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Two isomeric ketones - 3-pentanone (diethyl ketone) and 2-pentanone (methyl propyl ketone) - can be distinguished from each other by which of the following reagents?

1. I2 / NaOH (iodoform test)
2. NaHSO3 (sodium bisulfite)
3. NaCN / HCl (cyanohydrin formation)
4. 2,4-DNP (2,4-dinitrophenylhydrazine)

Correct answer: I2 / NaOH (iodoform test)

Solution

Iodoform test (I2/NaOH) gives a yellow precipitate only with methyl ketones (CH3COR) or acetaldehyde. 2-pentanone has a CH3CO- group and gives positive iodoform test. 3-pentanone has no CH3CO- group and gives no iodoform. The other reagents (NaHSO3, NaCN, 2,4-DNP) react with both ketones similarly and cannot distinguish them.

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