Aldehydes, Ketones and Carboxylic acid - IV

Oxidation of aldehydes and ketones:

(i) Aldehydes are oxidized to acids in presence of mild oxidising agents HNO₃, K₂Cr₂O₇,
KMnO₄.

(ii) Ketones are oxidized under drastic conditions i.e. with powerful oxidising agents like HNO₃, K₂Cr₂O₇/H₂SO₄, KMnO₄/H₂SO₄ at higher temperature.

In case of unsymmetrical ketones cleavage occurs in such a way that keto group stays with smaller alkyl group. This is known as Popoff’s rule.

(iii) Haloform reaction: Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom i.e. methyl ketones are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform.

Reactions of aldehydes and ketones due to a -hydrogen:

(i) Aldol condensation: Aldehydes and ketones having at least one a  -hydrogen undergo a self condensation in the presence of dilute alkali as catalyst to form a - hydroxy aldehydes (aldol) or a  -hydroxy ketones (ketol), respectively

(ii) Cross aldol condensation: Aldol condensation between two different aldehydes and ketones is called aldol condensation. If both of them contain a  -hydrogen atoms, it gives a mixture of four products.

Canizzaro reaction:

Aldehydes which do not have an hydrogen atom undergo self-oxidation and reduction (disproportionation) reaction on treatment with concentrated alkali to form alcohol and salt of acid.

Test to distinguish aldehydes and ketones:

1) Tollen’s test: When an aldehyde is heated with Tollen’s reagent it forms silver mirror. Tollen’s reagent is ammoniacal solution of AgNO₃

Ketones do not form silver mirror and hence do not give this test.

2) Fehling’s test: When an aldehyde is heated with Fehling’s reagent it forms reddish brown precipitates of cuprous oxide. Fehling’s reagent: Fehling solution A (aqueous solution of CuSO₄) + Fehling solution B (alkaline solution of sodium potassium tartarate)

Ketones do not give this test.

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