Question 12.7: Propose a two-step mechanism for the addition of HI to methy...

Solution
Step 1: Add a proton. In this step, the carbon–carbon double bond is a nucleophile and H^+ is the electrophile. Reaction of H^+ with the carbon– carbon double bond forms a new C—H bond with the carbon bearing the greater number of hydrogens and gives a 3° carbocation intermediate.

Step 2: Reaction of an electrophile and a nucleophile to form a new covalent bond. Reaction of the 3° carbocation intermediate with iodide ion completes the valence shell of carbon and gives the product