Nucleophilic Substitution Reactions: SN2 Mechanism
Understanding the bimolecular nucleophilic substitution mechanism, stereochemistry, and factors affecting reactivity.
What is Nucleophilic Substitution Reactions: SN2 Mechanism?
An electron-rich species (atom or molecule) that readily donates an electron pair to form a chemical bond.
Key formula / rule: SN2 Reaction Rate Law
Key points
- Describe the SN2 reaction mechanism.
- Explain the factors affecting the rate and feasibility of SN2 reactions.
- Predict the stereochemical outcome of an SN2 reaction.
- Differentiate SN2 reactions from other substitution mechanisms.
Common exam trap
Confusing SN2 with SN1 (unimolecular, two-step, racemization).
Definitions
- Term
Nucleophile
- Meaning
An electron-rich species (atom or molecule) that readily donates an electron pair to form a chemical bond.
- Term
Leaving Group
- Meaning
An atom or group that detaches from a molecule with its bonding electrons during a substitution or elimination reaction.
- Term
Walden Inversion
- Meaning
The inversion of configuration at a stereogenic center during a nucleophilic substitution reaction, characteristic of the SN2 mechanism.
- Term
Polar Aprotic Solvent
- Meaning
A solvent that has a high dielectric constant and is polar but does not have an acidic proton (e.g., DMSO, acetone, DMF).
Learning objectives
Describe the SN2 reaction mechanism.
Explain the factors affecting the rate and feasibility of SN2 reactions.
Predict the stereochemical outcome of an SN2 reaction.
Differentiate SN2 reactions from other substitution mechanisms.
Formulae
- Name
SN2 Reaction Rate Law
- Note
Indicates bimolecular nature, dependent on concentrations of both reactants.
- Expression
Rate = k[Substrate][Nucleophile]
Prerequisites
Understanding of organic functional groups (alkyl halides).
Knowledge of nucleophiles and leaving groups.
Basic concepts of reaction mechanisms (bond breaking/formation).
Understanding of stereochemistry (chirality, inversion).
Common mistakes
Confusing SN2 with SN1 (unimolecular, two-step, racemization).
Assuming SN2 occurs with tertiary substrates.
Not considering the steric hindrance of the substrate.
Incorrectly predicting the stereochemical outcome.
Using polar protic solvents, which hinder SN2 reactions by solvating the nucleophile.
Keywords
SN2
Nucleophilic Substitution
Bimolecular
Concerted Reaction
Backside Attack
Walden Inversion
Stereochemistry
Leaving Group
Nucleophile
Alkyl Halide
Polar Aprotic Solvent
Steric Hindrance
Practice preview
In an SN2 reaction, the nucleophile attacks the carbon atom from:…
easy
The reaction of (S)-2-bromobutane with sodium iodide in acetone is expected to yield:…
medium
Which of the following statements accurately describes the SN2 reaction mechanism?…
easy