What is the anticipated major elimination product (A or B) in the following reaction?[Fill in the blank]单项选择题
A
A
B
B

Question

What is the anticipated major elimination product (A or B) in the following reaction?[Fill in the blank]

BlackTom AI · Accepted Answer

Zaitsev’s rule predicts the more highly substituted (and therefore more stable) alkene will be the major E2 product, so product A is expected.

1) Relevant concepts
- E2 elimination: a base abstracts a β–H while the C–LeavingGroup bond breaks in one concerted step; an anti-periplanar geometry is required for the hydrogen that is removed.
- Zaitsev’s rule: the more substituted alkene (more alkyl groups attached to the C=C carbons) is lower in energy and normally predominates with small, non-bulky bases.
- Alkene stability increases with alkyl substitution due to hyperconjugation and inductive effects.

2) Step-by-step reasoning
- Identify the substrate: the bromide is on C2 of a 4‑carbon chain that bears a methyl on C3 (i.e., 3‑methyl‑2‑bromobutane). There are two sets of β‑hydrogens: the two equivalent hydrogens on C1 (removal gives the terminal alkene B) and the single hydrogen on C3 (removal gives the internal alkene A).
- E2 mechanism: ethoxide (CH3CH2O–) will abstract one of these β‑hydrogens while Br leaves; which β‑H is abstracted determines the alkene formed.

3) Intermediate “calculations” / substitution counts
- If H removed from C3 (giving double bond between C2–C3): the resulting double bond is tri‑substituted (three alkyl groups attached to the C=C carbons) — this is product A.
- If H removed from C1 (giving double bond between C1–C2): the resulting double bond is less substituted (disubstituted) — this is product B.

4) Why the correct answer is correct
- Ethoxide is a relatively small, strong base that favors the thermodynamically more stable (Zaitsev) alkene. The tri‑substituted alkene (A) is more stabilized by hyperconjugation and inductive effects than the less substituted alkene (B). Therefore formation of A is both kinetically accessible (anti‑periplanar β‑H on C3 is available) and thermodynamically favored.

5) Why the other option is incorrect
- B is the less substituted (less stable) alkene; with a small base under normal E2 conditions it will be the minor product. Only with very bulky bases or severe stereochemical constraints would B predominate.

6) Conclusion
- The major elimination product is A, which matches the provided answer (A).

What is the anticipated major elimination product (A or B) in the following reaction?[Fill in the blank]单项选择题

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