Compare SN1 and SN2 stereochemistry and ion-pair effects

Use a substrate with a labelled stereogenic reacting centre and preserve its three non-leaving substituents while following substitution.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Compare SN1 and SN2 stereochemistry and ion-pair effects

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.

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.

Follow the reacting carbon through the mechanism. Backside SN2 attack turns its tetrahedral arrangement inside out, whereas a planar SN1 carbocation can be attacked from either face; real product ratios need not be exactly 50:50 if the ion pair or surroundings shield one face.

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.

Try this

Draw the transition state and inverted alcohol skeleton with wedge/dash bonds.

Check your answer

A strong answer should include 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

Supplied reaction: C–Br ionisation of an enantiopure secondary bromide in water, with Br⁻ still close to the carbocation.

Sketch the geometry at the cationic carbon and its two faces, then decide which face the nearby Br⁻ blocks.

Try this

Sketch the carbocation, place Br⁻ on one face, and predict which face is more accessible to water.

Check your answer

The cationic carbon is planar with an empty p orbital open on both faces. Br⁻, ion-paired on the side it left, shields that face, so water approaches more from the opposite face and inversion exceeds retention.

Separate racemisation from zero rotation

Supplied data: an SN1 substrate gives 58% inverted and 42% retained product.

Explain why both configurations form, then why they are unequal, and decide whether the mixture is racemic. Optical rotation alone cannot identify the mechanism.

Try this

Explain why 58% inversion and 42% retention from an SN1 substrate is compatible with ion-pair effects.

Check your answer

Both configurations form because the planar carbocation can be attacked on either face. The excess of inversion fits ion pairing: the departing halide briefly shields its own face. At 58:42 the product is not racemic.

Common mistake: 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.

Try this

Correct: ‘SN2 always changes R to S, and SN1 always gives exactly 50:50 R:S.’

Check your answer

State geometric inversion plus fresh CIP assignment for SN2, and both-face but potentially biased capture for SN1.

Assess stereochemical mechanisms

Answer the check questions wedge/dash, face and ion-pair checks before trying the unseen isotope-labelled substrate.

Later, try the cyclic-substrate case; compare how substitution changes steric access and carbocation energy.

Draw the starting configuration and product relationship, then link inversion or both-face attack to the geometry of the relevant transition state or intermediate. Product labels alone are not a mechanism explanation.

Try this

Attempt the stereochemistry final practice question and mark geometry, electron flow, priority qualification and product ratio.

Check your answer

Use the observed geometry as evidence, then predict how methyl, primary, secondary and tertiary substrates alter each pathway.