The core question
How to Determine SN1 vs SN2: The Complete Decision Framework
The SN1 vs SN2 question is one of the most tested concepts in Orgo 1. Substrate class, nucleophile strength, leaving group ability, and solvent polarity all interact to determine which mechanism dominates.
| Factor | Favors SN2 | Favors SN1 | Why |
|---|
| Substrate | Primary (1°) | Tertiary (3°) · allylic · benzylic | Steric hindrance blocks backside attack |
| Nucleophile | Strong (CN⁻, I⁻, RS⁻, RO⁻) | Weak (H₂O, ROH) | SN2 rate depends on [Nu]; SN1 does not |
| Solvent | Polar aprotic (DMSO, DMF) | Polar protic (H₂O, MeOH, EtOH) | Aprotic leaves Nu reactive; protic stabilizes cations |
| Leaving group | Good LG needed | Good LG needed | Both mechanisms require good LG |
| Secondary (2°) | Strong Nu + polar aprotic | Weak Nu + polar protic | Conditions determine pathway |
| Rearrangement | Never | Possible (hydride/methyl shift) | Diagnostic for SN1 |
| Stereochemistry | Inversion (Walden) | Racemization | Most tested stereo topic |
| Rate law | k[sub][Nu] — bimolecular | k[sub] — unimolecular | Reflects rate-determining step |
Substrate class
How Substrate Class Determines the Mechanism
The single most important factor is the class of the carbon bearing the leaving group.
1°
Primary
CH₃Br, CH₃CH₂Br
Low steric hindrance. Primary carbocations too unstable.
Strongly favors SN22°
Secondary
2-bromobutane, cyclohexyl bromide
Moderate hindrance. Conditions determine pathway.
Conditions determine3°
Tertiary
(CH₃)₃CBr, tert-amyl chloride
SN2 impossible. Stable carbocations.
Strongly favors SN1
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Special Cases: Allylic and Benzylic
Allylic and benzylic substrates can undergo SN1 even at secondary level because resonance stabilizes the carbocation. Always check for these features before applying the standard rules.
Mechanism breakdown
SN1 and SN2 Mechanisms: Step-by-Step Comparison
SN2 is concerted (one step). SN1 is stepwise (leaving group departs first, nucleophile attacks second). This timing difference explains stereochemistry, rate law, and rearrangements.
Transition state
Pentacoordinate carbon, trigonal bipyramidal
Stereochemistry
100% inversion (Walden inversion)
Rate law
Rate = k[substrate][nucleophile]
Steps
Two steps (+ possible rearrangement)
Intermediate
Planar sp2 carbocation
Stereochemistry
Racemization
Rate law
Rate = k[substrate]
Rearrangements
Possible via 1,2-shifts
Stereochemical outcomes
Stereochemistry of SN1 and SN2 Reactions
↩️
SN2: Walden Inversion
Nucleophile attacks from the back face (180°). All substituents flip. R → S or S → R. 100% stereospecific.
SN2 · 100% inversion⚖️
SN1: Racemization
Flat carbocation attacked from both faces equally. 50:50 R and S mixture. Optically inactive product.
SN1 · racemic mixture
Solvent effects
Polar Aprotic vs Polar Protic Solvents
Polar aprotic solvents leave nucleophiles reactive (SN2). Polar protic solvents stabilize carbocations (SN1).
DMSOSN2
DMFSN2
AcetoneSN2
AcetonitrileSN2
THFSN2
WaterSN1
MethanolSN1
EthanolSN1
Acetic acidSN1
Formic acidSN1
Advanced topic
Carbocation Rearrangements in SN1 Reactions
When SN1 forms a secondary carbocation adjacent to a tertiary carbon, the carbocation rearranges via 1,2-hydride or methyl shift to the more stable form.
🔀
1,2-Hydride Shift
A hydrogen migrates from an adjacent carbon to the carbocation center, converting a secondary carbocation into a more stable tertiary one. This shift is extremely fast.
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1,2-Methyl Shift
A methyl group migrates when no hydride shift is possible. Produces a rearranged carbon skeleton.
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Exam Strategy
If the product carbon skeleton differs from the substrate, a rearrangement occurred, confirming SN1. SN2 can never give rearranged products.
Competition reactions
SN2 vs E2 and SN1 vs E1: When Does Elimination Compete?
Strong, non-bulky nucleophiles at lower temperatures favor SN2. Bulky bases promote E2. For tertiary substrates, SN1 and E1 compete, with E1 favored at higher temperatures.
Frequently asked questions
SN1 and SN2 Reaction Solver — FAQ
How do you determine SN1 or SN2? +
Primary + strong Nu + polar aprotic = SN2. Tertiary + weak Nu + polar protic = SN1. Secondary depends on conditions.
What is the stereochemical outcome? +
SN2: complete inversion (Walden inversion). SN1: racemization from flat carbocation.
What solvents favor SN2 vs SN1? +
SN2: polar aprotic (DMSO, DMF, acetone). SN1: polar protic (water, methanol, ethanol).
Can tertiary undergo SN2? +
No. Three alkyl groups completely block backside attack.
What is carbocation rearrangement? +
1,2-hydride or methyl shift in SN1 converts a less stable carbocation to a more stable one, changing the product skeleton. SN2 cannot rearrange.
SN2 vs E2 competition? +
Non-bulky nucleophiles favor SN2. Bulky bases favor E2. Higher temperature favors elimination. See the
elimination solver.
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