Identification of Aldehydes and Ketones

Experiment 4: Identification of Aldehydes and Ketones

• Aldehydes: Compounds with the general formula RCHO. R can be alkyl or aryl groups, and in formaldehyde, R is a hydrogen atom.
• Ketones: Compounds with the general formula RCOR'. R and R' can be alkyl or aryl groups.
• Carbonyl Group (C=O): A key functional group found in both aldehydes and ketones. It's responsible for many of their chemical and physical properties.
• Examples of Aldehydes: Formaldehyde, acetaldehyde, propionaldehyde, benzaldehyde, salicylaldehyde
• Examples of Ketones: Acetone, acetophenone, benzyl methyl ketone, benzophenone
• General Test (2,4-Dinitrophenylhydrazine): Both aldehydes and ketones react with 2,4-Dinitrophenylhydrazine to form a yellow or orange precipitate.
• Procedure (General Test): Add 2 drops of the compound to 3 drops of the reagent. Observe for a precipitate. If insoluble in water, dissolve compound in 1 ml methanol first, then add the reagent.

Differentiation Tests (Aldehyde vs. Ketone)

• Tollen's Test (Reduction of Ammonical Silver Nitrate): This reagent distinguishes aldehydes, which form a silver mirror, from ketones, which do not.
  • Procedure (Tollen's Test): Add 2-3 drops of the compound to 2-3 ml of Tollen's reagent. Formation of a silver mirror indicates an aldehyde. Warm the test tube in a water bath if no reaction occurs.
• Fehling's Test: Aldehydes, but not ketones, reduce Fehling's reagent (a deep blue solution), forming a red precipitate of cuprous oxide (Cu2O).
  • Procedure (Fehling's test): Add 5 drops of the compound to 1 ml of Fehling's solution. Heat in a water bath for 5 minutes (with shaking for water insoluble compounds). A change in color to green, orange, and then red precipitate or copper mirror suggests an aldehyde.
• lodoform Test: This test is specific for compounds containing a terminal methyl group (CH3) on a carbonyl group.
• Sodium Nitroprusside Test: Used for compounds with terminal methyl groups. A red color complex indicates a positive test.
• Polymerization Reaction: Formaldehyde and salicylaldehyde react with resorcinol and sulfuric acid to form a colored polymer.

Cannizzaro Reaction

• Cannizzaro Reaction: Aldehydes without alpha hydrogens can undergo self-oxidation-reduction reactions in a strong basic medium. The reaction produces an alcohol and a carboxylic acid salt
  • Procedure (Cannizzaro): Add a few drops of the aldehyde (e.g., benzaldehyde) to 0.5 ml of 30% sodium hydroxide, heat slowly, and add distilled water. Concentrated hydrochloric acid can be added to liberate benzoic acid as a precipitate.

This quiz focuses on the identification of aldehydes and ketones, two important classes of organic compounds. It covers their general formulas, chemical properties, examples, and a specific testing method using 2,4-Dinitrophenylhydrazine. Test your knowledge on the characteristics and reactions of these carbonyl compounds.