Question

How would you distinguish among primary, secondary, and tertiary alcohols?

Hint:

To distinguish alcohols, observe their reactivity with Lucas reagent and oxidation reactions: primary alcohols are easily oxidized, secondary alcohols are moderately oxidized, and tertiary alcohols are resistant to oxidation.

Answer 1

Step 1: Understanding alcohol classification.
Alcohols are classified based on the number of alkyl groups attached to the carbon that holds the hydroxyl group (-OH):
- Primary alcohols have the hydroxyl group attached to a carbon atom that is bonded to only one alkyl group.
- Secondary alcohols have the hydroxyl group attached to a carbon atom that is bonded to two alkyl groups.
- Tertiary alcohols have the hydroxyl group attached to a carbon atom that is bonded to three alkyl groups.
Step 2: Distinguishing tests.
1. Oxidation test:
- Primary alcohols can be oxidized to aldehydes and then to carboxylic acids (e.g., using potassium dichromate).
- Secondary alcohols can be oxidized to ketones but cannot be further oxidized to carboxylic acids.
- Tertiary alcohols do not undergo oxidation easily because the carbon bearing the -OH group is attached to three other carbon atoms, making it sterically hindered.
2. Reaction with Lucas reagent (a mixture of concentrated HCl and ZnCl₂):
- Primary alcohols react slowly with Lucas reagent.
- Secondary alcohols react moderately with Lucas reagent.
- Tertiary alcohols react rapidly with Lucas reagent, leading to the formation of an alkyl chloride.
3. Color reaction with ceric ammonium nitrate (Ce(NH₄)₂(NO₃)₆):
- Primary alcohols give a yellow to red color change when tested with ceric ammonium nitrate.
- Secondary alcohols show a weaker color change compared to primary alcohols.
- Tertiary alcohols do not give any color change.
Step 3: Conclusion.
Primary, secondary, and tertiary alcohols can be distinguished by their reaction with oxidation agents, Lucas reagent, and ceric ammonium nitrate. The number of alkyl groups attached to the carbon with the -OH group determines their classification.