Question

Give simple chemical test to distinguish between the following pair of compounds $\mathrm{Pentan}-2-\mathrm{one}$ and $\mathrm{Pentan}-3-\mathrm{one}$.

Answer 1

$\mathrm{Pentan}-2-\mathrm{one}$and $\mathrm{Pentan}-3-\mathrm{one}$:

1. The existence of an aldehyde or ketone in which a methyl group is one of the groups immediately linked to the carbonyl carbon is revealed by the iodoform test.
2. Iodine, a base, and a methyl ketone react to produce a yellow precipitate with an "antiseptic" odor.
3. Pentan-2-one is a methyl ketone and hence responds to the iodoform test, but Pentan-3-one is not. As a result, it does not respond to the iodoform test and no reaction occurs.

$\underset{\mathrm{Pentan}-2-\mathrm{one}}{{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{COCH}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{hypoiodite}}{3\mathrm{NaOI}\left(\mathrm{s}\right)}\to \underset{\mathrm{Sodium}\mathrm{butanoate}}{{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{COONa}\left(\mathrm{s}\right)}+\underset{\mathrm{Iodomethane}(\mathrm{Yellow}\mathrm{ppt})}{{\mathrm{CHI}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{hydroxide}}{2\mathrm{NaOH}\left(\mathrm{l}\right)}$

$\underset{\mathrm{Pentan}-3-\mathrm{one}}{{\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{COCH}}_2{\mathrm{CH}}_3\left(\mathrm{s}\right)}+\underset{\mathrm{Sodium}\mathrm{hypoiodite}}{3\mathrm{NaOI}\left(\mathrm{s}\right)}\to \mathrm{No}\mathrm{yellow}\mathrm{ppt}\mathrm{of}\mathrm{triiodomethane}$

Therefore, the Iodoform test is used to differentiate between $\mathrm{Pentan}-2-\mathrm{one}$ and $\mathrm{Pentan}-3-\mathrm{one}$.