Which test differentiates primary alcohols from tertiary alcohols?

Which test differentiates primary alcohols from tertiary alcohols?

• A. Nitroprusside test
• B. Iodoform test
• C. Tollens test
• D. Lucas test

Answer: (D)

This question hinges on how fast an alcohol reacts with Lucas reagent (concentrated HCl with ZnCl2) to form an alkyl chloride. The key idea is the reaction proceeds via an SN1 mechanism, where the alcohol loses water to form a carbocation, and the rate depends on how stable that carbocation would be. Tertiary alcohols form a very stable carbocation quickly, so the reaction proceeds at once and a cloudy mixture appears as the alkyl chloride forms. Primary alcohols would have to form an unstable primary carbocation, so at room temperature the reaction is extremely slow or essentially doesn’t occur, and no turbidity is seen for a long time. Because of this stark difference in reaction rate, the Lucas test distinguishes primary from tertiary alcohols.

This question hinges on how fast an alcohol reacts with Lucas reagent (concentrated HCl with ZnCl2) to form an alkyl chloride. The key idea is the reaction proceeds via an SN1 mechanism, where the alcohol loses water to form a carbocation, and the rate depends on how stable that carbocation would be.

Tertiary alcohols form a very stable carbocation quickly, so the reaction proceeds at once and a cloudy mixture appears as the alkyl chloride forms. Primary alcohols would have to form an unstable primary carbocation, so at room temperature the reaction is extremely slow or essentially doesn’t occur, and no turbidity is seen for a long time. Because of this stark difference in reaction rate, the Lucas test distinguishes primary from tertiary alcohols.