Notes for R3.4.2 Nucleophilic substitution reactions - IB | RevisionDojo
R3.4.2 Nucleophilic substitution reactions Notes
Nucleophilic Substitution Reactions: Mechanisms and Electron Movement
The General Mechanism of Nucleophilic Substitution
At its core, a nucleophilic substitution reaction involves an electron-rich species, called the nucleophile (Nu⁻), donating a pair of electrons to an electron-deficient carbon atom in a substrate molecule, R-X.
This carbon is bonded to a leaving group, X⁻, which gets displaced during the reaction.
The general equation for this reaction is: $$ \text{Nu}^- + \text{R-X} \longrightarrow \text{R-Nu} + \text{X}^- $$
Here’s a step-by-step breakdown of what happens:
Nucleophilic Attack:
The nucleophile uses its lone pair of electrons to form a new covalent bond with the carbon atom in the substrate.
Bond Breaking:
At the same time, the bond between the carbon and the leaving group breaks, transferring the bonding electrons to the leaving group.
Formation of Products:
The reaction produces a new molecule, R-Nu, and the leaving group, X⁻.
Key Players in the Reaction
Nucleophile (Nu⁻):
An electron-rich species, such as OH⁻, CN⁻, or NH₃, that donates a pair of electrons.
Substrate (R-X):
Often a halogenoalkane, where the carbon atom bonded to the halogen is electron-deficient due to the polar nature of the $C–X$ bond.
Leaving Group (X⁻):
A group that departs with the bonding electrons.
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A chemist dissolves bromoethane (CH₃CH₂Br) and hydroxide ions (OH⁻) in a solvent. In which solvent environment will the rate of nucleophilic substitution be fastest, all else being equal?
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Note
Introduction to Nucleophilic Substitution Reactions
A nucleophilic substitution reaction is like a chemical "swap" where one group in a molecule is replaced by another. This process is fundamental in organic chemistry and occurs in many natural and industrial processes.
A nucleophile is an electron-rich species that seeks out positive or electron-deficient areas.
The substrate is the molecule undergoing substitution, typically containing a carbon atom bonded to a leaving group.
The leaving group is the atom or group that departs, taking a pair of electrons with it.
AnalogyThink of a nucleophilic substitution reaction like a dance where one partner (the leaving group) steps out, allowing a new partner (the nucleophile) to step in and take their place.
DefinitionNucleophilic Substitution Reaction: A reaction where a nucleophile replaces a leaving group in a molecule.
ExampleIn the reaction between hydroxide ions (OH⁻) and chloromethane (CH₃Cl), the OH⁻ replaces the Cl⁻, forming methanol (CH₃OH).
