Concept 03 / 07Aldehydes, Ketones and Carboxylic Acids
Reduction and Oxidation of Aldehydes and Ketones: Clemmensen, Wolff-Kishner, Tollens, Fehling and Haloform
Reduction of carbonyl compounds to alcohols and to hydrocarbons (Clemmensen and Wolff-Kishner), oxidation of aldehydes by mild oxidants with Tollens' and Fehling's tests, oxidation of ketones under vigorous conditions, and the haloform (iodoform) reaction for methyl ketones and related alcohols.
Reduction.
To alcohols: aldehydes give primary and ketones secondary alcohols with , or catalytic hydrogenation.
To hydrocarbons (C=O to ):
Clemmensen reduction: zinc amalgam and concentrated HCl, (for acid-stable compounds).
Wolff-Kishner reduction: hydrazine forms the hydrazone, which is heated with KOH in a high-boiling solvent such as ethylene glycol: (for base-stable compounds).
Oxidation. Aldehydes, with a hydrogen on the carbonyl carbon, are oxidised to carboxylic acids even by mild oxidants (see figure): nitric acid, , , and also by:
Tollens' reagent (ammoniacal silver nitrate), giving a silver mirror: ;
Fehling's solution (Fehling A: aqueous ; Fehling B: alkaline sodium potassium tartrate), giving a red-brown precipitate of : . Aromatic aldehydes do not respond to Fehling's test; Benedict's solution works similarly.
Ketones resist mild oxidants. Strong oxidants at high temperature cleave the C-C bonds next to the carbonyl, giving a mixture of carboxylic acids with fewer carbons.
Haloform reaction. Aldehydes and ketones with at least one methyl group next to the carbonyl (, methyl ketones and ethanal) are oxidised by sodium hypohalite (NaOX, from + NaOH): the three -hydrogens of the methyl group are replaced by halogen, and the group is cleaved off as haloform, , leaving the sodium salt of an acid with one fewer carbon:
.
With iodine, yellow iodoform (, m.p. , characteristic smell) precipitates: the iodoform test. Compounds with the group (ethanol, propan-2-ol) also respond, because hypoiodite first oxidises them to . The C=C in the molecule, if present, is not attacked.
Key Takeaways:
- Clemmensen (Zn-Hg/HCl) and Wolff-Kishner (/KOH, glycol) convert C=O to .
- Tollens' (silver mirror) and Fehling's (red ) are positive for aldehydes, negative for ketones.
- Aromatic aldehydes give Tollens' but not Fehling's test.
- Iodoform test: or ; the acid formed has one carbon less.
- Pitfall: methanal gives neither the iodoform test nor, of course, a methyl ketone reaction, while ethanal does give iodoform.
Brain-Teaser 1: Which of these give a positive iodoform test: ethanal, propanal, pentan-2-one, pentan-3-one, ethanol, methanol, propan-2-ol, acetophenone?
Solution: Ethanal, pentan-2-one, ethanol, propan-2-ol and acetophenone (each has or ). Propanal, pentan-3-one and methanol do not.
Brain-Teaser 2: How would you distinguish (a) propanal from propanone, (b) benzaldehyde from acetaldehyde?
Solution: (a) Tollens' or Fehling's reagent: propanal gives a silver mirror / red precipitate; propanone does not. (Iodoform test also works: propanone positive, propanal negative.) (b) Fehling's test: acetaldehyde gives red ; benzaldehyde does not. (Iodoform: acetaldehyde positive, benzaldehyde negative.)
Brain-Teaser 3: Convert acetophenone to ethylbenzene by two different methods and state when each is preferred.
Solution: Clemmensen reduction (Zn-Hg, concentrated HCl) or Wolff-Kishner reduction (hydrazine, then KOH in ethylene glycol on heating). Clemmensen suits compounds stable to strong acid; Wolff-Kishner suits compounds that would be damaged by acid but are stable to strong base.
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