Halogenoalkanes A-Level Chemistry Syllabus

This page summarises the main syllabus content for halogenoalkanes in A-level Chemistry. Although wording differs slightly between exam boards, the core ideas are very similar.

1. What Halogenoalkanes Are

• Halogenoalkanes are compounds in which a halogen atom replaces a hydrogen atom in an alkane.
• They can be classified as primary, secondary, or tertiary.
• The classification depends on the number of carbon atoms attached to the carbon bonded to the halogen.

2. Bond Polarity

• The carbon-halogen bond is polar because halogens are more electronegative than carbon.
• This makes the carbon atom partially positive.
• As a result, halogenoalkanes are susceptible to attack by nucleophiles.

3. Nucleophilic Substitution

• Reaction with aqueous hydroxide ions produces alcohols.
• Reaction with cyanide ions produces nitriles.
• Reaction with ammonia produces primary amines.
• The reaction with cyanide ions is important because it increases the carbon chain length.

4. Elimination Reactions

• Halogenoalkanes can react with ethanolic potassium hydroxide to form alkenes.
• In this case, the hydroxide ion acts as a base rather than a nucleophile.
• Students often need to compare substitution and elimination conditions.

5. Mechanisms

• Students are expected to understand and draw nucleophilic substitution mechanisms.
• Curly arrows should be used correctly in mechanism diagrams.
• Mechanisms for reactions with hydroxide ions, cyanide ions, and ammonia are commonly required.

6. Reactivity and Hydrolysis Rates

• Students compare the reactivity of primary, secondary, and tertiary halogenoalkanes.
• Students also compare chloroalkanes, bromoalkanes, and iodoalkanes.
• Differences in reaction rate are explained using carbon-halogen bond enthalpy.

7. Practical Work and Tests

• Aqueous silver nitrate in ethanol is used to test halogenoalkanes and compare hydrolysis rates.
• Students should understand the observations, including precipitate formation.
• Practical work may include comparing relative reactivity experimentally.

8. Environmental Chemistry

• Some specifications include chlorofluorocarbons (CFCs) and ozone depletion.
• Students should understand how UV light breaks C-Cl bonds to form chlorine radicals.
• These radicals catalyse the decomposition of ozone in the stratosphere.

Simple Overall Summary

In short, the halogenoalkanes topic covers structure, polarity, substitution, elimination, mechanisms, reactivity trends, practical tests, and environmental impact.