Notes On Aldehydes And Ketones: Chemical Reactions - II - CBSE Class 12 Chemistry

Aldehydes and ketones differ in the manner in which they undergo oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents (or) even by very mild oxidising agents. This produces Carboxylic acids with the same number of carbon atoms as that of the parent aldehyde.

Ketones are oxidised only in the presence of strong oxidising agent and high temperatures. This produces a mixture of Carboxylic acids with fewer carbon atoms than the parent ketone.

This difference in oxidation is because of the difference in the structure of these compounds. An aldehyde has a hydrogen atom attached to the carbonyl carbon while a ketone has an alkyl (or) aryl group attached to the carbonyl carbon.

By using mild oxidising agents it can be possible to distinguish between aldehydes and ketones.

On heating an aldehyde with Fehling's or Benedict's reagent a reddish brown precipitate is obtained.

The reduction of aldehydes and ketones to hydrocarbons by the action of amalgamated zinc and concentrated hydrochloric acid is known as the Clemmensen reduction.

The reduction of aldehydes and ketones to hydrocarbons by the action of hydrazine followed by heating with sodium or potassium in a high boiling solvent such as ethylene glycol is known as the Wolff-Kishner reduction.

Aldehydes and ketones with at least one alpha hydrogen combine in the presence of a dilute alkali to form b-hydroxy aldehydes (or) aldols (or) b-hydroxy ketones or ketols. This reaction is called as aldol reaction.

When aldehydes that do not have an a-hydrogen are treated with concentrated alkali, they undergo self oxidation-reduction to yield a mixture of an alcohol and a salt of a carboxylic acid. This reaction is known as the Cannizaro reaction.

Summary

Aldehydes and ketones differ in the manner in which they undergo oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents (or) even by very mild oxidising agents. This produces Carboxylic acids with the same number of carbon atoms as that of the parent aldehyde.

Ketones are oxidised only in the presence of strong oxidising agent and high temperatures. This produces a mixture of Carboxylic acids with fewer carbon atoms than the parent ketone.

This difference in oxidation is because of the difference in the structure of these compounds. An aldehyde has a hydrogen atom attached to the carbonyl carbon while a ketone has an alkyl (or) aryl group attached to the carbonyl carbon.

By using mild oxidising agents it can be possible to distinguish between aldehydes and ketones.

On heating an aldehyde with Fehling's or Benedict's reagent a reddish brown precipitate is obtained.

The reduction of aldehydes and ketones to hydrocarbons by the action of amalgamated zinc and concentrated hydrochloric acid is known as the Clemmensen reduction.

The reduction of aldehydes and ketones to hydrocarbons by the action of hydrazine followed by heating with sodium or potassium in a high boiling solvent such as ethylene glycol is known as the Wolff-Kishner reduction.

Aldehydes and ketones with at least one alpha hydrogen combine in the presence of a dilute alkali to form b-hydroxy aldehydes (or) aldols (or) b-hydroxy ketones or ketols. This reaction is called as aldol reaction.

When aldehydes that do not have an a-hydrogen are treated with concentrated alkali, they undergo self oxidation-reduction to yield a mixture of an alcohol and a salt of a carboxylic acid. This reaction is known as the Cannizaro reaction.

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