Write the equation for ethanal reacting with HCN.

CH₃CHO + HCN → CH₃CH(OH)CN

Which atom of CN⁻ attacks the carbonyl carbon?

The carbon atom, using its lone pair.

What does the dilute acid do?

Provides H⁺ to protonate the O⁻ intermediate and form the OH group.

Why does the reaction give a mixture of enantiomers?

The C=O group is planar, so CN⁻ attacks from either side with equal probability, giving equal amounts of both enantiomers.

Which carbonyl compounds give a chiral product with HCN?

Aldehydes (except methanal) and unsymmetrical ketones.

Why does propanone give a product with no chiral centre?

Its product carbon has two identical CH₃ groups, so it is not bonded to four different groups.

Give two hazards of using KCN.

It is highly toxic, and it reacts with acid to release toxic hydrogen cyanide gas.

Nucleophilic addition with cyanideAQA A-Level Chemistry: Flashcards

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What class of compound forms when an aldehyde reacts with HCN?

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What class of compound forms when an aldehyde reacts with HCN?: A hydroxynitrile, containing OH and CN on the same carbon.
What are the reagents for making a hydroxynitrile from a carbonyl compound in the laboratory?: KCN, followed by dilute acid.
Write the equation for ethanal reacting with HCN.: CH₃CHO + HCN → CH₃CH(OH)CN
Name the product of ethanal with HCN.: 2-Hydroxypropanenitrile.
What is the nucleophile in this reaction?: The cyanide ion, CN⁻.
Which atom of CN⁻ attacks the carbonyl carbon?: The carbon atom, using its lone pair.
What type of mechanism is it?: Nucleophilic addition.
What does the dilute acid do?: Provides H⁺ to protonate the O⁻ intermediate and form the OH group.
Why does the reaction give a mixture of enantiomers?: The C=O group is planar, so CN⁻ attacks from either side with equal probability, giving equal amounts of both enantiomers.
Which carbonyl compounds give a chiral product with HCN?: Aldehydes (except methanal) and unsymmetrical ketones.
Why does propanone give a product with no chiral centre?: Its product carbon has two identical CH₃ groups, so it is not bonded to four different groups.
Give two hazards of using KCN.: It is highly toxic, and it reacts with acid to release toxic hydrogen cyanide gas.

Exam questions on Nucleophilic addition with cyanide

A student adds potassium cyanide solution to ethanal, then adds dilute sulfuric acid. A hydroxynitrile is formed in a nucleophilic addition reaction.
State the role of the dilute acid in the reaction and give one hazard of using KCN.2 marks
A chemist is investigating the reactions of several carbonyl compounds with KCN followed by dilute acid. The products are hydroxynitriles, and the chemist wants to know which of the products contain a chiral centre.
Butanone, CH₃COCH₂CH₃, reacts with HCN. Write the overall equation for this reaction and name the organic product.2 marks
Propanal is added to a solution of KCN in a fume cupboard. Dilute acid is then added to the mixture. The organic product is a hydroxynitrile.
Write the overall equation for the reaction of propanal with HCN, name the organic product, and name the type of mechanism.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).