H3 Chemistry 9813 · Study focus: H3 Chemistry: Evaluate competition between SN1 and SN2
H3 Chemistry: Evaluate competition between SN1 and SN2
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Evaluate competition between SN1 and SN2
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
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: the 9813 boundary 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.
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.
| Reactants | Reacting-carbon substitution | Nucleophile / concentration | Rate response | Profile maxima | Stereochemical evidence | Supported pathway |
|---|---|---|---|---|---|---|
| CH₃I + OH⁻ | methyl | strong; measured | depends on both | 1 | backside inversion where chiral | SN2 |
| 1-bromopropane + CN⁻ | primary | strong; measured | depends on both | 1 | backside attack | SN2 |
| 2-bromobutane + N₃⁻ | secondary | measured | must decide from data | one or two | measure product | evidence required |
| tert-butyl bromide + H₂O | tertiary | weak; solvent abundance | independent of [H₂O] | 2 | two-face capture | SN1 |
Text alternative: The text alternative reads each row as a complete mechanism argument and explicitly states that no solvent evidence is used.
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.
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.
- Write the two curved arrows and explain why raising [CN−] shifts the measured substitution channel.
Open the feedback checkpoint after attempting
- Arrow CN−→carbon and C–Br→Br; increased [CN−] raises the bimolecular rate while the hypothetical unimolecular ionisation rate is unchanged.
Tertiary water capture
tert-Butyl bromide cannot accommodate backside SN2 attack but can ionise to a stabilised tertiary carbocation.
Water concentration need not appear in the rate law when ionisation is rate determining.
- Choose the pathway and profile for tert-butyl bromide plus water without invoking solvent polarity.
Open the feedback checkpoint after attempting
- Choose SN1; draw two maxima and a tertiary carbocation minimum, with rate = k[tert-butyl bromide].
Resolve a secondary case from data
2-Bromobutane with N3− gives a rate that changes with [N3−] and a dominant inverted product.
Those independent observations outweigh a mechanism-free ‘secondary’ label.
- Decide the dominant pathway and state what alternative evidence would instead favour SN1.
Open the feedback checkpoint after attempting
- Choose SN2; SN1 alternatives include nucleophile-independent rate, a carbocation profile, or both configurations from planar capture.
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.
- Correct: ‘Always inspect solvent first; without it SN1/SN2 competition cannot be answered.’
Open the feedback checkpoint after attempting
- State that solvent effects are excluded here and make the decision from the supplied substrate, nucleophile, leaving group, rate and stereochemical evidence.
Assess competition without leakage
Complete the twelve mechanism-choice diagnostics before opening the unseen pair of secondary substrates.
After the different delayed methyl/tertiary pair, apply three-dimensional bond alignment to syn/anti elimination.
- Attempt the final competition check and mark every supplied factor and relevant evidence.
Open the feedback checkpoint after attempting
- Name a dominant tendency rather than claiming exclusivity, then identify the C–H and C–X bonds that can align for elimination.