(D) (CH₃)₂CHI

Introduction

The SN² (bimolecular nucleophilic substitution) reaction is a fundamental concept in organic chemistry. The rate of an SN² reaction strongly depends on the structure of the alkyl halide, especially steric hindrance around the carbon atom undergoing substitution.

In this article, we analyze the given iodides and determine which reacts most slowly with cyanide ion (CN⁻) as a nucleophile in an SN² reaction.

Correct Answer

✅ Option (D): (CH₃)₂CH–I (Secondary alkyl iodide)

Key Concept: SN² Reaction Rate

Important Factors Affecting SN² Rate

1. Steric hindrance (most important)

2. Nature of nucleophile (CN⁻ is strong)

3. Leaving group ability (I⁻ is excellent)

4. Structure of alkyl halide

SN² Reactivity Order

Methyl>1∘>2∘≫3∘\text{Methyl} > 1^\circ > 2^\circ \gg 3^\circMethyl>1∘>2∘≫3∘

More steric hindrance → slower SN² reaction

Option-wise Explanation

Option (A): CH₃CH₂CH₂CH₂I (n-Butyl iodide)

• Primary alkyl iodide

• Minimal steric hindrance

• Excellent for SN² reactions

✔ Reacts fast
❌ Not the slowest

Option (B): C₆H₅CH₂I (Benzyl iodide)

• Benzylic carbon

• Transition state stabilized by resonance

• Extremely reactive in SN²

✔ Reacts very fast
❌ Not the slowest

Option (C): Cyclopropyl iodide

• Primary carbon

• Slight ring strain

• Still allows backside attack

✔ Faster than secondary iodides
❌ Not the slowest

Option (D): (CH₃)₂CH–I (Isopropyl iodide)

• Secondary alkyl iodide

• Significant steric hindrance

• Backside attack by CN⁻ is difficult

❌ Slowest SN² reaction

✅ Correct Answer

Why Secondary Iodide Reacts Most Slowly

✔ Two alkyl groups block nucleophile approach
✔ Increased steric hindrance
✔ Less favorable transition state

Reactivity Comparison Summary

Final Conclusion

In an SN² reaction, steric hindrance is the dominant factor. Among the given iodides, the secondary alkyl iodide has the most crowded reaction center, making nucleophilic backside attack by CN⁻ the most difficult.

Final Answer

🟢 Correct Option: (D) (CH₃)₂CH–I