AS · Practice questions · Reactions of alcohols

React the –OH.

Six original Cambridge-style questions on the reactions of alcohols.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Na, halogenation, dehydration, esterification.

Hydrogen bonding: high b.p., soluble in water.
Na: alkoxide + H₂.
PCl₅: steamy HCl fumes.
Esterification: acid + alcohol, conc. H₂SO₄, reversible.

Explain

[3 marks]

Explain why propan-1-ol (M_r 60) has a much higher boiling point than butane (M_r 58).

Both have similar numbers of electrons, so similar id-id forces. ✓
Propan-1-ol also forms hydrogen bonds between –OH groups (H on O, lone pairs on O). ✓
Hydrogen bonds are much stronger, so more energy is needed to separate the molecules. ✓

Recall

[3 marks]

Write equations for the reactions of ethanol with (a) sodium, (b) PCl₅, (c) ethanoic acid.

(a) 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. ✓
(b) C₂H₅OH + PCl₅ → C₂H₅Cl + POCl₃ + HCl. ✓
(c) CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. ✓

Analysis

[3 marks]

Butan-2-ol is heated with concentrated phosphoric acid. Name all the organic products and explain why there is more than one.

But-1-ene, cis-but-2-ene and trans-but-2-ene. ✓
The H can be removed from C1 or C3 next to the C–OH carbon, giving different positions of C=C. ✓
But-2-ene has restricted rotation with two different groups on each carbon, so it exists as cis and trans isomers. ✓

Calculation

[2 marks]

4.60 g of ethanol reacts with excess propanoic acid to give 6.12 g of ethyl propanoate (M_r 102.0). Calculate the percentage yield.

n(ethanol) = 4.60 ÷ 46.0 = 0.100 mol, so the maximum is 0.100 mol = 10.2 g ester. ✓
Yield = 6.12 ÷ 10.2 × 100 = 60.0%. ✓

Recall

[2 marks]

Give two methods of making ethanol, with reagents and conditions.

Hydration of ethene: steam, H₃PO₄ catalyst, 300 °C, 6 MPa. ✓
Hydrolysis of bromoethane: NaOH(aq), heat under reflux (or reduction of ethanal with NaBH₄). ✓

Explain

[2 marks]

Explain why the solubility of alcohols in water decreases as the carbon chain gets longer.

The –OH group can hydrogen bond to water. ✓
A longer non-polar hydrocarbon chain cannot, and disrupts water's hydrogen bonding, so solubility falls. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.