Evaluate competition between SN1 and SN2

Evaluate substrate crowding/carbocation stability, nucleophile identity and concentration, and leaving-group ability together.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Evaluate competition between SN1 and SN2

Treat competition as a matched decision

Evaluate substrate crowding/carbocation stability, nucleophile identity and concentration, and leaving-group ability together.

Do not import solvent effects: this course explicitly says they are not required.

State tendencies, not universal switches

Methyl and unhindered primary substrates with effective concentrated nucleophiles favour SN2 because backside access is open.

Tertiary substrates with weak neutral nucleophiles favour SN1 because ionisation is viable and backside attack is blocked.

Use secondary substrates as evidence cases

Secondary substrates can support either pathway; nucleophile concentration dependence, profile topology and stereochemical products resolve the competition.

A better leaving group can accelerate both pathways and therefore does not select SN1 or SN2 on its own.

Mechanism choice is a balance of evidence, not a single-label rule. Check substrate crowding, carbocation stability, nucleophile strength and concentration, leaving-group ability, rate law and stereochemistry, giving greatest weight to observations supplied in the question.

Primary cyanide substitution

1-Bromopropane with concentrated CN⁻ has rate proportional to both concentrations and gives backside substitution.

Primary access and effective nucleophile participation jointly favour SN2 over primary-carbocation SN1.

Try this

Write the two curved arrows and explain why raising [CN⁻] shifts the measured substitution channel.

Check your answer

Arrow CN⁻→carbon and C–Br→Br; increased [CN⁻] raises the bimolecular rate while the hypothetical unimolecular ionisation rate is unchanged.

Tertiary water capture

Supplied reaction: tert-butyl bromide with water.

Judge backside access at the tertiary carbon and the stability of the cation formed on ionisation, then decide which species appear in the rate law.

Try this

Choose the pathway and profile for tert-butyl bromide plus water without invoking solvent polarity.

Check your answer

SN1. Three methyl groups block backside attack, but ionisation gives a stabilised tertiary carbocation. Draw two maxima with a carbocation minimum between them; rate = k[tert-butyl bromide], with no water term because ionisation is rate determining.

Resolve a secondary case from data

Supplied data: 2-bromobutane with N₃⁻ gives a rate that changes with [N₃⁻] and a mainly inverted product.

Decide which pathway each observation supports; the ‘secondary’ label alone decides nothing. Then state what observations would point the other way.

Try this

Decide the dominant pathway and state what alternative evidence would instead favour SN1.

Check your answer

SN2: a rate that depends on [N₃⁻] puts the nucleophile in the rate-determining step, and inversion shows backside attack. SN1 would instead show a rate independent of [N₃⁻], a carbocation minimum on the profile, or both configurations from capture on either face.

Remove the solvent shortcut

A defensible 9813 comparison can be made using substrate, nucleophile, leaving group, kinetics and stereochemistry alone.

A tertiary substrate does not undergo SN2 merely because the nucleophile is strong; geometrical blockage remains decisive.

Try this

Correct: ‘Always inspect solvent first; without it SN1/SN2 competition cannot be answered.’

Check your answer

State that solvent effects are excluded here and make the decision from the supplied substrate, nucleophile, leaving group, rate and stereochemical evidence.

Bring the evidence together

Complete the mechanism-choice check questions before trying the unseen pair of secondary substrates.

Later, try the different methyl/tertiary pair; apply three-dimensional bond alignment to syn/anti elimination.

Organise a competition answer as evidence for SN1, evidence for SN2 and a final qualified conclusion. Explain any mixed products rather than pretending that one pathway must be completely absent.

Try this

Attempt the competition final practice question and mark every supplied factor and relevant evidence.

Check your answer

Name a dominant tendency rather than claiming exclusivity, then identify the C–H and C–X bonds that can align for elimination.