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Oxidation in Aldehydes

(a) Oxidation of aldehydes: aldehydes are easily oxidized to carboxylic acids containing the same number of carbon atoms, as in parent aldehydes.

The reason for this easy oxidation is the presence of a hydrogen atom on the carbonyl carbon which can be converted into –OH group without involving the cleavage of any other bond. Hence, aldehydes are oxidized not only by strong oxidizing agents like KMnO₄ and K₂Cr₂O₇ but also by weak oxidizing agents like bromine water, Ag₂, Cu²⁺ etc. as a result, aldehydes act as strong reducing agents. They reduce:

(i) Tollen’s reagent (ammonical AgNO₃) to metallic silver.

(ii) Fehling solution CuSO₄ solution) to red precipitate of Cu₂O and

(iii) Benedict’s solution (alkaline CuSO₄ solution containing citrate ions) to red phosphorus of Cu₂O as described below:

1. Reduction of Tollen’s reagent: Tollen’s reagent is ammonical solution of silver nitrate. On warming with this reagent aldehydes form a silver mirror on the walls of the container. The reaction is also known as silver mirror test for aldehydes. The chemical reactions involved are:


2. Reduction of Fehling solution: Fehling’s solution is an alkaline solution of copper sulphate containing sodium potassium tartrate (Rochelle salt) as the complexing agent. Aldehydes on warming with this solution, give a red precipitate of cuprous oxide.


3. Reduction of Benedict’s solution: Benedict’s solution is alkaline copper sulphate containing citrate ions as complexing agent. Aldehydes on warming with this solution, give brick red precipitates.

It must be noted that Fehling solution and Benedict’s solution are very weak oxidizing agents. These can oxidize only aliphatic aldehydes but are not capable of oxidizing aromatic aldehydes such as benzaldehyde.

Since ketones are not oxidized by weak oxidizing agents, such as Tollen’s reagent or Fehling’s solution, therefore, such reagents may used to distinguish between aldehydes and ketones.

(b) Oxidation of ketones: ketones are oxidized only under vigorous conditions using powerful oxidizing agents such as conc. HNO₃, KMnO₄/H₂SO₄, K₂Cr₃O₇/H₂SO₄ etc. oxidation of ketones involves cleavage bond between carbonyl carbon and –carbon on either side of keto group giving a mixture of carboxylic acids.

In case unsymmetrical ketones, the point of cleavage is such that keto group stays with smaller alkyl group preferentially (Popoff’s rule).

(c) Oxidation with Sodium Hypohallic (NaOX) or (X₂ + NaOH): this reaction is given by acetaldehyde or methyl ketones and is known as haloform reaction. For example, when acetaldehyde or methyl ketone is treated with sodium hypoiodite (I₂ + aqueous NaOH) a yellow precipitate of iodoform is obtained.