The correct answer is Option (2) → Conc. $HCl$ and anhydrous $ZnCl_2$
Lucas test is used to distinguish primary, secondary, and tertiary alcohols based on their rate of formation of alkyl chlorides.
- Alcohols react with Lucas reagent ($\text{conc. HCl} + \text{ZnCl}_2$) via an SN1 mechanism.
- Stability of carbocation formed decides the speed: $\text{3}^\circ \text{ alcohol} > \text{2}^\circ \text{ alcohol} > \text{1}^\circ \text{ alcohol}$
- Turbidity appears due to formation of insoluble alkyl chloride.
Explanation (option-wise)
- $\text{H}_2\text{SO}_4$: Sulfuric acid mainly acts as a dehydrating agent and is used in elimination reactions or ester formation. It does not provide chloride ions needed to form alkyl chlorides; hence, it is not used in Lucas test.
- $\text{Conc. HCl}$ and anhydrous $\text{ZnCl}_2$: This is Lucas reagent. $\text{ZnCl}_2$ is a Lewis acid that activates the $-\text{OH}$ group, making it easier to leave as water. The alcohol forms a carbocation, which reacts with $\text{Cl}^-$ to form alkyl chloride, producing turbidity.
- Anhyd. $\text{AlCl}_3$: $\text{AlCl}_3$ is used in Friedel–Crafts alkylation/acylation in aromatic chemistry. It does not convert alcohols to alkyl chlorides in this context.
- $\text{CrO}_3$: Chromium trioxide is a strong oxidizing agent used to oxidize alcohols to aldehydes, ketones, or acids. Lucas test involves substitution, not oxidation, so $\text{CrO}_3$ is not involved.
|
