Nucleophilic substitution and elimination reactions form the backbone of introductory organic chemistry, and distinguishing between SN1, SN2, E1, and E2 pathways is one of the most heavily tested skills in university assignments. This guide builds on the bonding concepts from chemical bonding and molecular geometry and the rate laws from chemical kinetics to give you a systematic, example-driven approach to identifying and predicting these mechanisms.
1. The Four Reaction Types at a Glance
| Feature | SN2 | SN1 | E2 | E1 |
|---|---|---|---|---|
| Rate law | k[Nu][Substrate] | k[Substrate] | k[Base][Substrate] | k[Substrate] |
| Mechanism | Concerted, one step | Two steps (carbocation intermediate) | Concerted, one step | Two steps (carbocation intermediate) |
| Stereochemistry | Inversion (backside attack) | Racemization | Anti-periplanar geometry required | No specific requirement |
| Best substrate | Methyl, 1° | 3°, resonance-stabilized 2° | 2°, 3°, unhindered base | 3°, 2° |
| Nucleophile/Base | Strong Nu, weak base preferred | Weak Nu (often the solvent) | Strong, bulky base | Weak base |
| Solvent | Polar aprotic | Polar protic | Any (typically with strong base) | Polar protic |
2. The SN2 Mechanism
SN2 (Substitution, Nucleophilic, 2nd order) is a concerted, one-step reaction in which the nucleophile attacks the electrophilic carbon from the side directly opposite the leaving group (backside attack), while the leaving group departs simultaneously.
2.1 Key Features
- Rate law: Rate = k[Nucleophile][Substrate] — bimolecular, second order overall.
- Stereochemistry: Backside attack causes inversion of configuration at the stereocenter (Walden inversion) — like an umbrella flipping inside out in the wind.
- Steric effects dominate: SN2 works best on methyl and primary substrates; tertiary substrates essentially never undergo SN2 because the bulky groups block backside attack.
- Favored by: strong, unhindered nucleophiles (e.g., OH⁻, CN⁻, OCH₃⁻, I⁻) and polar aprotic solvents (e.g., acetone, DMSO, DMF), which do not hydrogen-bond to and “cage” the nucleophile.
Worked Example: Predict the product and stereochemistry of (R)-2-bromobutane reacting with NaCN in acetone.
CN⁻ is a strong nucleophile, the solvent is polar aprotic, and 2-bromobutane is a secondary substrate that can still undergo SN2. Backside attack causes inversion, so the (R) starting material yields the (S)-2-methylbutanenitrile product (the actual priority-based descriptor depends on substituents, but the key answer is that configuration inverts).
3. The SN1 Mechanism
SN1 (Substitution, Nucleophilic, 1st order) proceeds through a two-step mechanism: the leaving group departs first, forming a planar carbocation intermediate, and then the nucleophile attacks from either face.
3.1 Key Features
- Rate law: Rate = k[Substrate] — the nucleophile does not appear because it is not involved in the rate-determining step (ionization), directly connecting to the rate-determining step concept from chemical kinetics.
- Stereochemistry: Because the carbocation intermediate is planar (sp² hybridized), the nucleophile can attack from either face, giving a racemic mixture (partial or complete racemization) rather than clean inversion.
- Carbocation stability governs rate: stability order is 3° > 2° > 1° > methyl, due to hyperconjugation and inductive electron donation from alkyl groups; SN1 essentially never occurs at primary carbons.
- Favored by: weak nucleophiles (often the solvent itself, e.g., water or an alcohol acting as nucleophile in solvolysis), and polar protic solvents (e.g., water, ethanol), which stabilize the carbocation intermediate and the leaving group through solvation.
- Rearrangements are possible: if a hydride or alkyl shift produces a more stable carbocation, rearrangement will occur before the nucleophile attacks — a very common source of “surprise” products in assignments.
Worked Example: Predict the product(s) of 2-bromo-2-methylbutane reacting with methanol (solvolysis).
The substrate is tertiary, favoring SN1. The C–Br bond breaks heterolytically to form a tertiary carbocation. Methanol (weak nucleophile, polar protic solvent) attacks the planar carbocation from either face, giving a racemic mixture of the methyl ether product. No rearrangement occurs here since the carbocation is already tertiary (already maximally stable).
Worked Example — Rearrangement: 3-bromo-2,2-dimethylbutane undergoes solvolysis in ethanol. The initial secondary carbocation is adjacent to a carbon bearing two methyl groups, so a methyl shift occurs, converting the secondary carbocation into a more stable tertiary carbocation before the nucleophile attacks — producing a rearranged product that a student who forgets to check for rearrangement would miss entirely.
4. The E2 Mechanism
E2 (Elimination, 2nd order/bimolecular) is a concerted, one-step reaction in which a strong base removes a β-hydrogen while the leaving group departs simultaneously, forming a double bond.
4.1 Key Features
- Rate law: Rate = k[Base][Substrate] — bimolecular, second order overall.
- Stereochemistry requirement: requires anti-periplanar geometry — the β-hydrogen being removed and the leaving group must be in the same plane, on opposite sides (180° dihedral angle), which is why E2 reactions are highly stereospecific.
- Favored by: strong, often bulky bases (e.g., tert-butoxide, (CH₃)₃CO⁻), which favor elimination over substitution because their bulk hinders SN2 backside attack.
- Zaitsev’s rule: the more substituted (more stable) alkene is usually the major product, unless a bulky base is used, in which case the less hindered (Hofmann) product often predominates due to steric constraints during the anti-periplanar transition state.
Worked Example: Predict the major product of 2-bromobutane reacting with sodium ethoxide (NaOEt, a moderately strong, non-bulky base).
E2 elimination occurs, removing a β-hydrogen anti-periplanar to bromine. Following Zaitsev’s rule, the major product is the more substituted alkene, 2-butene (predominantly the more stable E-isomer), over the less substituted 1-butene.
Worked Example — Bulky Base: 2-bromo-2-methylbutane reacting with potassium tert-butoxide (bulky base) gives predominantly the Hofmann product, the less substituted alkene, because the bulky base preferentially removes the more accessible (less hindered) hydrogen.
5. The E1 Mechanism
E1 (Elimination, 1st order/unimolecular) proceeds through the same carbocation intermediate as SN1; after the carbocation forms, a base removes a β-hydrogen to form the alkene, rather than a nucleophile attacking the cationic carbon.
5.1 Key Features
- Rate law: Rate = k[Substrate] — unimolecular, matching SN1’s rate-determining ionization step.
- Zaitsev’s rule applies: since there is no anti-periplanar geometric constraint (the carbocation can rotate freely before deprotonation), the more stable, more substituted alkene is favored.
- Always competes with SN1: because both reactions share the same carbocation intermediate, SN1 and E1 products typically form together as a mixture, with the ratio depending on temperature (higher temperature favors elimination, since E1 has a higher entropy of activation) and the specific nucleophile/base strength.
- Rearrangements are possible, just as in SN1, since both proceed through the same carbocation.
Worked Example: Heating 2-bromo-2-methylbutane in ethanol produces a mixture of SN1 substitution product (ether) and E1 elimination product (alkene, favoring the Zaitsev product 2-methyl-2-butene over the Hofmann product 2-methyl-1-butene).
6. A Systematic Decision Framework for Assignments
When an assignment gives you a substrate, nucleophile/base, and solvent, work through these questions in order:
- What is the substrate class? Methyl/1° → SN2 (or E2 with a strong/bulky base); 2° → could be SN2, SN1, E2, or E1 depending on conditions; 3° → SN1/E1 only (SN2/E2 sterically blocked, except E2 with very strong, small bases in some cases).
- Is the nucleophile/base strong or weak? Strong nucleophile/base → favors bimolecular pathways (SN2/E2). Weak nucleophile/base (often just the solvent) → favors unimolecular pathways (SN1/E1).
- Is the nucleophile/base bulky? Bulky bases favor elimination (E2) over substitution (SN2) because they struggle to perform backside attack but can still abstract an accessible proton.
- What is the solvent? Polar aprotic → favors SN2 (nucleophile is “naked” and highly reactive). Polar protic → favors SN1/E1 (stabilizes cationic intermediate and leaving group via solvation).
- What is the temperature? Higher temperature favors elimination (E1/E2) over substitution, because elimination has a more favorable (more positive) entropy of activation, connecting back to the ΔG = ΔH − TΔS relationship in chemical thermodynamics.
7. Worked Comparative Problem
Question: For each scenario, identify the most likely mechanism and major product.
(a) 1-bromopropane + NaOH in ethanol (high concentration, strong nucleophile/base, primary substrate) Answer: SN2 dominates because the substrate is primary (backside attack unhindered) and OH⁻ is a strong nucleophile; product is 1-propanol.
(b) 2-bromo-2-methylpropane (tert-butyl bromide) + H₂O (weak nucleophile, polar protic solvent, tertiary substrate) Answer: SN1/E1 mixture — tertiary substrate cannot undergo SN2/E2 efficiently, and water is a weak nucleophile favoring the carbocation pathway; expect a mixture of tert-butanol (SN1) and isobutylene (E1).
(c) 2-bromobutane + potassium tert-butoxide (bulky, strong base) Answer: E2, favoring the Hofmann (less substituted) product, 1-butene, due to the base’s steric bulk.
8. Common Assignment Pitfalls
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- Forgetting that tertiary substrates cannot undergo SN2 due to steric hindrance — a very common error.
- Assuming SN1 always gives complete racemization — in practice, there is often a slight preference for inversion because the leaving group can partially shield one face of the carbocation, though full racemization is the standard textbook assumption.
- Forgetting to check for carbocation rearrangements in SN1/E1 problems whenever a more stable carbocation is accessible via a 1,2-hydride or alkyl shift.
- Confusing “strong nucleophile” with “strong base” — many species (like ethoxide) are both, but bulkiness specifically favors elimination regardless of base strength.
- Forgetting the anti-periplanar requirement for E2, which explains why certain stereoisomers of cyclic substrates cannot undergo E2 easily (e.g., in some substituted cyclohexanes).
Mastering these four mechanisms requires connecting concepts from chemical bonding (hybridization of the carbocation, orbital overlap in backside attack) and chemical kinetics (rate laws, rate-determining steps). Practice classifying at least 15–20 different substrate/nucleophile/solvent combinations using the decision framework in Section 6 — this systematic approach is what separates confident answers from guesswork on organic chemistry exams.
