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The Lucas test is a classical method in organic chemistry for distinguishing between primary, secondary, and tertiary alcohols based on their reactivity with Lucas reagent (a mixture of concentrated hydrochloric acid and anhydrous zinc chloride, HCl + ZnCl₂).
The test works by converting an alcohol (R–OH) into the corresponding alkyl chloride (R–Cl) through a substitution reaction. The reaction mechanism depends on the stability of the carbocation that forms during the process:
Tertiary alcohols [(CH₃)₃COH]:
Undergo reaction via the SN1 mechanism.
The tertiary carbocation formed is highly stable due to inductive and hyperconjugation effects.
As a result, the substitution occurs almost instantly at room temperature, producing turbidity (formation of insoluble alkyl chloride).
Therefore, tertiary alcohols give an immediate positive Lucas test.
Secondary alcohols [(CH₃)₂CHOH]:
Reaction proceeds moderately, usually within 5–10 minutes at room temperature.
The secondary carbocation is less stable than a tertiary carbocation but more stable than a primary one.
Primary alcohols (CH₃CH₂OH or long-chain CH₃(CH₂)OH):
Very slow or no reaction under normal conditions.
The primary carbocation is extremely unstable; hence, the SN1 pathway is unfavorable.
They may only react under strong heating conditions, making the Lucas test ineffective for primary alcohols.
Key Result of Lucas Test:
Tertiary alcohols: immediate turbidity (fastest).
Secondary alcohols: delayed turbidity.
Primary alcohols: no turbidity at room temperature.
Conclusion:
Among the given options, (CH₃)₃COH (tertiary alcohol) reacts the fastest with Lucas reagent because of the stability of the tertiary carbocation formed.
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