6.4.8Additional lessons
Relative Rates of Nucleophilic Substitution — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 155 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Effect of the Halogen
Breaking the carbon-halogen bond is part of the rate-determining step in both mechanisms.
Bond strength
- Bond strength The C–F bond enthalpy is 485 kJ mol⁻¹, C–Cl is 346, C–Br is 290 and C–I is 228, so the C–I bond is the weakest.
Leaving group
- Leaving group A weaker carbon-halogen bond breaks more easily, so iodoalkanes react fastest and fluoroalkanes slowest.
Rate order
- Rate order RI > RBr > RCl > RF for both SN1 and SN2 reactions.
Experiment
- Experiment Silver nitrate in ethanol gives a precipitate fastest with iodoalkanes: yellow AgI, cream AgBr, white AgCl.
Section 2 of 4Effect of Structure
1
For SN1 the rate depends on how easily a carbocation forms, and alkyl groups stabilise the positive charge by donating electron density.
2
Tertiary carbocations are therefore the most stable, so the SN1 rate order is tertiary > secondary > primary.
3
For SN2 the nucleophile must reach the carbon from the back side, and bulky alkyl groups block this approach.
4
The SN2 rate order is therefore primary > secondary > tertiary, the opposite of SN1.
5
A halogenoalkane that can react by both mechanisms, such as a secondary one, shows an intermediate rate.
Section 3 of 4Nucleophile and Solvent
Nucleophile and Solvent
Nucleophile strength
- Nucleophile strength A stronger nucleophile such as OH⁻ increases the rate of SN2 but does not change the rate of SN1.
Polar solvent
- Polar solvent Polar solvents stabilise ions and the carbocation, so they favour SN1 reactions.
Concentration
- Concentration Doubling the nucleophile concentration doubles an SN2 rate but leaves an SN1 rate unchanged.
Temperature
- Temperature Raising the temperature increases the rate of both mechanisms by increasing the fraction of molecules above the activation energy.
| Halogenoalkane | Dominant mechanism | Relative rate |
|---|---|---|
| Primary | SN2 | Depends on halogen and nucleophile |
| Secondary | SN1 and SN2 | Intermediate |
| Tertiary | SN1 | Fastest by SN1 route |
Section 4 of 4Exam Checklist & Tips
6.4.8 – Key Points for the Exam
- Leaving group The group that departs with the bonding pair during substitution.
- Carbocation stability Tertiary carbocations are more stable than secondary, which are more stable than primary.
- Steric hindrance Blocking of a reaction site by large neighbouring groups.
- Polar solvent A solvent whose molecules have a permanent dipole, helping to stabilise ions.
Common Mistakes
- Saying C–F reacts fastest because fluorine is most electronegative; bond strength matters.
- Assuming tertiary halogenoalkanes are fastest in every mechanism.
- Forgetting that the nucleophile concentration does not affect an SN1 rate.
- Mixing up inductive stabilisation with steric hindrance.
Last updated:
\n