CIE A Level Chemistry Halogen Compounds Exam Questions

About CIE A Level Chemistry Halogen Compounds

Halogen compounds in Cambridge International 9701 focuses on halogenoalkanes as useful intermediates in synthesis. At AS you classify them as primary, secondary or tertiary, then study nucleophilic substitution with aqueous hydroxide, cyanide ions and ammonia, and elimination with ethanolic hydroxide to form alkenes. You need to describe and explain the SN1 and SN2 mechanisms and relate reactivity to the strength of the carbon-halogen bond, tested by hydrolysis with aqueous silver nitrate. The A Level extension compares halogenoalkanes with halogenoarenes such as chlorobenzene, which are much less reactive. The topic links to kinetics and organic synthesis, and examiners regularly ask for mechanisms and the conditions that decide substitution or elimination.

What you need to know

• Classification Primary, secondary and tertiary halogenoalkanes are classified by the number of carbon atoms attached to the carbon bonded to the halogen.
• Nucleophilic substitution reactions Aqueous NaOH, heat under reflux gives an alcohol. KCN in ethanol, heat gives a nitrile, increasing chain length by one carbon. Excess NH3 in ethanol, heated in a sealed tube, gives an amine.
• SN2 mechanism Typical of primary halogenoalkanes: the nucleophile attacks the delta positive carbon as the C-X bond breaks, through a single transition state. Rate = k[RX][OH-].
• SN1 mechanism Typical of tertiary halogenoalkanes: the C-X bond breaks first to form a stable tertiary carbocation, which is then attacked by the nucleophile. Rate = k[RX].
• Elimination Ethanolic NaOH, heat gives an alkene and HX. OH- acts as a base, removing an H from the carbon next to the C-X carbon.
• Reactivity Rate of hydrolysis increases from C-Cl to C-Br to C-I because C-X bond energy decreases. With aqueous AgNO3 in ethanol, AgI precipitates fastest.
• Halogenoarenes (A Level) Chlorobenzene does not undergo nucleophilic substitution easily because a lone pair on chlorine overlaps with the delocalised ring, strengthening the C-Cl bond, and the ring repels nucleophiles.

Common mistakes in Halogen Compounds

• Saying reactivity depends on bond polarity; C-F is most polar but least reactive, because bond strength is the deciding factor.
• Using aqueous NaOH when elimination is required; elimination needs ethanolic NaOH.
• Drawing the SN2 transition state without a negative charge or partial bonds.
• Forgetting that substitution with KCN increases the carbon chain length, which is why it is useful in synthesis.
• Writing that the nucleophile attacks the halogen atom; it attacks the delta positive carbon.

Try three Halogen Compounds exam-style questions