Question

Which of the following pair(s) can be distinguished by using Lucas Test?

$\square$ PhCH2OH, CH3CH2OH $\square$ PhCH2OH, PhOH $\square$ (CH3)2CHOH, CH3CH2CH2OH $\square$ CH3CH2CH2OH, CH3CH2OH

• A. PhCH2OH, CH3CH2OH
• B. PhCH2OH, PhOH
• C. (CH3)2CHOH, CH3CH2CH2OH
• D. CH3CH2CH2OH, CH3CH2OH

Answer 1

Answer: (A), (B), (C)

PhCH2OH, CH3CH2OH; PhCH2OH, PhOH; (CH3)2CHOH, CH3CH2CH2OH

Answer 2

The Lucas Test is used to distinguish between primary, secondary, and tertiary alcohols based on their reactivity with Lucas Reagent (anhydrous ZnCl2 in concentrated HCl). The test involves the formation of an alkyl chloride, which appears as turbidity or an oily layer due to its insolubility in the aqueous medium.

The general reactivity order is:

• Tertiary (3°) alcohols: React immediately (turbidity appears within 1-2 minutes).
• Secondary (2°) alcohols: React within 5-10 minutes (turbidity appears within this timeframe).
• Primary (1°) alcohols: Do not react significantly at room temperature (no turbidity within a reasonable time, usually 15-20 minutes).

Important Exception: Benzylic and allylic alcohols, regardless of their primary, secondary, or tertiary classification, react rapidly with Lucas reagent. This is because the carbocations formed from these alcohols are resonance-stabilized, facilitating the SN1 reaction pathway.

Let's analyze each pair:

1. PhCH2OH (Benzyl alcohol) and CH3CH2OH (Ethanol)

   • PhCH2OH (Benzyl alcohol): This is a primary alcohol, but it is also a benzylic alcohol. Due to the resonance stabilization of the benzylic carbocation (PhCH2+), it reacts rapidly with Lucas reagent, showing turbidity quickly (similar to a tertiary or secondary alcohol).
   • CH3CH2OH (Ethanol): This is a primary aliphatic alcohol. It does not react with Lucas reagent at room temperature (no turbidity).
   • Conclusion: Since PhCH2OH reacts rapidly and CH3CH2OH does not react, this pair can be distinguished by the Lucas Test.

2. PhCH2OH (Benzyl alcohol) and PhOH (Phenol)

   • PhCH2OH (Benzyl alcohol): As explained above, it is a primary benzylic alcohol and reacts rapidly with Lucas reagent, showing quick turbidity.
   • PhOH (Phenol): Phenols are not alcohols and do not react with Lucas reagent. No turbidity will be observed.
   • Conclusion: Since PhCH2OH reacts rapidly and PhOH does not react, this pair can be distinguished by the Lucas Test.

3. (CH3)2CHOH (Propan-2-ol) and CH3CH2CH2OH (Propan-1-ol)

   • (CH3)2CHOH (Propan-2-ol): This is a secondary aliphatic alcohol. It will react with Lucas reagent within 5-10 minutes, producing turbidity.
   • CH3CH2CH2OH (Propan-1-ol): This is a primary aliphatic alcohol. It will not react with Lucas reagent at room temperature (no turbidity).
   • Conclusion: Since Propan-2-ol reacts within minutes and Propan-1-ol does not react, this pair can be distinguished by the Lucas Test.

4. CH3CH2CH2OH (Propan-1-ol) and CH3CH2OH (Ethanol)

   • CH3CH2CH2OH (Propan-1-ol): This is a primary aliphatic alcohol. It will not react with Lucas reagent at room temperature.
   • CH3CH2OH (Ethanol): This is also a primary aliphatic alcohol. It will not react with Lucas reagent at room temperature.
   • Conclusion: Since both are primary aliphatic alcohols and show no reaction, this pair cannot be distinguished by the Lucas Test.

Therefore, the pairs that can be distinguished by the Lucas Test are PhCH2OH and CH3CH2OH, PhCH2OH and PhOH, and (CH3)2CHOH and CH3CH2CH2OH.