Question

Which alcohol will give immediate turbidity on shaking with HCl at room temperature?
a.) 3-Methylpentan-2-ol
b.) 2-Methylbutan-1-ol
c.) Butan-3-ol
d.) 2-Methylpropan-2-ol

Answer 1

Hint: In order to solve this question, we will use the property of alcohol. The Lucas reagent reacts to alcohol and gives alkyl halide together with water. The response follows the SN1 mechanism, and an intermediate carbocation is formed, whose stability determines the reaction rate. The formed alkyl halide is insoluble, and its formation causes turbidity, i.e. Cloudy.

Complete step by step solution:
Now tertiary alcohols (three-degree carbocations) and alcoholics which cause very stable carbocations after protonation react quickly and immediately give turbidity. Eg: t-butyl alcohol.
As we know that tertiary alcohol give immediately turbidity on shaking with Lucas reagent
$\begin{array}{*{20}{c}}
  {}&{\begin{array}{*{20}{c}}
  {C{H_3}} \\
  |
\end{array}}&{} \\
  {C{H_3}}&{C - }&{C{H_3}} \\
  {}&{\begin{array}{*{20}{c}}
  | \\
  {OH}
\end{array}}&{}
\end{array}\xrightarrow[{}]{{conc.HCl + Anhy.ZnC{L_2}}}\begin{array}{*{20}{c}}
  {}&{\begin{array}{*{20}{c}}
  {C{H_3}} \\
  |
\end{array}}&{} \\
  {{H_3}C}&{ - C - }&{C{H_3}} \\
  {}&{\begin{array}{*{20}{c}}
  | \\
  {Cl}
\end{array}}&{}
\end{array}$ Secondary alcohols react a bit slower as they are not as stable (2 degrees) as they are above. After about 5-10 minutes, they cause turbidity. E.g. isopropyl alcohol
The primary alcohols react very slowly, and their turbidity comes after more than 45 minutes, (1 degree carbocation is highly unstable). This is why no turbidity is said to produce in the reaction mixture. Eg. Ethanol
Above methods are used to determine the degree of alcohol.

From the above it is clear that the correct option is D.

Note: In order to solve these types of questions, you need to learn about the chemical reactions of the alcohol and its properties. The lucas agent is a combination of the reagents like HCl and Anhydrous $ZnC{L_2}$ . The steps involved in the above reaction is protonation of alcohol with H+ from HCl. The second step is formation of carbocation of cleavage of C-O bond. This step is slow and the last step is Nucleophilic attack of Cl- to form alkyl chloride.