Aldehydes and ketones

AS Level Chemistry · Carbonyl compounds. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Aldehydes and ketones both contain the carbonyl group, C=O. In an aldehyde the carbonyl carbon has a hydrogen atom attached (–CHO, at the end of a chain). In a ketone it is joined to two carbon atoms.

The C=O bond is polar, with a δ+ carbon, so nucleophiles attack it. The typical reaction is nucleophilic addition. With HCN, the CN− ion attacks the δ+ carbon and the π bond breaks, putting a negative charge on the oxygen. The O− then takes H+ from HCN to give a hydroxynitrile.

Aldehydes are easily oxidised to carboxylic acids. Ketones are not. This difference is how the two are told apart.

Rules to remember

Common mistake

Students use 2,4-DNPH to tell an aldehyde from a ketone. It only shows that a C=O group is present; Tollens' reagent or Fehling's solution is needed to tell which one it is.

Worked example

A compound C5H10O gives an orange precipitate with 2,4-DNPH, no change with Tollens' reagent and no precipitate with alkaline aqueous iodine. Identify it.

  1. The orange precipitate shows a carbonyl group, so it is an aldehyde or a ketone.
  2. No silver mirror with Tollens' reagent, so it is a ketone.
  3. No yellow precipitate, so there is no CH3CO– group: this rules out pentan-2-one and 3-methylbutan-2-one.
  4. The only ketone left is CH3CH2COCH2CH3.

Answer: Pentan-3-one.

Practice questions

Try each one, then open the answer.

1. Which conditions convert ethanol into ethanal?

  1. A
    acidified K2Cr2O7, heat under reflux
  2. B
    acidified K2Cr2O7, heat and distil off the product as it forms
  3. C
    NaBH4 in water
  4. D
    concentrated H2SO4, heat
Show answer

Answer: B. Ethanal boils at 21 °C, far below ethanol (78 °C), so it can be distilled out of the reaction mixture before it is oxidised further. Refluxing gives ethanoic acid; NaBH4 is a reducing agent.

2. In the nucleophilic addition of HCN to ethanal, what is the first step of the mechanism?

  1. A
    the cyanide ion attacks the δ+ carbon of the C=O group, breaking the π bond
  2. B
    a hydrogen ion adds to the carbonyl oxygen
  3. C
    HCN adds across the C–H bond of the methyl group
  4. D
    the C=O π bond is broken by heat
Show answer

Answer: A. CN− (from KCN) is the nucleophile; it donates its lone pair to the carbonyl carbon and the π electrons move onto oxygen. The resulting alkoxide ion then picks up H+ to give 2-hydroxypropanenitrile.

3. Butanone is reduced with sodium tetrahydridoborate, NaBH4. What is the product?

  1. A
    butan-1-ol
  2. B
    butanal
  3. C
    butan-2-ol
  4. D
    butanoic acid
Show answer

Answer: C. Reduction of a ketone gives a secondary alcohol: the C=O on C2 becomes CH(OH), so butanone gives butan-2-ol.

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