H3 Chemistry 9813 · Study focus: H3 Chemistry: Compare SN1 and SN2 profiles, rate laws and steady-state reasoning
H3 Chemistry: Compare SN1 and SN2 profiles, rate laws and steady-state reasoning
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Compare SN1 and SN2 profiles, rate laws and steady-state reasoning
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Separate one-step and two-step pathways
SN2 is concerted: nucleophile attack and leaving-group departure occur through one transition state with no intermediate.
SN1 is stepwise: slow ionisation forms a carbocation, followed by faster nucleophile capture.
Link molecularity and rate law
The simple SN2 law is rate = k[RX][Nu], first order in each reactant and second order overall.
The simple SN1 law is rate = k[RX], first order overall because the nucleophile is absent from the ionisation step.
In an SN1 steady-state treatment, the carbocation remains at low concentration because its rate of formation is approximately balanced by its rates of consumption.
9813 requires this qualitative account; deriving a mathematical steady-state rate expression is outside the stated requirement.
| Pathway | Independent change | Rate response | Rate law | k units |
|---|---|---|---|---|
| SN2 | double [RX] | rate doubles | k[RX][Nu] | dm³ mol⁻¹ s⁻¹ |
| SN2 | double [Nu] | rate doubles | k[RX][Nu] | dm³ mol⁻¹ s⁻¹ |
| SN1 | double [RX] | rate doubles | k[RX] | s⁻¹ |
| SN1 | double [Nu] | unchanged | k[RX] | s⁻¹ |
Text alternative: The text alternative lists every maximum and minimum in sequence and states each concentration-rate comparison without relying on line colour or shape alone.
Use the steady-state statement correctly
In an SN1 steady-state treatment, the carbocation remains at low concentration because its rate of formation is approximately balanced by its rates of consumption.
9813 requires this qualitative account; deriving a mathematical steady-state rate expression is outside the stated requirement.
Initial-rate discrimination
Runs for an alkyl bromide give: doubling [RX] doubles rate; doubling [CN−] also doubles rate.
The evidence supports rate = k[RX][CN−] and a bimolecular rate-determining event.
- Calculate the predicted rate factor when [RX] is tripled and [CN−] is halved.
Open the feedback checkpoint after attempting
- The rate changes by 3 × 1/2 = 1.5; credit explicit use of both first-order dependencies.
Read the energy profiles
An SN2 profile has one maximum connecting reactants directly to products.
An SN1 profile has two maxima separated by a carbocation minimum; the first ionisation maximum is commonly higher.
- Label reactants, both transition states, carbocation and products on a two-step substitution profile.
Open the feedback checkpoint after attempting
- Credit two maxima, one intermediate minimum, and the first step as C–X ionisation rather than nucleophile attack.
Interpret a low intermediate concentration
A low measured carbocation concentration does not mean that the species is absent.
Rapid trapping can keep a real intermediate near steady state while product continues to form.
- Explain how a carbocation can be mechanistically essential yet remain difficult to detect during an SN1 reaction.
Open the feedback checkpoint after attempting
- Its formation is slow and its consumption is fast, so it is present at low steady concentration; do not replace it by a transition state.
Repair shared-rate-law reasoning
SN1 and SN2 form substitution products through different elementary sequences and therefore have different concentration dependencies.
A good leaving group can speed either mechanism without making their rate laws identical.
- Correct: ‘Both mechanisms contain nucleophile and haloalkane, so both rates must be k[RX][Nu].’
Open the feedback checkpoint after attempting
- State that SN2 includes nucleophile in its single rate-determining step, whereas SN1 ionisation precedes nucleophile capture and gives k[RX].
Assess kinetic convergence
Complete the twelve rate/profile diagnostics before opening the unseen bromide data table.
After the delayed chloride profile, use stereochemical evidence and ion-pair effects to distinguish pathways.
- Attempt the stored kinetics assessment and mark rate orders, k units, profile topology and steady-state statement.
Open the feedback checkpoint after attempting
- No mathematical steady-state derivation is required; next compare inversion with two-face capture and qualify any product ratio.