Concept 04 / 07Aldehydes, Ketones and Carboxylic Acids
Reactions Involving the Alpha-Hydrogen: Aldol Condensation, Cannizzaro Reaction and Ring Substitution
Acidity of alpha-hydrogens and enolate formation, aldol addition and condensation of aldehydes and ketones, cross aldol and Claisen-Schmidt condensations, the Cannizzaro reaction (including crossed Cannizzaro) for aldehydes without alpha-hydrogen, and electrophilic substitution in aromatic carbonyl compounds (meta orientation).
Acidity of -hydrogens. Hydrogens on the carbon next to the carbonyl group (-hydrogens) are weakly acidic ( about to ), because the carbonyl group withdraws electrons and the conjugate base (enolate ion) is resonance-stabilised, with negative charge shared between carbon and oxygen.
Aldol reaction (see figure). Aldehydes and ketones with at least one -hydrogen react in dilute alkali to give -hydroxy aldehydes (aldols) or -hydroxy ketones (ketols):
(3-hydroxybutanal).
(4-hydroxy-4-methylpentan-2-one).
Mechanism: the base removes an -hydrogen to form an enolate, which adds as a nucleophile to the carbonyl carbon of a second molecule; the alkoxide is protonated by water.
On heating, the aldol loses water (dehydration) to give an -unsaturated carbonyl compound (aldol condensation): but-2-enal from ethanal; 4-methylpent-3-en-2-one from acetone.
Cross aldol condensation: between two different carbonyl compounds, each with -hydrogens, a mixture of four products forms (ethanal and propanal give four -hydroxy aldehydes). It is useful when one partner has no -hydrogen (it can only accept): Claisen-Schmidt condensation, (benzalacetone), or benzaldehyde with acetophenone giving chalcone.
Cannizzaro reaction. Aldehydes without -hydrogen (methanal, benzaldehyde, 2,2-dimethylpropanal) undergo self oxidation-reduction (disproportionation) when heated with concentrated alkali: one molecule is reduced to the alcohol, another oxidised to the carboxylate salt:
;
.
Mechanism: adds to one aldehyde, and the resulting anion transfers a hydride ion to a second aldehyde molecule.
Crossed Cannizzaro: with methanal and another non-enolisable aldehyde, methanal (more reactive) is always oxidised to formate, and the other aldehyde is reduced to its alcohol: .
Electrophilic substitution. The carbonyl group of aromatic aldehydes and ketones is deactivating and meta-directing: nitration of benzaldehyde gives 3-nitrobenzaldehyde.
Key Takeaways:
- -H is acidic; enolates are nucleophiles.
- Aldol: dilute base, -H needed; heating gives -unsaturated carbonyls.
- Cross aldol gives mixtures unless one partner lacks -H (Claisen-Schmidt).
- Cannizzaro: no -H, concentrated base; alcohol + carboxylate.
- In crossed Cannizzaro, HCHO is oxidised; the other aldehyde is reduced.
- Pitfall: an aldehyde with -H in concentrated base gives aldol products (and resins), not a Cannizzaro reaction.
Brain-Teaser 1: Which of these undergo aldol condensation and which the Cannizzaro reaction: methanal, ethanal, benzaldehyde, 2-methylpropanal, 2,2-dimethylpropanal, propanone?
Solution: Aldol (-H present): ethanal, 2-methylpropanal (one -H), propanone. Cannizzaro (no -H): methanal, benzaldehyde, 2,2-dimethylpropanal.
Brain-Teaser 2: How many aldol products (ignoring stereoisomers) form in a cross aldol reaction of ethanal and propanal? Name them.
Solution: Four: ethanal + ethanal gives 3-hydroxybutanal; propanal + propanal gives 3-hydroxy-2-methylpentanal; ethanal (enolate) + propanal gives 3-hydroxypentanal; propanal (enolate) + ethanal gives 3-hydroxy-2-methylbutanal.
Brain-Teaser 3: What products form when a mixture of benzaldehyde and methanal is heated with concentrated NaOH?
Solution: Crossed Cannizzaro: methanal is oxidised to sodium methanoate (formate), and benzaldehyde is reduced to benzyl alcohol, .
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