6.4.7Additional lessons
Nucleophilic Substitution in Halogenoalkanes — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 154 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.
Part 1 of 4The SN2 Mechanism
SN2 is typical of primary halogenoalkanes such as bromoethane.
One step
- One step The nucleophile attacks the carbon and the leaving group departs in a single concerted step.
Transition state
- Transition state A short-lived state forms in which the carbon is partially bonded to both the nucleophile and the halogen.
Rate equation
- Rate equation Rate = k[RX][Nu⁻], so the step is bimolecular and first order in each reactant.
Stereochemistry
- Stereochemistry The nucleophile attacks from the side opposite the leaving group, which inverts the configuration of a chiral carbon.
Part 2 of 4The SN1 Mechanism
1
In the slow first step the C–Br bond breaks heterolytically to form a carbocation and a bromide ion.
2
This step involves only the halogenoalkane, so the rate equation is Rate = k[RX], first order and unimolecular.
3
In the second, fast step the nucleophile attacks the planar carbocation and forms the product.
4
The nucleophile can approach from either face, so a chiral starting material gives a racemic mixture.
5
SN1 is typical of tertiary halogenoalkanes such as 2-bromo-2-methylpropane because their carbocations are stable.
| Feature | SN1 | SN2 |
|---|---|---|
| Steps | Two | One |
| Rate equation | k[RX] | k[RX][Nu⁻] |
| Favoured by | Tertiary | Primary |
| Intermediate | Carbocation | Transition state only |
| Stereochemistry | Racemisation | Inversion |
Part 3 of 4Choosing the Mechanism
Choosing the Mechanism
Primary
- Primary Primary halogenoalkanes react mainly by SN2 because the carbon is little hindered and the carbocation would be unstable.
Tertiary
- Tertiary Tertiary halogenoalkanes react mainly by SN1 because alkyl groups stabilise the carbocation and block backside attack.
Secondary
- Secondary Secondary halogenoalkanes may react by a mixture of both mechanisms, depending on conditions.
Rate-determining step
- Rate-determining step The slowest step controls the rate, which is why SN1 depends only on the halogenoalkane concentration.
Part 4 of 4Exam Checklist & Tips
6.4.7 – Key Points for the Exam
- SN2 Bimolecular nucleophilic substitution in a single step.
- SN1 Unimolecular nucleophilic substitution through a carbocation intermediate.
- Rate-determining step The slowest step in a mechanism, which controls the overall rate.
- Racemic mixture An equal mixture of two enantiomers that does not rotate plane-polarised light.
Common Mistakes
- Saying SN1 occurs in one step and SN2 in two, which is the wrong way round.
- Drawing the transition state as an intermediate with no partial bonds.
- Forgetting that SN2 gives inversion of configuration.
- Writing SN1 rate as depending on the nucleophile concentration.
Last updated:
\n