Factors Influencing Substitution Rate
The rate of nucleophilic substitution is dictated by two primary factors: carbon-halogen bond enthalpy (leaving group ability) and the nature of the nucleophile.
Effect of the Halogen (Leaving Group)
For halogenoalkanes with the same carbon skeleton, the rate of substitution depends on which halogen is bonded to the carbon.
Bond Enthalpy Trend
| Bond | Bond enthalpy / kJ mol⁻¹ | Rate of substitution |
|---|---|---|
| C-F | 484 (strongest) | Slowest |
| C-Cl | 338 | Slow |
| C-Br | 276 | Moderate |
| C-I | 238 (weakest) | Fastest |
Rate vs Carbon-Halogen Bond Enthalpy
C-X Bond Enthalpy Dominance:
\[\text{C-F } (492\text{ kJ/mol}) > \text{C-Cl } (324\text{ kJ/mol}) > \text{C-Br } (285\text{ kJ/mol}) > \mathbf{\text{C-I } (228\text{ kJ/mol})}\]Because \(\text{C}-\text{I}\) is the weakest bond, it breaks fastest: Iodoalkanes react fastest, while fluoroalkanes are practically inert.
Leaving Group Ability
A good leaving group is a stable anion (a weak base that does not want to re-form the bond). The order of leaving group ability is:
I⁻ is the best leaving group because it is the largest halide ion and can best stabilise the negative charge over its large electron cloud.
Effect of the Nucleophile
Stronger nucleophiles react faster in SN2 reactions. Nucleophile strength depends on:
- Charge: Negatively charged nucleophiles (OH⁻) are stronger than neutral ones (H₂O)
- Electronegativity: Less electronegative atoms hold their lone pair less tightly and donate more readily
The Silver Nitrate Test
The relative rates of halogenoalkane hydrolysis can be investigated experimentally using silver nitrate solution (AgNO₃).
- Equal amounts of 1-chlorobutane, 1-bromobutane, and 1-iodobutane are added to separate test tubes containing ethanol and silver nitrate
- AgNO₃ precipitates the halide ions as they are produced: Ag⁺(aq) + X⁻(aq) → AgX(s)
- The tube with 1-iodobutane forms a precipitate first (yellow AgI), because C-I is the weakest bond
- The tube with 1-chlorobutane forms a precipitate last (white AgCl), because C-Cl is the strongest of the three
Fluoroalkane Stability and Inertness
Fluoroalkanes are extremely unreactive towards nucleophilic substitution because the \(\text{C}-\text{F}\) bond has an exceptionally high bond enthalpy (\(492\text{ kJ mol}^{-1}\)), creating a prohibitively high activation energy.
Bond Enthalpy vs Bond Polarity Pitfall
- Bond enthalpy vs Polarity: Although \(\text{C}-\text{F}\) is the most polar bond, bond enthalpy dominates rate (\(\text{C}-\text{I}\) reacts fastest).
- Leaving group ranking: \(\text{I}^- > \text{Br}^- > \text{Cl}^- > \text{F}^-\) (best to worst).
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