Comparison of SN1, SN2, SNi, SN-NGP and SNAr Reactions
Nucleophilic substitution reactions are among the most important topics in organic chemistry. For competitive exams like JEE Main, JEE Advanced and NEET, students must clearly understand the differences between various substitution mechanisms.
This article provides a clear and exam-oriented comparison of the five major nucleophilic substitution mechanisms: SN1, SN2, SNi, SN-NGP and SNAr.
Brief Overview of Each Mechanism
- SN1: Unimolecular nucleophilic substitution via carbocation formation
- SN2: Bimolecular nucleophilic substitution via backside attack
- SNi: Internal nucleophilic substitution with retention of configuration
- SN-NGP: Substitution assisted by neighbouring group participation
- SNAr: Nucleophilic aromatic substitution via Meisenheimer complex
Key Differences Between SN1, SN2, SNi, SN-NGP and SNAr
| Feature | SN1 | SN2 | SNi | SN-NGP | SNAr |
|---|---|---|---|---|---|
| Rate Law | First order | Second order | First order | First order (fast) | Second order |
| Intermediate | Carbocation | None | Chlorosulfite ester | Cyclic intermediate | Meisenheimer complex |
| Nucleophile | External | External | Internal | Internal assistance | External |
| Substrate Type | Tertiary & secondary | Primary & methyl | Alcohols | Secondary substrates | Aryl halides |
| Stereochemistry | Racemization | Inversion | Retention | Retention / partial retention | No stereochemical change |
| Solvent | Polar protic | Polar aprotic | Non-aqueous | Polar protic | Polar aprotic |
| Reaction Speed | Moderate | Fast | Fast | Very fast | Slow |
Examples of Each Mechanism
SN1 Reaction
Hydrolysis of tert-butyl chloride forming tert-butyl alcohol via carbocation intermediate.
SN2 Reaction
Reaction of methyl bromide with hydroxide ion showing backside attack and inversion.
SNi Reaction
Conversion of alcohol to alkyl chloride using SOCl2 without pyridine, showing retention.
SN-NGP Reaction
Solvolysis of 2-bromoethanol where –OH group assists via neighbouring group participation.
SNAr Reaction
Substitution of fluorine in nitro-substituted chlorobenzene by OH−.
Why This Comparison is Important for JEE & NEET
- Frequently asked in MCQs and assertion-reason questions
- Tests understanding of reaction mechanism
- Important for stereochemistry-based questions
- Helps predict reaction rate and product
Quick Memory Tips
- SN1 → Carbocation → Racemization
- SN2 → Backside attack → Inversion
- SNi → Internal nucleophile → Retention
- SN-NGP → Neighbouring group → Fast reaction
- SNAr → Nitro group + aryl halide
Conclusion
Although all these reactions involve nucleophilic substitution, their mechanisms, rate laws, intermediates and stereochemical outcomes are fundamentally different.
A clear comparison of SN1, SN2, SNi, SN-NGP and SNAr enables students to solve mechanism-based questions confidently and score high in organic chemistry.
— Chemca | Chemistry Made Easy
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