Draw the expected major elimination product and identify the mechanism.
The hint: The elimination product should be an alkene. Zaitsev\’s rule says the more stable alkene will be formed. The presence of a strong or weak base determines E1 vs E2 mechanisms in tertiary alkyl halides.
The correct answer and explanation is:
The expected major elimination product and mechanism will depend on the alkyl halide and base involved. Let’s break it down systematically.
Major Elimination Product
The major elimination product is the more substituted alkene, following Zaitsev’s Rule, which states that elimination reactions typically favor the formation of the most thermodynamically stable (more substituted) alkene. For example, if the starting material is 2-bromo-2-methylbutane, the major product would be 2-methyl-2-butene, as it is more substituted than the alternative product, 2-methyl-1-butene.
Mechanism: E1 vs. E2
The elimination mechanism depends on two main factors:
- The nature of the substrate (e.g., tertiary, secondary, or primary alkyl halide).
- The strength of the base used.
- E1 Mechanism:
- Occurs with tertiary alkyl halides or substrates that can form a stable carbocation.
- Favored in the presence of a weak base (e.g., water or ethanol).
- It proceeds in two steps:
- Step 1: The leaving group (e.g., Br⁻) departs, forming a carbocation.
- Step 2: A proton from a β-hydrogen is removed by the base, forming the alkene.
- Example: 2-bromo-2-methylpropane in ethanol produces 2-methylpropene via the E1 mechanism.
- E2 Mechanism:
- Favored by a strong, bulky base (e.g., NaOH, NaOEt, or t-BuOK).
- Occurs in one concerted step: The base abstracts a β-hydrogen while the leaving group departs.
- Example: When 2-bromo-2-methylbutane reacts with NaOEt, the product is 2-methyl-2-butene.
Conclusion
For a tertiary alkyl halide, if a strong base is used, the reaction will proceed via the E2 mechanism and give the Zaitsev product as the major alkene. Conversely, a weak base will favor the E1 mechanism, also leading to the Zaitsev product. The stereochemistry of the product depends on the specific mechanism.