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AS & A-Level Chemistry 09 — Carbonyls, acids, esters and nitrogen compounds

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Independent Deckloop AS Chemistry study material aligned with Cambridge International 9701 (2025–2027). Deck 9 of 18: Carbonyls, acids, esters and nitrogen compounds. Original explanations, worked applications and practice. Not affiliated with or endorsed by Cambridge International Education.

Chemistry EN A-Level
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Synthesis and Reactions of Aldehydes and Ketones

Aldehydes and ketones are principal carbonyl compounds with distinct synthesis pathways and reactions. Aldehydes are produced by the oxidation of primary alcohols using acidified potassium dichromate(VI)(\mathrm{VI}) (K2Cr2O7\text{K}_2\text{Cr}_2\text{O}_7) or acidified potassium manganate(VII)(\mathrm{VII}) (KMnO4\text{KMnO}_4). Crucially, the mixture must be distilled immediately to prevent further oxidation to a carboxylic acid. Ketones are similarly synthesised by the oxidation of secondary alcohols using the same reagents and distillation.

Both classes of carbonyl compounds can be reduced back to their corresponding alcohols using reducing agents such as sodium tetrahydridoborate (NaBH4\text{NaBH}_4) or lithium tetrahydridoaluminate (LiAlH4\text{LiAlH}_4). Aldehydes yield primary alcohols, whilst ketones yield secondary alcohols.

Carbonyls undergo nucleophilic addition with hydrogen cyanide (HCN\text{HCN}). Because HCN\text{HCN} is a weak acid and dissociates poorly, a KCN\text{KCN} catalyst and heat are required to provide a sufficient concentration of the cyanide nucleophile (CN−\text{CN}^-). The mechanism begins with a curly arrow from the lone pair on the CN−\text{CN}^- ion to the partially positive carbonyl carbon, whilst a second arrow shows the C=O\text{C=O} π\pi-bond breaking and electrons moving to the oxygen atom. This forms a negatively charged alkoxide intermediate. Finally, a lone pair on the intermediate's oxygen attacks the partially positive hydrogen of an HCN\text{HCN} molecule, regenerating the CN−\text{CN}^- catalyst and forming a hydroxynitrile. This reaction is valuable for extending the carbon chain by one carbon atom.

Key points

  • Primary alcohols oxidise to aldehydes using H+/K2Cr2O7\text{H}^+/\text{K}_2\text{Cr}_2\text{O}_7 or H+/KMnO4\text{H}^+/\text{KMnO}_4 and distillation.
  • Secondary alcohols oxidise to ketones using H+/K2Cr2O7\text{H}^+/\text{K}_2\text{Cr}_2\text{O}_7 or H+/KMnO4\text{H}^+/\text{KMnO}_4 and distillation.
  • Reduction with NaBH4\text{NaBH}_4 or LiAlH4\text{LiAlH}_4 converts aldehydes to primary alcohols and ketones to secondary alcohols.
  • Aldehydes and ketones react with HCN\text{HCN}, requiring a KCN\text{KCN} catalyst and heat, to form hydroxynitriles.
  • The nucleophilic addition mechanism features a CN−\text{CN}^- ion attacking the carbonyl carbon, followed by protonation of the resulting alkoxide oxygen.

Worked example

Question

Write the equation for the formation of 2-hydroxy-2-methylpropanenitrile from propanone, detailing the structural changes.

Solution

1. Propanone is CH3COCH3\text{CH}_3\text{COCH}_3.

2. The CN−\text{CN}^- nucleophile attacks the carbonyl carbon, adding a nitrile group (−C≡N-\text{C}\equiv\text{N}).

3. The carbonyl oxygen atom becomes an O−\text{O}^- intermediate, which is protonated to form a hydroxyl group (−OH-\text{OH}).

CH3COCH3+HCN→CH3C(OH)(CN)CH3\text{CH}_3\text{COCH}_3 + \text{HCN} \rightarrow \text{CH}_3\text{C(OH)(CN)CH}_3. The reaction requires heat and a KCN\text{KCN} catalyst.

Common pitfalls

  • Assuming HCN\text{HCN} reacts rapidly with carbonyls on its own. Correct reasoning: HCN\text{HCN} is a very weak acid, meaning the concentration of CN−\text{CN}^- is too low without a KCN\text{KCN} catalyst to initiate the nucleophilic attack effectively.
  • Drawing the curly arrow in the mechanism starting from the carbon atom's nucleus or a bond. Correct reasoning: Curly arrows must start precisely from the lone pair of electrons on the carbon atom of the CN−\text{CN}^- ion.

Prerequisites

  • Study Organic foundations and hydrocarbons first.
  • Study Halogenoalkanes and alcohols first.