Answer: Concentrated H 2 SO 4.
- A Dilute HCl as frequently observed in practice
- B Concentrated H<sub>2</sub>SO<sub>4</sub>
- C NaOH in many documented cases
- D Br<sub>2</sub> water according to conventional understanding
Correct answer: B. Concentrated H<sub>2</sub>SO<sub>4</sub>
Explanation: Concentrated sulfuric acid causes dehydration of alcohols to alkenes.
The phenoxide ion's negative charge spreads into the aromatic ring through resonance, lowering its energy and making phenol give up its proton more easily; an alkoxide ion (from a plain alcohol) has nowhere to delocalise the charge, making alcohols far less acidic.
Concept context
Oxygen-containing organic compounds. Understand the acidic nature of phenols, reactions of alcohols (dehydration, oxidation, esterification), and properties of ethers. Includes industrial applications and named reactions.