Chemistry Reaction Schemes

Aldol reaction: base-catalysed addition of ethanal

Hydrogens on the carbon next to a carbonyl group are weakly acidic. A base removes one to make an enolate, a carbon nucleophile, which attacks the carbonyl carbon of a second aldehyde molecule. This makes a new C–C bond.

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Aldol addition (base-catalysed)

Hydroxide removes an α-hydrogen from ethanal to form an enolate ion. The enolate attacks the carbonyl carbon of a second ethanal molecule, and protonation of the resulting alkoxide gives 3-hydroxybutanal, regenerating hydroxide.

Overall reaction

ethanal (2 molecules)
dilute NaOH
cold
3-hydroxybutanal

Mechanism — 3 steps

  1. 1Enolate formationdeprotonation · reversible
    hydroxide ion
    +
    ethanal
    enolate ion
    +
    water

    Electron movement

    1. Hydroxide removes a hydrogen from the carbon next to the C=O (the α-carbon).
    2. The C–H electrons form a C=C bond.
    3. The C=O π electrons move onto oxygen, giving the enolate ion.

    The α-hydrogen is weakly acidic because the negative charge in the enolate is delocalised onto the electronegative oxygen.

  2. 2Enolate attacks a second ethanal moleculenucleophilic addition (C–C bond formation) · reversible
    enolate ion
    +
    ethanal
    alkoxide intermediate

    Electron movement

    1. A lone pair on the enolate oxygen reforms the C=O bond.
    2. The C=C electrons attack the carbonyl carbon of the second ethanal, forming a new C–C bond.
    3. The C=O π electrons of the second ethanal move onto its oxygen.

    The enolate reacts through carbon, its nucleophilic end, forming the key carbon–carbon bond.

  3. 3Protonation of the alkoxideproton transfer · reversible
    alkoxide intermediate
    +
    water
    3-hydroxybutanal
    +
    hydroxide ion

    Electron movement

    1. The alkoxide oxygen takes a proton from water.
    2. The O–H bond of water breaks, regenerating hydroxide.

    Hydroxide is regenerated, so the reaction is base-catalysed. On heating, 3-hydroxybutanal loses water to give but-2-enal (aldol condensation).

Stereochemistry

A new stereocentre forms at the CH(OH) carbon. Attack can occur at either face of the planar carbonyl, so the product is racemic.

Key concepts

  • Acidity of α-hydrogens
  • Enolates as carbon nucleophiles
  • C–C bond formation
  • Aldol addition vs aldol condensation

Common mistakes

  • Removing a hydrogen from the aldehyde CHO carbon instead of the α-carbon
  • Having the enolate attack through oxygen
  • Forgetting to regenerate the base

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