Which of the following bases should be used to give the product of reaction 1 and reaction 2?

Question

BlackTom AI · Accepted Answer

A bulky base favors Hofmann elimination (the less substituted alkene) while a small, unhindered base favors Zaitsev elimination (the more substituted alkene). Using that core concept gives the correct reagent assignments.

1) Relevant knowledge points and concepts
- E2 elimination regioselectivity: small, strong bases (e.g., ethoxide, EtO−) typically give the more substituted, thermodynamically more stable alkene (Zaitsev product). Bulky bases (e.g., tert‑butoxide or similarly hindered alkoxides) preferentially remove the more accessible, less substituted β‑hydrogen and give the less substituted (Hofmann) alkene.  
- Nucleophiles like CN− are good nucleophiles but relatively poor bases for E2 on a hindered (likely tertiary) substrate; CN− typically gives substitution (SN2/SN1 dependent) rather than the observed elimination pattern.

2) Step‑by‑step reasoning
- Look at the two elimination products shown: Reaction 1 produces the less substituted terminal alkene (Hofmann product). Reaction 2 produces the more substituted internal alkene (Zaitsev product).  
- Therefore the base used in Reaction 1 must be the bulky base that favors Hofmann elimination. The drawn reagents show Y as a sterically hindered alkoxide (bulky). So Reaction 1 uses Y.  
- Reaction 2 must use the small unhindered alkoxide X (ethoxide, EtO−), which favors formation of the more substituted alkene. So Reaction 2 uses X.

3) Intermediate steps / mechanistic note
- For E2, the base abstracts a β‑hydrogen anti‑periplanar to the leaving group and the C–Br bond breaks concurrently to form the C=C. A bulky base will avoid crowded β‑hydrogens and remove a less hindered hydrogen, giving the Hofmann product; a small base can access the more substituted β‑hydrogen to give the Zaitsev product.

4) Why the correct answer is correct
- Reaction 1’s product is the less substituted alkene → matches behavior of bulky base Y. Reaction 2’s product is the more substituted alkene → matches small base X. This matches fundamental steric control in E2 eliminations.

5) Why other options are incorrect
- Z (CN−) would favor nucleophilic substitution or fail to give the specific E2 regioselectivity patterns shown; it would not selectively give the Hofmann alkene in Reaction 1 or the Zaitsev alkene in Reaction 2. Swapping X and Y would give the opposite regioselectivity and not match the drawn products.

6) Conclusion
- Reaction 1 uses reagent Y (bulky base), Reaction 2 uses reagent X (small ethoxide). This confirms the provided answer: "Reaction 1 uses reagent Y, Reaction 2 uses Reagent X."

Which of the following bases should be used to give the product of reaction 1 and reaction 2?Single choice

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