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

Solution :

Given Compounds for Distinction

The objective is to distinguish between Benzaldehyde ($C_{6}H_{5}CHO$) and Acetophenone ($C_{6}H_{5}COCH_{3}$). While both are carbonyl compounds, they differ in their structural characteristics: Benzaldehyde is an aromatic aldehyde, whereas Acetophenone is an aromatic ketone with a methyl keto group ($CH_{3}CO-$).

Step 1: Selecting the Diagnostic Test (Iodoform Test)

The most precise chemical test to distinguish between these two is the Iodoform Test. [Per the chemical requirement that compounds containing the $CH_{3}CO-$ group or $CH_{3}CH(OH)-$ group react with iodine in the presence of an alkali to form a yellow precipitate of iodoform ($CHI_{3}$)].

Step 2: Analysis of Chemical Reactivity

  • Acetophenone ($C_{6}H_{5}COCH_{3}$): Contains a methyl ketone group. Upon treatment with $I_{2}$ and $NaOH$, it undergoes haloform reaction.
  • Benzaldehyde ($C_{6}H_{5}CHO$): Does not contain a methyl ketone group; therefore, it does not undergo the haloform reaction.

Step 3: Execution and Observation

Compound Reagents ($I_{2} + NaOH$) Observation
Benzaldehyde $I_{2} + NaOH$ (aq) No reaction; solution remains clear/unchanged.
Acetophenone $I_{2} + NaOH$ (aq) Formation of yellow precipitate of iodoform ($CHI_{3}$).

Step 4: Chemical Equations

The reaction for Acetophenone is as follows:

$C_{6}H_{5}COCH_{3} + 3I_{2} + 4NaOH \rightarrow C_{6}H_{5}COONa + CHI_{3} (\downarrow \text{yellow ppt}) + 3NaI + 3H_{2}O$

Acetophenone I2 / NaOH Yellow PPT (CHI3)

Alternative Test: Tollen's Test

Alternatively, Tollen's Reagent can be used. [Per the principle that aldehydes are easily oxidized to carboxylic acids, while ketones are resistant to mild oxidation].

  • Benzaldehyde: Reacts with Tollen's reagent ($[Ag(NH_{3})_{2}]^{+}$) to form a "silver mirror" ($Ag(s)$).
  • Acetophenone: Does not react with Tollen's reagent.

Final Solution: Acetophenone can be distinguished from Benzaldehyde using the Iodoform test, where Acetophenone yields a yellow precipitate of $CHI_{3}$, whereas Benzaldehyde does not. Alternatively, Benzaldehyde gives a positive silver mirror test with Tollen's reagent, while Acetophenone does not.


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