Chloroethane can be made by reacting ethane with chlorine in the presence of ultraviolet light. Describe what happens during this reaction and explain why the product is classified as a halogenoalkane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethane reacts with chlorine in the presence of ultraviolet light to form chloroethane and hydrogen chloride: C₂H₆ + Cl₂ → C₂H₅Cl + HCl
Model answer (5 marks)
Ultraviolet light supplies the energy to break the Cl–Cl bond, producing two chlorine radicals. A chlorine radical abstracts a hydrogen atom from ethane, forming HCl and an ethyl radical. The ethyl radical then reacts with another chlorine radical to give chloroethane and regenerate a chlorine radical, propagating the chain reaction.
Chloroethane is a halogenoalkane because it is an alkane (ethane) in which one hydrogen atom has been replaced by a chlorine atom, i.e. a halogen bonded to a carbon chain.
Chloroethane is a halogenoalkane because it is an alkane (ethane) in which one hydrogen atom has been replaced by a chlorine atom, i.e. a halogen bonded to a carbon chain.
Examiner tips
- Mention UV light as the initiator and the radical chain mechanism; include HCl as a by‑product; state the definition of a halogenoalkane and why chloroethane fits it.
- Use the exact wording ‘hydrogen atom is substituted by a chlorine atom’ and ‘halogen atom bonded to an alkane’.
- Show the reaction steps clearly to demonstrate understanding of the mechanism.
Common mistakes
- Failing to state that UV light initiates the reaction; Not recognising HCl as a by‑product; Describing chloroethane as a ‘chlorinated hydrocarbon’ without linking it to the definition of a halogenoalkane.
Mark scheme (5 marks)
- Ultraviolet light provides the energy needed to initiate / start the reaction
- A hydrogen atom in ethane is substituted / replaced by a chlorine atom
- Hydrogen chloride (HCl) is also produced as a by-product
- A halogenoalkane contains a halogen atom bonded to an alkane / carbon chain
- Chloroethane is a halogenoalkane because it contains a chlorine atom (a halogen) bonded to an ethane / alkane carbon chain
Key terms in this question
halogenoalkane · ultraviolet light · halogen
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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