Catalysts in ethanol reactions

Steps:

• Identify ethanol to ethene as acid-catalyzed dehydration: CH3CH2OH → CH2=CH2 + H2O, using concentrated H2SO4 at 170°C.
• Confirm standard lab conditions require H2SO4 as catalyst for dehydration to remove water efficiently.
• Identify ethanol to bromoethane as nucleophilic substitution: CH3CH2OH + HBr → CH3CH2Br + H2O, proceeding without catalyst.
• Verify halogenation uses reagent like HBr directly, no additional catalyst needed under lab conditions.

Why B is correct:

• Dehydration follows the mechanism where H2SO4 acts as a catalyst by protonating the alcohol, facilitating water loss per organic chemistry principles.

Why the others are wrong:

• A: Bromoethane formation uses HBr as reactant, not catalyst.
• C: Ethene requires acid catalyst for dehydration.
• D: Ethene dehydration mandates catalyst use.

Final answer: B