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Addition of Hydroiodic Acid to Alkenes to Give Alkyl Iodides

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).