H3 Chemistry 9813 · Study focus: H3 Chemistry: Apply Zaitsev/Hofmann regioselectivity
H3 Chemistry: Apply Zaitsev/Hofmann regioselectivity
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Apply Zaitsev/Hofmann regioselectivity
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Inventory the beta sites
Regioselectivity is meaningful only after locating every β-carbon that actually bears a removable hydrogen.
Compare alkene substitution, β-H accessibility, base steric demand and reaction conditions before predicting a major product.
Thermodynamic and kinetic tendencies
A Zaitsev product is the more substituted alkene and is often the more thermodynamically stable accessible regioisomer.
A Hofmann product is the less substituted alkene and can be favoured kinetically when a bulky base removes the least hindered accessible β-H.
With 2-bromo-2-methylbutane, removal from the ethyl CH2 gives 2-methylbut-2-ene, whereas removal from either equivalent methyl group gives 2-methylbut-1-ene.
Ethoxide commonly increases the internal Zaitsev product; tert-butoxide can increase the terminal Hofmann product, but geometry, substrate structure and measured product ratios take priority over a slogan.
| β-site removed | Alkene | Substitution | Ethoxide product / % | tert-Butoxide product / % | Major tendency |
|---|---|---|---|---|---|
| ethyl CH₂ | 2-methylbut-2-ene | trisubstituted | 82 | 34 | small base favours internal |
| methyl | 2-methylbut-1-ene | disubstituted | 18 | 66 | bulky base favours terminal |
Text alternative: Every β-site, product name, substitution count, base identity and numerical ratio is written in text rather than encoded by colour.
Keep the rule conditional
With 2-bromo-2-methylbutane, removal from the ethyl CH2 gives 2-methylbut-2-ene, whereas removal from either equivalent methyl group gives 2-methylbut-1-ene.
Ethoxide commonly increases the internal Zaitsev product; tert-butoxide can increase the terminal Hofmann product, but geometry, substrate structure and measured product ratios take priority over a slogan.
Map a branched bromide
For 2-bromo-2-methylbutane, label the ethyl CH2 and the two equivalent methyl groups as β-sites.
The internal alkene 2-methylbut-2-ene is trisubstituted; terminal 2-methylbut-1-ene is disubstituted.
- Predict the tendency with sodium ethoxide in ethanol at a fixed temperature.
Open the feedback checkpoint after attempting
- Credit the accessible trisubstituted Zaitsev alkene as commonly major, while stating this is a matched-condition tendency.
Introduce bulky tert-butoxide
The tert-butoxide oxygen is surrounded by three methyl groups, making approach to a crowded β-H more difficult.
Removal at an exposed methyl β-site can be faster and increases 2-methylbut-1-ene even though it is less substituted.
- Explain why potassium tert-butoxide can change the product ratio for the same bromide.
Open the feedback checkpoint after attempting
- Require steric access, faster removal at the less hindered β-site and an increased Hofmann fraction.
Read product-ratio evidence
A reaction gives 82% 2-methylbut-2-ene and 18% 2-methylbut-1-ene with ethoxide, but 34% and 66% with tert-butoxide under stated matched conditions.
The data support a Zaitsev tendency for the small base and a Hofmann shift for the bulky base; neither minor product is impossible.
- Interpret both ratios without calling either rule universal.
Open the feedback checkpoint after attempting
- Name the major regioisomer in each run and connect the shift to kinetic β-H accessibility.
Do not count carbon atoms blindly
Alkene substitution counts carbon groups directly attached to the two C=C atoms, not the total molecular carbon count.
The most substituted alkene cannot form from a β-carbon with no hydrogen or from a geometry unavailable to concerted elimination.
- Repair: ‘Zaitsev product always forms exclusively.’
Open the feedback checkpoint after attempting
- Replace ‘always’ with a conditional stability tendency and test β-H availability, anti geometry, base size and observed selectivity.
Support a regioselectivity claim
Draw every constitutional alkene, label its substitution level, identify the removed β-H and cite stability or access evidence.
Next separate these product tendencies from the rate laws and energy profiles that distinguish E1 and E2.
- State four items needed for a Zaitsev/Hofmann comparison.
Open the feedback checkpoint after attempting
- Require β-sites, product structures, substitution counts and condition-specific stability/access reasoning.