Topic 5 of 12

Halogenoalkanes and alcohols

Water favours substitution of bromoethane; ethanolic base promotes elimination.

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

Conditions choose the reaction route

Water favours substitution of bromoethane; ethanolic base promotes elimination.

Elimination removes atoms or groups from adjacent positions to form a multiple bond. In contrast, substituting Br with OH keeps the carbon-carbon skeleton saturated. For bromoethane, the solvent and conditions make the requested product clear; writing only "NaOH" leaves the answer incomplete.

Route map from bromoethane and ethanol
Route	Reagents and essential conditions	Balanced change
Bromoethane → ethanol	Aqueous NaOH; heat, usually under reflux.	CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr
Bromoethane → ethene	NaOH in ethanol; heat.	CH₃CH₂Br + NaOH → CH₂=CH₂ + NaBr + H₂O
Ethanol → ethene	Concentrated H₃PO₄ catalyst; heat.	CH₃CH₂OH → CH₂=CH₂ + H₂O
Ethanol → ethanoic acid	Acidified K₂Cr₂O₇ or acidified KMnO₄; heat under reflux.	CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
Ethanol complete combustion	Oxygen; ignition.	C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

A reflux condenser returns vaporised volatile material to the reaction vessel, permitting sustained heating without continually losing that material. When oxidising ethanol to the acid, sufficient oxidant and continued contact allow oxidation to proceed through the aldehyde stage to the carboxylic acid. Acidified dichromate changes from orange towards green; acidified permanganate loses its purple colour as it is reduced under the stated conditions.

Organic oxidation often increases the number of bonds from carbon to oxygen and/or decreases the number of C-H bonds. Reduction does the reverse. [O] and [H] represent oxygen or hydrogen equivalents in an organic redox equation; they are bookkeeping symbols, not bottles of isolated atoms.

Worked example

Plan a two-step conversion

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

CH₃CH₂CH₂Br → CH₃CH₂CH₂OH using aqueous NaOH and heat: substitution replaces Br by OH.
Heat the primary alcohol with acidified K₂Cr₂O₇ or acidified KMnO₄ under reflux to form CH₃CH₂COOH.
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 understandingWhy is ethanol to ethene an elimination rather than substitution?Think it through, then reveal the answer

A small molecule, water, is removed overall: OH from one carbon and H from an adjacent carbon are lost while C=C forms. No incoming group replaces OH in the saturated carbon skeleton.