H3 Chemistry 9813 · Study focus: H3 Chemistry: Apply base effects in E2/SN2 competition
H3 Chemistry: Apply base 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 base effects in E2/SN2 competition
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Hold the substrate constant
Compare bases on the same haloalkane with leaving group, concentrations, solvent and temperature stated.
Carbon attack depends on nucleophilicity and steric access, whereas β-H removal depends on basicity, access to H and the E2 geometric requirement.
Strength, nucleophilicity and bulk
A strong base readily accepts a proton; an effective nucleophile rapidly donates an electron pair to carbon under the stated conditions.
Steric bulk around the donor atom obstructs backside carbon attack more than approach to an exposed β-H, often shifting a strong bulky reagent toward E2.
CN− is a relatively unhindered carbon nucleophile and with 1-bromobutane strongly supports SN2 substitution to pentanenitrile.
Ethoxide can give both paths; tert-butoxide is a strong bulky base whose carbon attack is hindered, so β-H abstraction and E2 are enhanced, especially for a secondary substrate at higher temperature.
| Reagent | T / K | Donor-site access | Butenes / % | Substitution / % | Required curved arrows |
|---|---|---|---|---|---|
| ethoxide | 323 | small | 46 | 54 | SN2: Nu→Cα; C–Br→Br. E2: base→Hβ; C–H→C=C; C–Br→Br. |
| tert-butoxide | 323 | bulky | 81 | 19 | E2: base→Hβ; C–H→C=C; C–Br→Br. SN2 channel sterically reduced. |
Text alternative: Each reagent, temperature, percentage and curved-arrow source/destination is written explicitly in the table alternative.
Compare cyanide, ethoxide and tert-butoxide
CN− is a relatively unhindered carbon nucleophile and with 1-bromobutane strongly supports SN2 substitution to pentanenitrile.
Ethoxide can give both paths; tert-butoxide is a strong bulky base whose carbon attack is hindered, so β-H abstraction and E2 are enhanced, especially for a secondary substrate at higher temperature.
Fix 2-bromobutane
At 298 K with ethoxide, 2-bromobutane can give ethoxy substitution and butene elimination products.
Replacing ethoxide by equal-concentration tert-butoxide increases steric obstruction to α-carbon attack while retaining strong proton acceptance.
- Predict the direction of the product-fraction change.
Open the feedback checkpoint after attempting
- Credit a higher butene fraction and lower substitution fraction, qualified to the matched conditions.
Use a cyanide control
1-Bromobutane plus CN− undergoes attack from the carbon end of cyanide at the primary carbon while C–Br electrons move to Br.
Do not call CN− incapable of basic behaviour universally; use the fixed primary substrate and observed substitution evidence.
- Draw the SN2 arrows and name the organic product.
Open the feedback checkpoint after attempting
- Require CN carbon lone pair→C1, C–Br→Br and CH3CH2CH2CH2CN (pentanenitrile).
Add temperature evidence
For one secondary bromide/base pair, raising temperature changes the measured alkene:substitution ratio from 42:58 to 67:33.
This supports increased elimination contribution under those conditions, but temperature alone does not identify E2 without base dependence or concerted evidence.
- Interpret the ratio change and state its limit.
Open the feedback checkpoint after attempting
- Credit the numerical E2 shift and require corroborating mechanism evidence.
Do not merge basicity with nucleophilicity
A bulky reagent may be strongly basic yet poor at backside attack because its donor atom is shielded.
A small reagent may be effective at carbon attack; substrate crowding and solvent still belong to any fully controlled comparison.
- Repair: ‘The strongest base must give the fastest SN2 reaction.’
Open the feedback checkpoint after attempting
- Reject it; SN2 needs accessible nucleophilic attack, while strong bulky bases can favour E2 proton removal.
Reconcile reagent and product data
Record base charge, strength, donor-site bulk, substrate access, β-H geometry, temperature and separate substitution/alkene yields.
After the delayed comparison, return to the registered H3 pathway hub for whole-course synthesis.
- State the base-effect evidence needed for an E2/SN2 judgement.
Open the feedback checkpoint after attempting
- Require a fixed substrate, controlled conditions, steric/basicity reasoning, electron-flow target and product ratio.