↻

The haloform, or iodoform test.

The iodoform test, a subset of the haloform reaction, is a classical chemical test that’s been used in organic chemistry for many decades. Its primary function is to detect the presence of methyl ketones, particularly propanone, but can also indicate the presence of compounds that can be oxidized to methyl ketones, such as secondary alcohols.

Chemical Basis of the Test:

When a methyl ketone (like propanone) reacts with iodine in the presence of a base (usually sodium or potassium hydroxide), a yellow precipitate of iodoform (CHI₃) is formed. Iodoform has an antiseptic smell. The mechanism of the reaction involves the halogenation of the methyl group followed by hydrolysis to give a carboxylate ion and iodoform.

Procedure of the Iodoform Test:

  1. To the sample in question, a solution of iodine is added.
  2. This is then followed by the addition of an aqueous solution of sodium hydroxide (or potassium hydroxide).
  3. The mixture is shaken well.
  4. If a yellow precipitate forms, it indicates a positive result for the iodoform test.

Compounds That Give a Positive Iodoform Test:

  1. Methyl ketones, like acetone (CH₃COCH₃).
  2. Secondary alcohols that can be oxidized to form methyl ketones. For instance, isopropyl alcohol (2-propanol) can be oxidized to form acetone.
  3. Some β-keto esters and β-diketones.

Importance in Organic Chemistry:

The iodoform test is particularly valuable in organic chemistry labs for the following reasons:

  1. Identification of Compounds: As mentioned to the left, the formation of a yellow precipitate confirms the presence of specific functional groups, aiding in the identification of unknown compounds.
  2. Distinctive Reaction: The production of the yellow iodoform precipitate is a rather distinct and easily identifiable reaction. As such, it serves as a quick and straightforward method for chemists to determine the presence of specific functional groups.
  3. Historical Significance: The iodoform test has been used for over a century and has played a significant role in the history of organic chemistry as one of the first functional group tests.

methyl ketones or 2-ones

Molecules with a carbonyl group that has a methyl group attached to the carbon of the carbonyl group.
These molecules give a positive iodoform test.
This means that all ketones with the carbonyl group on the second carbon.
It also means the aldehyde ethanal (CH3CHO).

CH3COCH3  +  3I2   →   CH3COCI3  +  3HI
CH3COCI3   +  NaOH   →   CH3COOH  +  CHI3

Some alcohols also give a positive iodoform as they are oxidised in the test to a carbonyl with the structure above.
This means one primary alcohol, ethanol, as it is oxidised to ethanal.

CH3CH2OH  +  [O]  →  CH3CHO  +  H2O

It also means secondary alcohols with the OH group on the second carbon, such as propan-2-ol.

CH3CH(OH)CH3  +  [O]  →  CH3COCH3 +  H2O

Scroll to Top