How is the reducing agent written in equations for these reactions?

As [H], so the reduction of propanone is written CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃.

React a carbonyl with HCN over KCN — what is the product?

A hydroxynitrile, in which a CN group and an OH group are both attached to what was the carbonyl carbon. Ethanal gives 2-hydroxypropanenitrile.

Why is KCN used alongside HCN in this reaction?

HCN is a weak acid and supplies too few cyanide ions on its own. KCN raises the concentration of the nucleophile, so the reaction proceeds at a useful rate.

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Reduction and nucleophilic addition of HCN

3 things to know
Papers 2 and 3

Overview

Reducing aldehydes to primary alcohols and ketones to secondary alcohols with lithium tetrahydridoaluminate, LiAlH₄, in dry ether, writing the reducing agent as [H], the nucleophilic addition of HCN with KCN to give a hydroxynitrile, and why the product is a racemic mixture.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

Describe the mechanism of the reaction between a carbonyl compound and cyanide ions.

Key term

What reagent reduces aldehydes and ketones, and under what conditions?

Key term

What is formed when an aldehyde is reduced, and what when a ketone is reduced?

Questions from this subtopic

Each one with the model answer underneath.

1
What reagent reduces aldehydes and ketones, and under what conditions?
Model answer
Lithium tetrahydridoaluminate, LiAlH₄, in dry ether. Water must be excluded because the reagent reacts violently with it.
2
What is formed when an aldehyde is reduced, and what when a ketone is reduced?
Model answer
An aldehyde gives a primary alcohol; a ketone gives a secondary alcohol.
3
How is the reducing agent written in equations for these reactions?
Model answer
As [H], so the reduction of propanone is written CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃.
4
React a carbonyl with HCN over KCN — what is the product?
Model answer
A hydroxynitrile, in which a CN group and an OH group are both attached to what was the carbonyl carbon. Ethanal gives 2-hydroxypropanenitrile.
5
Why is KCN used alongside HCN in this reaction?
Model answer
HCN is a weak acid and supplies too few cyanide ions on its own. KCN raises the concentration of the nucleophile, so the reaction proceeds at a useful rate.
6
Describe the mechanism of the reaction between a carbonyl compound and cyanide ions.
Model answer
A lone pair on the cyanide ion attacks the partially positive carbonyl carbon, and the pi bond breaks with both electrons going to the oxygen. The resulting negatively charged oxygen then takes a proton from HCN or water to give the hydroxyl group.

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