Free tool
SN1 vs SN2 Predictor
Predict whether SN1 or SN2 is favored from the substrate, nucleophile, and solvent — then see the reasoning step by step. For the full tutorial, read the complete SN1 vs SN2 guide.
How the SN1 vs SN2 predictor works
This tool uses a deterministic rule engine — not AI — to rank substitution and elimination pathways. You classify the carbon bearing the leaving group, describe the reagent as a nucleophile or base, identify the solvent, and optionally add temperature or beta-hydrogen information. The engine returns a qualitative prediction with reasons, warnings, and what would change the outcome.
It does not predict every organic reaction. Vinyl and aryl halides, for example, fall outside the standard alkyl SN1/SN2 flowchart. Secondary substrates may stay ambiguous until the reagent and solvent are specified.
What determines SN1 vs SN2?
Substrate
Methyl and primary substrates typically favor SN2 because backside attack is accessible. Tertiary substrates block normal SN2 and can support SN1 under ionizing conditions. Secondary substrates are the most context-dependent case.
Nucleophile
Good nucleophiles support SN2. Weak neutral nucleophiles can still participate in SN1 because they do not appear in the rate-determining ionization step. Strong or bulky bases introduce elimination competition.
Solvent
Polar protic solvents stabilize ions and favor carbocation pathways. Polar aprotic solvents keep many anionic nucleophiles reactive and support SN2.
Steric hindrance
SN2 requires backside access to the electrophilic carbon. Crowded tertiary centers make that geometry unrealistic, which is why sterics are evaluated before nucleophile strength alone.
Why the tool sometimes predicts E1 or E2
It would be chemically misleading to force every reaction into SN1 or SN2. Strong bases can favor E2, especially when substitution is sterically hindered. SN1 and E1 often compete because both proceed through carbocation intermediates. Tertiary substrates with strong bases therefore return E2 rather than a false SN1 answer.
SN1 vs SN2 quick reference
| Factor | SN1 | SN2 |
|---|---|---|
| Mechanism | Stepwise | Concerted, one step |
| Rate law | v = k[RX] | v = k[RX][Nu−] |
| Intermediate | Carbocation | None |
| Substrate | Tertiary favored | Methyl / primary favored |
| Nucleophile | Can be weak | Usually strong |
| Solvent | Polar protic | Polar aprotic |
| Stereochemistry | Substantial racemization | Inversion |
| Rearrangements | Possible | Not via carbocation |
Related pages: SN1 vs SN2 guide, reaction mechanism solver, organic chemistry solver, organic chemistry AI, homework help, organic chemistry tutor.
SN1 vs SN2 predictor FAQ
How do you predict SN1 vs SN2?
The predictor follows the same order students use in class: classify the substrate first, then evaluate the nucleophile or base, then the solvent, and finally optional conditions such as temperature and beta hydrogens. Each factor adds or removes steric, ionization, and elimination pathways.
Does a primary substrate favor SN1 or SN2?
Primary substrates usually favor SN2 because backside attack is still accessible. Normal SN1 is strongly disfavored unless resonance or special stabilization changes the story.
Does a tertiary substrate favor SN1 or SN2?
Normal SN2 is strongly blocked at a tertiary center. SN1 can occur under ionizing protic conditions, while strong bases often favor E2 instead of substitution.
Which solvent favors SN2?
Polar aprotic solvents such as DMSO, DMF, acetonitrile, and acetone keep many anionic nucleophiles relatively reactive and support SN2.
Which solvent favors SN1?
Polar protic solvents such as water and alcohols stabilize ions and support carbocation formation, which favors SN1/E1 conditions.
Can SN1 and E1 happen together?
Yes. They often compete because both can proceed through a carbocation intermediate, especially under polar protic conditions and with heat.
Why does a strong base favor E2?
Strong bases can remove beta hydrogens efficiently. That matters most when substitution is sterically hindered or when the reagent behaves more like a base than a nucleophile.
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