H3 Chemistry 9813 · Study focus: H3 Chemistry: Apply substituent effects to SN1 and SN2
H3 Chemistry: Apply substituent effects to SN1 and SN2
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Apply substituent effects to SN1 and SN2
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
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.
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.
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.
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.
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.
Open the feedback checkpoint after attempting
- Credit 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; nucleophile dependence, profile and stereochemistry are needed.
- List one observation that would favour each pathway for 2-bromobutane.
Open the feedback checkpoint after attempting
- SN2 evidence may be rate ∝ [Nu] or inversion; SN1 evidence may be nucleophile-independent rate, a carbocation minimum or both-face products.
Distinguish alpha and beta branching
Branching directly at or adjacent to the electrophilic carbon impedes backside approach, even when the reacting carbon is formally primary.
Carbocation stabilisation must concern charge at the reacting carbon; remote alkyl groups cannot be counted as if directly attached.
- Compare neopentyl bromide and tert-butyl bromide for both hypothetical SN2 attack and direct ionisation.
Open the feedback checkpoint after attempting
- Both resist SN2, but only tert-butyl bromide directly forms a tertiary carbocation; neopentyl bromide would initially form a primary carbocation.
Repair 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.’
Open the feedback checkpoint after attempting
- State tertiary favours SN1 ionisation but blocks SN2, whereas methyl favours SN2 and cannot support ordinary methyl-carbocation SN1.
Assess dual profiles
Complete the twelve paired-mechanism diagnostics before opening the unseen secondary/tertiary profile set.
After the delayed primary/neopentyl comparison, decide explicit SN1/SN2 competition from measured evidence.
- Attempt the stored substituent-mechanism assessment and mark both steric and electronic effects.
Open the feedback checkpoint after attempting
- Keep solvent effects outside the response, then compare steric access, carbocation stability and the supplied rate law.