SN1 vs SN2 Reaction Prediction
Predict whether a nucleophilic substitution reaction will proceed via SN1 or SN2 mechanism based on substrate, nucleophile, and solvent.
Problem Scenario
Predict the mechanism and major product for each reaction: (A) 2-bromobutane + NaCN in DMSO, (B) 2-bromo-2-methylpropane + CH3OH
Given Data
Requirements
- Analyze substrate structure (1°, 2°, or 3°)
- Evaluate nucleophile strength
- Consider solvent effects
- Predict mechanism (SN1 or SN2)
- Predict stereochemistry if applicable
Solution
Step 1:
Analyze Reaction A: 2-bromobutane is a secondary substrate. Secondary substrates can undergo both SN1 and SN2, so other factors determine the mechanism.
Step 2:
Reaction A analysis continued: CN- is a strong nucleophile and DMSO is a polar aprotic solvent (favors SN2 by not solvating the nucleophile). Strong nucleophile + polar aprotic solvent = SN2.
Step 3:
Reaction A mechanism: SN2 proceeds with backside attack, causing inversion of configuration. Product: 2-cyanobutane with inverted stereochemistry.
Step 4:
Analyze Reaction B: 2-bromo-2-methylpropane is a tertiary substrate. Tertiary substrates cannot undergo SN2 due to steric hindrance - backside attack is blocked.
Step 5:
Reaction B analysis: CH3OH is a weak nucleophile (neutral solvent), and it's also a polar protic solvent. Polar protic solvents stabilize carbocations and favor SN1.
Step 6:
Reaction B mechanism: SN1 proceeds via carbocation intermediate. The tertiary carbocation is stable. Nucleophile attacks from either face, giving racemization. Product: 2-methoxy-2-methylpropane (mixture of stereoisomers if chiral).
Final Answer
Reaction A: SN2 mechanism, product is 2-cyanobutane with inverted stereochemistry. Reaction B: SN1 mechanism, product is 2-methoxy-2-methylpropane. Key: A has strong nucleophile + polar aprotic solvent favoring SN2. B has tertiary substrate + weak nucleophile + polar protic solvent, only SN1 is possible.
Key Takeaways
- ✓Tertiary substrates only undergo SN1 (too sterically hindered for SN2)
- ✓Primary substrates favor SN2 (carbocations unstable)
- ✓Secondary substrates depend on nucleophile and solvent
- ✓Polar aprotic solvents favor SN2; polar protic favor SN1
Common Errors to Avoid
- ✗Forgetting that tertiary substrates cannot do SN2
- ✗Confusing polar protic and polar aprotic solvents
- ✗Not considering nucleophile strength
- ✗Forgetting stereochemical consequences (inversion vs racemization)
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Common questions about this problem type
With secondary substrates and borderline conditions, both may occur. Consider the dominant factor: strong nucleophile pushes toward SN2, tertiary-like substitution patterns push toward SN1.
E1 and E2 compete with SN1 and SN2 respectively. Strong bases favor elimination. Heat and bulky bases also favor elimination over substitution.