MCQ Detailed View

To distinguish between primary, secondary, and tertiary alcohols, chemists use a method known as the Lucas Test. This test involves treating the alcohol with concentrated hydrochloric acid (HCl) in the presence of anhydrous zinc chloride (ZnCl₂). The reagent used is called Lucas reagent.

The reaction is based on the different rates at which alcohols form alkyl chlorides when treated with Lucas reagent. In the reaction, the hydroxyl group (-OH) of the alcohol is replaced by a chlorine atom from HCl, producing an alkyl chloride. The speed of this substitution depends on the stability of the carbocation intermediate formed during the reaction.

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  Tertiary alcohols form carbocations very easily because they are stabilized by three alkyl groups. As a result, tertiary alcohols react almost instantly with Lucas reagent at room temperature, producing turbidity immediately.

  
  


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  Secondary alcohols are less stable than tertiary but more stable than primary. They react more slowly, typically showing turbidity within 5–10 minutes.

  
  


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  Primary alcohols form carbocations very slowly because they are least stable. Therefore, they either do not react under room temperature conditions or require heating to show turbidity.

  
  

The test is simple: after adding Lucas reagent, the solution is observed for turbidity or cloudiness. The cloudiness is due to the formation of insoluble alkyl chloride. The faster the turbidity appears, the more substituted the alcohol.

The other options in the question involve reagents like SOCl₂, CaCl₂, or PCl₂, which are not used for this particular distinction test. These may convert alcohols into chlorides, but they do not differentiate between primary, secondary, and tertiary alcohols based on reaction rates.

Thus, the correct answer is concentrated HCl with anhydrous ZnCl₂, which together form Lucas reagent, the standard test for classifying alcohols.