Which test allows us to distinguish between pentan-2-one and pentan-3-one?

Iodoform test

The correct answer is option 1. Iodoform test.

The iodoform test is a chemical test used to detect the presence of methyl ketones, specifically those containing a methyl group adjacent to the carbonyl group \((–CO–CH_3)\). This test is based on the reaction of the ketone with iodine \((I_2)\) and a base, such as sodium hydroxide \((NaOH)\), to form a yellow precipitate of iodoform \((CHI_3)\).

Here is a step-by-step explanation of the iodoform test and how it allows us to distinguish between pentan-2-one (2-pentanone) and pentan-3-one (3-pentanone):

Reaction with Iodine and Base:

In the presence of iodine and a base, methyl ketones undergo a series of reactions. The iodine is reduced to iodide ions \((I^-)\), and the methyl ketone is oxidized to a carboxylate anion \((RCOO^-)\) and a molecule of iodoform \((CHI_3)\).

Formation of Iodoform:

Pentan-2-one (2-pentanone) contains a methyl group \((–CH_3)\) attached to the second carbon atom of the carbonyl group \((–CO–CH_3)\). When treated with iodine and base, pentan-2-one undergoes oxidation and forms iodoform due to the presence of the methyl group adjacent to the carbonyl group.

In contrast, pentan-3-one (3-pentanone) lacks a methyl group adjacent to the carbonyl group. Therefore, it does not undergo the iodoform reaction and does not produce iodoform precipitate.

Observation of Yellow Precipitate:

If a yellow precipitate forms when the ketone is treated with iodine and base, it indicates the presence of a methyl ketone and a positive result for the iodoform test. This precipitate is characteristic of iodoform and confirms the presence of a methyl group adjacent to the carbonyl group in the ketone.

Conclusion:

The iodoform test is specific to methyl ketones containing a methyl group adjacent to the carbonyl group. Pentan-2-one (2-pentanone) gives a positive iodoform test and forms a yellow precipitate of iodoform, while pentan-3-one (3-pentanone) does not react and does not produce iodoform.

Therefore, the iodoform test allows us to distinguish between pentan-2-one and pentan-3-one based on their reactivity towards iodine and base, providing a useful method for identifying methyl ketones in organic chemistry.

So, to sum up:

Pentan-2-one (2-pentanone) gives a positive iodoform test.  Pentan-3-one (3-pentanone) does not give a positive iodoform test. This is why the iodoform test is used to differentiate between these two compounds.