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Lucas reagent is a widely used reagent in organic qualitative analysis to distinguish between primary, secondary, and tertiary alcohols. The reagent is a mixture of concentrated hydrochloric acid (HCl) and anhydrous zinc chloride (ZnCl₂). This combination creates a strong acidic and catalytic environment that promotes the conversion of alcohols into alkyl chlorides through a substitution reaction.
The test works based on the reactivity of different alcohol classes:
Tertiary alcohols react almost instantly with Lucas reagent at room temperature, forming alkyl chlorides. These are insoluble in water, causing immediate cloudiness or turbidity.
Secondary alcohols react more slowly, typically producing turbidity within a few minutes.
Primary alcohols react very slowly or not at all under room temperature because they form unstable carbocations.
In this reaction, ZnCl₂ acts as a Lewis acid, coordinating with the hydroxyl group (-OH) of the alcohol. This increases the electrophilic character of the carbon atom bonded to the -OH group. The chloride ion (Cl⁻) from concentrated HCl then replaces the hydroxyl group, forming the corresponding alkyl chloride (R-Cl).
The other options are incorrect:
Dil. HCl & hydrated ZnCl₂: Insufficient acid strength and catalytic activity to produce noticeable results.
Conc. HNO₃-based mixtures: Nitric acid is an oxidizing agent, not used for substitution reactions of alcohols.
MgCl₂: Does not act as a catalyst for this reaction.
Therefore, Lucas reagent = conc. HCl + anhydrous ZnCl₂, making Option 1 the correct choice. This test is a fundamental tool in organic chemistry for identifying alcohol types via their reactivity and the rate of alkyl chloride formation.
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