Question

Which one of the following alcohols reacts instantaneously with Lucas reagent?

• A. CH₃ – CH₂ – CH₂ – CH₂OH
• B.
• C.
• D.

Answer 1

The Correct Option is D

The reactivity of alcohols with Lucas reagent follows the order:

Tertiary ($ \text{3}^\circ $) > Secondary ($ \text{2}^\circ $) > Primary ($ \text{1}^\circ $)

This order of reactivity is due to the stability of the carbocation intermediate formed during the reaction. The mechanism involves the protonation of the alcohol by $ \text{HCl} $, followed by the loss of water to form a carbocation, which then reacts rapidly with the chloride ion. Tertiary carbocations are the most stable due to hyperconjugation and inductive effects from three alkyl groups, leading to the fastest reaction. Secondary carbocations are less stable, and primary carbocations are the least stable, often requiring heat or a longer time to react.

Analyzing the Given Alcohols:

(1) $ \text{CH}_3 - \text{CH}_2 - \text{CH}_2 - \text{CH}_2\text{OH} $: This is a primary alcohol (the carbon bearing the $ -\text{OH} $ group is attached to only one other carbon). Primary alcohols generally do not react or react very slowly with Lucas reagent at room temperature; no turbidity will be observed immediately.

(2) $ \text{CH}_3 - \text{CH}_2 - \underset{\underset{\text{CH}_3}{|}}{\text{CH}} - \text{OH} $: This is a secondary alcohol (the carbon bearing the $ -\text{OH} $ group is attached to two other carbons). Secondary alcohols react with Lucas reagent to form turbidity within 5 to 10 minutes.

(3) $ \text{CH}_3 - \underset{\underset{\text{CH}_3}{|}}{\text{CH}} - \text{CH}_2\text{OH} $: This is also a primary alcohol (the carbon bearing the $ -\text{OH} $ group is attached to only one other carbon). Similar to (1), it will not react instantaneously with Lucas reagent at room temperature.

(4) $ \underset{\underset{\text{CH}_3}{|}}{\overset{\overset{\text{CH}_3}{|}}{\text{CH}_3 - \text{C} - \text{OH}}} $: This is a tertiary alcohol (the carbon bearing the $ -\text{OH} $ group is attached to three other carbons). Tertiary alcohols react almost instantaneously with Lucas reagent to produce turbidity immediately due to the formation of a stable tertiary carbocation.

Conclusion:
The alcohol that reacts instantaneously with Lucas reagent is (4).

Final Answer:
The alcohol that reacts instantaneously with Lucas reagent is: $ \boxed{(4)} $

Answer 2

Step 1: Recall the Lucas Test Mechanism

The Lucas test detects tertiary alcohols, which react instantly with Lucas reagent (ZnCl₂ + HCl) to form a cloudy solution. 

Step 2: Analyze the Given Options

Option 1: A tertiary alcohol, reacts slowly.

Option 2: A secondary alcohol, reacts slowly.

Option 3: Another tertiary alcohol, reacts instantly.

Option 4: A primary alcohol, shows no immediate reaction.

Step 3: Conclusion

The tertiary alcohol in Option 4 reacts instantly with Lucas reagent.