19.2 Preparations of Aldehydes and Ketones | Carbonyl Compounds | Chemistry 11

This section covers the synthesis of aldehydes and ketones through the oxidation of primary and secondary alcohols, hydration of alkynes, and other industrial methods.

By the Oxidation of Primary and Secondary Alcohols

The oxidation of alcohols is a common method for preparing aldehydes and ketones. The outcome of the reaction depends on whether the starting alcohol is primary or secondary.

Oxidizing Agents: Strong oxidizing agents like acidified potassium permanganate ( $K M n O_{4}$ ) or potassium dichromate ( $K_{2} C r_{2} O_{7}$ ) are typically used.
Characteristic Reaction: The reaction of alcohols with acidified potassium dichromate ( $K_{2} C r_{2} O_{7}$ ) is easily observed by a color change in the solution from orange ( $C r_{2} O_{7}^{2 -}$ ) to green ( $C r^{3 +}$ ).

Do You Know? Many fragrances and perfumes contain carbonyl compounds, which contribute to their aroma. For instance, vanillin is a carbonyl compound that imparts the characteristic flavor of vanilla.

1. Oxidation of Primary Alcohols

A primary alcohol is first oxidized to an aldehyde. However, the aldehyde itself is susceptible to further oxidation, which converts it into a carboxylic acid.

Controlling the Reaction: To stop the reaction at the aldehyde stage, specific conditions are required. Since aldehydes have lower boiling points than their corresponding alcohols, the aldehyde can be distilled from the reaction mixture as soon as it is formed.
Temperature Control: The reaction temperature is typically kept slightly above the boiling point of the aldehyde (e.g., around $5 0^{\circ} C$ for acetaldehyde) to facilitate its immediate removal and prevent further oxidation.

Step 1: Alcohol to Aldehyde

The primary alcohol is oxidized to an aldehyde.

$C H_{3} C H_{2} O H + [O] K_{2} C r_{2} O_{7} + H_{2} S O_{4}, 5 0^{\circ} C Acetaldehyde C H_{3} C H O + H_{2} O$

Step 2: Aldehyde to Carboxylic Acid (Further Oxidation)

If the aldehyde is not removed, it will be further oxidized to a carboxylic acid.

$C H_{3} C H O + [O] K_{2} C r_{2} O_{7} + H_{2} S O_{4} Acetic Acid C H_{3} C O O H$

2. Oxidation of Secondary Alcohols

A secondary alcohol is oxidized under similar conditions to produce a ketone. Unlike aldehydes, ketones are resistant to further oxidation under these mild conditions. Therefore, controlling the reaction is much simpler.

$2-Propanol (a secondary alcohol) C H_{3} - CH (OH) - C H_{3} + [O] K_{2} C r_{2} O_{7} + H_{2} S O_{4} Propanone (a ketone) C H_{3} - C (= O) - C H_{3} + H_{2} O$

By Hydration of Alkynes

Alkynes react with water in the presence of mercuric sulfate ( $H g S O_{4}$ ) and sulfuric acid ( $H_{2} S O_{4}$ ) to produce carbonyl compounds. Ethyne produces acetaldehyde, while higher alkynes produce ketones.

$H C \equiv C H + H_{2} O H g S O_{4} / H_{2} S O_{4} [C H_{2} = C H - O H] \to C H_{3} C H O$

From Calcium Salts of Carboxylic Acids

Dry distillation of calcium salts of carboxylic acids is a traditional method for preparing aldehydes and ketones.

Distillation of calcium formate yields formaldehyde.
Distillation of a mixture of calcium formate and calcium acetate yields acetaldehyde.
Distillation of calcium acetate alone yields acetone.

Rosenmund Reduction

Aldehydes can be prepared by the reduction of acid chlorides with hydrogen in the presence of palladium catalyst supported on barium sulfate ( $P d / B a S O_{4}$ ), poisoned with quinoline or sulfur to prevent further reduction to alcohols.

$R - C O C l + H_{2} P d / B a S O_{4} R - C H O + H C l$

This section covers the synthesis of aldehydes and ketones through the oxidation of primary and secondary alcohols, hydration of alkynes, and other industrial methods.

By the Oxidation of Primary and Secondary Alcohols

The oxidation of alcohols is a common method for preparing aldehydes and ketones. The outcome of the reaction depends on whether the starting alcohol is primary or secondary.

Oxidizing Agents: Strong oxidizing agents like acidified potassium permanganate ( $K M n O_{4}$ ) or potassium dichromate ( $K_{2} C r_{2} O_{7}$ ) are typically used.
Characteristic Reaction: The reaction of alcohols with acidified potassium dichromate ( $K_{2} C r_{2} O_{7}$ ) is easily observed by a color change in the solution from orange ( $C r_{2} O_{7}^{2 -}$ ) to green ( $C r^{3 +}$ ).

1. Oxidation of Primary Alcohols

A primary alcohol is first oxidized to an aldehyde. However, the aldehyde itself is susceptible to further oxidation, which converts it into a carboxylic acid.

Controlling the Reaction: To stop the reaction at the aldehyde stage, specific conditions are required. Since aldehydes have lower boiling points than their corresponding alcohols, the aldehyde can be distilled from the reaction mixture as soon as it is formed.
Temperature Control: The reaction temperature is typically kept slightly above the boiling point of the aldehyde (e.g., around $5 0^{\circ} C$ for acetaldehyde) to facilitate its immediate removal and prevent further oxidation.

Step 1: Alcohol to Aldehyde

The primary alcohol is oxidized to an aldehyde.

$C H_{3} C H_{2} O H + [O] K_{2} C r_{2} O_{7} + H_{2} S O_{4}, 5 0^{\circ} C Acetaldehyde C H_{3} C H O + H_{2} O$

Step 2: Aldehyde to Carboxylic Acid (Further Oxidation)

If the aldehyde is not removed, it will be further oxidized to a carboxylic acid.

$C H_{3} C H O + [O] K_{2} C r_{2} O_{7} + H_{2} S O_{4} Acetic Acid C H_{3} C O O H$

Distillation of calcium formate yields formaldehyde.
Distillation of a mixture of calcium formate and calcium acetate yields acetaldehyde.
Distillation of calcium acetate alone yields acetone.

Rosenmund Reduction

$R - C O C l + H_{2} P d / B a S O_{4} R - C H O + H C l$