Methanol (CH₃OH) and bromoethane (C₂H₅Br) are examples of two different homologous series of organic compounds. Describe the key differences between an alcohol and a halogenoalkane, including the functional group present in each, and explain how a halogenoalkane can be converted back into an alcohol.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Organic chemists frequently interconvert alcohols and halogenoalkanes in the laboratory. Methanol belongs to the alcohol series and bromoethane belongs to the halogenoalkane series.
Model answer (5 marks)
Alcohols contain a hydroxyl (–OH) functional group.
Halogenoalkanes contain a halogen atom (F, Cl, Br or I) in place of a hydrogen.
A halogenoalkane can be converted into an alcohol by reacting with aqueous NaOH (hydrolysis with water or aqueous alkali). The halogen atom is replaced by a hydroxyl (–OH) group during this reaction, producing a halide ion (e.g. Br⁻) as a by‑product.
Halogenoalkanes contain a halogen atom (F, Cl, Br or I) in place of a hydrogen.
A halogenoalkane can be converted into an alcohol by reacting with aqueous NaOH (hydrolysis with water or aqueous alkali). The halogen atom is replaced by a hydroxyl (–OH) group during this reaction, producing a halide ion (e.g. Br⁻) as a by‑product.
Examiner tips
- Use the exact terms – hydroxyl group, halogen atom, hydrolysis with aqueous NaOH.
- Show the replacement of X by –OH and mention the halide ion as by‑product.
- Keep the answer concise and structured to match the 5 marks.
Common mistakes
- Writing the functional group of a halogenoalkane as –OH instead of a halogen.
- Omitting the halide ion as a by‑product.
- Using vague terms like ‘water’ without specifying aqueous NaOH or hydrolysis.
Mark scheme (5 marks)
- Alcohols contain the hydroxyl (–OH) functional group
- Halogenoalkanes contain a halogen atom (F, Cl, Br or I) as the functional group / in place of a hydrogen atom
- A halogenoalkane can be converted into an alcohol by reacting with aqueous sodium hydroxide (NaOH) / by hydrolysis with water or aqueous alkali
- The halogen atom is replaced by a hydroxyl (–OH) group during this reaction
- A halide ion (e.g. Br⁻) is produced as a by-product of the reaction
Key terms in this question
alcohol · halogenoalkane · functional group
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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