Question

Give a simple chemical test to distinguish between the following pair of compounds:

Pentan-2-one and Pentan-3-one

Can you distinguish pentan-2-one and pentan-3-one by iodoform test?

Answer 1

Pentan-2-one and pentan-3-one can be distinguished by the iodoform test.

Iodoform test: Pentan-2-one is a methyl ketone, so it responds to the iodoform test. However, pentan-3-one is not a methyl ketone and does not respond to this test. 

Pentan-2-one has a methyl ketone group (CH₃-CO-) at the second carbon. Compounds with a methyl ketone group give a positive iodoform test, producing a yellow precipitate of iodoform (CHI₃).

\[\begin{array}{cc}
\ce{O}\phantom{.................................................}\\
||\phantom{.................................................}\\
\ce{\underset{Pentan-2-one}{CH3CH2CH2 - C - CH3} + 3NaOI ->\underset{Sodium butanoate}{CH3CH2CH2COONa} + \underset{(yellow ppt)}{\underset{Iodoform}{CHI3\downarrow}} + 2NaOH}\
\end{array}\]

Pentan-3-one has the carbonyl group at the third position, so it does not have a methyl ketone structure. It does not give a positive iodoform test.

\[\begin{array}{cc}
\ce{O}\phantom{.......................................}\\
||\phantom{.......................................}\\
\ce{\underset{Pentan-3-one}{CH3CH2 - C - CH2CH3} + NaOI -> No yellow ppt of iodoform}\
\end{array}\]