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Q8.13(iii):
Give simple chemical tests to distinguish between the following pair of compounds. (iii) Phenol and Benzoic acid

Solution :

Step 1: Chemical Characteristics of Phenol and Benzoic Acid

To distinguish between Phenol ($C_6H_5OH$) and Benzoic acid ($C_6H_5COOH$), we must exploit the difference in their acidic strength and functional group reactivity.

  • Phenol: A weak acid ($pK_a \approx 9.95$). It does not react with sodium bicarbonate ($NaHCO_3$) to evolve $CO_2$ gas.
  • Benzoic acid: A stronger carboxylic acid ($pK_a \approx 4.20$). It reacts with $NaHCO_3$ to evolve $CO_2$ gas with effervescence [Per the Bronsted-Lowry acid-base theory, a stronger acid displaces a weaker acid/carbonic acid from its salts].

Step 2: The Sodium Bicarbonate Test (Primary Distinction)

Add a saturated solution of sodium bicarbonate ($NaHCO_3$) to both samples individually.

Compound Observation Chemical Conclusion
Benzoic Acid Brisk effervescence of $CO_2$ gas Positive test
Phenol No visible reaction Negative test

Chemical Equation for Benzoic Acid:

$C_6H_5COOH + NaHCO_3 \rightarrow C_6H_5COONa + H_2O + CO_2 \uparrow$

Step 3: Neutral Ferric Chloride Test (Secondary Confirmation)

Both compounds yield distinct colorations with neutral ferric chloride ($FeCl_3$).

  • Phenol: Forms a violet-colored complex [Formation of hexaaquairon(III) phenoxide complex].
  • Benzoic Acid: Forms a buff-colored (light brown/tan) precipitate of ferric benzoate.

Step 4: Visualization of Chemical Test Principles

Benzoic Acid + NaHCO3 Effervescence (CO2) Phenol + NaHCO3 No Reaction

Final Solution: The most definitive test is the Sodium Bicarbonate ($NaHCO_3$) test. Benzoic acid reacts to produce brisk effervescence due to the evolution of $CO_2$ gas, whereas Phenol shows no reaction. Alternatively, the neutral $FeCl_3$ test can be used, where Phenol produces a violet coloration and Benzoic acid produces a buff-colored precipitate.


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