The iodoform test is a test to determine if an organic compound is ethanol or contains a secondary alcohol (CH₃—CH(OH)R). Potassium iodide and sodium hypochlorite solution are added to the organic compound in the presence of sodium hydroxide solution. Potassium hypoiodite forms, which then oxidizes the CH₃—CH(OH)R group to CH₃COR. Potassium hypoiodite replaces the hydrogen attached to the carbon atom adjacent to the carbonyl group with iodine. The carbon carbon bond is cleaved to give the iodoform. The iodoform is a yellow color.

The reaction is as follows:

This test can also be used to determine whether an aldehyde or ketone with the structure CH₃COR is present in the organic compound. This test only works for the aldehyde ethanal. The iodoform test is a haloform reaction because a halogen in basic conditions reacts with an aldehyde or ketone.

Materials

• Test tubes

• Test tube rack

• 10% potassium iodide solution

• Sodium hypochlorite solution

• Dioxane

• Sodium hydroxide

• Potassium iodine iodine reagent: prepared by dissolving 20 g of potassium iodide and 10 g of iodine in 100 mL of water.

• Compound to be tested.

Safety

Sodium hydroxide is corrosive. Sodium hypochlorite is corrosive and an environmental hazard. Potassium iodide is harmful, a health hazard, and an environmental hazard. Dioxane is flammable, harmful, and a health hazard.

Procedure

There are 2 methods.

First method

1. Put 0.2 mL of the compound in a test tube.

2. Add 10 mL of 10% aqueous potassium iodide solution and 10 mL of freshly prepared sodium hypochlorite solution.

3. Warm gently.

4. 黄水晶的碘仿如果te分开st is positive.

Second method

1. Dissolve 0.1 g or 4 to 5 drops of the compound in 2 mL of water. If it does not dissolve, add dioxane drop by drop to get a homogeneous solution.

2. Add 2 mL of 5% sodium hydroxide solution followed by potassium iodide-iodine reagent dropwise with continuous shaking till a definite dark colour of iodine persists.

3. Allow the reactants to remain at room temperature for 2-3 minutes.

4. If no iodoform separates:

   • Warm the reaction mixture in a water bath at 60°C.
   • Add more drops of potassium iodide–iodine reagent.
   • If the colour of iodine disappears, continue addition of reagent till the colour of iodine persists even after two minutes of heating at 60°C.
   • Remove excess iodine by adding a few drops of sodium hydroxide solution with shaking.
   • Dilute the mixture with an equal volume of water and keep it at room temperature for 10-15 minutes.

5. A yellow precipitate of the iodoform is obtained if the test is positive.

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References:

• NCERT Lab Manual for Functional Group tests