Question

The compound which reacts fastest with Lucas reagent at room temperature, is
A. butan-1-ol
B. butan-2-ol
C. 2-methyl propan-1-ol
D. 2-methyl propan-2-ol

Answer 1

We were asked to find the compound which reacts fastest with Lucas reagent at room temperature. We first discuss Lucas ' reagent quickly. At that point we discuss whether primary alcohols or secondary alcohols or tertiary alcohols respond quickly with Lucas reagent.

Complete step by step answer:
Lucas' reagent is the solution of anhydrous zinc chloride in the concentrated hydrochloric acid. This solution is used to group alcohols of low sub-atomic weight. The reaction is a substitution chloride; the chloride replaces a hydroxyl group. A positive test is indicated by a change from clear and colourless to turbid, signalling formation of a chloroalkane. Additionally, the best results for this test are observed in tertiary alcohols, as they structure the respective alkyl halides fastest due to higher strength of the moderate tertiary carbocation. The test has since become somewhat obsolete with the availability of the various spectroscopic and chromatographic techniques for analysis. The Lucas test in alcohol is a test to differentiate between primary, secondary, and tertiary alcohols. It depends on the difference in reactivity of the three classes of alcohols with hydrogen halides through an $S{{N}_{1}}$ reaction
$ROH\text{ }+\text{ }HCl\text{ }\to \text{ }RCl\text{ }+\text{ }{{H}_{2}}O$
The differing reactivity reflects the formation of development of the comparing carbocation. Tertiary carbocation is far more stable than secondary carbocation, and primary carbocation is least stable due to hyper formation.
Hence, tertiary alcohol reacts fastest with Lucas reagent at room temperature, 2-methyl propan-2-ol is a tertiary alcohol, thus reacts immediately with Lucas reagent.
So, the correct answer is “Option D”.

Additional Information: An equimolar mixture of $ZnC{{l}_{2}}$ and concentrated $HCl$ is the reagent. The alcohol is protonated, the ${{H}_{2}}O$ group formed leaves, forming a carbocation, and the nucleophile $C{{l}^{-}}$ (which is present in excess) readily attacks the carbocation, forming the chloroalkane. Tertiary alcohols react quickly with Lucas reagent as proven by turbidity owing from the low solubility of the natural chloride in the watery mixture. Secondary alcohols react in five or so minutes (depends upon their dissolvability). primary alcohols don't react with Lucas reagent at room temperature.

Note: Lucas reagent reacts with alcohols is a type of $S{{N}_{1}}$ reaction. In this reaction chloride ion of $HCl$ substitutes a hydroxyl group of alcohols. It takes by following two steps:
-Loss of leaving the group and formation of carbocation.
-Nucleophilic attack.