Apply substituent effects to SN1 and SN2
Compare methyl, primary, secondary and tertiary substrates twice: once for concerted backside attack and once for ionisation to a carbocation.
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The core idea
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Learning objectives
- Apply substituent effects to SN1 and SN2
Put both mechanisms on one substrate axis
Compare methyl, primary, secondary and tertiary substrates twice: once for concerted backside attack and once for ionisation to a carbocation.
Keep the leaving group fixed so each profile change is attributed to substituents around the reacting carbon.
Name the affected transition state
In SN2, added alkyl groups destabilise the crowded five-coordinate transition-state arrangement and obstruct the Nu···C···X axis.
In SN1, added alkyl groups stabilise developing positive charge in the ionisation transition state and the carbocation intermediate.
Explain the opposite trends
Methyl and primary substrates have accessible backsides but would form very unstable carbocations.
Tertiary substrates stabilise carbocations but block the backside geometry; secondary substrates sit between these extremes and require other evidence.
The same alkyl groups push the two pathways in opposite directions: they crowd the SN2 approach but stabilise positive charge in SN1. Make two separate rankings instead of trying to learn one universal substrate order.
Methyl versus tert-butyl profiles
For CH₃Br, draw a low one-step SN2 barrier and an energetically prohibitive hypothetical SN1 carbocation pathway.
For tert-butyl bromide, draw a blocked/high SN2 barrier and a lower two-step SN1 ionisation profile.
Annotate the structural reason beside each of the four barriers.
Check your answer
A strong answer should include open versus shielded backside access and methyl versus tertiary carbocation stability; the leaving group remains Br in every panel.
Interpret a secondary substrate
2-Bromobutane has more SN2 crowding than a primary bromide but can form a more stable carbocation.
Its substitution mechanism cannot be selected from ‘secondary’ alone; name observable evidence for each pathway.
List one observation that would favour each pathway for 2-bromobutane.
Check your answer
SN2 evidence: rate proportional to [Nu], or inversion of configuration. SN1 evidence: a rate independent of [Nu], a carbocation minimum on the profile, or products from attack on both faces.
Distinguish alpha and beta branching
Supplied substrates: neopentyl bromide, (CH₃)₃CCH₂Br, and tert-butyl bromide, (CH₃)₃CBr.
For SN2, check branching at and next to the reacting carbon. For ionisation, classify the carbocation that would form at the reacting carbon itself, not at a neighbouring carbon.
Compare neopentyl bromide and tert-butyl bromide for both hypothetical SN2 attack and direct ionisation.
Check your answer
Both resist SN2: tert-butyl bromide is crowded at the reacting carbon, and neopentyl bromide next to it. Only tert-butyl bromide ionises directly to a tertiary carbocation; neopentyl bromide would initially form a primary carbocation.
Common mistake: one substituent rule for both pathways
Substituents do not have a single mechanism-free effect: steric congestion and positive-charge stabilisation act on different structures.
A mechanism comparison must show the relevant transition state or intermediate rather than cite ‘tertiary’ as a universal accelerator.
Correct: ‘Tertiary bromides react faster than methyl bromide by both SN1 and SN2.’
Check your answer
State tertiary favours SN1 ionisation but blocks SN2, whereas methyl favours SN2 and cannot support ordinary methyl-carbocation SN1.
Assess dual profiles
Complete the paired-mechanism check questions before trying the unseen secondary/tertiary profile set.
Later, try the primary/neopentyl comparison; decide explicit SN1/SN2 competition from measured evidence.
For each substrate, state backside accessibility and the stability of the carbocation it would form. Only after both checks should you decide which pathway the substituent pattern supports.
Attempt the substituent-mechanism final practice question and mark both steric and electronic effects.
Check your answer
Keep solvent effects outside the response, then compare steric access, carbocation stability and the supplied rate law.
Apply substituent effects to SN1 and SN2 scientific representation
The text alternative orders all substrates, states whether each line is one-step or two-step, and names steric or charge stabilisation beside every energy change.
About 5 minutes
Text alternative: The text alternative orders all substrates, states whether each line is one-step or two-step, and names steric or charge stabilisation beside every energy change.