In which of the following reaction, the major product alkane is formed by E1cb mechanism?

Options

A. `CH_(3)-CH_(2)-CH_(2)-CH_(2)-Br underset(Delta)overset(alc. KOH)to`
B. `CH_(3)-CH_(2)-underset(Br)underset(|)CH-CH_(3) underset(Delta)overset(alc. KOH)to`
C. `CH_(3)-CH_(2)-underset(F)underset(|)CH-CH_(3) underset(Delta)overset(alc. KOH)to`
D. `CH_(3)F underset(Delta)overset(alc. KOH)to`

Answer

To determine which reaction forms the major product alkane via the E1cb mechanism, we need to understand the mechanism itself and the conditions under which it occurs.

### Step-by-Step Solution:

1. **Understanding E1cb Mechanism**:
- E1cb stands for Elimination Unimolecular Conjugate Base. In this mechanism, the rate of reaction depends on the formation of a conjugate base, which is the intermediate species. The elimination process occurs in two steps: first, the formation of the conjugate base, followed by the elimination step.

2. **Identify the Reactants**:
- The reaction typically involves an alkyl halide (R-X) and a strong base (like alcoholic KOH). The alkyl halide must have a β-hydrogen for the elimination to occur.

3. **Formation of the Conjugate Base**:
- The strong base abstracts a β-hydrogen, leading to the formation of a carbanion (the conjugate base). This step is crucial as it determines whether the E1cb mechanism can proceed.

4. **Elimination Step**:
- Once the conjugate base is formed, the elimination step occurs where the leaving group (X) departs, resulting in the formation of a double bond (alkene). If the conditions are right, this can lead to the formation of an alkane as the final product.

5. **Consider the Substituents**:
- The stability of the intermediate carbanion and the final alkene product is influenced by the substituents on the β-carbon. More substituted alkenes are generally more stable due to hyperconjugation and the inductive effect.

6. **Identify the Major Product**:
- The major product will be the more substituted alkene, following Zaitsev's rule, which states that the more substituted alkene is favored in elimination reactions.

7. **Conclusion**:
- After analyzing the reactions, the one that fits the criteria for the E1cb mechanism and yields the major product alkane is the one where a stable conjugate base can form, and the elimination leads to a more substituted alkene.