1. Carbonyl Group Structure & Polarity
Aldehydes and ketones are carbonyl compounds containing the carbonyl functional group (C=O):
- Aldehydes: The carbonyl carbon is bonded to at least one hydrogen atom: R-CHO (located at the end of a carbon chain).
- Ketones: The carbonyl carbon is bonded to two alkyl or aryl groups: R-CO-R' (located within a carbon chain).
>C(δ+) = O(δ−)
The electrophilic δ+ carbon atom is highly susceptible to attack by nucleophiles (electron-pair donors).
2. Distinguishing Aldehydes and Ketones: Chemical Tests
Aldehydes are easily oxidised to carboxylic acids because the carbonyl carbon has an attached hydrogen atom. Ketones cannot be oxidised without breaking strong carbon-carbon bonds. This allows simple chemical tests to distinguish them:
| Reagent & Conditions | Reaction with Aldehyde (RCHO) | Observation with Aldehyde | Reaction with Ketone (RCOR') |
|---|---|---|---|
| Tollens' Reagent
[Ag(NH₃)₂]⁺ in aqueous ammonia; warm gently in a water bath |
Oxidised to carboxylate:
RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag(s) + 4NH₃ + 2H₂O |
Silver mirror forms on inside of test tube (or grey precipitate) | No reaction (solution remains clear and colourless) |
| Fehling's solution
Cu²⁺ complexed with tartrate in alkaline solution (Benedict's uses citrate); warm in water bath |
Oxidised to carboxylate; Cu²⁺ reduced to Cu⁺:
RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O(s) + 3H₂O |
Deep blue solution forms a brick-red precipitate of copper(I) oxide (Cu₂O) | No reaction (solution remains clear blue) |
| Acidified Potassium Dichromate (K₂Cr₂O₇ / H₂SO₄)
Warm gently |
Oxidised to carboxylic acid:
3RCHO + Cr₂O₇²⁻ + 8H⁺ → 3RCOOH + 2Cr³⁺ + 4H₂O |
Orange solution turns green (Cr³⁺) | No reaction (solution remains orange) |
Tollens' and Fehling's tests are part of Required Practical 5 (RP5): tests for alcohols, aldehydes, alkenes and carboxylic acids. The oxidation of alcohols to aldehydes and ketones is on the Unit 2 Alcohols page.
3. Reduction of Carbonyls with Sodium Borohydride (NaBH₄)
Aldehydes and ketones are readily reduced back to alcohols using sodium borohydride (NaBH₄) in aqueous solution:
- Aldehydes are reduced to Primary Alcohols:
RCHO + 2[H] → RCH₂OH
Example: Ethanal + 2[H] → Ethanol - Ketones are reduced to Secondary Alcohols:
RCOR' + 2[H] → RCH(OH)R'
Example: Propanone + 2[H] → Propan-2-ol
The active reducing agent is the hydride ion (:H⁻), which acts as a nucleophile attacking the δ+ carbonyl carbon.
In equations, [H] stands for the reducing agent:
CH₃CHO + 2[H] → CH₃CH₂OH
Mechanism: Nucleophilic Addition of H⁻
- The lone pair on H⁻ attacks the δ+ carbonyl carbon. The C=O π electrons move onto the oxygen. This gives an alkoxide ion (O⁻).
- A lone pair on the O⁻ takes H⁺ from a water molecule. This gives the alcohol and OH⁻.
4. Nucleophilic Addition with Potassium Cyanide (KCN)
Aldehydes and ketones react with potassium cyanide acidified with dilute acid (providing HCN) in a nucleophilic addition reaction to form 2-hydroxynitriles (cyanohydrins).
Reaction Conditions & Synthetic Importance
- Reagent: KCN followed by dilute acid (or acidified KCN). Pure HCN is avoided because it is an extremely toxic gas with a low boiling point (26 °C). The reaction must be carried out in a fume cupboard.
- Synthetic value: The reaction introduces a nitrile group (-C≡N), extending the carbon chain by one carbon atom. The nitrile can then be reduced to a primary amine. Beyond the OxfordAQA specification: a nitrile can also be hydrolysed to a carboxylic acid.
Overall Equation Using HCN
The overall reaction can be written with HCN:
CH₃CHO + HCN → CH₃CH(OH)CN
The product is 2-hydroxypropanenitrile.
Hazards of KCN
- KCN is highly toxic.
- With acid, KCN can release HCN gas, which is very toxic.
- Use a fume cupboard and wear gloves.
- KCN is used rather than HCN because HCN is a very toxic gas.
Why the Product Is a Racemic Mixture
- The carbonyl group is planar.
- CN⁻ can attack the carbonyl carbon from either side, with equal chance.
- So aldehydes (except methanal) and unsymmetrical ketones give a 50:50 mixture of the two enantiomers (a racemic mixture).
- The mixture is optically inactive. See Optical Isomerism.
5. Nucleophilic Addition Mechanism Diagram
The diagram shows the mechanism for ethanal with KCN, then dilute acid.
- The first curly arrow must start directly from the lone pair on the carbon atom of the cyanide ion (:CN⁻) and point directly to the δ+ carbonyl carbon.
- The second curly arrow must originate from the centre of the C=O double bond and terminate on the oxygen atom.
- The intermediate tetrahedral alkoxide ion must clearly show a negative charge and a lone pair on the oxygen atom.
- The third curly arrow must start from the oxygen lone pair and point directly to H⁺.
6. Worked Synthesis & Nomenclature
Step 1: Write the addition reaction:
Propanal is CH₃-CH₂-CHO (3 carbons). Cyanide adds one carbon: CH₃-CH₂-CH(OH)-CN.
Step 2: Identify the longest continuous carbon chain containing the nitrile carbon:
The carbon chain has 4 carbons: nitrile carbon (C₁), CH(OH) (C₂), CH₂ (C₃), CH₃ (C₄). The stem is butanenitrile.
Step 3: Number substituents:
The -OH group is located on Carbon-2. The IUPAC name is 2-hydroxybutanenitrile.
Step 1: Chain extension via nucleophilic addition:
React ethanal (CH₃CHO) with KCN acidified with dilute H₂SO₄ at room temperature in a fume cupboard.
Product: 2-hydroxypropanenitrile, CH₃-CH(OH)-CN.
Beyond the OxfordAQA specification: nitrile hydrolysis (Step 2) is not required.
Step 2: Hydrolysis of the nitrile group:
Reflux 2-hydroxypropanenitrile with dilute hydrochloric acid (HCl):
CH₃CH(OH)CN + HCl + 2H₂O → CH₃CH(OH)COOH + NH₄Cl.
Product: 2-hydroxypropanoic acid.
Go deeper
These UK AQA pages cover the same ideas in more detail. They may include content that is not in 9620.
7. Practice Questions
Try each question, then open the answer to check your mark points.
Question 1 (3 marks): Outline the mechanism for the reduction of propanone by NaBH₄. Name the organic product.
Show Answer & Mark Points
- Curly arrow from the lone pair on H⁻ to the δ+ carbonyl carbon, and an arrow from the C=O bond to the oxygen (1)
- Alkoxide intermediate with O⁻, then an arrow from a lone pair on O⁻ to H on H₂O (1)
- Product: propan-2-ol, CH₃CH(OH)CH₃ (1)
Question 2 (2 marks): Describe a test to tell propanal from propanone. Give the result for each.
Show Answer & Mark Points
- Warm with Tollens' reagent (or Fehling's solution) (1)
- Propanal gives a silver mirror (or a brick-red precipitate with Fehling's); propanone shows no change (1)
Question 3 (3 marks): Butanone reacts with KCN, then dilute acid. Name the product and explain why the product is optically inactive.
Show Answer & Mark Points
- 2-hydroxy-2-methylbutanenitrile, CH₃C(OH)(CN)CH₂CH₃ (1)
- The C=O group is planar, so CN⁻ attacks from either side with equal chance (1)
- A 50:50 (racemic) mixture of enantiomers forms, and their rotations of plane-polarised light cancel (1)
Question 4 (2 marks): Give the main hazard of using KCN in this reaction and one precaution.
Show Answer & Mark Points
- KCN is highly toxic, and with acid it can release very toxic HCN gas (1)
- Use a fume cupboard (and wear gloves) (1)