Reaction Mechanisms — Nucleophilic Substitution and Elimination
Understanding reaction mechanisms allows you to predict products, explain reactivity patterns, and design synthetic routes — essential skills for pharmaceutical chemistry. In the SAPC exam, SN1, SN2, E1, and E2 mechanisms are consistently tested, often through analysis of reaction conditions and substrate structure.
Bond Cleavage and Formation
Heterolytic Cleavage: Bond breaks unevenly — one atom receives both electrons → carbocation (electron-deficient, electrophile) and carbanion (electron-rich, nucleophile).
Homolytic Cleavage: Bond breaks evenly — each atom receives one electron → two free radicals (important in polymerization and halogenation).
Nucleophilic Substitution Reactions
SN2 Mechanism — Bimolecular Nucleophilic Substitution
Mechanism: Concerted, single step. The nucleophile attacks from the back side (180° from the leaving group), pushing the leaving group off the front — like an umbrella turning inside out in the wind.
Rate Law: Rate = k[Substrate][Nucleophile] — bimolecular
Characteristics:
- Inversion of configuration at the chiral centre (Walden inversion)
- Backside attack requires less steric hindrance — better with less substituted carbon
- Strong nucleophile required in polar aprotic solvent (acetone, DMSO, DMF)
- No intermediate — single transition state
- Steric hindrance dramatically slows the reaction
Favourable conditions: Methyl > 1° > 2° alkyl halides. 3° almost never undergoes SN2.
Good nucleophiles (strong, anionic): OH⁻, CN⁻, RO⁻, N₃⁻, I⁻, RS⁻
SN1 Mechanism — Unimolecular Nucleophilic Substitution
Mechanism: Two steps. Step 1 — slow ionization to form a carbocation intermediate. Step 2 — rapid nucleophilic attack on the carbocation.
Rate Law: Rate = k[Substrate] only — unimolecular
Characteristics:
- Racemization occurs at the chiral centre (if applicable) because the planar carbocation can be attacked from either face
- Rate depends only on substrate — carbocation stability is key
- More substituted carbocations are more stable (tertiary > secondary > primary > methyl)
- Polar protic solvent stabilizes the carbocation intermediate and the leaving group anion
- Rearrangements possible (hydride shift, methyl shift) when a more stable carbocation can form
Favourable conditions: Tertiary and secondary substrates. Polar protic solvents (water, alcohols). Weak nucleophiles (H₂O, ROH).
Carbocation Stability
| Type | Structure | Stability |
|---|---|---|
| Methyl | CH₃⁺ | Least stable |
| Primary (1°) | RCH₂⁺ | Unstable |
| Secondary (2°) | R₂CH⁺ | Moderate |
| Tertiary (3°) | R₃C⁺ | Most stable |
| Allylic | R-CH=CH-CH₂⁺ | Stabilized by resonance |
| Benzyllic | C₆H₅-CH₂⁺ | Very stable (resonance) |
Resonance-stabilized carbocations (allylic and benzylic) can form more readily than their non-resonance-stabilized counterparts.
Competition Between SN1 and SN2
| Factor | Favours SN2 | Favours SN1 |
|---|---|---|
| Substrate | Methyl, 1° | 2°, 3° |
| Nucleophile | Strong (anionic) | Weak |
| Solvent | Polar aprotic | Polar protic |
| Leaving group | Good (I⁻ > Br⁻ > Cl⁻ > F⁻) | Good |
| Steric hindrance | Low | High |
| Carbocation stability | Not relevant | Critical |
Elimination Reactions
E2 Mechanism — Bimolecular Elimination
Mechanism: Concerted, single step. A base removes a proton (β-hydrogen) while the leaving group departs simultaneously, forming a double bond.
Requirements:
- Anti-periplanar geometry — the hydrogen and leaving group must be 180° apart (trans-diaxial in cyclohexane)
- Strong base (or strong nucleophile acting as base)
- Good leaving group
Rate: Rate = k[Substrate][Base] — bimolecular
Zaitsev’s Rule: The more substituted (more stable) alkene is the major product. Exception: bulky bases (t-BuO⁻, DBU) give the Hofmann product (less substituted alkene) due to steric hindrance.
E1 Mechanism — Unimolecular Elimination
Mechanism: Same first step as SN1 — ionization to carbocation. Step 2 — base removes a β-hydrogen from the carbocation, forming a double bond.
Rate: Rate = k[Substrate] — unimolecular
Characteristics:
- Same conditions and substrate requirements as SN1
- Carbocation intermediate → rearrangements possible
- Competes directly with SN1 — polar protic solvent, weak base
- Regioselectivity follows Zaitsev’s Rule
Substitution vs. Elimination Competition
| Condition | Favours Substitution | Favours Elimination |
|---|---|---|
| Substrate | Methyl, 1° (SN2) | 3° (E2) |
| Base/nucleophile strength | Weak nucleophile | Strong base (especially bulky) |
| Solvent | Polar aprotic (SN2) | Polar protic (E1) |
| Temperature | Lower | Higher (endothermic, ΔS positive) |
| Steric hindrance | Low | High |
Leaving Group Ability
Better leaving groups = more stable anions when departed:
Good to Poor: I⁻ > Br⁻ > Cl⁻ >> F⁻
Tosylate (TsO⁻) and mesylate (MsO⁻) are excellent leaving groups — frequently used in pharmaceutical synthesis.
Pharmaceutical Chemistry Connections
- Drug metabolism Phase I reactions often involve nucleophilic attack on drug metabolites (e.g., cytochrome P450 oxidation creates electrophilic intermediates that react with nucleophiles in proteins/DNA)
- SN1-like reactions occur in the formation of oxonium ions during glycoside hydrolysis
- E2 reactions are used in the laboratory synthesis of pharmaceutical intermediates — understanding anti-periplanar requirements explains why some elimination reactions fail
SAPC Examination Tips
- Always check substrate first — 3° → E1/SN1; methyl/1° → SN2; 2° is the battleground (analyze conditions)
- Temperature matters — high temperature favours elimination over substitution (more positive entropy change for elimination)
- Drawing the mechanism matters — show arrows correctly: nucleophile → substrate, leaving group ← departing. Never draw arrows crossing atoms.
- Zaitsev vs. Hofmann — unless a bulky base is specified, draw the more substituted alkene as the major product.
- Anti-periplanar geometry — in cyclohexane E2, the H and leaving group must both be axial and trans to each other.