What Are Reaction Mechanisms?
A reaction mechanism describes the step-by-step sequence of bond-breaking and bond-forming events that converts reactants into products. Mechanisms show the movement of electrons using curved arrows — each arrow represents the movement of an electron pair from nucleophile to electrophile.
Understanding mechanisms is more valuable than memorizing reactions. Once you grasp the logic of nucleophilic substitution and elimination, you can predict products, explain stereochemistry, and design synthetic routes.
This guide focuses on four closely related mechanisms involving haloalkanes (alkyl halides): SN1, SN2, E1, and E2.
Key Terms
Before diving in, ensure you understand these concepts:
- Nucleophile: electron-rich species that donates electrons to form a bond. Examples: OH⁻, CN⁻, I⁻, NH₃, H₂O.
- Electrophile: electron-poor species that accepts electrons. The carbon bonded to the leaving group is electrophilic.
- Leaving group: an atom or group that departs with the bonding electron pair. Good leaving groups are stable anions: I⁻ > Br⁻ > Cl⁻ >> F⁻. Tosylate (OTs⁻) and mesylate (OMs⁻) are excellent leaving groups.
- Carbocation: a carbon with only three bonds and a positive charge. Stability order: tertiary > secondary > primary > methyl.
SN2: Bimolecular Nucleophilic Substitution
Mechanism
In SN2, the nucleophile attacks the electrophilic carbon simultaneously as the leaving group departs — a concerted, one-step process. The reaction is bimolecular: rate depends on both the nucleophile and the substrate.
Rate = k[substrate][nucleophile]
The nucleophile attacks from the backside (180° from the leaving group). As the transition state forms, the carbon undergoes inversion of configuration — like an umbrella turning inside out. This is called Walden inversion.
Stereochemistry
SN2 reactions at a stereocenter give complete inversion of configuration. An (R) starting material gives an (S) product (or vice versa).
Factors Favoring SN2
- Substrate structure: Methyl > primary > secondary >> tertiary. Tertiary substrates cannot undergo SN2 — too much steric hindrance blocks backside attack.
- Strong, unhindered nucleophile: CN⁻, I⁻, RS⁻, HO⁻ in polar aprotic solvents.
- Polar aprotic solvent: DMSO, acetone, acetonitrile. These solvents don't hydrogen-bond to nucleophiles, keeping them reactive. Protic solvents (water, alcohols) solvate and slow nucleophiles.
SN1: Unimolecular Nucleophilic Substitution
Mechanism
SN1 proceeds in two steps: 1. Ionization: The leaving group departs to form a carbocation intermediate. (Slow, rate-determining step) 2. Nucleophilic attack: The nucleophile attacks the planar carbocation.
Rate = k[substrate] (only substrate concentration matters)
Stereochemistry
The carbocation is sp² hybridized and planar. The nucleophile can attack from either face, giving a racemic mixture (equal amounts of both enantiomers) when starting from a single enantiomer. In practice, slight inversion is often observed because the leaving group partially blocks one face.
Factors Favoring SN1
- Substrate structure: Tertiary > secondary >> primary. Tertiary carbocations are most stable due to hyperconjugation and inductive stabilization by alkyl groups.
- Polar protic solvent: Water, alcohols. These solvents stabilize the ionic transition state and carbocation intermediate through solvation.
- Weak or absent nucleophile: SN1 doesn't require a strong nucleophile — water can act as nucleophile (solvolysis).
Carbocation Rearrangements
Carbocation intermediates can rearrange via hydride shifts or methyl shifts to form more stable carbocations. This can lead to unexpected products. For example, a secondary carbocation can rearrange to a tertiary one before nucleophilic attack.
E2: Bimolecular Elimination
Mechanism
E2 is a concerted one-step elimination. A strong base abstracts a proton from the β-carbon (adjacent to the leaving group) while the leaving group departs, forming a double bond.
Rate = k[substrate][base]
Stereochemistry
E2 requires an anti-periplanar arrangement: the H being removed and the leaving group must be on opposite sides of the molecule (180° dihedral angle). This is the anti elimination requirement. In cyclohexane systems, both groups must be axial (diaxial arrangement).
This stereochemical requirement can determine which alkene is formed when multiple β-H atoms are available.
Zaitsev's Rule
When elimination can give two or more alkenes, the major product is usually the more substituted (more stable) alkene — the one with more alkyl groups on the double bond. This is Zaitsev's rule.
However, with a bulky base (e.g., tert-butoxide, (CH₃)₃CO⁻), the less hindered proton is removed preferentially, giving the less substituted alkene (Hofmann product).
E1: Unimolecular Elimination
Mechanism
E1 is a two-step process: 1. Ionization to form a carbocation (same as SN1 step 1) 2. Base removes a proton from the β-carbon, forming the alkene
Rate = k[substrate]
E1 often competes with SN1 since both share the same carbocation intermediate. The product ratio depends on temperature and base concentration: higher temperature and stronger/more concentrated base favor elimination.
Stereochemistry
E1 has no stereoelectronic requirement (unlike E2) because the steps are separate. However, Zaitsev's rule still applies — the more stable alkene predominates.
Comparing the Four Mechanisms
| Feature | SN1 | SN2 | E1 | E2 |
|---|---|---|---|---|
| Steps | 2 | 1 | 2 | 1 |
| Rate law | k[substrate] | k[sub][Nu] | k[substrate] | k[sub][base] |
| Substrate | 3° > 2° | Methyl > 1° > 2° | 3° > 2° | 3° > 2° > 1° |
| Stereochemistry | Racemization | Inversion | Statistical | Anti elimination |
| Intermediate | Carbocation | None | Carbocation | None |
| Solvent | Polar protic | Polar aprotic | Polar protic | Any |
How to Predict the Mechanism
Use this decision tree:
- What is the substrate?
- Methyl or primary → SN2 (or E2 with strong bulky base)
- Secondary → SN2 or E2 (polar aprotic + strong Nu → SN2; strong base → E2)
-
Tertiary → SN1/E1 (polar protic) or E2 (strong base)
-
What is the nucleophile/base?
- Strong nucleophile, weak base (CN⁻, I⁻, RS⁻) → favors substitution
- Strong base (HO⁻, RO⁻, R₂N⁻) → favors elimination, especially E2
-
Weak nucleophile/base (H₂O, ROH) → SN1 or E1
-
What is the solvent?
- Polar protic → SN1/E1
- Polar aprotic → SN2
Real-World Applications
- Pharmaceutical synthesis: SN2 reactions with specific configurations are used to build chiral drug molecules with defined stereochemistry.
- Mustard gas (a chemical weapon) acts through an intramolecular SN1/SN2 reaction, alkylating DNA.
- Elimination reactions are used industrially to make alkenes (ethylene from ethanol via acid-catalyzed E1/E2).
- Enzyme mechanisms: Many enzyme-catalyzed reactions proceed through nucleophilic substitution at carbon or phosphorus centers.