Describe the one-step SN2 mechanism
Show simultaneous nucleophile attack and carbon–halogen bond breaking in a bimolecular substitution.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the named reagents, solvents, temperatures and pressure conditions; begin curly arrows at a bond or lone pair and distinguish typical mechanisms from absolute rules. Written practice is self-checked
Before you begin
- Use covalent formulas, nucleophiles, electrophiles, homolytic and heterolytic fission, and the alkene and alcohol pathways in Topics 13–14.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where does the first SN2 curly arrow begin?
What you will learn
- Identify the electron-pair source
- Show bond formation and cleavage in one step
- Interpret the bimolecular rate dependence
SN2 is a concerted substitution
In an SN2 mechanism, the nucleophile donates a lone pair to the carbon attached to X while the C–X bond pair moves to X. Bond formation and bond breaking occur in the same step.
Use a full curly arrow from the nucleophile lone pair to carbon and another from the C–X bond to X. The leaving group departs as X⁻; the nucleophile forms the new C–Nu bond.
Both reactants affect the rate
The rate depends on both halogenoalkane and nucleophile concentrations: rate ∝ [R–X][Nu]. The transition state involves both species, which is why the mechanism is called bimolecular.
Primary halogenoalkanes commonly favor SN2 because the carbon is less crowded. The course does not treat this preference as an absolute guarantee for every structure and condition.
Pause and explain
What happens to the C–X bond in one SN2 step?
Worked example
Outline the SN2 substitution of bromoethane by OH⁻.
Show the worked solution
- Start a curly arrow at an oxygen lone pair and point it to the carbon bonded to Br.
- At the same time, start a second arrow at the C–Br bond and point it to Br.
- The product is ethanol and Br⁻.
Answer One step: OH⁻ attacks the C–Br carbon as the C–Br bond pair moves to Br⁻, forming CH₃CH₂OH.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw or describe the two curly arrows for OH⁻ substitution of CH₃Br by SN2.
Give me a hint
Track the incoming lone pair and the leaving bond pair simultaneously.
Compare my reasoning
- Arrow 1 starts at an O lone pair and points to the C bonded to Br.
- Arrow 2 starts at the C–Br bond and points to Br.
- Show CH₃OH and Br⁻ after the one-step event.
OH⁻ lone pair → C; C–Br bond pair → Br; products CH₃OH + Br⁻.
Look for these in your work
- First arrow source/destination correct
- Second arrow source/destination correct
- Products identified
If [R–X] doubles while [Nu] is constant, what does the SN2 rate law predict?
Give me a hint
Use rate ∝ [R–X][Nu].
Compare my reasoning
- Only [R–X] changes.
- The rate is first order in [R–X].
- Doubling [R–X] doubles the predicted rate.
The rate doubles, provided all other conditions remain constant.
Look for these in your work
- Rate law used
- Order identified
- Change calculated
A proposed SN2 diagram first shows R–X → R⁺ + X⁻, then Nu⁻ attacks. Identify the mechanism mismatch.
Give me a hint
Check whether substitution is concerted or stepwise.
Compare my reasoning
- The proposed first step creates a carbocation.
- That is the two-stage feature of an SN1 pathway.
- SN2 instead combines attack and leaving-group departure in one step.
The drawing is SN1-like; SN2 has no carbocation intermediate and shows simultaneous bond formation and cleavage.
Look for these in your work
- Intermediate recognized
- SN1 distinguished
- SN2 corrected
Common mistakes
- Showing a carbocation intermediate in SN2
- Starting an arrow at a positive charge instead of an electron pair
- Calling SN2 unimolecular because only one product is formed
What can you explain now?
Where does the first SN2 curly arrow begin?
Compare with the explanation
At a nucleophile lone pair
The electron pair donated by the nucleophile forms the new bond.
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Make it stick
Why does the SN2 rate depend on both the halogenoalkane and nucleophile concentrations?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.