3.4.9 (HL)—Nucleophilic substitution mechanisms
- Syllabus
- First assessment 2025
- Objective
- 3.4.9
- Level
- HL
SN1 and SN2 Substitution
HL only| Mechanism | Typical substrate/context | Steps and stereochemical result |
|---|---|---|
| SN2 | primary; some secondary | one concerted backside attack; inversion at a stereogenic centre |
| SN1 | tertiary; some secondary | two steps through a planar carbocation; both configurations can form |
SN2 is a concerted backside attack: the nucleophile bonds as the carbon–halogen bond breaks. SN1 first forms a carbocation; the nucleophile then attacks the planar intermediate. Primary substrates generally favour SN2 and tertiary substrates generally favour SN1 because steric access and carbocation stability differ. Secondary substrates can support either mechanism, so use the conditions as well as the substrate class.
Do not treat SN1 and SN2 as labels that follow substrate class with no exceptions, or confuse inversion in SN2 with the mixture possible after planar SN1 attack.
Comparing SN1 and SN2
HL onlyAssessment in practice
Representative question
Question 1
[Maximum number: 4]
Sketch the mechanism of the reaction for step 1 in part (b), using curly arrows to show the movement of electron pairs.
Official SN2 mechanism shown in the figure.
- Curly arrow from the lone pair/negative charge on O in OH− to C.
- Curly arrow showing Cl leaving.
- Transition state with negative charge, square brackets and partial bonds.
- Correct products.
Accept OH− with or without the lone pair.
Do not allow curly arrows originating on H in OH−.
Accept curly arrows in the transition state.
Do not penalize if HO and Cl are not at 180∘.
Do not award M3 if the OH-C bond is represented.
If the answer in 3(c)(i) is correct, award [3 max] for an SN1 mechanism.
If the answer in 3(c)(i) is SN1, award [4] for an SN1 mechanism.
Electron-Pair Sharing Summary
Retrieve the route: classify nucleophiles and electrophiles, show heterolysis, write substitution and addition mechanisms, map Lewis coordination, compare SN1/SN2, and restore aromaticity in benzene substitution.
Check electron-pair arrow origin and destination, leaving-group departure, intermediate identity, carbocation stability and the assessed mechanism boundary.