Why is the C–Br bond polar?

Bromine is more electronegative than carbon, so carbon is δ+ and bromine is δ−.

What is a nucleophile?

A species that donates a lone pair of electrons to form a covalent bond, e.g. OH⁻, NH₃, CN⁻, H₂O.

What are the reagent and product when 1-bromopropane reacts with aqueous KOH?

Aqueous KOH heated under reflux gives propan-1-ol (nucleophilic substitution).

What does ethanolic KOH do to a halogenoalkane?

OH⁻ acts as a base and removes HBr (elimination), forming an alkene.

How does the solvent decide between substitution and elimination with KOH?

Aqueous KOH gives substitution (alcohol); ethanolic KOH gives elimination (alkene).

Why is KCN reacted in ethanol, not water?

Water would compete as a nucleophile and give some alcohol; ethanol gives only the nitrile.

Why is the halogenoalkane–cyanide reaction useful?

It lengthens the carbon chain by one carbon atom.

Why is excess ammonia used with a halogenoalkane?

The amine product is also a nucleophile; excess ammonia limits further substitution to secondary and tertiary amines.

What are the precipitate colours with AgNO₃ in ethanol?

AgCl white, AgBr cream, AgI yellow.

What is the role of water in the silver nitrate test?

It is the nucleophile that replaces the halogen, releasing the halide ion that precipitates with Ag⁺.

Halogenoalkanes and nucleophilic substitutionEdexcel International A Level Chemistry: Flashcards

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What is a primary halogenoalkane?

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What is a primary halogenoalkane?: One in which the carbon bonded to the halogen is attached to only one other carbon atom.
Name CH₃CHClCH₂CH₃.: 2-chlorobutane.
Why is the C–Br bond polar?: Bromine is more electronegative than carbon, so carbon is δ+ and bromine is δ−.
What is a nucleophile?: A species that donates a lone pair of electrons to form a covalent bond, e.g. OH⁻, NH₃, CN⁻, H₂O.
What are the reagent and product when 1-bromopropane reacts with aqueous KOH?: Aqueous KOH heated under reflux gives propan-1-ol (nucleophilic substitution).
What does ethanolic KOH do to a halogenoalkane?: OH⁻ acts as a base and removes HBr (elimination), forming an alkene.
How does the solvent decide between substitution and elimination with KOH?: Aqueous KOH gives substitution (alcohol); ethanolic KOH gives elimination (alkene).
Why is KCN reacted in ethanol, not water?: Water would compete as a nucleophile and give some alcohol; ethanol gives only the nitrile.
Why is the halogenoalkane–cyanide reaction useful?: It lengthens the carbon chain by one carbon atom.
Why is excess ammonia used with a halogenoalkane?: The amine product is also a nucleophile; excess ammonia limits further substitution to secondary and tertiary amines.
What are the precipitate colours with AgNO₃ in ethanol?: AgCl white, AgBr cream, AgI yellow.
What is the role of water in the silver nitrate test?: It is the nucleophile that replaces the halogen, releasing the halide ion that precipitates with Ag⁺.

Exam questions on Halogenoalkanes and nucleophilic substitution

A chemist is using the halogenoalkane CH₃CH₂CHBrCH₃ as the starting material in a multi-step synthesis in a research laboratory.
Explain why the carbon atom in the C–Br bond of a halogenoalkane is attacked by nucleophiles.2 marks
In a teaching laboratory, 1-bromopropane is warmed with aqueous potassium hydroxide in a flask fitted with a condenser, and the mixture is heated for about 30 minutes.
Explain why the mixture is heated under reflux rather than simply warmed in an open flask.2 marks
A student has a sample of 1-bromobutane and plans to convert it into two different organic products using two different reagents.
The first reaction uses potassium cyanide. State the reagent and solvent, name the organic product, and explain why this reaction is useful in synthesis.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).