Explain the dehydration of ethanol with its mechanism. How do 1°, 2° and 3° alcohols differ in (i) ease of dehydration and (ii) the Lucas test?

1. 1 · Overall reaction. C₂H₅OH → CH₂=CH₂ + H₂O, with conc H₂SO₄ at 443 K.
2. 2 · Mechanism.
   • Step 1: CH₃CH₂OH + H⁺ → CH₃CH₂O⁺H₂ (protonation)
   • Step 2: CH₃CH₂O⁺H₂ → CH₃CH₂⁺ + H₂O (slow, carbocation)
   • Step 3: CH₃CH₂⁺ → CH₂=CH₂ + H⁺
3. 3 · Ease of dehydration. 3° > 2° > 1°: 20% H₃PO₄ at 358 K, 85% H₃PO₄ at 440 K, conc H₂SO₄ at 443 K, following carbocation stability.
4. 4 · Lucas test. Conc HCl + anhydrous ZnCl₂: 3° turbid at once, 2° in about 1 to 5 minutes, 1° no turbidity at room temperature.
5. 5 · Close in one line. So both dehydration and the Lucas test depend on how easily a carbocation forms.

Scoring tip: Mark step 2 as 'slow, rate-determining'; that phrase is often worth a mark.