Final answer:
To add two water molecules across an alkene, a typical reaction is the hydration of the alkene using sulfuric acid as a catalyst, which results in the formation of an alcohol. For an anti addition, one might use a halohydrin formation followed by hydrolysis, although the question as stated typically refers to simple hydration rather than a strict anti addition.
Step-by-step explanation:
To add two water molecules across an alkene in an anti fashion, a typical reagent setup might involve a process similar to halohydrin formation followed by hydrolysis. However, if the goal is simply to hydrate an alkene in typical fashion (not necessarily anti), you would often use sulfuric acid (H₂SO₄) as a catalyst for the hydration of the alkene.
During the hydration process, the alkene undergoes an addition reaction where the double bond is broken and two components of a water molecule (H-OH) are added across the carbon atoms that were previously doubly bonded. With sulfuric acid as a catalyst, the reaction proceeds as follows: First, the alkene protonates to form a more stable carbocation. Then H₂O acts as a nucleophile and attacks the carbocation. Finally, another water molecule helps to deprotonate the newly formed oxonium ion, leading to the formation of the alcohol.
If a strict anti addition of water is required, one could envision the use of a reagent that forms a halohydrin intermediate, followed by an intramolecular displacement of the halide by a hydroxide ion, typically using water or hydroxide as the nucleophile in the presence of a suitable base. This results in the addition of two water molecules across the double bond in an anti fashion.