ORGANIC CHEMISTRY: THE HALOFORM TEST USING PARACETAMOL TABLETS


Paracetamol, or acetaminophen, a common painkiller
Image courtesy: Google

Among the many qualitative analytical tests in organic chemistry, the haloform test, or Lieben's test (specifically, the iodoform test) is quite common. The reagent used for the iodoform test is cold sodium hydroxide solution and iodine mixed together, which react to form sodium hypoiodite. The test detects the presence of an acetyl or an α-hydroxy methyl group. Exhaustive halogenation takes place at the α (methyl) position, which eventually leads to its cleavage as a molecule of haloform.

Iodoform, also known as triiodomethane, was once used as an antiseptic but now has been universally replaced, since the antiseptic properties were not characteristic of iodoform itself, but of the iodine liberated from it. It can have an unpleasant odour.

Paracetamol, or acetaminophen is a common painkiller that consists of a phenolic ring with an NH-acetyl group at the para position. Due to resonance in the phenolic ring, the nitrogen becomes more acidic than usual. It is poorly soluble in water.

REQUIREMENTS:

• Cold, concentrated sodium hydroxide solution

• Finely ground iodine crystals

• Paracetamol tablets. Instead of using paracetamol extract, acetone may also be used, but if done so, it should not be heated under any cost. The reaction is usually more impressive with paracetamol.

METHOD:

• Put some sodium hydroxide solution in a test tube, and add some finely ground iodine. Shake it vigorously till it has a slight tinge of yellow.

• In another test tube, put some pulverized paracetamol (around 250 milligrammes) and keep shaking it with hot water. After about half of the powder, or more, has dissolved, the setup is ready.

• Pour the contents of the first test tube into the second slowly. A reddish brown precipitate of iodoform appears.

Demonstration of the iodoform test

Be warned though, that sodium hydroxide solution can be highly corrosive. Also, if you're working with acetone, ensure that there are no flames around, because it is volatile and easily flares up.

Kindly leave your valuable feedback in the comments. Thank you.

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