Question

Lucas reagent produces cloudiness immediately with :

• A. 2-methyl-1-propanol
• B. Butan-2-ol
• C. 2-methyl-2-propanol
• D. Butan-1-ol

Hint:

Reactivity order for Lucas Test: \( 3^\circ \gt 2^\circ \gt 1^\circ \).
Anhydrous \( ZnCl_{2} \) acts as a Lewis acid catalyst, helping to break the \( C-O \) bond in the alcohol.

Answer 1

The Correct Option is C

Concept:

• Lucas reagent consists of a mixture of concentrated Hydrochloric acid (\( HCl \)) and anhydrous Zinc chloride (\( ZnCl_{2} \)).

• It is a classic laboratory reagent used to differentiate between primary (\( 1^\circ \)), secondary (\( 2^\circ \)), and tertiary (\( 3^\circ \)) alcohols.

• The test is based on the difference in the rate of formation of alkyl chlorides from alcohols.

• Alkyl chlorides are insoluble in the Lucas reagent, leading to the appearance of turbidity or "cloudiness" in the solution.
Step 1: Identify the structure and degree of each option
We must classify each alcohol based on the carbon atom attached to the hydroxyl (\( -OH \)) group:

(A) 2-methyl-1-propanol: The \( -OH \) is attached to a carbon bonded to only one other carbon (\( 1^\circ \) alcohol).
(B) Butan-2-ol: The \( -OH \) is attached to a carbon bonded to two other carbons (\( 2^\circ \) alcohol).
(C) 2-methyl-2-propanol: The \( -OH \) is attached to a carbon bonded to three other carbons (\( 3^\circ \) alcohol).
(D) Butan-1-ol: The \( -OH \) is attached to a carbon bonded to only one other carbon (\( 1^\circ \) alcohol).

Step 2: Analyze the reaction mechanism and stability
The reaction proceeds via an \( S_{N}1 \) mechanism involving a carbocation intermediate.
The rate of the reaction depends on the stability of the carbocation formed:
\[ \text{Tertiary carbocation} \gt \text{Secondary carbocation} \gt \text{Primary carbocation} \]
Tertiary alcohols form highly stable tertiary carbocations, making the reaction extremely fast.

Step 3: Correlate observations with alcohol types
1. Tertiary (\( 3^\circ \)) alcohols: Produce cloudiness immediately upon mixing.
2. Secondary (\( 2^\circ \)) alcohols: Produce cloudiness within 5 to 10 minutes at room temperature.
3. Primary (\( 1^\circ \)) alcohols: Do not produce cloudiness at room temperature; reaction only occurs upon heating.

Step 4: Final selection
Since 2-methyl-2-propanol is a tertiary alcohol, it will react with the Lucas reagent to produce cloudiness instantly.
The reaction is:
\[ (CH_{3})_{3}COH + HCl \xrightarrow{ZnCl_{2}} (CH_{3})_{3}CCl \downarrow (\text{cloudy}) + H_{2}O \]