Question

Write a simple chemical test to distinguish between Ethanal and Propanal.

Hint:

Iodoform test (I₂/NaOH) — positive for ethanal, negative for propanal.

Answer 1

Concept: A good test gives one clear result with one compound and a different result with the other. The iodoform test does exactly that here.

Step 1: Why the iodoform test works
The iodoform test gives a yellow solid only when a compound has the $\mathrm{CH_3CO\text{-}}$ group (a methyl group attached straight to a carbonyl). So we just check which of the two has this group.

Step 2: Apply it to both
Ethanal is $\mathrm{CH_3CHO}$. It has the $\mathrm{CH_3CO\text{-}}$ group, so with $\mathrm{I_2}$ and NaOH it gives a yellow precipitate of iodoform. Propanal is $\mathrm{CH_3CH_2CHO}$. Its methyl group is one carbon away from the $\mathrm{C{=}O}$, so it does not have the needed group and gives no yellow solid.

Step 3: Read the result
So the one that turns yellow is ethanal, and the one that stays clear is propanal. That tells the two apart.

Answer: Use the iodoform test ($\mathrm{I_2}$ + NaOH): ethanal gives a yellow iodoform precipitate, propanal gives none.