Nucleophilic substitution is one of the most important reaction types in IB Chemistry. It explains how molecules exchange functional groups through attack by electron-rich species. These reactions form the basis of many organic transformations, including preparing alcohols, halogenoalkanes, and amines. Understanding nucleophilic substitution is essential for exam mechanisms, reaction prediction, and arrow-pushing skills.
What Is Nucleophilic Substitution?
Nucleophilic substitution is a reaction where a nucleophile replaces a leaving group in a molecule.
Breaking it down:
- Nucleophile = electron-rich species that donates a lone pair (e.g., OH⁻, CN⁻, NH₃).
- Leaving group = atom or group that departs with electrons (commonly a halide: Cl⁻, Br⁻, I⁻).
- Substitution = one group replaces another.
This reaction occurs most commonly in halogenoalkanes because the carbon–halogen bond is polar, making carbon electrophilic and vulnerable to attack.
Why Nucleophilic Substitution Happens
The carbon atom bonded to a halogen carries a partial positive charge due to electronegativity differences:
C–δ⁺ — X–δ⁻
The nucleophile is attracted to this electron-poor carbon and attacks it.
The halogen (X⁻) acts as the leaving group and departs with the bonding electrons.
This electron flow is the foundation of the mechanism.
Two Types of Nucleophilic Substitution: SN1 and SN2
IB Chemistry requires you to know both mechanisms, their conditions, and how structure affects them.
SN2 Mechanism (Bimolecular Nucleophilic Substitution)
Key features:
- Occurs in primary halogenoalkanes
