The elimination of 2-bromo-2-methylbutane can be performed using
potassium tert-butoxide (KOtBu) as the base rather than KOH.
Explain how the results of the experiment would differ qualitatively if KOtBu were
used. A correct answer will identify how the product yield would differ and offer a
mechanistic explanation for the different result.
potassium tert-butoxide (KOtBu) as the base rather than KOH.
Explain how the results of the experiment would differ qualitatively if KOtBu were
used. A correct answer will identify how the product yield would differ and offer a
mechanistic explanation for the different result.
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This is a classic problem in which one compound can give rise to two different products (in this case, differently substituted olefins) based on the reaction conditions. The determining factor in both cases is transition state energy. Whichever product results from the more stable transition state will dominate. The other thing you need to know is that this is an E2 reaction. E2 reactions are endothermic; according to the Hammond postulate the transition theory resembles the product more than the reactant. This will become important later
With a base such as hydroxide, the more substituted product is formed. More substituted olefins are more stable because of hypoerconjugation. Because this product is more stable, its transition state was lower in energy. Thus with KOH the trisubstituted olefin is formed. This is known as Zaitzev's rule (as with many Russian names, there are other English spellings, so be aware if you search for it).
This effect is still present in the case of KOtBu, but the steric hindrance of the tBu group overrides the Zaitzev rule. Steric hindrance is minimized by approaching the least hindered beta hydrogen. This gives rise to the least-substituted product. This is also known as the Hoffman rule.
With a base such as hydroxide, the more substituted product is formed. More substituted olefins are more stable because of hypoerconjugation. Because this product is more stable, its transition state was lower in energy. Thus with KOH the trisubstituted olefin is formed. This is known as Zaitzev's rule (as with many Russian names, there are other English spellings, so be aware if you search for it).
This effect is still present in the case of KOtBu, but the steric hindrance of the tBu group overrides the Zaitzev rule. Steric hindrance is minimized by approaching the least hindered beta hydrogen. This gives rise to the least-substituted product. This is also known as the Hoffman rule.