SN1 vs. SN2: Choosing the Right Path

Distinguishing Between SN1, SN2, E1, and E2 Reactions

I. Summary of Key Factors

A. Substrate Structure

SN1 and E1: Favor tertiary (3°) > secondary (2°), allylic, and benzylic substrates due to carbocation stability.
SN2 and E2: Favor methyl > primary (1°) > secondary (2°). Tertiary substrates are too sterically hindered for SN2, but favor E2 elimination.

B. Nucleophile/Base Strength

SN1 and E1: Rate does not depend on nucleophile or base strength (rate-determining step is carbocation formation).
SN2: Requires a strong nucleophile.
E2: Requires a strong base to abstract a proton in a concerted elimination step.

C. Leaving Group Ability

All reactions: Favored by good leaving groups such as I⁻, Br⁻, Cl⁻, and tosylates (TsO⁻).

D. Solvent Effects

SN1 and E1: Favor polar protic solvents (e.g., water, alcohols) that stabilize carbocations and leaving groups.
SN2 and E2: Favor polar aprotic solvents (e.g., DMSO, DMF, acetone) that keep nucleophiles and bases reactive.

E. Mechanism and Stereochemistry

SN1: Two-step mechanism via carbocation intermediate; results in racemization at chiral centers.
SN2: One-step concerted mechanism with backside attack; results in inversion of configuration.
E1: Two-step elimination via carbocation intermediate; typically forms the more substituted (Zaitsev) alkene.
E2: One-step concerted elimination; requires anti-periplanar geometry and strong base; often forms Zaitsev product unless bulky base is used.

II. Decision-Making Strategies

A. Flowchart or Decision Tree

To decide which mechanism is most likely for a given reaction, follow the flowchart provided below. This flowchart guides you through key considerations such as substrate type, nucleophile or base strength, solvent choice, and temperature to predict whether the reaction proceeds via SN1, SN2, E1, or E2.

Flowchart: Decision guide for predicting SN1, SN2, E1, and E2 reaction mechanisms.

B. Summary Table

A comparison table summarizing the key features of SN1, SN2, E1, and E2 reactions:

Feature	SN1	SN2	E1	E2
Mechanism	Two-step; carbocation intermediate	One-step; concerted backside attack	Two-step; carbocation intermediate	One-step; concerted elimination
Rate Law	Rate = k [substrate]	Rate = k [substrate][nucleophile]	Rate = k [substrate]	Rate = k [substrate][base]
Substrate Preference	Tertiary > secondary > primary (rare)	Methyl > primary > secondary (tertiary unfavorable)	Tertiary > secondary > primary (rare)	Tertiary > secondary > primary
Nucleophile/Base Strength	Weak nucleophile sufficient	Strong nucleophile required	Weak base sufficient	Strong base required
Solvent	Polar protic (e.g., H₂O, ROH)	Polar aprotic (e.g., DMSO, DMF, acetone)	Polar protic	Polar aprotic or protic (usually aprotic preferred)
Leaving Group	Good leaving group essential	Good leaving group essential	Good leaving group essential	Good leaving group essential
Stereochemistry	Racemization or partial racemization	Inversion of configuration (Walden inversion)	Not applicable (elimination)	Not applicable (elimination)
Product Type	Substitution	Substitution	Alkene (elimination)	Alkene (elimination)
Typical Examples	Tertiary alkyl halides with weak nucleophiles	Primary alkyl halides with strong nucleophiles	Tertiary alkyl halides with weak bases	Secondary or tertiary alkyl halides with strong bases

III. Practice Problems

Work through practice problems that challenge you to identify whether given reactions proceed via SN1, SN2, E1, or E2 mechanisms. Predict the major products and describe the stereochemical and regiochemical outcomes.