Electrophilic vs Nucleophilic Substitution & Elimination (SN1, SN2, E1, E2)
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1. Quick Comparison Matrix
| Parameter | $S_N1$ | $S_N2$ | $E1$ | $E2$ |
|---|---|---|---|---|
| Kinetics | First Order: $k[\text{R-X}]$ | Second Order: $k[\text{R-X}][\text{Nu}^-]$ | First Order: $k[\text{R-X}]$ | Second Order: $k[\text{R-X}][\text{Base}]$ |
| Substrate Preference | $3^\circ > 2^\circ \gg 1^\circ$ (Carbocation stability) | $\text{CH}_3 > 1^\circ > 2^\circ \gg 3^\circ$ (Steric hindrance) | $3^\circ > 2^\circ \gg 1^\circ$ | $3^\circ > 2^\circ > 1^\circ$ |
| Nucleophile / Base | Weak nucleophile (e.g., $\text{H}_2\text{O}, \text{ROH}$) | Strong nucleophile (e.g., $\text{CN}^-, \text{I}^-, \text{RS}^-$) | Weak base + heat | Strong hindered base (e.g., $t\text{-BuO}^-$) |
| Solvent | Polar Protic (stabilizes ions) | Polar Aprotic (e.g., DMSO, Acetone) | Polar Protic | Polar Aprotic or Protic |
| Stereochemistry | Racemization (inversion + retention) | 100% Inversion of configuration (Walden) | Zaitsev / Hofmann mix | Anti-periplanar geometry required |
2. Key Regiochemical Rules
Zaitsev's Rule (Saytzeff)
In elimination reactions, the more substituted (more stable) alkene is the major product when using a small, unhindered base (e.g., $\text{NaOCH}_3$).
Hofmann Rule
When a bulky, sterically hindered base like potassium tert-butoxide ($t\text{-BuOK}$) is used, proton abstraction occurs at the least hindered carbon, giving the less substituted alkene as the major product.
3. High-Yield Exam Tips
- Primary Alkyl Halides ($1^\circ$): Almost always undergo $S_N2$ with good nucleophiles or $E2$ with strong, bulky bases. Never $S_N1$ or $E1$ under normal conditions due to unstable primary carbocations.
- Tertiary Alkyl Halides ($3^\circ$): Never undergo $S_N2$ due to extreme steric congestion. If a strong base is present, $E2$ dominates. With weak nucleophiles/bases and heat, a mixture of $S_N1$ and $E1$ occurs.