Question

Which of the following alcohols will react rapidly with Lucas reagent (HCl/ZnCl$_2$)?

1. 1-methylcyclohexan-1-ol
2. Cyclohexanol
3. Bicyclic Tertiary Alcohol
4. Cyclohex-2-en-1-ol

• A. 1, 3, and 4
• B. 1 and 3 only
• C. 2 and 4 only
• D. 1, 2, 3, and 4

Answer 1

Answer: (A)

1, 3, and 4

Answer 2

The reagent HCl/ZnCl$_2$ is known as Lucas reagent, which is used to differentiate between primary, secondary, and tertiary alcohols based on their reaction rates.

• Tertiary alcohols react rapidly with Lucas reagent to form alkyl chlorides, causing immediate turbidity.
• Secondary alcohols react slower, typically forming turbidity within 5-20 minutes, sometimes requiring gentle heating.
• Primary alcohols react very slowly or not at all.
• Allylic and benzylic alcohols react rapidly because the carbocation intermediates formed are resonance-stabilized, similar to tertiary alcohols.

Analyzing each structure:

1. 1-methylcyclohexan-1-ol: This is a tertiary alcohol. It reacts rapidly with Lucas reagent.
2. Cyclohexanol: This is a secondary alcohol. It reacts slowly with Lucas reagent.
3. Bicyclic Tertiary Alcohol: The carbon atom bearing the hydroxyl group is bonded to three other carbon atoms, classifying it as a tertiary alcohol. It reacts rapidly with Lucas reagent.
4. Cyclohex-2-en-1-ol: This is a secondary alcohol that is also allylic to the double bond. The formation of a resonance-stabilized allylic carbocation leads to a rapid reaction with Lucas reagent, comparable to tertiary alcohols.

Therefore, alcohols 1, 3, and 4 react rapidly with HCl/ZnCl$_2$.