What is the role of OH⁻ in nucleophilic substitution?

Nucleophile: it donates a lone pair of electrons to the δ+ carbon bonded to the halogen.

What is the role of OH⁻ in elimination?

Base: it accepts a proton (H⁺) from a carbon atom adjacent to the C–Br carbon.

Which conditions favour elimination from a halogenoalkane?

Potassium hydroxide dissolved in ethanol, heated (under reflux).

Which conditions favour substitution?

Aqueous potassium hydroxide with gentle warming.

Write the equation for elimination from 2-bromopropane with KOH.

CH₃CHBrCH₃ + KOH → CH₃CH=CH₂ + KBr + H₂O

What happens to the C–Br bond in elimination?

It breaks heterolytically; both electrons go to bromine, forming Br⁻.

What happens to the electrons of the C–H bond in elimination?

They move to form the C=C double bond.

Why is the carbon in a C–Br bond δ+?

Bromine is more electronegative than carbon, so it draws the bonding electrons towards itself.

How can you show the gaseous product of elimination is unsaturated?

It decolourises bromine water, orange to colourless.

Elimination from halogenoalkanesAQA A-Level Chemistry: Flashcards

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What are the two concurrent reactions of 2-bromopropane with hydroxide ions?

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What are the two concurrent reactions of 2-bromopropane with hydroxide ions?: Nucleophilic substitution (giving propan-2-ol) and elimination (giving propene).
What is the role of OH⁻ in nucleophilic substitution?: Nucleophile: it donates a lone pair of electrons to the δ+ carbon bonded to the halogen.
What is the role of OH⁻ in elimination?: Base: it accepts a proton (H⁺) from a carbon atom adjacent to the C–Br carbon.
Which conditions favour elimination from a halogenoalkane?: Potassium hydroxide dissolved in ethanol, heated (under reflux).
Which conditions favour substitution?: Aqueous potassium hydroxide with gentle warming.
Define a nucleophile.: An electron pair donor.
Define a base.: A proton (H⁺) acceptor.
Write the equation for elimination from 2-bromopropane with KOH.: CH₃CHBrCH₃ + KOH → CH₃CH=CH₂ + KBr + H₂O
What happens to the C–Br bond in elimination?: It breaks heterolytically; both electrons go to bromine, forming Br⁻.
What happens to the electrons of the C–H bond in elimination?: They move to form the C=C double bond.
Why is the carbon in a C–Br bond δ+?: Bromine is more electronegative than carbon, so it draws the bonding electrons towards itself.
How can you show the gaseous product of elimination is unsaturated?: It decolourises bromine water, orange to colourless.

Exam questions on Elimination from halogenoalkanes

Hydroxide ions, OH⁻, can react with a halogenoalkane in two different ways. Each oxygen atom in OH⁻ has lone pairs of electrons, and the ion can also accept a proton. Both reactions start with the polar carbon–halogen bond.
Explain how the role of the hydroxide ion in substitution differs from its role in elimination.2 marks
A student warms 2-bromopropane, CH₃CHBrCH₃, with aqueous potassium hydroxide and obtains mainly propan-2-ol. She repeats the experiment using a solution of potassium hydroxide in ethanol and heats the mixture strongly. This time the main product is a gas that decolourises bromine water.
Write an equation for the formation of the gas in the second experiment, using structural formulae for the organic species.2 marks
A technician needs to make propene from 2-bromopropane. A competing reaction, which gives propan-2-ol, takes place at the same time under some conditions, so the technician must choose the reagent and conditions with care.
Describe, in words, the mechanism for the elimination of HBr from 2-bromopropane by hydroxide ions, referring to the movement of electron pairs.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).