Question

Write the mechanism of SN1 and SN2 reactions.

Hint:

The key difference between SN1 and SN2 reactions lies in the mechanism: SN1 forms a carbocation intermediate, while SN2 occurs in a single, concerted step.

Answer 1

Step 1: SN1 Mechanism.
The SN1 (Substitution Nucleophilic Unimolecular) reaction involves two main steps: 1. Formation of carbocation: The leaving group (e.g., halide ion) departs from the carbon atom, forming a positively charged carbocation. 2. Nucleophilic attack: A nucleophile (e.g., water or alcohol) attacks the carbocation, leading to the substitution product. The rate-determining step is the formation of the carbocation, which is why SN1 reactions are typically favored in polar protic solvents. \[ \text{R-Cl} \xrightarrow{\text{polar solvent}} \text{R}^+ + \text{Cl}^- \text{and then} \text{R}^+ + \text{Nu}^- \rightarrow \text{R-Nu} \]

Step 2: SN2 Mechanism.
The SN2 (Substitution Nucleophilic Bimolecular) reaction occurs in a single step. The nucleophile directly attacks the electrophilic carbon while the leaving group departs simultaneously. This leads to the inversion of configuration at the carbon center. The rate-determining step involves both the nucleophile and the leaving group, so the rate of the reaction is affected by the concentration of both reactants. \[ \text{R-Cl} + \text{Nu}^- \rightarrow \text{R-Nu} + \text{Cl}^- \]

Step 3: Conclusion.
SN1 involves a two-step mechanism with a carbocation intermediate, whereas SN2 occurs in one step with a simultaneous bond formation and breaking.