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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

6.1.2 Carbonyl compounds

Characteristic tests for carbonyl compoundsReactions of carbonyl compounds

Reactions of carbonyl compounds

Specification Reference (a)–(e)

Quick Notes

  • Oxidation
    • Aldehydes can be oxidised to carboxylic acids using potassium dichromate(VI) in dilute sulfuric acid.
    • Observed colour change: orange → green (Cr2O72− → Cr3+).
    • Ketones do not undergo oxidation under these conditions.
  • Nucleophilic Addition Reactions
    • With NaBH4 (sodium borohydride): reduces carbonyls to alcohols.
      • Aldehyde forms a primary alcohol.
      • Ketone forms a secondary alcohol.
    • With HCN (NaCN/H+): adds across the C=O to form hydroxynitriles.
      • Carbonyl carbon is δ+, making it susceptible to nucleophilic attack.
      • Mechanism involves nucleophile attacking carbonyl C, followed by protonation.

Full Notes

Oxidation of Aldehydes

Aldehydes can be oxidised using acidified potassium dichromate(VI) (K2Cr2O7/H2SO4).

The aldehyde is oxidised to a carboxylic acid:

RCHO + [O] → RCOOH

OCR (A) A-Level Chemistry oxidation of aldehyde to carboxylic acid.

The dichromate ion (Cr2O72−) is reduced to Cr3+, resulting in an observable colour change from orange to green.

OCR (A) A-Level Chemistry colour change from orange to green when aldehydes are oxidised with acidified potassium dichromate.

Ketones resist oxidation due to the C=O bond being bonded to two alkyl groups, making the carbon less accessible and unable to be oxidised by Cr2O72− ions.

Nucleophilic Addition Reactions

Carbonyl groups (C=O) are polar, with the carbon being electrophilic. This makes them reactive toward nucleophiles.

Reduction with NaBH4

[H] in reactions represents the addition of hydrogen, indicating a reduction reaction. It is often used as shorthand for a reducing agent, such as sodium tetrahydridoborate (NaBH4) when reducing carbonyls.

Aldehydes can be reduced to primary alcohols.

Reagent: NaBH4 in aqueous solution.

OCR (A) A-Level Chemistry reaction scheme showing aldehyde reduced to primary alcohol with NaBH4.

Equation for aldehyde reduction:
CH3CHO + 2[H] → CH3CH2OH

Ketones can be reduced to secondary alcohols.
Reagent: NaBH4 in aqueous solution.

OCR (A) A-Level Chemistry reaction scheme showing ketone reduced to secondary alcohol with NaBH4.

Equation for ketone reduction:
CH3COCH3 + 2[H] → CH3CH(OH)CH3

Mechanism

The reduction reactions for both aldehydes and ketones follow a nucleophilic addition mechanism, with the nucleophile a hydride ion (H−) coming from NaBH4.

OCR (A) A-Level Chemistry nucleophilic addition mechanism for carbonyl reduction by hydride ion.

For Example Reductions

Ethanal → Ethanol
CH3CHO + 2[H] → CH3CH2OH

Propanone → Propan-2-ol
CH3COCH3 + 2[H] → CH3CH(OH)CH3

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Remember in organic chemistry that oxidation is the gaining of a bond to a more electronegative element (such as oxygen or nitrogen) and, or the loss of a bond to a less electronegative element (hydrogen). Reduction is the opposite of this — the losing of a bond to a more electronegative element (such as oxygen or nitrogen) and the gaining of a bond to hydrogen.

Reaction with HCN (NaCN/H+)

Aldehydes and ketones react with KCN followed by dilute acid to form hydroxynitriles.

Reagent: KCN followed by H+.

Mechanism: Nucleophilic addition of CN− to the carbonyl carbon.

OCR (A) A-Level Chemistry nucleophilic addition mechanism showing cyanide ion adding to a carbonyl carbon.

The cyanide (CN−) ion comes from KCN which dissociates to release K+ and CN−.

OCR (A) A-Level Chemistry diagram showing dissociation of KCN into K+ and CN- ions.

Extra Note: The product contains a new chiral centre, so if starting with an achiral carbonyl, a racemic mixture of enantiomers forms (see optical isomerism). The –CN group also increases carbon chain length.

Summary