SN1 & SN2 Reactions: Mechanism, Differences & Notes PDF
- SN1: Rate = k[Substrate] — 2 steps, unimolecular
- SN2: Rate = k[S][Nu] — concerted backside attack
- SN1 → racemization; SN2 → Walden inversion
- SN2 favors methyl/1°; SN1 favors 3° substrates
- SN2: polar aprotic solvent; SN1: polar protic
The Sₙ2 Reaction (Substitution Nucleophilic Bimolecular)
Sₙ2 Mechanism
Factors Favoring Sₙ2 Reactions:
The Sₙ1 Reaction (Substitution Nucleophilic Unimolecular)
Sₙ1 Mechanism
Stereochemistry in Sₙ1
Factors Favoring Sₙ1 Reactions:
Comparison Table: Sₙ1 vs. Sₙ2
| Feature | Sₙ1 Reaction | Sₙ2 Reaction |
|---|---|---|
| Molecularity | Unimolecular | Bimolecular |
| Kinetics | Rate = k[Substrate] | Rate = k[Substrate][Nucleophile] |
| Mechanism | Two steps | One concerted step |
| Intermediate | Carbocation formed | No intermediate |
| Substrate Reactivity | 3° > 2° >> 1° | Methyl > 1° > 2° >> 3° |
| Nucleophile | Weak favored | Strong required |
| Stereochemistry | Racemization | Inversion |
| Solvent | Polar protic | Polar aprotic |
- SN1: Rate = k[Substrate] — 2 steps, unimolecular
- SN2: Rate = k[S][Nu] — concerted backside attack
- SN1 → racemization; SN2 → Walden inversion
- SN2 favors methyl/1°; SN1 favors 3° substrates
- SN2: polar aprotic solvent; SN1: polar protic
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About SN1 & SN2 Reactions: Mechanism, Differences & Notes PDF
SN1 & SN2 Reactions: Mechanism, Differences & Notes PDF is a fundamental concept in organic chemistry. Understanding the mechanisms, reaction conditions, and stereo-chemical outcomes is crucial for mastering organic chemistry. Our curated resources provide step-by-step visualizations to help you excel.
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Practice MCQs
What is the rate-determining step of an SN1 reaction?
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SN1
& SN2 Reactions: Mechanism, Differences & Notes PDF FAQ
What is the difference between SN1 and SN2?
The rate depends on the substrate. Primary substrates undergo SN2 faster, while tertiary substrates undergo SN1 faster. Generally, SN2 reactions can be very fast because they are concerted, provided there is minimal steric hindrance.
Polar protic solvents (like water, alcohols) favor SN1 reactions because they can hydrogen-bond and stabilize both the carbocation intermediate and the leaving group.
SN2 stands for Substitution Nucleophilic Bimolecular.
How to know if it is SN1 or SN2?
It depends heavily on the substrate and conditions. SN2 is faster for methyl and primary substrates. SN1 is faster (and SN2 does not occur) for tertiary substrates.
Look for a tertiary substrate, a weak nucleophile (like H2O or ROH), and a polar protic solvent. These conditions strongly favor the SN1 pathway.
Look for a primary or methyl substrate, a strong negatively-charged nucleophile, and a polar aprotic solvent. These conditions strongly favor the SN2 pathway.
What does SN1 stand for?
SN2 is a 1-step concerted reaction. SN1 is a 2-step reaction.
SN1 stands for Substitution Nucleophilic Unimolecular. It means it is a substitution reaction where the rate-determining step involves only one molecule (the substrate breaking apart).
Is SN2 first or second order?
No, SN1 is a first-order reaction overall (Rate = k[Substrate]). SN2 is a second-order reaction (Rate = k[Substrate][Nucleophile]).
SN2 strongly prefers primary substrates over secondary substrates because primary carbons have much less steric hindrance, allowing the nucleophile to attack more easily.
Because the rate-determining step is a single concerted step involving the collision of two molecules: the nucleophile and the substrate. Thus, the rate depends on the concentrations of both.