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Q8.13(ii):
Give simple chemical tests to distinguish between the following pairs of compounds. (ii) Acetophenone and Benzophenone

Solution :

Step 1: Theoretical Basis of Identification

To distinguish between Acetophenone ($C_6H_5COCH_3$) and Benzophenone ($C_6H_5COC_6H_5$), we utilize the Iodoform Test. This test is specific for compounds containing the $CH_3CO-$ group (methyl ketone) or compounds that can be oxidized to a methyl ketone [Per the Haloform Reaction mechanism].

Step 2: Analysis of Molecular Structures

  • Acetophenone ($C_6H_5COCH_3$): Contains an acetyl group attached to a phenyl ring. It possesses the necessary $CH_3CO-$ moiety.
  • Benzophenone ($C_6H_5COC_6H_5$): Consists of two phenyl rings attached to a carbonyl group. It lacks the $CH_3CO-$ methyl ketone unit.

Step 3: Execution of the Chemical Test

Add an aqueous solution of iodine ($I_2$) and sodium hydroxide ($NaOH$) to each sample, followed by gentle warming. This generates the haloform reagent in situ: $I_2 + 2NaOH \rightarrow NaI + NaOI + H_2O$.

Step 4: Observation and Chemical Equations

Case A: Acetophenone
Acetophenone reacts to form a yellow precipitate of Iodoform ($CHI_3$).
$C_6H_5COCH_3 + 3NaOI \rightarrow C_6H_5COONa + CHI_3 \downarrow (\text{yellow ppt}) + 2NaOH$ [By the Nucleophilic Substitution at the $\alpha$-carbon followed by cleavage].

Case B: Benzophenone
Benzophenone does not contain the $CH_3CO-$ group, therefore it does not react with sodium hypoiodite ($NaOI$). No yellow precipitate is formed.

Step 5: Visual Representation of the Test

Acetophenone + I2/NaOH Result: Yellow Ppt Benzophenone + I2/NaOH Result: No Reaction

Step 6: Comparative Summary Table

Compound Iodoform Test ($I_2/NaOH$) Observation
Acetophenone Positive Yellow precipitate of $CHI_3$
Benzophenone Negative No change

Final Solution: Acetophenone can be distinguished from Benzophenone by the Iodoform test, where Acetophenone gives a yellow precipitate of Iodoform due to the presence of the methyl ketone group, whereas Benzophenone gives no reaction.


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