Key Concept
In a nucleophilic substitution reaction, a nucleophile donates an electron pair to form a new bond, while another bond breaks and a leaving group departs with the bonding pair. One group is substituted for another.
What Happens in the Reaction?
A nucleophile attacks an electrophilic carbon (a carbon bonded to an electronegative atom like a halogen). The halogen leaves as a halide ion, taking the bonding electrons with it.
General Equation
Nu: + R-X → R-Nu + X⁻
Where Nu: = nucleophile, R = carbon chain, X = halogen (leaving group)
The Role of Each Species
Nucleophile
Donates its lone pair to the δ+ carbon. Starts the reaction.
Substrate
The halogenoalkane (R-X). Contains the δ+ carbon that is attacked.
Leaving Group
The halide ion (X⁻). Departs with the bonding pair.
Curly Arrows
Curly arrows show the movement of electron pairs during a reaction. In nucleophilic substitution:
- One curly arrow goes from the lone pair on the nucleophile to the δ+ carbon (new bond forming)
- A second curly arrow goes from the C-X bond to the halogen (bond breaking, leaving group departing)
Common Nucleophilic Substitution Reactions
| Nucleophile | Equation | Product |
|---|---|---|
| OH⁻ | R-X + OH⁻ → R-OH + X⁻ | Alcohol |
| CN⁻ | R-X + CN⁻ → R-CN + X⁻ | Nitrile |
| NH₃ | R-X + NH₃ → R-NH₂ + HX | Amine |
| H₂O | R-X + H₂O → R-OH + HX | Alcohol |
Note for HL Students
The detailed mechanisms of nucleophilic substitution (SN1 and SN2, including rate laws and stereochemistry) are covered in R3.4.9. At SL, you only need to understand the general concept and be able to deduce equations with curly arrows showing electron-pair movement.
Think About It
Why does the nucleophile always attack the carbon atom bonded to the halogen, rather than any other carbon in the chain?
The halogen is more electronegative than carbon, so it pulls electron density away from the C atom. This creates a partial positive charge (δ+) on the carbon. The electron-rich nucleophile is attracted to this electron-deficient centre.
Common Exam Mistakes
- Forgetting to show both curly arrows: one for bond formation, one for bond breaking
- Drawing the curly arrow starting from the carbon. It must start from the nucleophile
- Not showing that the leaving group takes the bonding pair with it (it leaves as X⁻, not X)
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