Learning Arc 02 · Build electron, stereochemical and substitution grammar
Distinguish SN1 and SN2 independently
- Board Questions ask for distinctions, while unfamiliar Questions mix structural and kinetic clues.
- A reliable answer must separate support from proof.
Foundation for this Concept6 symbols · 2 recovery lessons · 2 entry checksOpen
Start here: words and symbols used in this Concept
Open only the recovery material you need before the Mystery.
Read these words and symbols first
- evidence bundle
- Two or more independent observations tested against the same proposed pathway.
- One substrate label or solvent word is not a complete bundle.
- mechanism verdict
- The pathway conclusion supported by every supplied observation under the stated conditions.
- A verdict belongs to the stated experiment, not permanently to a molecule name.
- insufficient evidence
- The supplied observations do not yet separate the competing models.
- It means another measurement is needed, not that chemistry is unknowable.
- conflicting evidence
- At least two supplied observations support incompatible models under the claimed controls.
- First check whether temperature, medium or species identity actually changed.
- decisive measurement
- A controlled observation that gives different predictions for the competing models.
- It must be named precisely, such as nucleophile-order or an intermediate minimum.
- matched conditions
- Every variable except the one being compared is held fixed.
- Unmatched cases cannot isolate the cause of a rate difference.
Foundation 1
Recall both complete models
- SN2 has one coordinated event.
- SN1 ionises before capture.
- Each model also predicts its own rate, energy and geometry records.
Do
Place the SN2 and SN1 timing statements in separate columns before comparing any new case.
- Conditions
- fixed temperature · supplied polar aprotic medium · tracked 2-bromobutane centre
- Conditions
- fixed temperature · water as polar protic medium
Foundation 2
Keep solvent and nucleophile separate
To test nucleophile order, vary a separately dissolved nucleophile while solvent composition, temperature, substrate and identities stay fixed.
Do
Circle the one changed quantity and underline every held-constant condition.
Entry check 1
State the complete taught SN2 evidence model.
Show the recovery
One coordinated backside event, rate = k[RX][Nu], one maximum with no intermediate, and inversion when a labelled tetrahedral centre can be tracked.
Return to Concept 2.2.
Entry check 2
State the complete taught SN1 evidence model.
Show the recovery
Ionisation before capture, rate = k[RX], two maxima with one carbocation minimum, and two-face capture.
Return to Concept 2.3.
1 · ObjectiveOpen
What you will understand
Choose between the two substitution models only from a complete controlled evidence set.
Problems this Concept unlocks
- construct an HSC distinction table
- choose a pathway from rate and energy data
- refuse an underdetermined mechanism claim
- diagnose conflicting evidence
- request a decisive measurement
2 · MysteryOpen
The problem to explain
Read these words and symbols first
- evidence bundle
- Two or more independent observations tested against the same proposed pathway.
- One substrate label or solvent word is not a complete bundle.
- mechanism verdict
- The pathway conclusion supported by every supplied observation under the stated conditions.
- A verdict belongs to the stated experiment, not permanently to a molecule name.
- insufficient evidence
- The supplied observations do not yet separate the competing models.
- It means another measurement is needed, not that chemistry is unknowable.
- conflicting evidence
- At least two supplied observations support incompatible models under the claimed controls.
- First check whether temperature, medium or species identity actually changed.
- Two substitutions form the same bond type.
- Case S uses labelled 2-bromobutane and has rate = k[RX][Nu], one energy maximum and inversion at the tracked carbon.
- Case T has rate = k[RX], two maxima with a carbocation minimum and two-face capture.
- Conditions
- fixed temperature · supplied polar aprotic medium · tracked 2-bromobutane centre
- Conditions
- fixed temperature · water as polar protic medium
Which complete evidence set supports each pathway, and why would product, substrate class or solvent alone be insufficient?
Your decision
Classify every evidence row before choosing SN1, SN2, insufficient evidence or conflicting evidence.
3 · InvestigationOpen
Investigation
Investigation 1
One decision algorithm
Read these words and symbols first
- evidence bundle
- Two or more independent observations tested against the same proposed pathway.
- One substrate label or solvent word is not a complete bundle.
- mechanism verdict
- The pathway conclusion supported by every supplied observation under the stated conditions.
- A verdict belongs to the stated experiment, not permanently to a molecule name.
Do
Rewrite one decision algorithm as one evidence-to-conclusion arrow.
- Conditions
- fixed temperature · supplied polar aprotic medium · tracked 2-bromobutane centre
- Conditions
- fixed temperature · water as polar protic medium
Check
Name the evidence and its boundary in separate statements.
Investigation 2
Electron-timing row
Read these words and symbols first
- evidence bundle
- Two or more independent observations tested against the same proposed pathway.
- One substrate label or solvent word is not a complete bundle.
Do
Rewrite electron-timing row as one evidence-to-conclusion arrow.
Check
Name the evidence and its boundary in separate statements.
Investigation 3
Rate row
Read these words and symbols first
- evidence bundle
- Two or more independent observations tested against the same proposed pathway.
- One substrate label or solvent word is not a complete bundle.
Do
Rewrite rate row as one evidence-to-conclusion arrow.
Check
Name the evidence and its boundary in separate statements.
Investigation 4
Energy row
Read these words and symbols first
- evidence bundle
- Two or more independent observations tested against the same proposed pathway.
- One substrate label or solvent word is not a complete bundle.
Do
Rewrite energy row as one evidence-to-conclusion arrow.
Check
Name the evidence and its boundary in separate statements.
Investigation 5
Geometry row
Read these words and symbols first
- evidence bundle
- Two or more independent observations tested against the same proposed pathway.
- One substrate label or solvent word is not a complete bundle.
Do
Rewrite geometry row as one evidence-to-conclusion arrow.
Check
Name the evidence and its boundary in separate statements.
Investigation 6
Insufficient verdict
Read these words and symbols first
- insufficient evidence
- The supplied observations do not yet separate the competing models.
- It means another measurement is needed, not that chemistry is unknowable.
- decisive measurement
- A controlled observation that gives different predictions for the competing models.
- It must be named precisely, such as nucleophile-order or an intermediate minimum.
Do
Rewrite insufficient verdict as one evidence-to-conclusion arrow.
Check
Name the evidence and its boundary in separate statements.
Investigation 7
Conflict verdict
Read these words and symbols first
- conflicting evidence
- At least two supplied observations support incompatible models under the claimed controls.
- First check whether temperature, medium or species identity actually changed.
- matched conditions
- Every variable except the one being compared is held fixed.
- Unmatched cases cannot isolate the cause of a rate difference.
Do
Rewrite conflict verdict as one evidence-to-conclusion arrow.
Check
Name the evidence and its boundary in separate statements.
4 · MeaningOpen
Build the meaning
Meaning 1
One decision algorithm
Classify the stated substitution comparison, then test electron timing, rate, energy and labelled geometry before issuing a verdict.
Check
Name the evidence and its boundary in separate statements.
Meaning 2
Electron-timing row
- SN2 coordinates bond making and breaking in one event.
- SN1 ionises before nucleophile capture.
Check
Name the evidence and its boundary in separate statements.
Meaning 3
Rate row
- SN2 uses rate = k[RX][Nu] in the supplied case.
- SN1 uses rate = k[RX] in the supplied case.
Check
Name the evidence and its boundary in separate statements.
Meaning 4
Energy row
- One maximum with no intermediate supports the taught SN2 case.
- Two maxima with a carbocation minimum support the taught SN1 case.
Check
Name the evidence and its boundary in separate statements.
Meaning 5
Geometry row
- Backside SN2 gives labelled inversion.
- Planar-carbocation SN1 allows two-face capture without an exact-ratio claim.
Check
Name the evidence and its boundary in separate statements.
Meaning 6
Insufficient verdict
If substrate class, solvent or product is supplied alone, state the support and name the missing decisive measurement.
Check
Name the evidence and its boundary in separate statements.
Meaning 7
Conflict verdict
If independent records support incompatible models under claimed controls, audit the controls and withhold the pathway name.
Check
Name the evidence and its boundary in separate statements.
5 · ExamplesOpen
Worked examples
Example 1 of 6
Build the SN2 row
Case S uses labelled 2-bromobutane and has two-reactant rate dependence, one maximum and inversion.
Key evidence
- rate depends on RX and Nu
- one maximum
- four distinguishable attachments reverse their tracked arrangement
One lawful move at a time
One move at a time
Step 1 of 4
- Action
- read rate
- Why
- rate depends on RX and Nu
- Result
- Established: rate depends on RX and Nu.
Answer
SN2 is supported for case S.
Independent check
The rate, energy and labelled-geometry records agree.
Counter-pattern
The secondary label alone would have been insufficient.
6 · QuestionsOpen
Questions
Question 1 of 14
Distinguish one coordinated event from ionisation followed by capture.
Which timing comparison is correct for the two taught models?
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