Nucleophilic Substitution Reactions: SN1 and SN2 Mechanisms
What Are Nucleophilic Substitution Reactions?
Definition
Nucleophilic substitution reactions
Nucleophilic substitution reactions occur when a nucleophile (an electron-rich species) attacks an electrophile (an electron-deficient species) and replaces a leaving group.
- The reaction can proceed via two distinct mechanisms: SN2 (Substitution, Nucleophilic, Bimolecular) or SN1 (Substitution, Nucleophilic, Unimolecular).
- 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
- The SN2 mechanismis a one-step, concerted process.
- Here, the nucleophile attacks the electrophilic carbon atom directly from the opposite side of the leaving group.
- 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
- Bimolecular Reaction:
- The rate-determining step involves both the nucleophile and the halogenoalkane.
- The rate equation is: $$\text{rate} = k[\text{halogenoalkane}][\text{nucleophile}]$$
- Transition State:
- A high-energy, unstable arrangement forms, where the nucleophile and leaving group are partially bonded to the same carbon atom.
- Inversion of Configuration:
- The nucleophile attacks at 180° to the leaving group, flipping the molecule’s stereochemistry (like an umbrella turning inside out).
Reaction of Bromoethane with Hydroxide
The reaction between bromoethane ($ \text{CH}_3\text{CH}_2\text{Br} $) and hydroxide ions ($ \text{OH}^- $) is a classic SN2 reaction:
- The hydroxide ion attacks the electron-deficient carbon atom in bromoethane.
- A transition state forms, with partial bonds between the carbon, bromine, and hydroxide.
- Bromine leaves as a bromide ion ($ \text{Br}^- $), and ethanol ($ \text{CH}_3\text{CH}_2\text{OH} $) forms.
- Reaction equation: $$\text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{Br}^-$$
SN2 reactions are favored by strong nucleophiles and polar aprotic solvents (e.g., acetone or DMSO) that do not form hydrogen bonds with the nucleophile.
SN1 Mechanism: Two-Step Reaction for Tertiary Halogenoalkanes
How It Works
- The SN1 mechanism occurs in two steps.
- First, the bond between the carbon atom and the leaving group breaks, forming a carbocation intermediate.
- In the second step, the nucleophile attacks the positively charged carbocation, forming the final product.
Key Features
- Unimolecular Reaction:
- The rate-determining step involves only the halogenoalkane.
- The rate equation is:$$\text{rate} = k[\text{halogenoalkane}]$$
- Carbocation Intermediate:
- A positively charged carbon species forms after the leaving group departs.
- Racemization:
- If the carbon is chiral, the planar structure of the carbocation allows the nucleophile to attack from either side, leading to a racemic mixture of products.
