Energy Profiles: SN1 vs SN2
Mapping the journey from reactants to products through Transition States and Intermediates.
In organic chemistry, a Potential Energy vs. Reaction Coordinate graph provides a visual "map" of a reaction's mechanism. By observing the number of peaks and valleys, we can instantly determine if a reaction is concerted (one-step) or stepwise, making these graphs vital for distinguishing between SN2 and SN1 mechanisms.
1 The SN2 Mechanism (Concerted)
The SN2 (Substitution Nucleophilic Bimolecular) reaction is a concerted, one-step process. The nucleophile attacks the electrophilic carbon exactly as the leaving group departs. There are no intermediate molecules formed.
- Graph Shape: Features exactly one peak.
- Transition State (TS): The peak represents the highly unstable pentacoordinate transition state where bonds are half-formed and half-broken. It cannot be isolated.
- Activation Energy ($E_a$): The vertical distance from the reactants to the transition state peak.
SN2 Energy Profile
A single, smooth hurdle (one transition state).
2 The SN1 Mechanism (Stepwise)
The SN1 (Substitution Nucleophilic Unimolecular) reaction is a two-step process. First, the leaving group departs to form a Carbocation Intermediate. Second, the nucleophile attacks the carbocation.
- Graph Shape: Features two peaks separated by a valley.
- The Valley (Intermediate): Represents the Carbocation. It has a finite lifetime and sits at a local energy minimum.
- The Peaks (TS1 & TS2): The first peak ($E_{a1}$) is for the breaking of the C-LG bond. The second peak ($E_{a2}$) is for the formation of the C-Nu bond.
- Rate-Determining Step (RDS): Forming the unstable carbocation requires massive energy, making Step 1 the slow step. Therefore, Peak 1 is always taller than Peak 2.
SN1 Energy Profile
Two hurdles separated by an intermediate valley.
Quick Comparison Summary
| Feature | SN2 Reaction | SN1 Reaction |
|---|---|---|
| Mechanism Steps | Concerted (One step) | Stepwise (Two steps) |
| Graph Shape (Peaks) | 1 Peak | 2 Peaks (1 Valley) |
| Intermediate? | None | Yes (Carbocation) |
| Rate Determining Step | The only step | Step 1 (Formation of C⁺) |
| Rate Law | Rate = $k[\text{Sub}][\text{Nu}]$ | Rate = $k[\text{Sub}]$ |
Knowledge Check
10 Practice MCQs on Energy Profiles
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