The Influence of the Leaving Group on the Rate of Nucleophilic Substitution Reactions
Understanding the Role of the Leaving Group
What is a Leaving Group?
Definition
Leaving group
A leaving group is an atom or group of atoms that detaches during a substitution reaction.
In nucleophilic substitution reactions, the nucleophile donates an electron pair to the electron-deficient carbon atom of the halogenoalkane, while the leaving group departs with the bonding electrons.
Example$$ \text{CH}_3\text{CH}_2\text{Cl} + \text{OH}^- \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{Cl}^- $$
Here, the hydroxide ion ($ \text{OH}^- $) replaces the chlorine atom, and the chloride ion ($ \text{Cl}^- $) becomes the leaving group.
What Makes a Good Leaving Group?
- The effectiveness of a leaving group depends on its ability to stabilize the negative charge it carries after leaving.
- A good leaving group typically:
- Stabilizes the negative charge:
- When the group departs, it takes the bonding electrons, becoming an anion.
- The more stable this anion, the easier it is for the group to leave.
- Forms a weak bond with carbon:
- Weaker bonds break more easily, facilitating faster reactions.
- Stabilizes the negative charge:
- Halide ions (e.g., $ \text{F}^- $, $ \text{Cl}^- $, $ \text{Br}^- $, $ \text{I}^- $) are common leaving groups in halogenoalkanes.
- Their leaving ability increases down Group 17 of the periodic table due to decreasing bond strength and increasing anion stability.
When evaluating leaving groups, prioritize both bond strength and the stability of the anion formed after leaving.
The Rate of Substitution and the Leaving Group
Bond Strength: The Carbon-Halogen Bond
- The rate of nucleophilic substitution reactions is closely tied to the strength of the carbon-halogen bond.
- Bond strength is measured by bond enthalpy, which represents the energy required to break the bond.
The table below shows the bond enthalpies for carbon-halogen bonds:
| Bond | Bond enthalpy ($\text{kJ mol}^{-1}$) |
|---|---|
| C-F | 492 |
| C-Cl | 324 |
| C-Br | 285 |
| C-I | 228 |
- From the table, you can see that the C–F bond is the strongest, while the C–I bond is the weakest.
- This means that fluoroalkanes are highly resistant to nucleophilic substitution, while iodoalkanes react much more quickly.
