Aldehydes

• The functional group of an aldehyde is H-C=O.
• An aldehyde can be oxidised to a carboxylic acid using potassium dichromate (VI) and dilute sulfuric acid. This process requires heating under reflux conditions.
• During the oxidation of aldehydes, there is a colour change from the orange dichromate ion (Cr2O4 2-) which becomes reduced to the green Cr3+ ion.
• There are two different test for aldehydes - Tollen's Reagent and Fehling's solution.
• Tollen's reagent is a combination of aqueous ammonia and silver nitrate.
• The active ion in Tollen's reagent is (Ag(NH3)2)+.
• In order to test for aldehydes using Tollen's reagent, the sample should be heated gently. During this reaction, aldehydes are oxidised to carboxylic acids, and silver (I) ions are reduced to silver atoms.
• If aldehyde is present with Tollen's reagent, a silver mirror will form on the inside of the tube. There will be no change for ketones.
• The active ion in Fehling's solution is Cu2+.
• When testing for aldehydes using Fehling's solution, you must heat the sample gently. During this reaction, aldehydes are oxidised to carboxylic acids and copper ions are reduced to copper (I) oxide.
• If aldehyde is present using Fehling's solution, the blue Cu2+ ions in solution will change to a red precipitate of Cu2O. This solution will not react with ketones.
• Aldehydes can be reduced to primary alcohols using lithium aluminium hydride (LiAlH4) in dry ether. Room temperature and pressure are required for this reaction.