Theory — SN1, SN2, E1, E2
The Four Mechanisms
Alkyl halides (R–X) react with nucleophiles and bases by four canonical mechanisms that differ in the number of steps, the order of the rate law, and the stereochemistry. Which one dominates depends on the substrate, the nucleophile or base, the solvent, and the temperature.
| Mechanism | Steps | Rate law | Stereo | Regio | Key feature |
|---|---|---|---|---|---|
| SN2 | 1 concerted | k[RX][Nu] | Inversion (Walden) | — | Backside attack; 2 arrows |
| SN1 | 2 (ionise → trap) | k[RX] | Racemisation | — | Planar carbocation intermediate |
| E2 | 1 concerted | k[RX][B] | Anti-periplanar | Zaitsev (Hofmann with bulky base) | 3 arrows in one step |
| E1 | 2 (ionise → β-H loss) | k[RX] | — | Zaitsev | Shares step 1 with SN1 |
Curved Arrows — The Language of Mechanism
A curved arrow represents the movement of two electrons. The tail of the arrow starts where the electrons come from — a lone pair on an electron-rich atom, or a bond that is about to break. The head of the arrow ends where the electrons go — onto an electron-poor atom (forming a new bond or neutralising a charge) or into a new bond forming between two atoms.
Head TO: an atom (form a new bond / accept charge) or another bond (form a π-bond)
Mechanism 1 — SN2 (concerted, 2 arrows)
The nucleophile attacks carbon from the back side of the leaving group. In a single concerted transition state, the Nu–C bond forms while the C–X bond breaks.
Arrow 2: C–X bond → X (X takes the bonding electrons as it leaves)
Mechanism 2 — SN1 (stepwise, 2 arrows total)
Step 1 is the slow heterolysis of the C–X bond to give a carbocation and the leaving group. Step 2 is the fast attack of the nucleophile on the planar cation.
SN1 — step 2 (fast) Arrow: Nu: lone pair → C⁺ (Nu captures the cation)
Mechanism 3 — E2 (concerted, 3 arrows)
In a single concerted step, a base removes a β-hydrogen while the leaving group departs. The C–H bond electrons form the new C=C π-bond.
Arrow 2: Cβ–H bond → Cα–Cβ bond (forms the π bond)
Arrow 3: Cα–X bond → X
Mechanism 4 — E1 (stepwise, 3 arrows total)
Step 1 is identical to SN1 (ionisation to a carbocation). In step 2, a base — often the solvent — removes a β-H and the electrons form the new C=C π-bond.
E1 — step 2 (fast) Arrow 1: B: lone pair → β-H
Arrow 2: Cβ–H bond → Cα⁺ (forms π bond, neutralises cation)
Choosing the Mechanism — Four Factors
Substrate
Methyl: SN2 only.
1°: SN2 (E2 with bulky base).
2°: any — depends on other factors.
3°: SN1, E1, or E2. Never SN2.
Nucleophile / Base
Strong Nu, weak base (I⁻, CN⁻, N₃⁻, RS⁻): SN2.
Strong Nu AND strong base (HO⁻, RO⁻): SN2 (1°), E2 (2°/3°).
Bulky strong base (tBuO⁻, DBU): E2 → Hofmann.
Weak Nu and weak base (H₂O, ROH — solvent only): SN1/E1.
Solvent
Polar aprotic (DMSO, DMF, acetone): favours SN2 — the anion is unsolvated and maximally reactive.
Polar protic (H₂O, ROH): favours SN1/E1 — stabilises cations/anions by H-bonding.
Temperature
Higher temperature favours elimination over substitution (entropy: one molecule → two fragments makes ΔS‡ more positive for elimination). Low T favours substitution.
Instructions — Running the Virtual Experiment
Section I — Classification Sorter
Section II — Interactive Mechanism Builder
Section III — Predict Products
Simulation
Select a mechanism example
Kinetics & rate law
Energy diagram
Stereo / regio outcome
Reaction setup
Major product (pick one)
Team Questions
Example Lab Report
Substitution and Elimination Mechanisms
Chemistry 221 | Section: [Your Section] | Date: [Date]
Lab Members: [Names of all members present]
Purpose
To distinguish between the four canonical mechanisms of alkyl halide reactions (SN1, SN2, E1, E2) from substrate, nucleophile/base, solvent, and temperature; to draw complete curved-arrow mechanisms showing electron flow at each step; to relate the number of steps and rate law to the energy diagram; and to predict major products including stereochemistry and regiochemistry.
Theory
Alkyl halides R–X react with nucleophiles or bases by four mechanisms that differ in molecularity. In SN2 and E2 the rate-determining step is a single concerted transition state; both the substrate and the Nu/base appear in the rate law, giving 2nd-order kinetics. In SN1 and E1, ionisation of the C–X bond to a carbocation is the slow step; the nucleophile/base is involved only in a fast subsequent step, so the rate is 1st order in substrate only. The dominant mechanism is controlled by the substrate (methyl/1°/2°/3°), the nucleophile/base (strong Nu weak base → SN2; strong base → E2; bulky strong base → E2 Hofmann; weak Nu/base → SN1/E1), the solvent (polar aprotic → SN2, polar protic → SN1/E1), and the temperature (heat → elimination).
Curved arrows show the flow of electron pairs. Each arrow starts from a lone pair or a bond (the electron source) and ends at an atom or a bond (the destination). SN2 uses two arrows in one step; SN1 uses two arrows across two steps; E2 uses three arrows in one concerted step; E1 uses three arrows across two steps.
Stereochemistry: SN2 gives inversion at the α-carbon (Walden inversion — the Nu attacks from the back face of the LG). SN1 goes via a planar sp² carbocation that is attacked on either face, giving racemic product. E2 requires the β-H and LG to be anti-periplanar (180° dihedral). Regiochemistry: with small bases, E1 and E2 give the more-substituted (Zaitsev) alkene; with bulky bases (tBuO⁻, DBU), E2 gives the less-substituted (Hofmann) alkene.
Calculations / Analysis
Sample analysis — (R)-2-bromobutane + NaCN in DMSO at 25 °C:
Substrate: 2° (2-BrBu). Nu: CN⁻ (strong Nu, weak base). Solvent: DMSO (polar aprotic). T: RT.
Strong Nu, weak base, polar aprotic → SN2. Concerted backside attack gives inversion: (R) → (S).
Rate law: rate = k [RX][CN⁻] (2nd order). Product: (S)-2-cyanobutane, enantiopure.
Energy diagram: one hump (single TS). No intermediate. Arrows: (1) CN⁻ lone pair → α-C; (2) C–Br bond → Br.
Sample analysis — (CH₃)₃CBr + H₂O:
Substrate: 3°. Nu: water (weak). Solvent: water (polar protic). T: RT.
Tertiary substrate with weak Nu in protic solvent → SN1. Two-step: ionise to (CH₃)₃C⁺ + Br⁻; water traps the cation from either face (racemisation, though not observable on an achiral substrate); deprotonation gives alcohol.
Rate law: rate = k [(CH₃)₃CBr]. Arrows step 1: C–Br → Br. Arrows step 2: water lone pair → C⁺.
Energy diagram: two humps, TS1 > TS2, valley is the carbocation intermediate.
Results Table
Section I — classification of 12 reactions
| Reaction | Substrate | Nu/Base | Solvent | T | Mechanism |
|---|---|---|---|---|---|
| 1-BrBu + NaCN | 1° | Strong Nu | DMSO | RT | SN2 |
| (R)-2-BrBu + N₃⁻ | 2° | Strong Nu | DMF | RT | SN2 |
| MeI + NaSH | methyl | Strong Nu | acetone | RT | SN2 |
| tBuBr + H₂O | 3° | Weak Nu | H₂O | RT | SN1 |
| 3-Br-3-Me-hexane + MeOH | 3° | Weak Nu | MeOH | RT | SN1 |
| 2-Cl-2-Me-propane + EtOH | 3° | Weak Nu | EtOH | RT | SN1 |
| 2-BrBu + NaOEt (hot) | 2° | Strong base | EtOH | 80°C | E2 |
| 1-ClPr + tBuOK | 1° | Bulky base | tBuOH | RT | E2 |
| CyBr + NaOH | 2° | Strong base | EtOH | RT | E2 |
| tBuI + EtOH (hot) | 3° | Weak base | EtOH | 60°C | E1 |
| 2-I-2-Me-butane + AcOH | 3° | Weak base | AcOH | 80°C | E1 |
| 3-Me-3-BrC₅ + H₂O (hot) | 3° | Weak base | H₂O | 70°C | E1 |
Section II — mechanism summary for the 12 built examples
| Mechanism | Steps | Rate law | Total arrows | Stereochemistry |
|---|---|---|---|---|
| SN2 | 1 | k [RX][Nu] | 2 | Inversion |
| SN1 | 2 | k [RX] | 2 | Racemisation |
| E2 | 1 | k [RX][B] | 3 | Anti-periplanar |
| E1 | 2 | k [RX] | 3 | Planar cation, Zaitsev |
Discussion
The sorter in Section I demonstrated how the four factors combine to determine mechanism. Primary substrates with strong non-basic nucleophiles in polar aprotic solvents (1-BrBu + NaCN/DMSO, MeI + NaSH/acetone) are the clearest SN2 cases. Tertiary substrates in polar protic solvents with no strong base (tBuBr + H₂O, 3-Br-3-Me-hexane + MeOH) give SN1. Adding a strong base to a tertiary substrate tips to E2 (tBuBr + NaOEt hot), while a tertiary substrate heated with only solvent acting as base gives E1 (tBuI + EtOH hot). The trickiest case — primary substrate with a bulky base (1-ClPr + tBuOK) — gives E2 instead of SN2 because tBuO⁻ is too large to approach the α-carbon.
Section II's interactive arrow placement showed that all four mechanisms can be described in the same "curved arrow" language, and the number of arrows equals the number of electron-pair movements. SN2 uses 2 arrows in 1 step. SN1 uses 2 arrows in 2 steps. E2 uses 3 arrows in 1 step (base grabs β-H, C-H electrons form π-bond, C-X breaks). E1 uses 3 arrows in 2 steps (first step same as SN1 ionisation, then 2 arrows in step 2 to eliminate).
The kinetics and energy diagram directly follow: concerted mechanisms (SN2, E2) have one TS → 2nd-order kinetics; stepwise (SN1, E1) have two TSs with a carbocation intermediate between them → 1st-order kinetics (because ionisation, the RDS, only involves the substrate). Section III confirmed that once mechanism is known, the major product follows from its stereochemistry and regiochemistry rules.
Conclusion
Classified all 12 reactions correctly using the four-factor analysis (substrate, Nu/base, solvent, T). Built full arrow-pushing mechanisms for 12 examples (3 each for SN2, SN1, E2, E1) showing 2 arrows in 1 step (SN2), 2 arrows in 2 steps (SN1), 3 arrows in 1 step (E2), and 3 arrows in 2 steps (E1). Predicted products for 6 reactions with correct stereo/regio outcomes (inversion for SN2, racemic for SN1, Zaitsev for E2/E1 small base, Hofmann for E2 bulky base). Results consistent with textbook predictions throughout.