingroup$ Is this in clayden @user55119 ? And can this happen in other like vicinal chlorides / bromides ? $\endgroup$
    – WizardMath
    Mar 26, 2021 at 3:19
  • $\begingroup$ Hey i found an interesting observation, C-C single bond length is 134 pm. While van der wall radius of Iodine is 198 pm and that of chlorine is 175 pm. That means even if we count opposite orientation in Iodine, van der wall repulsions are too significant unlike in case of Chlorine. $\endgroup$
    – Swadhin
    Mar 26, 2021 at 7:43
  • $\begingroup$ Could you share a reference for the mechanism? $\endgroup$ May 7, 2021 at 9:39
  • $\begingroup$ Ashish Ahuja: Try this. pubs.acs.org/doi/pdf/10.1021/ja01607a037 $\endgroup$
    – user55119
    May 7, 2021 at 16:01

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