Select substrate class, nucleophile/base strength, solvent polarity, and temperature to predict which mechanism (SN1, SN2, E1, E2) dominates — with full step-by-step reasoning.
Predicting which mechanism — SN1, SN2, E1, or E2 — actually dominates for a given set of reaction conditions is one of organic chemistry's genuinely decision-tree-shaped skills, and this predictor is built specifically around that decision framework rather than mechanism animation itself.
Substrate class is the first and often most decisive factor: primary substrates strongly favor SN2 (the backside nucleophilic attack has no steric hindrance) and essentially never proceed through SN1 or E1 (a primary carbocation is far too unstable to form). Tertiary substrates favor the opposite: SN2 is sterically blocked entirely by three bulky substituents crowding the backside approach, so tertiary substrates react exclusively through SN1/E1 pathways via a relatively stable tertiary carbocation intermediate. Secondary substrates are genuinely ambiguous and can go either way depending on the other three factors.
Nucleophile/base strength is the second major factor: strong nucleophiles favor the bimolecular SN2 and E2 pathways (which require the nucleophile or base to directly attack in the rate-determining step), while weak nucleophiles favor the unimolecular SN1 and E1 pathways (where the rate-determining step is simply the substrate ionizing on its own, independent of nucleophile strength). Whether the reagent acts primarily as a nucleophile (attacking carbon) or a base (removing a proton) further determines substitution versus elimination.
Solvent polarity governs the third factor: polar aprotic solvents (like DMSO or acetone, which cannot hydrogen bond to the nucleophile) leave nucleophiles "naked" and highly reactive, favoring SN2; polar protic solvents (like water or alcohols, which hydrogen bond to and stabilize both the nucleophile and any carbocation intermediate) favor SN1/E1 by stabilizing the ionization step.
Temperature is the final tiebreaker between substitution and elimination pathways specifically: higher temperature favors elimination (entropically favored, producing more product particles) over substitution when both are mechanistically plausible for a given substrate.
This predictor lets you select substrate class, nucleophile/base strength, solvent polarity, and temperature, then predicts the dominant mechanism with full step-by-step reasoning — turning a four-variable decision into a checkable, explainable prediction rather than memorized case-by-case rules.