Alkene hydration as electrophilic addition

Steps:

• Identify reactants: ethene (C₂H₄) has a C=C double bond; steam provides H₂O.
• Examine product: ethanol (C₂H₅OH) forms by incorporating H and OH across the double bond.
• Recall reaction type: alkenes undergo electrophilic addition where the pi bond breaks and atoms add.
• Confirm mechanism: H₂SO₄ catalyzes H⁺ addition to C=C, followed by H₂O, yielding alcohol.

Why B is correct:

• Addition reactions involve reagents adding to unsaturated bonds like C=C, converting CH₂=CH₂ + H₂O to CH₃CH₂OH without substituents.

Why the others are wrong:

• A: Acid-base reactions transfer protons (e.g., neutralization), not add across double bonds.
• C: Hydrolysis cleaves bonds in compounds like esters using water, not adds to alkenes.
• D: Substitution replaces one atom/group with another (e.g., halogenation of alkanes), preserving saturation.

Final answer: B