Conversion of Benzaldehyde to Benzoic Acid

To convert benzaldehyde to benzoic acid, a two-step oxidation reaction is required. Benzaldehyde, an aromatic aldehyde, undergoes oxidation to form benzoic acid, a carboxylic acid. The reaction involves the conversion of the aldehyde functional group to a carboxylic acid functional group.

Step 1: Oxidation of Benzaldehyde to Benzoic Acid using Tollens' Reagent

1. Prepare Tollens' reagent by dissolving silver nitrate (AgNO₃) in distilled water, followed by adding a few drops of sodium hydroxide (NaOH) solution. The solution turns alkaline.
2. Take benzaldehyde in a clean test tube or flask.
3. Add Tollens' reagent dropwise to the benzaldehyde while continuously shaking or stirring the mixture.
4. Observe the reaction mixture. Initially, a silver mirror will form on the inner surface of the container. This is due to the reduction of silver ions (Ag⁺) to metallic silver (Ag) by benzaldehyde.
5. Continue adding Tollens' reagent until the silver mirror disappears completely.
6. The reaction is complete when the silver mirror disappears, indicating the conversion of benzaldehyde to benzoic acid.

Step 2: Acidification to Convert the Silver Mirror to Benzoic Acid

1. After the silver mirror disappears, add dilute hydrochloric acid (HCl) dropwise to the reaction mixture.
2. The silver mirror will dissolve in the presence of hydrochloric acid, forming a clear solution.
3. Heat the solution gently to evaporate excess water and concentrate the benzoic acid.
4. Allow the solution to cool and crystallize. Crystals of benzoic acid will form as the solution cools down.
5. Collect the benzoic acid crystals by filtration using a Buchner funnel or filter paper.
6. Wash the crystals with a small amount of cold water to remove any impurities.
7. Dry the benzoic acid crystals by placing them in an oven or desiccator to remove any remaining moisture.

Summary

By following these two oxidation steps, benzaldehyde can be converted to benzoic acid. The first step involves the oxidation of benzaldehyde using Tollens' reagent to form a silver mirror, which is then acidified to convert it into benzoic acid. The final product is obtained as crystalline benzoic acid after filtration and drying.