Addition of HI twice to alkynes to give geminal diiodides
Description: Addition of two equivalents of hydrogen iodide (HI) to alkynes results in geminal di-iodides.
Notes: This reaction proceeds with Markovnikoff selectivity, meaning that the iodide ends up on the more substituted carbon. “Geminal” means that the two iodides are attached to the same carbon. Note that this reaction is essentially identical to the double addition of HCl and HBr to alkynes.
Notes: Note how in all cases the iodides add to the more substituted carbon (Markovnikoff selectivity)
Mechanism: Attack of the alkyne π bond on H–I leads to formation of the vinyl carbocation at the more substituted position (Markovnikoff selectivity) (Step 1, arrows A and B). Attack of the carbocation by iodide (Step 2, arrow C) provides the vinyl iodide. When a second equivalent of HI is present, the process then repeats itself. Protonation of the alkene (Step 3, arrows D and E) give a carbocation, which is then attacked by iodide (Step 4, arrow F). [Note that this second addition also follows Markovnikoff’s rule].