Compare E1 and E2 regioselectivity

E1 and E2 can begin from the same haloalkane yet apply different controls to the available β-sites.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Compare E1 and E2 regioselectivity

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.

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.

E1 usually compares the stability of alkenes formed after a flexible carbocation has appeared. E2 compares the rates of particular concerted removals, so an anti hydrogen, ring geometry or bulky base can outweigh the simple substitution trend.

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.

Try this

Explain why the same carbon skeleton need not give the same major product by E1 and bulky-base E2.

Check your answer

Your answer should include carbocation/alkene stability for E1 and concerted β-H access for E2.

Apply trans-diaxial control

Supplied chair: 1-bromo-2-methylcyclohexane with Br axial at C1 and the C2 methyl axial on the opposite face.

For E2, check each β-carbon (C2 and C6) for an axial H anti to Br before considering which alkene would be more substituted.

Try this

Predict the available E2 regioisomer for the fixed chair.

Check your answer

At C2 the axial position holds the methyl, so there is no β-H anti to Br there. C6 has an axial H, so E2 forms only the C1=C6 alkene (3-methylcyclohexene), even though C1=C2 would be more substituted.

Read matched product ratios

Supplied data: a tertiary substrate gives 76% internal alkene under weak-base E1 conditions, but 69% terminal alkene with a strong bulky base under E2 conditions.

Identify the major product in each run, then link each to what controls product formation in that mechanism. Keep the conclusion to these specified conditions.

Try this

Interpret the two major products and their controlling factors.

Check your answer

E1: the internal, more substituted alkene is major (76%), because β-H loss from the carbocation favours the more stable alkene. E2 with a bulky base: the terminal alkene is major (69%), because the base removes the more accessible β-H. These are tendencies under these conditions, not fixed distributions for every E1 or E2.

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.

Try this

Better reasoning: ‘E1 and E2 must give the same alkene because they remove HX.’

Check your answer

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.

Choose E1 or E2 first, then apply only that pathway’s product controls. State why an apparently more stable alkene may be inaccessible or slower in the actual reacting geometry.

Try this

State the evidence for different E1/E2 regioselectivity.

Check your answer

Your answer should include a fixed substrate, mechanism, β-site map, geometry/base constraint and qualified product tendency.