See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Solution: Fastest Reaction with HI **Reaction Mechanism**: Alcohols react with HI via an $S_N1$ mechanism (for most alcohols), where the rate depends on **carbocation stability**. $$R-OH + HI \rightarrow R-I + H_2O$$ The $S_N1$ mechanism involves: 1. Protonation of $-OH$ group 2. Loss of water to form carbocation 3. Attack by $I^-$ **Carbocation Stability Analysis**: - **(A) Cyclopropanol**: Forms primary cyclopropyl carbocation — **highly strained and unstable** - **(B) Cyclopropylmethanol**: Forms **primary alkyl carbocation stabilized by ring strain relief** — carbocation has some allylic character from adjacent cyclopropane - **(C) Allyl alcohol**: Forms primary allylic carbocation — moderately stable (resonance stabilization) - **(D) Benzyl alcohol**: Forms benzyl carbocation — **very stable** (resonance with benzene ring), but more congested **Why (B) is fastest**: Option (B) forms a primary carbocation adjacent to a cyclopropane ring. The **ring strain in cyclopropane is relieved upon carbocation formation**, providing significant stabilization. This makes the carbocation formation step occur faster than in (A), while being more readily formed than the sterically hindered benzyl cation in (D). **Answer: (B)** ✓