Nucleophilic Substitution Reactions
Quick Notes
- In a nucleophilic substitution reaction, a nucleophile donates an electron pair to a carbon atom bonded to a leaving group.
- A new bond forms as the old bond breaks, removing the leaving group.
- The reaction involves electron pair movement, shown with curly arrows.
- Typical leaving groups include halide ions like Cl−, Br−, I−.
- Common nucleophiles: OH−, CN−, NH3.
- Nucleophilic substitution is common in halogenoalkane reactions.
Full Notes
In a nucleophilic substitution reaction, a nucleophile donates an electron pair to form a new bond, as another bond breaks producing a leaving group.
Overview of the Reaction

The nucleophile attacks the carbon atom that is bonded to the leaving group, forming a new covalent bond and displacing the leaving group.
Mechanism
Nucleophilic substitution can be represented using a curly arrow mechanism, where each full arrow shows the movement of an electron pair.
The arrow starts from the electron source (typically a lone pair or bond) and points to where the electrons go – forming or breaking bonds.
One common such mechanism is the SN2 pathway. This occurs in primary and some secondary halogenoalkanes, shown below.

- Curly arrow from nucleophile to δ+ carbon.
- Curly arrow from C–X bond to halogen (X− leaves).
- New bond forms between nucleophile and carbon.
For Example:Halogenoalkane with OH–

Reagent: Aqueous NaOH/KOH.
Conditions: Warm, reflux.
Summary
- Nucleophilic substitution involves a nucleophile replacing a leaving group.
- Curly arrows show electron pair movement.
- Common in halogenoalkane chemistry.
- Reagents include OH−, CN−, and NH3.