Halogenoalkanes

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Recall the reactions (reagents and conditions) by which halogenoalkanes can be produced:

    • (a) Free-radical substitution of alkanes by Cl₂ or Br₂ in the presence of ultraviolet light, as exemplified by the reactions of ethane
    • (b) Electrophilic addition of an alkene with a halogen, X₂, or hydrogen halide, HX(g), at room temperature
    • (c) Substitution of an alcohol by various reagents

  2. Classify halogenoalkanes into primary, secondary and tertiary

  3. Describe nucleophilic substitution reactions with NaOH(aq), KCN, NH₃, and aqueous silver nitrate

  4. Describe the elimination reaction with NaOH in ethanol and heat to produce an alkene

  5. Describe the SN1 and SN2 mechanisms of nucleophilic substitution including the inductive effects of alkyl groups

  6. Recall which halogenoalkanes react via SN1 or SN2 mechanisms

  7. Describe and explain the different reactivities of halogenoalkanes with reference to C–X bond strengths

1. What Are Halogenoalkanes?

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Halogenoalkanes are organic compounds derived from alkanes (saturated hydrocarbons containing only carbon and hydrogen) where one or more hydrogen atoms have been replaced by halogen atoms. The halogens are fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

Example: Chloroethane (CH₃CH₂Cl) is a halogenoalkane where one hydrogen atom in ethane has been replaced by a chlorine atom.

2. How Halogenoalkanes Are Produced

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There are three main ways to make halogenoalkanes:

Method 1: Free-Radical Substitution of Alkanes

This reaction happens when an alkane reacts with chlorine (Cl₂) or bromine (Br₂) in the presence of ultraviolet (UV) light. UV light is a type of light energy that we cannot see with our eyes but has enough energy to break chemical bonds.

Example: Reaction of ethane with chlorine

Overall equation: CH₃CH₃ + Cl₂ → CH₃CH₂Cl + HCl (ethane + chlorine → chloroethane + hydrogen chloride)

How it works - the mechanism:

This reaction happens in three stages:

Stage 1: Initiation

  • UV light provides energy to break the Cl–Cl bond
  • The bond breaks by homolytic fission (each chlorine atom takes one electron from the shared pair)
  • This produces two free radicals - chlorine atoms with an unpaired electron, written as Cl•

Cl–Cl → 2Cl•

Stage 2: Propagation (the chain reaction)

  • Step 1: A chlorine radical attacks an ethane molecule, removing a hydrogen atom

    • CH₃CH₃ + Cl• → CH₃CH₂• + HCl
    • This creates a new free radical (the ethyl radical, CH₃CH₂•)

  • Step 2: The ethyl radical then reacts with a chlorine molecule

    • CH₃CH₂• + Cl₂ → CH₃CH₂Cl + Cl•
    • This makes the product (chloroethane) and regenerates a chlorine radical
    • The chlorine radical can now attack another ethane molecule, continuing the chain

Stage 3: Termination

  • The chain reaction stops when two free radicals meet and combine
  • Possible termination reactions:
    • CH₃CH₂• + Cl• → CH₃CH₂Cl (chloroethane)
    • CH₃CH₂• + CH₃CH₂• → CH₃CH₂CH₂CH₃ (butane)
    • Cl• + Cl• → Cl₂ (chlorine)

Key points:

  • The reaction needs UV light to start (initiation)
  • Once started, it continues as a chain reaction (propagation)
  • Multiple substitution can occur, producing dichloroethane, trichloroethane, etc.

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