SN1 vs SN2 Reactions

The Core Difference

SN1 and SN2 reactions both replace a leaving group, but they do it by different pathways. SN2 is concerted: the nucleophile attacks the carbon as the leaving group departs. SN1 is stepwise: the leaving group leaves first, forming a carbocation, and the nucleophile attacks afterward.

That difference changes everything students usually test: rate law, substrate preference, nucleophile strength, solvent effect, and stereochemical result. If you can identify the pathway, most exam questions become pattern recognition rather than memorization.

How To Decide Quickly

Start with the substrate. Methyl and primary substrates usually point toward SN2 because backside attack is accessible. Tertiary substrates block backside attack and stabilize carbocations, so they point toward SN1. Secondary substrates are the decision zone and require checking nucleophile strength, solvent, and competing elimination.

Strong, negatively charged nucleophiles in polar aprotic solvents favor SN2. Weak neutral nucleophiles in polar protic solvents favor SN1 when a stable carbocation can form.

Common Mistakes

Do not call every tertiary reaction SN1 automatically. A strong bulky base can drive E2 instead. Do not call every strong nucleophile SN2 either; if the substrate is tertiary, the geometry can prevent substitution. Always check whether elimination is available.

For stereochemistry, SN2 gives inversion at the reacting stereocenter. SN1 often gives a mixture because the carbocation is planar, although ion pairing can make one side slightly favored.

Useful Site Tools

Use the Reaction Mechanism Visualizer page for mechanism planning and the Functional Group Identifier page when you need to recognize the reactive center. These tools are listed in the Tools menu as coming-soon resources, while the current blog article gives the decision framework.