Complete the mechanism for the electrophilic addition when the alkene is treated with water in acid.

CH₃CH == CH₂  +  H₂O → H⁺ →  CH₃CH(OH)CH₃  +  H⁺

The reaction of propene and water in acidic medium produces isopropyl alcohol.

Explanation:

Let's propose the following reaction:

Propene + H₂O in H⁺ medium  →  Alcohol

CH₃CH == CH₂  +  H⁺ →  CH₃C⁺CH₂

Electrons from the double bond attack the proton. So the double bond is broken and you form a carbocation.

Carbocation will be attacked by the electrons from the oxygen on the molecule of water.

CH₃C⁺CH₂  + H₂O: :  →  CH₃ _CH _ CH₃  

                                                   |

                                                ⁺OHH

It is called electrophilic addition cause the proton which is the electrophile is added to the sp₂ carbon in the alkene (where many H are bonded),  and the nucleophile, water is added to the other sp₂ carbon.

This last attack is so fast that the carbocation is combined with any nucleophile which it collides first. In the hydration reaction, there are two nucleophiles: water and the anion from the acid (let's think Cl⁻, cause our medium can be HCl).    The product of this collision is a protonated alcohol. Since we know that protonated alcohols are very strong acids, this protonated alcohol loses a proton, and the end product of the addition reaction is an alcohol.

In the first step, a proton adds to the alkene, but is returned to the reaction mixture during the final step, when the alcohol is deprotonated.

 CH₃ _CH _ CH₃   →   CH₃ _CH _ CH₃   + H⁺

             |                                 |

          ⁺OHH                          OH