Question

How would you distinguish between the following compounds by chemical tests?
(i) Formaldehyde and Acetaldehyde
(ii) Acetaldehyde and Acetone

Hint:

Schiff’s reagent → detects formaldehyde. Iodoform test → confirms methyl ketone (–COCH$_3$) or ethanal.

Answer 1

(i) Formaldehyde and Acetaldehyde
Test: Schiff’s Reagent Test
- Formaldehyde gives a deep pink colour with Schiff’s reagent.
- Acetaldehyde gives only a light pink colour.
\[ \text{HCHO} + \text{Schiff’s reagent} \rightarrow \text{Deep pink colour} \] (ii) Acetaldehyde and Acetone
Test: Iodoform Test
- Acetaldehyde (CH$_3$CHO) gives a yellow precipitate of iodoform (CHI$_3$).
- Acetone (CH$_3$COCH$_3$) also gives the iodoform test but formaldehyde does not.
\[ \text{CH}_3\text{CHO} + 3\text{I}_2 + 4\text{NaOH} \rightarrow \text{CHI}_3 + \text{HCOONa} + 3\text{NaI} + 3\text{H}_2\text{O} \] Step 3: Observation.
Formation of yellow precipitate of CHI$_3$ confirms the presence of a –COCH$_3$ or –CHOCH$_3$ group.