6.2.9Additional lessons
Reduction of Carboxylic Acids, Aldehydes and Ketones — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 92 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Reducing Agents
Reduction of the carbonyl group adds hydrogen and forms alcohols.
Lithium aluminium hydride
- Lithium aluminium hydride LiAlH₄ in dry ether is a strong reducing agent that reduces carboxylic acids, aldehydes and ketones.
Sodium borohydride
- Sodium borohydride NaBH₄ is milder and reduces aldehydes and ketones but not carboxylic acids.
Dry conditions
- Dry conditions LiAlH₄ reacts violently with water, so the reaction is carried out in dry ether and later treated with acid.
[H] notation
- [H] notation The reducing agent is shown simply as [H] to balance organic equations.
Part 2 of 6Products of Reduction
Products of Reduction
Aldehyde
- Aldehyde CH₃CHO + 2[H] → CH₃CH₂OH; aldehydes give primary alcohols.
Ketone
- Ketone CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃; ketones give secondary alcohols.
Carboxylic acid
- Carboxylic acid CH₃COOH + 4[H] → CH₃CH₂OH + H₂O; carboxylic acids give primary alcohols.
| Starting compound | Product | Example |
|---|---|---|
| Aldehyde | Primary alcohol | Propanal → propan-1-ol |
| Ketone | Secondary alcohol | Butanone → butan-2-ol |
| Carboxylic acid | Primary alcohol | Propanoic acid → propan-1-ol |
Part 3 of 6Linking Oxidation and Reduction
1
Oxidation of a primary alcohol gives an aldehyde and then a carboxylic acid.
2
Reduction reverses these steps and returns the primary alcohol.
3
A secondary alcohol is oxidised to a ketone, and reduction of the ketone returns the same secondary alcohol.
4
Tertiary alcohols cannot be made this way, because carbonyl compounds always have a hydrogen or two carbon groups on the carbonyl carbon.
Part 4 of 6Practical Notes and Mechanism Idea
Practical Notes and Mechanism Idea
Hydride ion
- Hydride ion The reducing agent behaves as a source of hydride ions, H⁻, which attack the δ+ carbon of the carbonyl group.
Work-up
- Work-up After the hydride addition, dilute acid is added to protonate the oxygen and give the alcohol.
Carbonyl polarity
- Carbonyl polarity The carbon of C=O is δ+ because oxygen is more electronegative, so it is attacked by nucleophiles.
Safety
- Safety LiAlH₄ must be kept away from water and handled with care because it reacts violently and releases hydrogen.
Part 5 of 6Worked Example
Question Name the organic product when pentanal, pentan-3-one and propanoic acid are each reduced by LiAlH₄ in dry ether.
- Pentanal is an aldehyde, so it gives a primary alcohol with the same carbon chain.
- Pentan-3-one is a ketone, so it gives a secondary alcohol.
- Propanoic acid is a carboxylic acid, so it gives a primary alcohol and water.
Answer Pentan-1-ol, pentan-3-ol and propan-1-ol respectively.
Part 6 of 6Exam Checklist & Tips
6.2.9 – Key Points for the Exam
- Carbonyl group The C=O functional group present in aldehydes and ketones.
- LiAlH₄ A strong reducing agent used in dry ether.
- [H] Symbol used for hydrogen supplied by a reducing agent.
- Reduction in organic chemistry Gain of hydrogen or loss of oxygen by an organic compound.
Common Mistakes
- Using aqueous conditions for LiAlH₄.
- Saying that ketones produce primary alcohols.
- Forgetting that carboxylic acids need four [H] and produce water as well.
- Believing NaBH₄ reduces carboxylic acids.
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