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Step-by-Step Explanation
1. Understanding the Lucas Reagent
The Lucas reagent is a mixture of concentrated hydrochloric acid ($\text{HCl}$) and zinc chloride ($\text{ZnCl}_2$). It is used to classify low molecular weight alcohols into primary, secondary, or tertiary based on their reactivity. The key to this test lies in how quickly a carbocation intermediate can form when the alcohol reacts with $\text{HCl}$ in the presence of $\text{ZnCl}_2$.
2. Mechanism of the Reaction
• In the SN1 mechanism, the reaction proceeds via carbocation formation. Hence, the faster an alcohol can form a stable carbocation, the faster it will react.
• In the SN2 mechanism, the nucleophile attacks directly, which is less favored for sterically hindered alcohols like tertiary ones.
3. Stability of the Carbocation
Tertiary carbocations ($3^\circ$) are the most stable because of maximum alkyl group substitution. Secondary ($2^\circ$) carbocations are less stable than tertiary, and primary ($1^\circ$) carbocations are the least stable.
4. Reason for Fastest Reaction of Tertiary Alcohol
Because tertiary alcohols easily form a stable tertiary carbocation via the SN1 mechanism, they react most rapidly with the Lucas reagent. In contrast, primary and secondary alcohols form less stable carbocations or follow an SN2 process, resulting in slower reactions.
5. Conclusion
Therefore, the alcohol that reacts the fastest with Lucas reagent is a tertiary alcohol, and it proceeds via an SN1 mechanism.
