Apply Zaitsev/Hofmann regioselectivity

Regioselectivity is meaningful only after locating every β-carbon that actually bears a removable hydrogen.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Apply Zaitsev/Hofmann regioselectivity

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.

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.

First list the alkenes that can actually form; only then compare them. A small base often reaches the pathway to the more substituted Zaitsev alkene, while a bulky base may remove the more exposed hydrogen faster and increase the Hofmann product.

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.

Try this

Predict the tendency with sodium ethoxide in ethanol at a fixed temperature.

Check your answer

A strong answer should include the accessible trisubstituted Zaitsev alkene as commonly major, while stating this is a matched-condition tendency.

Introduce bulky tert-butoxide

Supplied comparison: the same tertiary bromide, which can form 2-methylbut-2-ene or 2-methylbut-1-ene, with ethoxide and with potassium tert-butoxide.

Compare the crowding around each base's oxygen, then decide which β-H each base can reach more easily.

Try this

Explain why potassium tert-butoxide can change the product ratio for the same bromide.

Check your answer

tert-Butoxide's oxygen is surrounded by three methyl groups, so it removes the exposed CH₃ β-H faster than the more hindered CH₂ β-H. That raises the fraction of the less substituted 2-methylbut-1-ene, the Hofmann product.

Read product-ratio evidence

Supplied data under matched conditions: ethoxide gives 82% 2-methylbut-2-ene and 18% 2-methylbut-1-ene; tert-butoxide gives 34% and 66%.

Name the major regioisomer in each run, then explain the shift using base access to β-H.

Try this

Interpret both ratios without calling either rule universal.

Check your answer

Ethoxide: 2-methylbut-2-ene is major (82%), the more substituted Zaitsev product. tert-Butoxide: 2-methylbut-1-ene is major (66%), the Hofmann product, because the bulky base reaches the terminal CH₃ hydrogens more easily. Both products form in both runs, so neither rule is universal.

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.

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Better reasoning: ‘Zaitsev product always forms exclusively.’

Check your answer

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.

A strong regioselectivity answer identifies every β-site, draws each possible alkene, counts substitution and then applies base access and geometry to the stated conditions.

Try this

State four items needed for a Zaitsev/Hofmann comparison.

Check your answer

Your answer should include β-sites, product structures, substitution counts and condition-specific stability/access reasoning.