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8873 / 2027
Polymers and Organic Chemistry

Lesson 5 of 12

Halogenoalkanes and alcohols

Water favours substitution of bromoethane; ethanolic base promotes elimination.

A-Level 8873, revised syllabus (2026-2027)

  • I can use organic reaction terminology.Syllabus

    Syllabus 8873, 9(b). Use organic reaction terminology.

  • I can apply the specified organic reactions, conditions and analogous product predictions.Syllabus

    Syllabus 8873, 9(g). Apply the specified organic reactions, conditions and analogous product predictions.

Conditions choose the reaction route

Water favours substitution of bromoethane; ethanolic base promotes elimination.

Make a guess. You are not marked.

Bromoethane is heated with sodium hydroxide dissolved in ethanol, not water. What is the main organic product?

  1. Ethene, by elimination of HBr
  2. No reaction, as ethanol is the solvent
  3. Ethanol, by substitution of Br
Show the answer

Ethene, by elimination of HBr

In ethanol, hydroxide acts mainly as a base and removes HBr, so elimination gives an alkene.

Elimination removes atoms or groups from two neighbouring carbons and forms a double bond. Substituting Br with OH is different: the carbon chain stays saturated. For bromoethane, the solvent and conditions decide the product, so writing only "NaOH" is not a complete answer.

Route map from bromoethane and ethanol
RouteReagents and essential conditionsBalanced change
Bromoethane → ethanolAqueous NaOH; heat, usually under reflux.CH3CH2Br + NaOH → CH3CH2OH + NaBr
Bromoethane → etheneNaOH in ethanol; heat.CH3CH2Br + NaOH → CH2=CH2 + NaBr + H2O
Ethanol → etheneConcentrated H3PO4 catalyst; heat.CH3CH2OH → CH2=CH2 + H2O
Ethanol → ethanoic acidAcidified K2Cr2O7 or acidified KMnO4; heat under reflux.CH3CH2OH + 2[O] → CH3COOH + H2O
Ethanol complete combustionOxygen; ignition.C2H5OH + 3O2 → 2CO2 + 3H2O

A reflux condenser cools the vapour so that it drips back into the flask. This lets you heat for a long time without losing volatile substances. When ethanol is oxidised to the acid, enough oxidant and a long heating time let the reaction go past the aldehyde to the carboxylic acid. Acidified dichromate turns from orange to green. Acidified permanganate loses its purple colour as it is reduced.

In organic chemistry, oxidation usually adds bonds from carbon to oxygen or removes C-H bonds. Reduction does the opposite. [O] and [H] stand for the oxygen or hydrogen supplied in a balanced equation. They are a way of keeping count, not real single atoms in a bottle.

Worked examplePlan a two-step conversion

Convert 1-bromopropane into propanoic acid using analogous reactions from this course.

Try it first. Then show the worked stepsHide the worked steps
  1. CH3CH2CH2Br → CH3CH2CH2OH using aqueous NaOH and heat: substitution replaces Br by OH.
  2. Heat the primary alcohol with acidified K2Cr2O7 or acidified KMnO4 under reflux to form CH3CH2COOH.
  3. Check that the carbon skeleton remains three carbons in both steps. Ethanolic NaOH would instead divert the first step towards propene.
Answer

Use aqueous NaOH and heat, then an acidified oxidising agent and heat under reflux.

Check your understanding

Why is ethanol to ethene an elimination, not a substitution?

  1. Two ethanol molecules join together, giving ethene and a water molecule.
  2. An H atom replaces the OH group on the carbon, giving a new compound.
  3. Water is removed (OH from one carbon, H from the next) and a C=C forms.
Show the answer

Water is removed (OH from one carbon, H from the next) and a C=C forms.

In elimination a small molecule is lost and a double bond forms. In substitution, another group would replace OH on the same carbon.