Nucleophilic Substitution Principles
In a nucleophilic substitution reaction, a nucleophile donates an electron pair to form a new bond with an electron-deficient carbon, while a leaving group bond breaks heterolytically:
\[\text{Nu:}^- + \text{R-X} \rightarrow \text{R-Nu} + \text{X}^-\]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.
The Role of Each Species
The Three Key Components
Nucleophile (Nu:)
Donates lone pair to \(\delta+\) carbon to initiate reaction.
Substrate (R-X)
Contains the polarised \(\text{C}^{\delta+}-\text{X}^{\delta-}\) bond under attack.
Leaving Group (X⁻)
Halide ion departs with the bonding electron 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 |
SN1 and SN2 Mechanism Scope
Detailed mechanisms for nucleophilic substitution (\(\text{S}_\text{N}1\) and \(\text{S}_\text{N}2\)), stereochemical Walden inversion vs racemisation, and rate laws are covered in R3.4.9 and R3.4.10.
Carbon-Halogen Bond Polarity
The nucleophile always attacks the carbon bonded to the halogen because the difference in electronegativity (\(\Delta\chi\)) polarises the bond (\(\text{C}^{\delta+}-\text{X}^{\delta-}\)), making the carbon electron-deficient.
Curly Arrow Drawing Rules
- Two curly arrows required: Arrow 1 (lone pair → \(\text{C}^{\delta+}\)), Arrow 2 (\(\text{C}-\text{X}\) bond → \(\text{X}\)).
- Arrow origin: Arrow 2 must start from the centre of the bond, not from the carbon atom.
- Product charges: Ensure charge balance (neutral reactants → neutral organic product + \(\text{X}^-\)).
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