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Q8.13(vi):
Give simple chemical tests to distinguish between the following pair of compounds. (vi) Benzaldehyde and Acetophenone

Solution :

Given Compounds

The objective is to distinguish between Benzaldehyde ($C_6H_5CHO$) and Acetophenone ($C_6H_5COCH_3$).

Step 1: Theoretical Basis for Differentiation

Benzaldehyde is an aromatic aldehyde, whereas Acetophenone is an aromatic ketone containing a methyl ketone group ($-COCH_3$). These structural differences allow for differentiation via specific chemical tests: Iodoform Test and Tollens' Test.

Step 2: The Iodoform Test (Specific for Acetophenone)

Acetophenone contains a $CH_3CO-$ group, which undergoes the haloform reaction to produce a yellow precipitate of iodoform ($CHI_3$). Benzaldehyde lacks the methyl ketone group and does not give this test.

Reaction:
$C_6H_5COCH_3 + 3I_2 + 4NaOH \rightarrow C_6H_5COONa + CHI_3 \downarrow (\text{yellow ppt}) + 3NaI + 3H_2O$

[Justification: Per the haloform reaction mechanism, methyl ketones react with iodine in the presence of base to yield a tri-iodomethane precipitate.]

Step 3: Tollens' Test (Specific for Benzaldehyde)

Aldehydes act as reducing agents. Benzaldehyde reacts with Tollens' reagent (ammoniacal silver nitrate) to form a silver mirror, while ketones (like Acetophenone) do not reduce Tollens' reagent.

Reaction:
$C_6H_5CHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow C_6H_5COO^- + 2Ag \downarrow (\text{silver mirror}) + 4NH_3 + 2H_2O$

[Justification: Aldehydes are easily oxidized to carboxylate ions, whereas ketones are resistant to mild oxidation.]

Comparative Analysis Table

Test Benzaldehyde Acetophenone
Iodoform Test Negative Positive (Yellow ppt)
Tollens' Test Positive (Silver mirror) Negative

Visual Representation: Tollens' Test Mechanism

Benzaldehyde (C6H5CHO) Tollens' Reagent Ag Mirror Formation (Observation)

Final Solution: Benzaldehyde and Acetophenone can be distinguished using the Iodoform test (Acetophenone yields a yellow precipitate, Benzaldehyde does not) or the Tollens' test (Benzaldehyde forms a silver mirror, Acetophenone does not).


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