Unit 4: CH04 Syllabus Node

Aldehydes & Ketones (Carbonyl Chemistry)

Carbonyl group polarity, distinguishing oxidation tests (Tollens, Fehling, dichromate), NaBH4 reduction, and nucleophilic addition mechanism with KCN for OxfordAQA A-Level Chemistry.

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).
Carbonyl Bond Polarity Oxygen is substantially more electronegative than carbon (3.44 vs 2.55). Consequently, the pi and sigma electron density of the double bond is strongly pulled towards oxygen:
>C(delta+) = O(delta-)
The electrophilic delta+ 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 / Benedict's Solution
Aqueous Cu2+ complexed with tartrate in alkaline solution; 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 + Cr2O7 2- + 8H+ -> 3RCOOH + 2Cr3+ + 4H2O		 Orange solution turns green (Cr3+) No reaction (solution remains orange)

3. Reduction of Carbonyls with Sodium Borohydride (NaBH4)

Aldehydes and ketones are readily reduced back to alcohols using sodium borohydride (NaBH4) dissolved in aqueous ethanol:

Reduction Principles
  • 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 delta+ carbonyl carbon.

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 degrees 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 subsequently be hydrolysed to a carboxylic acid or reduced to a primary amine.

5. Nucleophilic Addition Mechanism Diagram

Nucleophilic Addition Mechanism: Carbonyl + Cyanide Ion Step 1: Nucleophilic Attack on Planar Carbonyl CH3 C delta+ H O delta- :C ≡N - Step 2: Protonation CH3 C :O - H CN H+ Final Product OH CH3 - C* - H CN 2-hydroxypropanenitrile
Curly Arrow Precision in Nucleophilic Addition
  1. 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 delta+ carbonyl carbon.
  2. The second curly arrow must originate from the center of the C=O double bond and terminate on the oxygen atom.
  3. The intermediate tetrahedral alkoxide ion must clearly show a negative charge and a lone pair on the oxygen atom.
  4. The third curly arrow must start from the oxygen lone pair and point directly to H+.

6. Worked Synthesis & Nomenclature

Worked Example 1: Naming Hydroxynitriles
Draw the structure and give the IUPAC name of the product formed when propanal reacts with acidified potassium cyanide.

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.

Worked Example 2: Multi-Step Synthesis with Carbon Chain Extension
Outline a two-step reaction pathway to convert ethanal (2 carbons) into 2-hydroxypropanoic acid (lactic acid, 3 carbons).

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.

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.

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