Addition of Hydroiodic Acid to Alkenes to Give Alkyl Iodides

by James

Description: Treatment of alkenes with hydroiodic acid (HI) leads to the formation of alkyl iodides. Note that the iodine always ends up on the more substituted carbon.

Notes: Note that this reaction is Markovnikoff selective. Since it goes through a carbocation, rearrangements are possible in some situations.


Notes: When a secondary carbocation is formed adjacent to a tertiary or quaternary carbon, rearrangements are possible. Note the third example – where Markovnikoff’s rule gives no clear preference a mixture will be obtained.

Mechanism: Electrons from the C1-C2 π bond attack the hydrogen of HI, expelling the iodide ion and leading to the formation of a carbocation (Step 1, arrows A and B). Note that the most stable carbocation is formed preferentially. The iodide anion then attacks the carbocation, leading to formation of the alkyl iodide (Step 2, arrow C).



{ 4 comments… read them below or add one }


Hi James,

I think you meant to switch the Cl’s with the I’s?



Yep – thanks!



Um…I might be wrong but as far as I know Iodine has the greatest atomic size of all the members of the halogen groups so shouldn’t it try to detach itself from the organic compound thus making the compound highly unstable



Alkyl iodides can be unstable under certain conditions, but alkyl iodides can still be made this way.


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