Final answer:
In the hydration of an alkene, the alkene itself is the nucleophile in the first step. Water acts as the nucleophile in the second step, adding to the carbocation intermediate formed from the first step.
Step-by-step explanation:
In the first step of the hydration of an alkene, the alkene itself acts as the nucleophile. This is because alkenes have electron-rich double bonds (pi bonds) that are attracted to electrophiles and capable of donating their electrons to form a new bond. During hydration, as the alkene contacts the acid catalyst, typically sulfuric acid, it undergoes protonation forming a carbocation intermediate.
This positively charged intermediate is then attacked by water (H2O), which acts as the nucleophile in the second step of the hydration mechanism. The overall process results in the addition of water across the double bond, converting the alkene into an alcohol.