All subjects · Chemistry · WCH12 (Unit 2)

Halogenoalkanes: substitution and elimination

Halogenoalkanes contain a polar C-X bond. Nucleophiles attack the δ+ carbon in substitution, while strong base in ethanol causes elimination.

Naming and classifying

Name with the prefix fluoro-, chloro-, bromo- or iodo-: CH3CHBrCH3 is 2-bromopropane. Primary: the C bearing X is bonded to one other carbon; secondary: two; tertiary: three. The same classification applies to alcohols.

Reactions and conditions

KOH(aq), heat under reflux: substitution to an alcohol: CH3CH2Br + OH- -> CH3CH2OH + Br-
KOH in ethanol, heat: elimination to an alkene: CH3CH2Br + KOH -> CH2=CH2 + KBr + H2O
Ammonia in ethanol, heated in a sealed tube: substitution to an amine: CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br
KCN in ethanol, heat under reflux: substitution to a nitrile, adding one carbon to the chain: CH3CH2Br + CN- -> CH3CH2CN + Br-
Silver nitrate in ethanol (aqueous): water hydrolyses the halogenoalkane and Ag+ precipitates the halide.

Nucleophilic substitution mechanism

A nucleophile is an electron-pair donor (OH-, NH3, CN-). For a primary halogenoalkane with OH-: a curly arrow from a lone pair on OH- to the δ+ carbon, and a curly arrow from the C-Br bond to Br, which leaves as Br-. With NH3, the same attack forms CH3CH2NH3+, then a second NH3 removes H+ to give the amine and NH4+.

Substitution or elimination?

OH- can act as a nucleophile or as a base. Water as solvent favours substitution; ethanol as solvent and higher temperature favour elimination. Tertiary halogenoalkanes eliminate more readily. 2-bromobutane can eliminate to give but-1-ene and both E and Z but-2-ene.

Worked example

Give the product, type of reaction and conditions for 1-bromopropane with potassium cyanide.

CN- is a nucleophile and replaces Br.
Conditions: KCN dissolved in ethanol, heated under reflux.

Answer: Butanenitrile, CH3CH2CH2CN, by nucleophilic substitution; the chain grows from 3 to 4 carbons.

Key idea: KOH(aq): alcohol (substitution). KOH in ethanol: alkene (elimination). NH3 in ethanol, sealed tube: amine. KCN in ethanol: nitrile. Nucleophiles attack the δ+ carbon.

Check yourself

What forms when 2-bromopropane is heated with KOH dissolved in ethanol?
- Propan-2-ol
- Propylamine
- Propene
- Propane
Show answer
Propene. Ethanolic KOH acts as a base, causing elimination of HBr.
What forms when 1-chlorobutane is heated under reflux with KOH(aq)?
- Butan-1-ol
- But-1-ene
- Butane
- Butanal
Show answer
Butan-1-ol. Aqueous OH- substitutes for Cl.
What is a nucleophile?
- An electron-pair acceptor
- A species with an unpaired electron
- An electron-pair donor
- A positive ion
Show answer
An electron-pair donor. Nucleophiles attack δ+ atoms with a lone pair.
Which is a tertiary halogenoalkane?
- 1-chlorobutane
- 2-chloro-2-methylpropane
- 2-chlorobutane
- 1-chloro-2-methylpropane
Show answer
2-chloro-2-methylpropane. The C bonded to Cl is attached to three other carbons.
Why is the reaction with KCN useful in synthesis?
- It produces an alkene
- It makes an alcohol
- It removes the halogen without replacement
- It increases the length of the carbon chain by one
Show answer
It increases the length of the carbon chain by one. The CN group adds a new C-C bond.
Which conditions convert a halogenoalkane into an amine?
- Dilute aqueous ammonia at room temperature
- Excess ammonia in ethanol, heated in a sealed tube
- Ammonia gas with a nickel catalyst
- Ammonium chloride solution, reflux
Show answer
Excess ammonia in ethanol, heated in a sealed tube. Excess concentrated ammonia in ethanol under pressure gives the primary amine.
In the mechanism of OH- with bromoethane, where does the first curly arrow start?
- The C-Br bond
- The bromine atom
- A C-H bond
- A lone pair on the oxygen of OH-
Show answer
A lone pair on the oxygen of OH-. The nucleophile's lone pair attacks the δ+ carbon.

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Last updated 7 November 2026. Spotted a mistake? Tell us.