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Chemistry Question 61 – NEET-UG 2024

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Which one of the following alcohols reacts instantaneously with Lucas reagent?

The Lucas reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid, used to classify alcohols based on their reactivity.

Step 1: Understand Lucas Reagent Test✦ Active

The Lucas reagent (concentrated HCl and anhydrous ZnCl2) is used to distinguish between primary, secondary, and tertiary alcohols. The reaction involves the formation of an alkyl halide, which is insoluble in the reagent and appears as turbidity.

Step 2: Recall Reactivity Order○ Expand

The order of reactivity of alcohols with Lucas reagent is: Tertiary alcohols (instantaneous turbidity) > Secondary alcohols (turbidity in 5-10 minutes) > Primary alcohols (no turbidity at room temperature).

💡 Teacher's Secret Hint

The stability of the carbocation intermediate formed during the reaction dictates the reactivity order (3∘>2∘>1∘).

Step 3: Classify Given Alcohols○ Expand

Let's classify each alcohol:

(1) CH3−CH2−CH(CH3)−OH: This is 2-butanol, a secondary alcohol.

(2) CH3−CH(CH3)−CH2OH: This is 2-methylpropan-1-ol, a primary alcohol.

(3) CH3−C(CH3)2−OH: This is 2-methylpropan-2-ol (or tert-butyl alcohol), a tertiary alcohol.

(4) CH3−CH2−CH2−CH2OH: This is butan-1-ol, a primary alcohol.

Since tertiary alcohols react instantaneously with Lucas reagent, 2-methylpropan-2-ol (option 3) is the correct answer.

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