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Q8.13(vii):
Give simple chemical tests to distinguish between the following pair of compounds. (vii) Ethanal and Propanal

Solution :

Theoretical Foundation & Basis of Distinction

To distinguish between Ethanal ($CH_3CHO$) and Propanal ($CH_3CH_2CHO$), we utilize the Iodoform Test. This test is specifically reactive toward methyl ketones or compounds containing a $CH_3CO-$ group, including acetaldehyde (ethanal). Propanal, lacking the terminal methyl-carbonyl structure, does not yield a positive result under these conditions.

Step 1: The Chemical Reaction (Iodoform Test)

Ethanal contains a methyl group attached directly to the carbonyl carbon ($CH_3-CO-$), making it susceptible to haloform reaction when treated with iodine in the presence of an alkaline solution (e.g., $NaOH$).

Reaction for Ethanal:
$CH_3CHO + 3I_2 + 4NaOH \rightarrow CHI_3(s) + HCOONa + 3NaI + 3H_2O$

[Justification: Per the iodoform reaction mechanism, the $\alpha$-hydrogens of the methyl group are replaced by iodine, followed by cleavage to form yellow iodoform precipitates.]

Step 2: Observation & Analysis

Reagent Ethanal ($CH_3CHO$) Propanal ($CH_3CH_2CHO$)
$I_2 / NaOH$ (Iodoform Test) Formation of a yellow precipitate of iodoform ($CHI_3$). No yellow precipitate is formed.

Visual Representation of Molecular Structure

Ethanal (CH3CHO) CH3 C=O H Propanal (CH3CH2CHO) CH3-CH2 C=O H Positive Iodoform Test

Conclusion of Test Logic

The distinction is predicated on the structural presence of the $CH_3C=O$ moiety. Ethanal satisfies the requirement for the formation of triiodomethane ($CHI_3$), which is a solid at room temperature and appears as a yellow precipitate. Propanal, containing an ethyl group attached to the carbonyl carbon, cannot undergo the final cleavage step required for $CHI_3$ formation.

Final Solution: The Iodoform test serves as the definitive chemical distinction; Ethanal produces a yellow precipitate of $CHI_3$ upon treatment with $I_2$ and $NaOH$, whereas Propanal does not.


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