Acidified Potassium Dichromate(VI): When heated with , aldehydes reduce the orange ions to green ions. Ketones show no reaction, remaining orange.
Tollens' Reagent: This contains the diamminesilver(I) complex, . Aldehydes reduce the silver(I) ions to metallic silver, forming a silver mirror on the test tube wall, while the aldehyde is oxidized to a carboxylate ion.
Fehling's and Benedict's Solutions: These contain ions (complexed with tartrate or citrate). Aldehydes reduce the blue to a brick-red precipitate of copper(I) oxide (), while ketones remain blue.
Structural Requirement: The iodoform test specifically identifies the methyl carbonyl group (). It also gives a positive result for alcohols that can be oxidized to this group, such as ethanol and secondary alcohols with a methyl group on the carbon bearing the hydroxyl group.
Reaction Conditions: The sample is heated with iodine in the presence of an alkali (NaOH). The reaction involves halogenation of the methyl group followed by hydrolysis.
Observation: A positive result is indicated by the formation of a pale yellow precipitate of tri-iodomethane (), which has a characteristic medicinal smell.
| Test Reagent | Aldehyde Result | Ketone Result | Specificity |
|---|---|---|---|
| 2,4-DNPH | Orange Precipitate | Orange Precipitate | All compounds |
| Tollens' | Silver Mirror | No Reaction | Aldehydes only |
| Fehling's | Red Precipitate | No Reaction | Aldehydes only |
| Iodoform | Yellow Ppt (if ) | Yellow Ppt (if ) | Methyl carbonyls |
Sequence of Testing: Always use 2,4-DNPH first to confirm the compound is a carbonyl before attempting to distinguish between an aldehyde and a ketone. Using Tollens' on an unknown that isn't a carbonyl is a waste of time.
Distinguishing Isomers: If two compounds have the same molecular formula (e.g., propanal and propanone), use Tollens' or Fehling's to identify the aldehyde. If both are ketones, use the iodoform test to see if one is a methyl ketone.
Common Error: Students often forget that ethanal is the only aldehyde that gives a positive iodoform test because it is the only aldehyde containing the group.
Observation Accuracy: Ensure you specify 'precipitate' for Fehling's and Iodoform tests; simply saying 'color change' is often insufficient for full marks.
