Question

Primary, secondary and tertiary alcohols can be distinguished by:

• A. Lucas test
• B. Fehling's test
• C. Tollens' test
• D. Hinsberg's test

Hint:

{Lucas Test Memory Trick:} 3° alcohol → Instant cloudiness 2° alcohol → Slow cloudiness 1° alcohol → No cloudiness (cold conditions)

Answer 1

The Correct Option is A

To distinguish between primary, secondary, and tertiary alcohols, chemists frequently use the Lucas test. This test is based on the reactivity of alcohols with Lucas reagent, which is a mixture of concentrated hydrochloric acid (HCl) and zinc chloride (ZnCl₂). Here’s a detailed explanation of why the Lucas test is suitable and how it works:

1. Lucas test mechanism:
   • Lucas reagent interacts with alcohols to form alkyl chlorides through an SN1 reaction, which is faster in tertiary alcohols compared to secondary and primary, due to the stability of the carbocation intermediates.
2. Reactivity:
   • Tertiary alcohols: React rapidly, causing turbidity immediately or within 5 minutes at room temperature.
   • Secondary alcohols: Form turbidity in about 5-10 minutes.
   • Primary alcohols: Do not react at room temperature, showing no immediate turbidity; they require heating to show any change.
3. Conclusion: Based on the speed and conditions under which turbidity appears, you can distinguish between primary, secondary, and tertiary alcohols using the Lucas test.

Here’s why other options do not serve the purpose:

1. Fehling's test: Typically used to detect reducing sugars, aldehydes, and not applicable for distinguishing alcohols.
2. Tollens' test: Used to identify aldehydes by forming a silver mirror and is irrelevant in classifying alcohols.
3. Hinsberg's test: Utilized to differentiate amines, not alcohols.

Thus, the correct answer is the Lucas test because it is specifically designed to distinguish between the three types of alcohols based on reactivity differences.