Key Concept
The rate of nucleophilic substitution depends on the identity of the halogen (leaving group ability and C-X bond enthalpy) and the identity 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 |
The Key Explanation
Weaker C-X bonds are easier to break, so the reaction proceeds faster. The C-I bond is the weakest because iodine is the largest halogen, with the poorest orbital overlap with carbon. Therefore, iodoalkanes react fastest and fluoroalkanes react slowest.
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⁻ > Br⁻ > Cl⁻ > F⁻
Best leaving group → Worst leaving group
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
Nucleophile Strength Ranking
CN⁻ ~ OH⁻ > NH₃ > H₂O
Strongest → Weakest
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
Think About It
Fluoroalkanes are essentially unreactive towards nucleophilic substitution. Why are fluoroalkanes so stable?
The C-F bond has the highest bond enthalpy of all C-X bonds (484 kJ mol⁻¹) because fluorine is very small and has excellent orbital overlap with carbon. Also, F⁻ is a very poor leaving group because it is a strong base that readily forms bonds.
Common Exam Mistakes
- Saying C-F reacts fastest because F is the most electronegative. Electronegativity makes the bond more polar, but the key factor is bond enthalpy. The C-F bond is too strong to break easily.
- Confusing leaving group ability with nucleophile strength. I⁻ is the best leaving group but a weak nucleophile.
- Forgetting to mention bond enthalpy in your explanation. "Weaker bond, easier to break, faster reaction."
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