AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
These reduction reactions are examples of nucleophilic addition.
Full specification wording
Aldehydes and ketones (A-level only): These reduction reactions are examples of nucleophilic addition.
Sodium tetrahydridoborate(III), NaBH₄, in aqueous solution supplies hydride ion, H⁻, which has a lone pair and a negative charge and therefore behaves as a nucleophile.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Sodium tetrahydridoborate(III), NaBH₄, in aqueous solution supplies hydride ion, H⁻, which has a lone pair and a negative charge and therefore behaves as a nucleophile. The carbonyl group is polar because oxygen is more electronegative than carbon, so the carbon is δ⁺ and is the point of attack. The hydride's lone pair forms a bond to that carbon while the π electrons of the C=O shift onto the oxygen, giving a negatively charged alkoxide intermediate; this then takes a hydrogen ion from the water, completing the hydroxyl group. Two atoms have been added across the C=O and nothing has been removed, and the first step was attack by a nucleophile, so the mechanism is nucleophilic addition. Aldehydes give primary alcohols and ketones give secondary alcohols.
What earns the marks
- Naming the mechanism as nucleophilic addition.
- Identifying H⁻ as the nucleophile that attacks the δ⁺ carbon.
- Labelling the C=O as δ⁺ on carbon and δ⁻ on oxygen, with electronegativity as the reason.
- The second step, in which the negatively charged intermediate gains H⁺ from water.
Common mistakes and how to put them right
Naming the mechanism nucleophilic addition–elimination, which belongs to the acyl chlorides.
Showing NaBH₄ itself as the attacking species rather than H⁻.
Putting the δ⁺ on the oxygen of the carbonyl group.
Forgetting that the second hydrogen comes from water, not from the reducing agent.
Exam tips
- AQA requires the nucleophile to be shown as H⁻, so write the charge and the lone pair.
- Name the product class as well as the mechanism: primary alcohol from an aldehyde, secondary from a ketone.
- Check you have added two atoms in total across the C=O and removed none.
Try the lesson
The start of the Aldehydes and ketones (A-level only): These reduction reactions are examples of nucleophilic addition. lesson, made from checked teaching notes for this specification point.
Have a go first
Which type of reaction mechanism is involved when an aldehyde is reduced by a hydride ion?
Listen
Sodium tetrahydridoborate(III), NaBH₄, in aqueous solution supplies hydride ion, H⁻, which has a lone pair and a negative charge and therefore behaves as a nucleophile. The carbonyl group is polar because oxygen is more electronegative than carbon, so the carbon is δ⁺ and is the point of attack. The hydride's lone pair forms a bond to that carbon while the π electrons of the C=O shift onto the oxygen, giving a negatively charged alkoxide intermediate; this then takes a hydrogen ion from the water, completing the hydroxyl group. Two atoms have been added across the C=O and nothing has been removed, and the first step was attack by a nucleophile, so the mechanism is nucleophilic addition. Aldehydes give primary alcohols and ketones give secondary alcohols. A strong answer earns marks for these points. naming the mechanism as nucleophilic addition. identifying H⁻ as the nucleophile that attacks the δ⁺ carbon. labelling the C=O as δ⁺ on carbon and δ⁻ on oxygen, with electronegativity as the reason. the second step, in which the negatively charged intermediate gains H⁺ from water. Exam tip: AQA requires the nucleophile to be shown as H⁻, so write the charge and the lone pair.
Key words
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reduction with NaBH₄
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hydride as nucleophile
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polar carbonyl group
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alkoxide intermediate
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See every definition
- reduction with NaBH₄
- Sodium tetrahydridoborate(III), NaBH₄, in aqueous solution supplies hydride ion, H⁻, which has a lone pair and a negative charge and therefore behaves as a nucleophile.
- hydride as nucleophile
- The hydride's lone pair forms a bond to the δ⁺ carbon while the π electrons of the C=O shift onto the oxygen.
- polar carbonyl group
- The carbonyl group is polar because oxygen is more electronegative than carbon, so the carbon is δ⁺ and is the point of attack.
- alkoxide intermediate
- A negatively charged alkoxide intermediate is formed, which then takes a hydrogen ion from the water, completing the hydroxyl group.
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