H3 Chemistry 9813 · Study focus: H3 Chemistry: Apply substrate effects in E2/SN2 competition
H3 Chemistry: Apply substrate effects in E2/SN2 competition
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Apply substrate effects in E2/SN2 competition
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Hold the reagent constant
Compare substrates with the same strong nucleophile/base, leaving group, concentration, solvent and temperature to isolate substitution at carbon versus β-H removal.
SN2 requires backside access to the α-carbon; E2 requires at least one β-H and a reactive periplanar arrangement.
Competing concerted pathways
SN2 forms a nucleophile–carbon bond as the leaving group departs; α-carbon crowding strongly raises its barrier.
E2 removes β-H as C=C forms and the leaving group departs; substitution can shift competition toward E2, but a substrate without β-H cannot eliminate.
Methyl halides have no β-carbon and undergo SN2 rather than E2; unhindered primary halides normally retain strong SN2 access.
Secondary halides show substantial competition, while tertiary halides block backside SN2 and can favour E2 with strong base if an accessible β-H exists.
| Substrate | α-class | β-H status | SN2 | E2 | Decisive reason |
|---|---|---|---|---|---|
| CH₃Br | methyl | no β-carbon | accessible | unavailable | no β-H |
| CH₃CH₂Br | primary | present | strong | possible | open α-carbon |
| 2-bromobutane | secondary | present | competes | competes | both channels accessible |
| tert-butyl bromide | tertiary | present | blocked | supported | backside shielding |
| neopentyl bromide | primary | no H on quaternary β-C | slowed | excluded | β-branching and no β-H |
Text alternative: Every substrate is named, with α-substitution, β-H status and pathway conclusion written in separate text columns.
Follow methyl to tertiary
Methyl halides have no β-carbon and undergo SN2 rather than E2; unhindered primary halides normally retain strong SN2 access.
Secondary halides show substantial competition, while tertiary halides block backside SN2 and can favour E2 with strong base if an accessible β-H exists.
Compare bromoethane and tert-butyl bromide
With ethoxide under matched warm conditions, bromoethane offers an accessible primary carbon and can substitute.
tert-Butyl bromide blocks SN2 approach but has nine β-H, so E2 formation of 2-methylpropene gains strongly.
- Predict which substrate has greater SN2 and which greater E2 tendency.
Open the feedback checkpoint after attempting
- Credit bromoethane for SN2 and tert-butyl bromide for E2, with access and β-H evidence.
Place a secondary substrate
2-Bromobutane has a hindered secondary α-carbon and β-H on both sides.
Ethoxide can therefore attack carbon or remove β-H; measured substitution and alkene product fractions are needed for a quantitative claim.
- Explain why 2-bromobutane shows stronger competition than bromoethane.
Open the feedback checkpoint after attempting
- Require increased backside hindrance plus available β-H, without declaring one path exclusive.
Use a no-beta-hydrogen control
Neopentyl bromide, (CH3)3CCH2Br, has a highly hindered primary reaction centre and its adjacent quaternary carbon bears no H.
E2 is structurally impossible at that side, while SN2 is unusually slow because adjacent branching obstructs backside approach.
- Evaluate E2 and SN2 for neopentyl bromide with ethoxide.
Open the feedback checkpoint after attempting
- Exclude E2 for no β-H and describe hindered, not impossible, SN2.
Do not say substitution helps both equally
Increasing α-substitution progressively blocks the precise backside trajectory required by SN2.
E2 can tolerate a substituted α-carbon, but still depends on β-H geometry and the chosen base.
- Repair: ‘A tertiary halide reacts fastest by SN2 because it has more alkyl groups.’
Open the feedback checkpoint after attempting
- Reject it; tertiary crowding blocks backside attack and strong-base reaction with β-H instead supports E2.
Audit substrate evidence
Record α-substitution, backside access, every β-carbon, β-H availability and any periplanar constraint before comparing product pathways.
Next hold the substrate fixed and vary base strength, nucleophilicity and steric bulk.
- State the substrate checklist for E2/SN2 competition.
Open the feedback checkpoint after attempting
- Require reaction-centre substitution, backside sterics, β-H existence, geometry and matched-condition product evidence.