Which Compounds Give the Iodoform Test: Iodoform Test Description (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • The iodoform test is a qualitative chemical test for the identification of ketones and aldehydes carrying an alpha-methyl group.
  • Iodoform test is used to check the presence of carbonyl compounds with the structure or alcohols with the structure in each unknown substance.
  • The reagents used in this test are iodine and sodium hydroxide. Only methyl ketones or alcohols with this feature undergo this test.
  • The reaction of iodine, a base and a methyl ketone give a yellow precipitate along with an “antiseptic” smell.
  • It tests positive for a few specific secondary alcohols that contain at least one methyl group in the alpha position.

Iodoform Test Description

Illustration 2 for Iodoform_Test_Description
  • When Iodine and sodium hydroxide are added to a compound that contains either a methyl ketone or a secondary alcohol with a methyl group in the alpha position, a pale-yellow precipitate of iodoform or triiodomethane is formed.
  • It can be used to identify aldehydes or ketones.
  • If an aldehyde gives a positive iodoform test, then it must be acetaldehyde since it is the only aldehyde with a group.

Compounds That Give Positive Iodoform Test

  • Acetaldehyde
  • Methyl Ketones
  • Ethanol
  • Secondary Alcohols that contain Methyl Groups in Alpha Position

Iodoform Test Mechanism

Iodoform Test Mechanism
  • First, the Hydroxide ion removes an acidic alpha hydrogen. This results in the formation of an enolate ion.
  • Enolate anion then goes on to displace an iodide ion from the iodine molecule.
  • This process repeats twice to give . Now, a hydroxide ion forms a bond with the carbonyl carbon.
  • Which leads to the reformation of the carbonyl group and the elimination of the anion?
  • A group is also formed.
  • The carboxylic acid group and the basic - ion neutralize each other. Thus, the iodoform is precipitated.
  • So, the pale-yellow precipitate of iodoform is formed, which can be identified by its characteristic “antiseptic” smell.
  • Presence of the methyl ketone is confirmed.
  • The iodoform test is a very useful method to identify the presence of these methyl ketones or acetaldehyde in an unknown compound.