Predict reagents needed to complete this E1 elimination reaction. A NaH heat Cl B (CH
)
COK heat C CH
OH D CH
OH heat E H
O
heat
The Correct Answer and Explanation is:
Correct Answer: D. CH₃OH, heat
Explanation
The image shows a classic E1 (elimination unimolecular) reaction. The starting material is a secondary alkyl halide (2-chloro-3-methylbutane), and the product is an alkene (2-methyl-2-butene), indicating an elimination reaction has occurred.
Understanding the E1 Mechanism:
The E1 mechanism proceeds in two steps:
- Carbocation Formation: The chloride ion (Cl⁻), a good leaving group, departs from the molecule, forming a carbocation intermediate.
- Deprotonation: A base (often the solvent itself in E1) removes a proton (H⁺) from a β-carbon, forming a double bond (C=C) and yielding the alkene product.
E1 reactions are favored by:
- Stable carbocations (secondary or tertiary),
- Weak bases (such as alcohols or water),
- Polar protic solvents (which stabilize both carbocations and leaving groups),
- Heat, which drives elimination over substitution.
Why Option D is Correct:
- CH₃OH (methanol) is a polar protic solvent, ideal for stabilizing the developing carbocation.
- The heat promotes the elimination over competing SN1 substitution.
- Methanol can also act as a weak base to abstract the β-hydrogen.
- The secondary carbocation formed here is sufficiently stable, and the final product is a more substituted alkene (Zaitsev product), which is also thermodynamically favored.
Why the Other Options Are Incorrect:
- A (NaH, heat): NaH is a strong base, which favors an E2 mechanism, not E1.
- B ((CH₃)₃COK, heat): Also a strong, bulky base → favors E2.
- C (CH₃OH without heat): Lacks the thermal energy needed to favor elimination.
- E (H₃O⁺, heat): Acidic water usually favors rearrangements or SN1/E1 in aqueous phase, but CH₃OH is better in organic contexts.
Conclusion:
Option D (CH₃OH, heat) provides the best environment for an E1 elimination: a weak base, polar protic solvent, and heat to favor the formation of the alkene.
