See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Dehydration Reactivity of Alcohols **Key Principle:** Dehydration ease follows carbocation stability: **3° > 2° > 1° > aromatic** **Analysis of each compound:** **(I) Phenol** ($C_6H_5OH$) - $OH$ attached to aromatic ring - Forms unstable phenyl carbocation (aromatic ring breaks) - **Extremely difficult to dehydrate** — least reactive **(II) Cyclohexanol** (saturated, 2° alcohol) - Secondary alcohol → forms stable 2° carbocation - Moderate reactivity **(III) Cyclohexene-ol** (allylic alcohol, 2° alcohol) - Secondary alcohol with adjacent $C=C$ - Forms resonance-stabilized allylic carbocation - **More reactive than (II)** **(IV) Cyclopentadiene-ol or unsaturated cyclic alcohol** - Can form highly stabilized carbocation (allylic/conjugated) - Very reactive intermediate possible - **More reactive than (III)** but less than (I) in reverse order **Reactivity Order:** $$\text{Phenol} < \text{Cyclohexanol} < \text{Allylic-2°} < \text{Highly unsaturated}$$ $$\boxed{\text{I} < \text{IV} < \text{II} < \text{III}} \quad \text{(Option D)}$$ Phenol is **least reactive** (aromatic stability prevents dehydration), while the allylic alcohol (III) is **most reactive** due to resonance stabilization of the carbocation intermediate.