H3 Chemistry 9813 · Study focus: H3 Chemistry: Compare E1 and E2 regioselectivity
H3 Chemistry: Compare E1 and E2 regioselectivity
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Compare E1 and E2 regioselectivity
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Choose the pathway before the product
E1 and E2 can begin from the same haloalkane yet apply different controls to the available β-sites.
E1 product tendency follows carbocation formation and alkene stability; E2 must additionally satisfy base access and β-H/leaving-group geometry in its concerted step.
Pathway-specific regioselectivity
E1 commonly favours the more substituted stable alkene when its carbocation can be deprotonated at more than one accessible β-site, but a supplied product distribution remains decisive.
E2 regioselectivity is kinetic: each product requires an available β-H in the required periplanar geometry and a base able to reach it.
A flexible tertiary bromide with β-H on two sides can rotate before E1 deprotonation and commonly gives the more substituted alkene as major.
In a cyclohexane E2, only trans-diaxial H/X pairs react; a nominal Zaitsev alkene is absent if its β-carbon offers no axial anti H in the reactive chair.
| Case | β-site | Alkene | Substitution | E1 access | E2 anti-periplanar access | Base steric access |
|---|---|---|---|---|---|---|
| acyclic | more substituted β-C | internal | higher | accessible | if anti conformer exists | lower for bulky base |
| acyclic | less substituted β-C | terminal | lower | accessible | if anti conformer exists | higher for bulky base |
| cyclohexane, Br axial-down C1 | C2 | nominal C1=C2 | specified | deprotonation possible | no axial H; blocked | not decisive |
| cyclohexane, Br axial-down C1 | C6 | C1=C6 | specified | deprotonation possible | axial-up H anti; allowed | accessible |
Text alternative: All chair positions, missing/present hydrogens, product bonds and pathway conclusions are stated in text cells.
Compare flexible and rigid substrates
A flexible tertiary bromide with β-H on two sides can rotate before E1 deprotonation and commonly gives the more substituted alkene as major.
In a cyclohexane E2, only trans-diaxial H/X pairs react; a nominal Zaitsev alkene is absent if its β-carbon offers no axial anti H in the reactive chair.
Contrast one bromide under two pathways
2-Bromo-2-methylbutane can give 2-methylbut-2-ene or 2-methylbut-1-ene.
E1 commonly favours the internal stable alkene; E2 with tert-butoxide can favour the terminal alkene because removal at an exposed methyl β-site is faster.
- Explain why the same carbon skeleton need not give the same major product by E1 and bulky-base E2.
Open the feedback checkpoint after attempting
- Require carbocation/alkene stability for E1 and concerted β-H access for E2.
Apply trans-diaxial control
In 1-bromo-2-methylcyclohexane, inspect the chair with Br axial and identify axial β-H at C2 and C6.
If the C2 axial site is occupied by methyl, only C6 supplies an anti β-H, so E2 forms C1=C6 even if C1=C2 would be more substituted.
- Predict the available E2 regioisomer for the fixed chair.
Open the feedback checkpoint after attempting
- Credit the C1=C6 alkene and exclusion of C1=C2 for missing trans-diaxial H.
Read matched product ratios
A tertiary substrate under weak-base E1 conditions gives 76% internal alkene, whereas strong bulky-base E2 conditions give 69% terminal alkene.
The change is mechanistic evidence only when substrate, products and conditions are specified; it does not prove every E1 or E2 has that distribution.
- Interpret the two major products and their controlling factors.
Open the feedback checkpoint after attempting
- Assign stability-controlled E1 tendency and accessibility-controlled E2 tendency with numerical evidence.
Do not transfer a major product blindly
A Zaitsev product predicted for flexible E1 may be geometrically inaccessible to a rigid E2 substrate.
Conversely, anti geometry does not by itself guarantee a product is major if several anti β-H sites compete at different rates.
- Repair: ‘E1 and E2 must give the same alkene because they remove HX.’
Open the feedback checkpoint after attempting
- Reject it and compare intermediacy, conformational requirement, β-H access and base sterics.
Make a pathway-product matrix
For every β-site record E1 accessibility, E2 periplanar geometry, product substitution and measured or expected kinetic access.
Next hold base conditions fixed and isolate how substrate substitution shifts competition between E2 and backside SN2.
- State the evidence for different E1/E2 regioselectivity.
Open the feedback checkpoint after attempting
- Require a fixed substrate, mechanism, β-site map, geometry/base constraint and qualified product tendency.