Organic chemistry study guide · Reaction Mechanism MCAT Tutor
SN1 vs SN2: how to pick the mechanism and predict stereochemistry
Substitution reactions swap a leaving group for a nucleophile. Exams rarely ask you to name the mechanism in isolation — they give you a substrate, solvent, and nucleophile and expect the major product and stereochemistry. SN2 is one concerted step with backside attack; SN1 goes through a carbocation and is often racemizing at the electrophilic carbon.
Written for undergraduate orgo, pre-med, and MCAT organic chemistry review — United States exam-style framing.
Short lesson
SN2 — one step, backside attack
The nucleophile attacks the electrophilic carbon from the side opposite the leaving group. Bond-making and bond-breaking happen together, so steric hindrance on the carbon slows SN2 dramatically.
Stereochemistry: expect inversion of configuration (Walden inversion) at the stereocenter that reacts. A single enantiomer substrate typically gives a single enantiomer product with opposite R/S assignment.
Rate law: first order in both substrate and nucleophile (second order overall). Strong nucleophiles and polar aprotic solvents (DMSO, acetone, DMF) favor SN2.
SN1 — carbocation intermediate
The leaving group departs first to form a carbocation. The nucleophile attacks afterward, so the rate depends only on substrate concentration (unimolecular).
Stereochemistry: the planar sp² carbocation can be attacked from either face, so a stereocenter at the reaction site often gives racemization (or partial racemization if the carbocation is long-lived).
Substrate trend: tertiary > secondary >> primary for SN1. Polar protic solvents (water, alcohols) stabilize the carbocation and favor SN1.
Quick decision checklist
Primary alkyl halide + strong nucleophile + polar aprotic → think SN2.
Tertiary halide + weak nucleophile (often solvent) + polar protic → think SN1.
If elimination competes, check for bulky base or high temperature — that pushes E2 over SN2 on many secondary systems.
Common traps on exams
Assuming SN1 always gives a 50:50 racemic mixture
Carbocations that react before they can rotate, or neighboring-group participation, can give stereospecific or partially stereospecific outcomes. Don't default to “racemic” without checking substitution pattern and solvent.
Calling every primary halide SN2-only
Primary centers can still undergo SN1 if the leaving group is excellent and the carbocation is stabilized (allylic/benzylic). Read the whole structure, not just “primary.”
Ignoring nucleophile vs base
A strong base on a hindered secondary substrate often gives E2, not SN2. If the prompt says t-BuOK or LDA, elimination may beat substitution.
Forgetting stereochemistry on SN2
If the question marks a chiral center with wedges and dashes, SN2 requires inversion. Drawing retention is a common wrong answer.
Practice-style prompts
Try these on paper, then check product and mechanism in OrgoMech.
- (R)-2-bromobutane + NaSH in DMSO — predict the substitution product and absolute configuration at C2.
- tert-Butyl bromide in aqueous methanol — SN1 or SN2? What happens to stereochemistry at the tertiary carbon?
- 1-bromobutane vs 2-bromo-2-methylpropane with NaI in acetone — which reacts faster in SN2 and why?
- Would CH₃CH₂Cl undergo SN1 in water? Justify using carbocation stability.
- A student predicts retention for SN2 on a chiral secondary iodide — explain the error in one sentence.
Student questions
Common questions undergraduates ask about this topic before exams.
- How do I quickly decide between SN1 and SN2?
- Check substrate (primary → SN2; tertiary → often SN1), nucleophile strength (strong → SN2), and solvent (polar aprotic favors SN2; polar protic favors SN1). Bulky base on a hindered substrate often means elimination instead.
- Does SN2 always invert stereochemistry?
- At a stereocenter that undergoes SN2, yes — backside attack gives Walden inversion. If no stereocenter is present, there is no R/S change to track.
- Why does SN1 often give racemic product?
- The carbocation is planar sp², so nucleophile attack from either face can occur, producing a mixture of enantiomers when the electrophilic carbon was chiral.
- Can primary alkyl halides ever go SN1?
- Rarely for simple primary centers, but allylic or benzylic substrates can stabilize a carbocation enough for SN1 pathways. Always read the full structure.
- What solvents favor SN2?
- Polar aprotic solvents such as DMSO, acetone, and DMF stabilize ions less and keep nucleophiles more reactive, which helps bimolecular substitution.
- Is SN1 or SN2 faster for tert-butyl bromide in water?
- SN1 is favored — tertiary substrate, polar protic solvent, weak nucleophile (water). Expect substitution with carbocation chemistry, not backside SN2.
- How is SN1/SN2 tested on the MCAT?
- Passages and discrete questions often pair conditions with product stereochemistry. Practice predicting both mechanism type and the correct enantiomer or racemic outcome.
Further reading (external)
Open textbooks and reference sites for the same topics covered above. We link out to established science education publishers — not affiliate pages.
- The SN2 mechanism (LibreTexts) — LibreTexts
- OpenStax — Substitution reactions of alkyl halides — OpenStax
- PubChem — NIH NLM
Continue on this site
Drill this topic in OrgoMech
Predict the major product for your structure, then order the curved-arrow mechanism — the same skills tested on homework, finals, and the MCAT orgo section. Predict the major product, then order the curved arrows. Organic chemistry practice for reactions, mechanisms, and the days left before your final.