Iodoform (Tri iodomethane) CHI₃

Preparation

Laboratory preparation

It is prepared in the laboratory by the action of iodine on ethyl alcohol or acetone, in the presence of alkali. This is called haloform reaction.

From ethyl alcohol (ethanol)

With ethyl alcohol, the reactions take place in three steps as:

Oxidation occurs to give:

Iodination takes place:

Finally cleavage occurs to form iodoform:

The complete reaction may be written as:

From propanone (acetone)

The reactions taking place are similar to the above reactions, finally giving rise to iodoform:

Sodium carbonate can also be used in place of NaOH (or KOH). If Na₂CO₃ is used the complete reactions are:

Physical Properties of Iodoform

• It is a yellow colored crystalline solid with melting point 392 K.

• It has a characteristic unpleasant odor.

• It is insoluble in water but readily dissolves in ethyl alcohol and ether.

• Due to liberation of free iodine it has an antiseptic action.

Chemical Properties

Chemical properties of iodoform are:

Stability

On heating, iodoform decomposes to give iodine vapor. This reaction is accelerated by moisture, air or light.

Reduction

Iodoform can be reduced with P and HI to give methylene iodide.

Hydrolysis

On boiling with aqueous or alcoholic KOH, iodoform gives potassium formate.

Carbylamine reaction

When iodoform is warmed with primary amine and alcoholic KOH, it forms isocyanide or carbylamine, which has very unpleasant smell.

With silver powder

Iodoform, when heated with silver powder gives acetylene.

Uses

• As an antiseptic and this nature is due to iodine that it liberates. However, because of its very unpleasant smell, it has now been replaced by better antiseptics.

• In the manufacture of pharmaceuticals.

Iodoform test

Formation of yellow precipitate of iodoform is used as a test for certain aldehydes and ketones that have methyl group bonded to carbonyl group CH₃CO- or compounds that can form this group on oxidation e.g.

This test is performed by heating the substance with aqueous sodium carbonate and iodine solution. On warming the reaction mixture, a yellow precipitate of iodoform indicates positive iodoform test. For example, ethanol (CH₃CH₂OH), ethanal (CH₃CHO), propanone (CH₃COCH₃), 2-pentanone (CH₃COCH₂CH₂CH₃) etc. give this test.