H3 Chemistry 9813 · Study focus: H3 Chemistry: Compare SN1 and SN2 stereochemistry and ion-pair effects
H3 Chemistry: Compare SN1 and SN2 stereochemistry and ion-pair effects
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 stereochemistry and ion-pair effects
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Track one stereogenic carbon
Use a substrate with a labelled stereogenic reacting centre and preserve its three non-leaving substituents while following substitution.
Assign attack geometry before translating any drawing into R/S; a descriptor can change or remain depending on product priority order.
Backside inversion and planar capture
SN2 backside attack gives inversion of tetrahedral geometry at the reacting centre.
SN1 forms a trigonal planar carbocation that can be attacked from either face, but a nearby leaving-group ion can shield one face and prevent exact racemisation.
A single concerted SN2 event is stereospecific in geometry because frontside approach is blocked by the leaving group and electron-density arrangement.
SN1 products can contain both configurations; unequal amounts are chemically defensible when ion-pair interactions bias face access.
Text alternative: The text alternative names every group retained at carbon, describes front and rear faces, and states the predicted product relationship and ion-pair bias.
Read stereochemical evidence cautiously
A single concerted SN2 event is stereospecific in geometry because frontside approach is blocked by the leaving group and electron-density arrangement.
SN1 products can contain both configurations; unequal amounts are chemically defensible when ion-pair interactions bias face access.
Inversion at a labelled centre
Draw (R)-CH3CH(Br)CH2CH3 with the C–Br bond toward the viewer and OH− approaching directly opposite it.
The product tetrahedron inverts; assign the product descriptor only after recalculating priorities for OH versus Br.
- Draw the transition state and inverted alcohol skeleton with wedge/dash bonds.
Open the feedback checkpoint after attempting
- Credit collinear Nu···C···Br geometry, partial C–O/C–Br bonds, and inverted spatial arrangement; do not award an automatic R→S claim without priority checking.
Planar carbocation faces
After C–Br ionisation from an enantiopure secondary substrate, the empty p orbital presents two faces.
Attack from opposite faces produces opposite tetrahedral arrangements, while Br− retained near one face can reduce attack there.
- Sketch the carbocation, place Br− on one face, and predict which face is more accessible to water.
Open the feedback checkpoint after attempting
- Credit a planar carbon, an ion-paired bromide shielding its side, and preferential water approach from the opposite face.
Separate racemisation from zero rotation
An unequal 60:40 enantiomer mixture is not racemic even though both configurations form.
Observed optical rotation also depends on matched measurement conditions and cannot identify mechanism without structural and kinetic evidence.
- Explain why 58% inversion and 42% retention from an SN1 substrate is compatible with ion-pair effects.
Open the feedback checkpoint after attempting
- Both-face attack follows carbocation planarity, while the leaving-group ion biases access; the mixture is not exactly racemic.
Repair automatic descriptor swapping
Inversion describes spatial geometry, whereas R/S depends on the priority order in the product.
SN1 need not give a perfect 50:50 pair because ion-pair shielding and incomplete separation can favour one face.
- Correct: ‘SN2 always changes R to S, and SN1 always gives exactly 50:50 R:S.’
Open the feedback checkpoint after attempting
- State geometric inversion plus fresh CIP assignment for SN2, and both-face but potentially biased capture for SN1.
Assess stereochemical mechanisms
Complete the twelve wedge/dash, face and ion-pair checks before opening the unseen isotope-labelled substrate.
After the delayed cyclic-substrate case, compare how substitution changes steric access and carbocation energy.
- Attempt the stored stereochemistry assessment and mark geometry, electron flow, priority qualification and product ratio.
Open the feedback checkpoint after attempting
- Use the observed geometry as evidence, then predict how methyl, primary, secondary and tertiary substrates alter each pathway.