Halogenoalkanes are organic compounds in which one or more hydrogen atoms in an alkane have been replaced by a halogen atom. Chloroethane (C₂H₅Cl) is an example of a halogenoalkane. Explain how chloroethane is formed from ethane, and describe the type of reaction that occurs, giving the name of the process and the conditions required.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Halogenoalkanes can be produced from alkanes by substitution reactions in the presence of ultraviolet light. In these reactions, a hydrogen atom is replaced by a halogen atom. Chloroethane, for example, can be made from ethane and chlorine.
Model answer (5 marks)
1. A hydrogen atom in ethane is replaced by a chlorine atom.
2. The reaction is a free‑radical substitution (photochemical substitution).
3. Ultraviolet light is required to initiate the reaction.
4. UV light homolytically cleaves the Cl–Cl bond, giving two chlorine radicals.
5. A chlorine radical abstracts a hydrogen from ethane to give HCl and an ethyl radical; the ethyl radical then reacts with Cl₂ to produce chloroethane and another chlorine radical, propagating the chain.
2. The reaction is a free‑radical substitution (photochemical substitution).
3. Ultraviolet light is required to initiate the reaction.
4. UV light homolytically cleaves the Cl–Cl bond, giving two chlorine radicals.
5. A chlorine radical abstracts a hydrogen from ethane to give HCl and an ethyl radical; the ethyl radical then reacts with Cl₂ to produce chloroethane and another chlorine radical, propagating the chain.
Examiner tips
- Use the word "replace" to show substitution. Mention UV light as the energy source. Include the homolytic cleavage of Cl–Cl. Show the chain‑propagation step with HCl formation.
Common mistakes
- Forgetting to mention UV light. Describing the reaction as ionic instead of free‑radical. Omitting the homolytic cleavage of the Cl–Cl bond.
Mark scheme (5 marks)
- A hydrogen atom in ethane is replaced by a chlorine atom (substitution reaction named or implied by 'replacement')
- The reaction is called free-radical substitution (or photochemical substitution)
- Ultraviolet (UV) light is required as the condition / energy source
- The Cl–Cl bond in chlorine is broken homolytically (each atom receives one electron), producing chlorine free radicals
- A chlorine free radical reacts with ethane, removing a hydrogen atom to form HCl and an ethyl free radical, which then reacts with Cl₂ to form chloroethane (chain propagation described)
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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