Nucleophilic Substitution Reactions
A detailed look at $S_N1$ and $S_N2$ pathways.
By chemca Team • Updated Jan 2026
Nucleophilic Substitution involves the replacement of a leaving group ($L$) on an $sp^3$ hybridized carbon by a nucleophile ($Nu^-$). The two primary mechanisms are $S_N2$ (Substitution Nucleophilic Bimolecular) and $S_N1$ (Substitution Nucleophilic Unimolecular).
1. $S_N2$ Mechanism (Bimolecular)
Concerted Process
This is a one-step reaction. The nucleophile attacks the substrate from the side opposite to the leaving group (Backside Attack), leading to a pentavalent Transition State (TS).
$$ Nu^- + C-L \rightarrow [Nu \cdots C \cdots L]^\ddagger \rightarrow Nu-C + L^- $$
Characteristics:
- Kinetics: Second Order. Rate $= k[RX][Nu]$.
- Stereochemistry: Complete Inversion of Configuration (Walden Inversion). Imagine an umbrella turning inside out.
- Reactivity Order: $CH_3X > 1^\circ > 2^\circ > 3^\circ$.
Reason for Reactivity Order: Steric Hindrance. Bulky groups hinder the backside attack of the nucleophile. Tertiary halides are too crowded for $S_N2$.
2. $S_N1$ Mechanism (Unimolecular)
Two-Step Process
This reaction occurs in two steps. The first step (ionization) determines the rate.
Step 1 (Slow/RDS): Formation of Carbocation.
$$ R-L \rightleftharpoons R^+ + L^- $$
Step 2 (Fast): Attack of Nucleophile on Carbocation.
$$ R^+ + Nu^- \rightarrow R-Nu $$
Characteristics:
- Kinetics: First Order. Rate $= k[RX]$. Independent of nucleophile concentration.
- Stereochemistry: Racemization (Retention + Inversion) because the carbocation is planar ($sp^2$), allowing attack from both sides.
- Reactivity Order: $3^\circ > 2^\circ > 1^\circ > CH_3X$.
Stability Factor: The rate depends on the stability of the carbocation intermediate. Benzyl and Allyl halides also react fast via $S_N1$ due to resonance stabilization of the cation.
3. Factors Affecting Substitution
A. Nature of Leaving Group:
Better leaving groups increase the rate of both $S_N1$ and $S_N2$.
Order: $I^- > Br^- > Cl^- > F^-$ (Weaker bases are better leaving groups).
Better leaving groups increase the rate of both $S_N1$ and $S_N2$.
Order: $I^- > Br^- > Cl^- > F^-$ (Weaker bases are better leaving groups).
B. Solvent Effect:
- Polar Protic Solvents (Water, Alcohol): Favor $S_N1$. They stabilize the carbocation and leaving group through solvation (H-bonding).
- Polar Aprotic Solvents (Acetone, DMSO, DMF): Favor $S_N2$. They solvate cations well but leave anions (nucleophiles) "naked" and highly reactive.
C. Nucleophile Strength:
- Strong nucleophiles favor $S_N2$.
- Weak nucleophiles (often the solvent itself) favor $S_N1$.
4. $S_N1$ vs $S_N2$ Comparison
| Feature | $S_N2$ | $S_N1$ |
|---|---|---|
| Steps | One (Concerted) | Two (Ionization + Attack) |
| Rate Law | $k[RX][Nu]$ | $k[RX]$ |
| Intermediate | Transition State | Carbocation |
| Stereochemistry | Inversion | Racemization |
| Reactivity | $1^\circ > 2^\circ > 3^\circ$ | $3^\circ > 2^\circ > 1^\circ$ |
| Rearrangement | No | Possible |
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