What is the product of a halogenoalkane with CN⁻?

A nitrile, with one more carbon atom in the chain.

Write the equation for bromoethane with ammonia.

CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br

In a nucleophilic substitution mechanism, where does the first curly arrow start and end?

From the lone pair on the nucleophile to the δ+ carbon.

What does the second curly arrow show?

The C–X bond breaking, with both electrons going to the halogen to form X⁻.

Why are two moles of NH₃ needed per mole of halogenoalkane?

The second NH₃ removes H⁺ from the intermediate to form NH₄⁺.

Why is a large excess of ammonia used?

So that further substitution to secondary and tertiary amines is less likely.

Which carbon-halogen bond is the weakest?

C–I, with a bond enthalpy of 238 kJ mol⁻¹.

Which halogenoalkane reacts fastest, RCl, RBr or RI?

RI, because the C–I bond is the weakest.

Why does C–Cl react more slowly than C–I even though it is more polar?

The rate depends on bond enthalpy, and the C–Cl bond is stronger, so it needs more energy to break.

Why is the reaction with CN⁻ useful in synthesis?

It increases the length of the carbon chain by one carbon atom.

Nucleophilic substitution in halogenoalkanesAQA A-Level Chemistry: Flashcards

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Why are halogenoalkanes attacked by nucleophiles?

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Why are halogenoalkanes attacked by nucleophiles?: The C–X bond is polar, so the carbon atom is δ+.
What is a nucleophile?: An electron pair donor.
What is the product of a halogenoalkane with aqueous OH⁻?: An alcohol.
What is the product of a halogenoalkane with CN⁻?: A nitrile, with one more carbon atom in the chain.
What is the product of a halogenoalkane with excess NH₃?: A primary amine.
Write the equation for bromoethane with ammonia.: CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br
In a nucleophilic substitution mechanism, where does the first curly arrow start and end?: From the lone pair on the nucleophile to the δ+ carbon.
What does the second curly arrow show?: The C–X bond breaking, with both electrons going to the halogen to form X⁻.
Why are two moles of NH₃ needed per mole of halogenoalkane?: The second NH₃ removes H⁺ from the intermediate to form NH₄⁺.
Why is a large excess of ammonia used?: So that further substitution to secondary and tertiary amines is less likely.
Which carbon-halogen bond is the weakest?: C–I, with a bond enthalpy of 238 kJ mol⁻¹.
Which halogenoalkane reacts fastest, RCl, RBr or RI?: RI, because the C–I bond is the weakest.
Why does C–Cl react more slowly than C–I even though it is more polar?: The rate depends on bond enthalpy, and the C–Cl bond is stronger, so it needs more energy to break.
Why is the reaction with CN⁻ useful in synthesis?: It increases the length of the carbon chain by one carbon atom.

Exam questions on Nucleophilic substitution in halogenoalkanes

1-Bromobutane, CH₃CH₂CH₂CH₂Br, is warmed with aqueous sodium hydroxide, NaOH. Butan-1-ol is formed by a nucleophilic substitution reaction.
Write an equation for the reaction of 1-bromobutane with hydroxide ions.2 marks
A chemist converts 1-bromopropane, CH₃CH₂CH₂Br, into butanenitrile, CH₃CH₂CH₂CN, by heating it under reflux with a solution of potassium cyanide, KCN, in ethanol.
Suggest why this reaction is useful in organic synthesis.2 marks
A student compares the rates of reaction of three halogenoalkanes with water. Each of 1-chlorobutane, 1-bromobutane and 1-iodobutane is warmed with aqueous silver nitrate in ethanol and the time taken for a precipitate of the silver halide to form is measured. The bond enthalpies are: C–Cl 338 kJ mol⁻¹, C–Br 276 kJ mol⁻¹ and C–I 238 kJ mol⁻¹.
Predict which halogenoalkane forms a precipitate first and explain your answer.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).