SN1, SN2, E1, E2: How to Tell Them Apart

This is the topic. The one that makes or breaks Orgo 1 grades. Four mechanisms, overlapping conditions, competing pathways, and an exam question format that essentially asks you to predict which mechanism wins. Students stare at a problem and freeze because all four seem possible. The solution isn't to memorize more; it's to have a systematic decision framework that narrows four options to one. Here's the framework I teach, and it works.

Step 1: Look at the substrate.

This is always your starting point. Methyl or primary carbon: SN2 or E2. The carbon bearing the leaving group is minimally hindered, so backside nucleophilic attack (SN2) can proceed without steric interference, and strong bases can access the β-hydrogen for E2. Tertiary carbon: SN1 or E1. The carbon is too sterically crowded for backside attack, so SN2 is effectively off the table. Instead, the leaving group departs first to form a carbocation, and then the nucleophile or base reacts with the intermediate. Secondary carbon: all four mechanisms are possible, and this is where the remaining steps become critical tiebreakers.

Step 2: Look at the nucleophile or base.

Strong nucleophile (CN⁻, RS⁻, I⁻, N₃⁻) or strong base (EtO⁻, tBuO⁻, LDA): the reaction goes through a bimolecular mechanism, either SN2 or E2. A strong, bulky base (like tert-butoxide) specifically favors E2 over SN2 because its size prevents it from acting as a nucleophile at the electrophilic carbon. Weak or neutral nucleophile (H₂O, ROH, no added nucleophile): the reaction goes through a unimolecular mechanism, either SN1 or E1. Without a strong nucleophile to drive a concerted mechanism, the leaving group departs on its own.

Step 3: Look at the solvent.

Polar aprotic solvents (DMSO, DMF, acetone) favor SN2. They dissolve the ionic nucleophile but don't solvate it heavily, leaving it "naked" and highly reactive. Polar protic solvents (water, methanol, ethanol) favor SN1 and E1. They stabilize the carbocation intermediate through solvation and ion-dipole interactions, which is essential for unimolecular mechanisms that generate charged intermediates.

Step 4: Temperature (the tiebreaker).

High temperature favors elimination (E1 or E2). Elimination has a higher activation energy than substitution and benefits more from added thermal energy. Low or room temperature favors substitution (SN1 or SN2). When everything else, substrate, nucleophile, and solvent, points to two mechanisms, temperature breaks the tie. If the problem says "reflux" or "heat," think elimination.

The framework in action: three examples.

1-bromobutane + NaCN in DMSO: Primary substrate (SN2 or E2) + strong nucleophile (CN⁻) + polar aprotic solvent = SN2. The cyanide ion attacks the primary carbon via backside attack, displacing bromide in a single concerted step.

2-bromo-2-methylpropane in water: Tertiary substrate (SN1 or E1) + weak nucleophile (water) + polar protic solvent = SN1. The bromide leaves first to form a tertiary carbocation, then water attacks the carbocation.

2-bromobutane + NaOEt in ethanol at reflux: Secondary substrate (all four possible) + strong base (ethoxide) + high temperature = E2. The base abstracts a β-hydrogen and the leaving group departs in a concerted, anti-periplanar transition state.

The secondary substrate: where it gets interesting.

Secondary substrates are the reason this topic is hard. With primary and tertiary substrates, the substrate alone narrows your options to two mechanisms. With secondary substrates, all four are on the table, and you have to work through Steps 2–4 carefully. Here's a quick decision map for secondary substrates: Strong nucleophile + polar aprotic solvent = SN2. Strong bulky base + heat = E2. Weak nucleophile + polar protic solvent = SN1 (substitution product) or E1 (elimination product), and you often get a mixture of both, with E1 favored at higher temperatures. Most exam questions on secondary substrates are really testing whether you can identify which factor dominates.

One more thing worth emphasizing: SN1 and E1 are always in competition because they share the same first step, the formation of the carbocation. Once that carbocation forms, it can either be attacked by a nucleophile (SN1) or lose a proton (E1). That's why SN1/E1 reactions typically give product mixtures, and why temperature is the tiebreaker between them.

This four-step framework handles the vast majority of SN1/SN2/E1/E2 problems you'll encounter. Learn it, drill it, and the topic that breaks most students becomes one of your strengths.