A chemist treats 2-bromobutane with potassium tert-butoxide (t-BuOK), a bulky base. Which product predominates and why?
A2-butene, because t-BuOK is a strong base that favors the more stable alkene
B1-butene, because the bulky tert-butyl group cannot reach the internal beta-hydrogen and abstracts the terminal one instead
CButane, because t-BuOK is too hindered to initiate elimination
DA 1:1 mixture of 1-butene and 2-butene, because base size does not affect regioselectivity
t-BuOK is a bulky base — the three methyl groups create significant steric bulk. When approaching 2-bromobutane, the bulky base cannot reach the internal beta-hydrogens (adjacent to C2) because the carbon backbone blocks access. It can only abstract the more accessible terminal hydrogens on C1, yielding 1-butene (the less substituted alkene). This is the anti-Zaitsev or Hofmann product. Base size, not base strength, controls regioselectivity — both NaOEt and t-BuOK are strong bases, but only t-BuOK is bulky.
Question 2 Multiple Choice
Which factor is the primary determinant of whether an E2 elimination gives the Zaitsev or anti-Zaitsev product?
AThe strength of the base — stronger bases give Zaitsev products
BThe temperature — higher temperatures favor the anti-Zaitsev product
CThe size of the base — bulky bases favor anti-Zaitsev products by directing abstraction to less hindered hydrogens
DThe leaving group — better leaving groups give Zaitsev products
The key variable is base size. A small, non-bulky base (like NaOEt) can reach both internal and terminal beta-hydrogens and preferentially abstracts the internal one because the transition state leading to the more substituted alkene is lower in energy (Zaitsev product). A bulky base (like t-BuOK) cannot reach the hindered internal hydrogens and is forced to abstract the terminal ones (anti-Zaitsev product). Neither base strength, temperature, nor leaving group identity is the primary determinant of this regioselectivity difference.
Question 3 True / False
Zaitsev's rule predicts the major E2 product is the more substituted alkene because alkyl substituents destabilize the double bond through steric strain.
TTrue
FFalse
Answer: False
This reverses the logic. Alkyl substituents actually stabilize double bonds through hyperconjugation — the overlap of adjacent C–H sigma bonds with the pi system donates electron density and lowers the energy of the alkene. More substituted alkenes are more stable. Zaitsev's rule predicts the more substituted alkene is the major product precisely because it is more stable (lower energy), and the E2 transition state has partial double-bond character that benefits from this stability.
Question 4 True / False
Using a bulky base instead of a small base in an E2 elimination changes which beta-hydrogen is abstracted without changing the overall E2 mechanism.
TTrue
FFalse
Answer: True
The mechanism remains E2 in both cases — concerted, anti-periplanar, with a strong base abstracting a beta-hydrogen as the leaving group departs. What changes is only which beta-hydrogen is accessible to the base. A bulky base is sterically prevented from reaching hindered (internal) beta-hydrogens and abstracts the more accessible terminal ones instead. The concertedness, the anti-periplanar requirement, and the role of the leaving group are all unchanged. Hofmann elimination is the same mechanism with a different regioselectivity outcome due to steric effects.
Question 5 Short Answer
A synthetic chemist needs to make 1-butene (not 2-butene) from 2-bromobutane via E2 elimination. Describe what base should be used and why the choice achieves the desired regioselectivity.
Think about your answer, then reveal below.
Model answer: The chemist should use a bulky base such as potassium tert-butoxide (t-BuOK). The large tert-butyl group creates steric bulk around the basic oxygen, preventing it from reaching the internal beta-hydrogens adjacent to the bromine. Instead, the base abstracts the terminal beta-hydrogen on C1, forming 1-butene as the major product — the anti-Zaitsev (Hofmann) product. A small base like sodium ethoxide (NaOEt) would give 2-butene as the major product because it can access the internal hydrogens and the more substituted alkene is thermodynamically preferred.
This tests whether the student can apply the principle to a synthetic design problem rather than just recalling which rule gives which product. The key insight is that base SIZE (not strength or type) is the adjustable parameter that controls regioselectivity in E2 eliminations. Both NaOEt and t-BuOK are strong enough bases to drive E2; only their steric bulk differs.