4. CH3CHO and C6H5CH2CHO can be distinguished chemically by:
a. Tollen’s reagent test
b. Fehling solution test
c. Benedict test
d. Iodoform test
CH3CHO (acetaldehyde) and C6H5CH2CHO (phenylacetaldehyde) can be distinguished chemically by the iodoform test, where only acetaldehyde gives a positive result.
Compounds Overview
Acetaldehyde (CH3CHO) features a methyl group directly attached to the carbonyl, enabling specific reactions. Phenylacetaldehyde (C6H5CH2CHO), or benzyl aldehyde, has a CH2 group between phenyl and carbonyl, classifying it as aliphatic but lacking the CH3CO- structure.
Test Analysis
Both compounds react similarly in most aldehyde tests but differ in iodoform specificity.
| Test | CH3CHO Result | C6H5CH2CHO Result | Distinguishes? |
|---|---|---|---|
| Tollen’s reagent | Silver mirror (positive) | Silver mirror (positive) | No |
| Fehling solution | Red Cu2O precipitate (positive) | Red Cu2O precipitate (positive) | No |
| Benedict test | Red Cu2O precipitate (positive) | Red Cu2O precipitate (positive) | No |
| Iodoform test | Yellow CHI3 precipitate (positive) | No reaction (negative) | Yes |
Why Iodoform Works
The iodoform test detects CH3CO- groups or acetaldehyde, which oxidizes to form methyl ketone intermediate under I2/NaOH. Acetaldehyde (CH3CHO) yields CHI3, while phenylacetaldehyde lacks this structure, giving no precipitate. This makes option d. Iodoform test correct for CSIR NET preparation.
