Leaving Groups and Substitution Rates HL Only
Quick Notes:
- The rate of nucleophilic substitution depends on the quality of the leaving group.
- Better leaving groups = faster substitution.
- Halide ions (Cl⁻, Br⁻, I⁻) vary in effectiveness as leaving groups.
- General trend: I⁻ > Br⁻ > Cl⁻
- Iodide ion (I⁻) is a better leaving group than chloride (Cl⁻) because:
- It is larger, more polarizable, and more stable after leaving.
- Solvent effects and mechanism types (SN1 vs SN2) are not assessed here.
Full Notes:
Role of the Leaving Group
In nucleophilic substitution, the leaving group breaks away as the nucleophile forms a new bond.
Good leaving groups:
- Form stable anions (weak bases)
- Are large and polarizable
- Stabilize negative charge after departure
Comparison of Common Halogenoalkanes
| Halogen | C–X Bond Strength | Stability of Leaving Group (X⁻) | Substitution Rate |
|---|---|---|---|
| Cl | Strongest | Least stable | Slowest |
| Br | Intermediate | Moderately stable | Moderate |
| I | Weakest | Most stable | Fastest |
Iodine forms the weakest C–X bond due to its large size and longer bond length, making it easier to break. The I⁻ ion is also more stable in solution than Cl⁻ or Br⁻, making it the best leaving group among common halogens.
This trend explains why iodoalkanes react faster than chloro- or bromoalkanes in both SN1 and SN2 mechanisms.
Summary
- The identity of the halogen (X) in R–X significantly affects substitution rate.
- Iodide is a better leaving group than bromide or chloride.
- Iodide leaves more readily because of lower bond strength and greater charge stabilization.
- Substitution rate trend: RI > RBr > RCl.
Linked Course Question
Why is the iodide ion a better leaving group than the chloride ion?
The iodide ion (I⁻) is larger and more polarizable than the chloride ion (Cl⁻), which allows it to better stabilise the negative charge after leaving. I⁻ is also a weaker base than Cl⁻, making it less likely to re-attack the substrate and more stable in solution. This stability makes I⁻ a much better leaving group than Cl⁻ in substitution reactions.