AS · Practice questions · Halogenoalkanes

Swap the halogen.

Six original Cambridge-style questions on halogenoalkanes.

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

Aqueous substitutes; ethanolic eliminates.

NaOH(aq) → alcohol; KCN/ethanol → nitrile (+1 C).
NH₃/ethanol, pressure → amine.
NaOH/ethanol → alkene.
Rate: C–I > C–Br > C–Cl (bond strength).

Recall

[4 marks]

Give the reagents, conditions and product for converting 1-bromopropane into (a) propan-1-ol, (b) butanenitrile, (c) propylamine, (d) propene.

(a) NaOH(aq), heat under reflux: propan-1-ol. ✓
(b) KCN in ethanol, heat under reflux: CH₃CH₂CH₂CN. ✓
(c) excess NH₃ in ethanol, heat in a sealed tube: CH₃CH₂CH₂NH₂. ✓
(d) NaOH in ethanol, heat: CH₃CH=CH₂. ✓

Recall

[2 marks]

Give two ways of making 1-chloropropane from propan-1-ol, with equations.

PCl₅: CH₃CH₂CH₂OH + PCl₅ → CH₃CH₂CH₂Cl + POCl₃ + HCl. ✓
SOCl₂: CH₃CH₂CH₂OH + SOCl₂ → CH₃CH₂CH₂Cl + SO₂ + HCl. ✓

Explain

[3 marks]

Describe an experiment to compare the rates of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane, and explain the results.

Add each to aqueous silver nitrate in ethanol at the same temperature (about 50 °C) and time the appearance of a precipitate. ✓
Order: iodo (yellow) fastest, then bromo (cream), then chloro (white) slowest. ✓
The C–I bond is weakest and breaks most easily; bond strength matters more than polarity. ✓

Analysis

[2 marks]

Classify 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane as primary, secondary or tertiary.

1-bromobutane: primary; 2-bromobutane: secondary. ✓
2-bromo-2-methylpropane: tertiary. ✓

Explain

[2 marks]

Explain why the reaction of a halogenoalkane with KCN is useful in synthesis.

It adds one carbon atom to the chain, forming a nitrile. ✓
The nitrile can then be converted into other compounds, e.g. hydrolysed to a carboxylic acid or reduced to an amine. ✓

Explain

[2 marks]

Explain why the carbon atom of the C–Br bond is attacked by nucleophiles.

Bromine is more electronegative than carbon, so the bond is polar, C^δ+–Br^δ−. ✓
The δ+ carbon attracts nucleophiles, which donate a lone pair. ✓

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.