IB Chemistry R3.4.9 SN1 and SN2 Mechanisms Notes

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Nucleophilic Substitution Reactions: SN1 and SN2 Mechanisms

1. The reaction can proceed via two distinct mechanisms: SN2 (Substitution, Nucleophilic, Bimolecular) or SN1 (Substitution, Nucleophilic, Unimolecular).
2. The choice of mechanism depends on factors such as the type of halogenoalkane (primary, secondary, or tertiary) and the reaction conditions.

SN2 Mechanism: One-Step Reaction for Primary Halogenoalkanes

How It Works

1. The SN2 mechanism is a one-step, concerted process.
2. Here, the nucleophile attacks the electrophilic carbon atom directly from the opposite side of the leaving group.
3. This simultaneous attack and departure create a transition state, where the carbon is partially bonded to both the nucleophile and the leaving group.

Key Features

1. Bimolecular Reaction:
   1. The rate-determining step involves both the nucleophile and the halogenoalkane.
   2. The rate equation is: $$\text{rate} = k[\text{halogenoalkane}][\text{nucleophile}]$$
2. Transition State:
   1. A high-energy, unstable arrangement forms, where the nucleophile and leaving group are partially bonded to the same carbon atom.
3. Inversion of Configuration:
   1. The nucleophile attacks at 180° to the leaving group, flipping the molecule’s stereochemistry (like an umbrella turning inside out).

SN1 Mechanism: Two-Step Reaction for Tertiary Halogenoalkanes

How It Works