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Learn9701Paper 2 · AS Level Structured QuestionsHalogen compoundsHalogenoalkanes
Halogen compounds · Subtopic 1 of 1

Halogenoalkanes

The C–X bond is polar (C δ+), so halogenoalkanes are attacked by nucleophiles. Nucleophilic substitution: aqueous NaOH, heat under reflux → alcohol; KCN in ethanol → nitrile (chain lengthened by one carbon); excess NH₃ in ethanol, heat in a sealed tube → amine. SN2 (one step, transition state) for primary; SN1 (via carbocation) for tertiary. With ethanolic NaOH (hot), elimination gives an alkene. Reactivity: C–I > C–Br > C–Cl, because the C–I bond is weakest (bond enthalpy matters more than polarity). Hydrolysis rate can be compared with aqueous AgNO₃ in ethanol: AgI (yellow) forms fastest.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH12AQA 7405 Paper 2≈ 50 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Classify halogenoalkanes.
  • Describe substitution reactions with OH⁻, CN⁻ and NH₃, and elimination.
  • Draw SN1 and SN2 mechanisms.
  • Explain reactivity in terms of bond enthalpy.
Pause and think

The C–F bond is the most polar carbon–halogen bond, yet fluoroalkanes are the least reactive. Why?

Section 2 of 14

Core idea

Start hereSwap or snap

Hydroxide ions can either swap places with a halogen or knock off a hydrogen to make a double bond. Change the solvent and you change the reaction.

The big idea

The polar C–X bond makes carbon δ+ and open to nucleophiles. Substitution or elimination depends on conditions; rate depends on C–X bond strength.

1. SN1 vs SN2

Two substitution mechanisms

Structure decides the route.

Worked exampleChoosing conditions

Starting from 1-bromopropane, give reagents and conditions for (a) propan-1-ol, (b) propene, (c) butanenitrile.

Goal
Three sets.
Strategy
Match nucleophile and solvent.
  1. (a) NaOH(aq), reflux\text{(a) NaOH(aq), reflux}
    Substitution.
  2. (b) NaOH in ethanol, heat\text{(b) NaOH in ethanol, heat}
    Elimination.
  3. (c) KCN in ethanol, reflux\text{(c) KCN in ethanol, reflux}
    Substitution; chain + 1.
AnswerAs above.
✓ CheckNitrile has 4 carbons ✓.
Common misconceptionPolarity trap C–F is the most polar, but the strongest bond, so the least reactive. Reactivity follows bond enthalpy.
Pause and think

Why is excess ammonia used to make a primary amine?

Summary

  • OH⁻(aq), CN⁻, NH₃: substitution.
  • OH⁻ in ethanol: elimination.
  • SN2 primary; SN1 tertiary.
  • C–I fastest.
Key words
Nucleophilic substitution
A nucleophile replaces the halogen.
Elimination
Removal of HX to form a C=C.
Reflux
Heating with a condenser so vapours return to the flask.
HistoryIngold and Hughes. Christopher Ingold and Edward Hughes distinguished SN1 and SN2 mechanisms in the 1930s by measuring how rates depended on concentrations.
Fun factTeflon. PTFE (Teflon) is made of C–F bonds, so strong that almost nothing sticks to or reacts with non-stick pans.
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