Preparation of Aldehydes and Ketones, Chemistry tutorial

Introduction:

Aldehydes and ketones can be made as using different methods ranging from the oxidation to reduction of appropriate compounds.

Preparation of Aldehydes and ketones:

Aldehydes and ketones can be made by using the different methods illustrated below:

Oxidation of primary and secondary alcohols:

Aldehydes and ketones are made by the oxidation of primary and secondary alcohols correspondingly by using a mild oxidizing agent like acidified solution of potassium heptadicromate (VI) or tetraoxomanganate (VII).

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Fig: Oxidation of primary and secondary alcohols

If potassium heptaoxodichromate (VI) acidified by dilute tetraoxosulphate (VI) acid is employed, a change in the colour of the solution from orange solution having the dichromate (VI) ions is reduced to the green solution having chromium (III) ions. The total effects is that an oxygen from the oxidizing agent eliminates one hydrogen atom from the -OH group of the alcohol and the other one from the carbon to which it is linked to form water.

Though in the oxidation of primary alcohols to form aldehydes, there is a problem. The aldehydes generated can be oxidized further to a carboxylic acid via the acidified dichromate (VI) solution employed as the oxidizing agent. This though can be prevented by employing surplus alcohol and limited oxidizing agent. As well by eliminating the aldehydes as soon as it is formed, this can be controlled.

Oxidative cleavage of alkanes (ozonolysis):

Ozone, if bubbled via a solution of an alkene in 1, 1, 1-trichloromethane, followed through the hydrolysis of the ozonide formed, in the presence of zinc and ethanoic acid provides aldehydes and ketones. The starting alkene finds out the product, if aldehydes or ketones. 

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Fig: Oxidative cleavage of alkanes

Dehydration of alcohols:

The lower members of aldehydes and ketones are made industrially by passing the alcohol vapor over hot copper catalyst.

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Fig: Dehydration of alcohols

A primary alcohol provides aldehyde whereas a secondary alcohol provides ketone.

Decarboxylaton of calcium salts:

Aldehydes are prepared by heating a mixture of calcium methanoate and calcium carboxylate at 400oC. If calcium methanoate is not utilized, then a ketone is formed.

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Fig: Decarboxylaton of calcium salts

Reduction of acyl chloride using a 'poisoned catalyst':

This reaction is termed as Rosenmund Reaction. This is only suitable for aldehydes. Acyl chlorides are reduced to aldehydes by treatment with hydrogen in the presence of Palladised barium tetraoxosulphate (VI), poisoned through barium tetraoxosulphate (VI). The catalyst is poisoned to prevent further reduction to the alcohol.

RCOCl + H2 + (Pd/BaSO4)/heat → RCHO + HCl

Friedel-Craft acylation:

The condensation of benzene by an acyl chloride, RCOCl, or acid anhydride, (RCO)2O, in the presence of anhydrous aluminum chloride-a Lewis acid provides a good yield of the aromatic ketone. The reaction mixture is refluxed on the water bath at around 50oC

RCOCl + AlCl3 → (at 50oC) → RC+O + [AlCl4]-

Or if the acid anhydride is employed,

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Fig: Friedel-Craft acylation

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