CBSE Class 12 · 2004 · All India · Alcohols Phenols and EthersShort Answer

What happens when ethanol is heated with concentrated sulphuric acid at 453 K? Explain the mechanism of this reaction.

Solution

C H_{3} C H_{2} OH conc. H_{2} S O_{4} 443 - 453 K C H_{2} = C H_{2} + H_{2} O

Mechanism:
Step 1 — protonation: CH

_{3}

_{2}

OH + H

^{+}

→ CH

_{3}

_{2}

\overset{+}{O}H

_{2}

.
Step 2 (slow) — loss of water: CH

_{3}

_{2}

\overset{+}{O}H

_{2}

→ CH

_{3}

_{2}^{+}

+ H

_{2}

O.
Step 3 — a β-hydrogen is removed (by HSO

_{4}^{-}

): CH

_{3}

_{2}^{+}

→ CH

_{2}

=CH

_{2}

+ H

^{+}

.
The acid is regenerated — it is a catalyst.

CBSE Class 12 Board · 2004 · All India · Subjective