Question

How is the iodoform test carried out?

Answer 1

The iodoform test is performed by reacting a suspected compound with iodine in a basic solution, typically with sodium hydroxide. A positive result is the formation of a pale yellow precipitate of iodoform, accompanied by a characteristic antiseptic odor.

Principle of the test: The iodoform test, or haloform reaction, is used to detect the presence of one of two functional groups:

1. Methyl ketones: Compounds containing a methyl group directly attached to a carbonyl group.
2. Secondary alcohols: Compounds containing a methyl group and a hydroxyl group on the same carbon. Ethanol is the only primary alcohol that gives a positive result.

Compounds with these structures are first oxidized (if an alcohol), and then undergo exhaustive iodination followed by cleavage to produce iodoform.

Procedure: There are two common variations for carrying out the iodoform test in a laboratory setting.

1. Using iodine and sodium hydroxide:
   1. Prepare the sample: Place a small amount of the compound to be tested (a few drops of liquid or a few milligrams of solid) into a clean test tube.
   2. Add reagents: Add a few millilitres of iodine-potassium iodide (KI-I₂) solution to the test tube. Next, add a 10% sodium hydroxide (NaOH) solution dropwise until the solution's dark brown color disappears or becomes very pale yellow.
   3. Observe the reaction: If no precipitate forms at room temperature after 2-3 minutes, gently warm the test tube to about 60°C. If the brown color returns, add a few more drops of NaOH.
   4. Confirm the result: The appearance of a pale yellow precipitate of iodoform (CHI₃) after heating and standing confirms a positive test.
2. Using potassium iodide and sodium hypochlorite:
   1. Prepare the sample: Place about 0.2 mL of the test compound in a test tube.
   2. Add reagents: Add 10 mL of 10% aqueous potassium iodide.
   3. Warm and observe: Gently warm the test tube for a few minutes. The formation of yellow iodoform crystals indicates a positive result.