6.2.9Additional lessons
Reduction of Carboxylic Acids, Aldehydes and Ketones — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 140 of 172
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.
Section 1 of 4Reducing Agents
Reduction in organic chemistry usually means gaining hydrogen or losing oxygen.
Lithium aluminium hydride
- Lithium aluminium hydride LiAlH₄ in dry ether is a powerful reducing agent that reduces carboxylic acids, aldehydes and ketones.
Sodium borohydride
- Sodium borohydride NaBH₄ is a milder reagent that reduces aldehydes and ketones but does not reduce carboxylic acids.
Hydride ion
- Hydride ion Both reagents act as a source of H⁻, which attacks the partially positive carbon of the C=O bond.
Dry conditions
- Dry conditions LiAlH₄ reacts violently with water, so the reaction uses dry ether and a careful acid work-up afterwards.
Section 2 of 4Products of Reduction
1
A carboxylic acid is reduced first to an aldehyde and then to a primary alcohol, with LiAlH₄ taking it all the way.
2
The overall change is CH₃COOH + 4[H] → CH₃CH₂OH + H₂O, giving ethanol from ethanoic acid.
3
An aldehyde is reduced to a primary alcohol: CH₃CHO + 2[H] → CH₃CH₂OH.
4
A ketone is reduced to a secondary alcohol: CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃, which is propan-2-ol.
5
The alcohol made can be oxidised back, so reduction and oxidation of these compounds are reverse processes.
| Starting compound | Product | Reagent |
|---|---|---|
| Carboxylic acid | Primary alcohol | LiAlH₄ |
| Aldehyde | Primary alcohol | LiAlH₄ or NaBH₄ |
| Ketone | Secondary alcohol | LiAlH₄ or NaBH₄ |
Section 3 of 4Choosing and Using the Reagent
Choosing and Using the Reagent
Selectivity
- Selectivity NaBH₄ can reduce a carbonyl group in a molecule while leaving a carboxylic acid group unchanged.
Safety
- Safety LiAlH₄ is flammable and reacts with moisture, so it is handled in dry apparatus away from water.
Reflux
- Reflux The reduction of a carboxylic acid is usually carried out under reflux in dry ether to ensure complete reaction.
Link to oxidation
- Link to oxidation Oxidising the alcohols formed with acidified dichromate regenerates the original carbonyl compounds.
Section 4 of 4Exam Checklist & Tips
6.2.9 – Key Points for the Exam
- Reduction Gain of hydrogen, loss of oxygen or gain of electrons.
- Hydride ion The H⁻ ion, which acts as the nucleophile in these reductions.
- [H] Shorthand for hydrogen supplied by the reducing agent in balancing equations.
- Carbonyl group The C=O functional group present in aldehydes and ketones.
Common Mistakes
- Saying NaBH₄ reduces carboxylic acids, which it does not under normal conditions.
- Using water or aqueous solvents with LiAlH₄, which causes a violent reaction.
- Producing a primary alcohol from a ketone; a ketone gives a secondary alcohol.
- Forgetting that reducing a carboxylic acid needs four [H] and releases water.
Last updated:
\n