A : `CH_(3)-CH_(2)-underset(F)underset(|)CH-CH_(3)` on reaction with `KNH_(2)` gives but-1-ene as major product.
R : It follows `E_(1)CB` mechanism.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. if Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the question regarding the reaction of 2-fluorobutane with KNH2 and the mechanism involved, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Identify the Reactant**:
- The compound given is 2-fluorobutane, which has the structure:
\[ CH_3-CH_2-CH(F)-CH_3 \]
- Here, the fluorine (F) is attached to the second carbon of the butane chain.

2. **Understand the Reagent**:
- KNH2 (potassium amide) is a strong base. It dissociates in solution to give K⁺ and NH2⁻ (amide ion), where NH2⁻ is the active species for the reaction.

3. **Mechanism Overview**:
- The E1CB (Elimination Unimolecular Conjugate Base) mechanism is favored here due to the presence of a poor leaving group (fluorine) and a strong base (NH2⁻).

4. **First Step - Deprotonation**:
- The strong base (NH2⁻) will first deprotonate a hydrogen atom adjacent to the carbon bearing the fluorine (the β-hydrogen). This leads to the formation of a carbanion.
- The structure after deprotonation will be:
\[ CH_3-CH_2-CH^(-)-CH_3 \]
- Here, the negative charge is on the β-carbon.

5. **Stability of the Carbanion**:
- The stability of the carbanion is crucial. The carbanion formed can be stabilized by the adjacent alkyl groups. In this case, the carbanion formed at the β-carbon is more stable due to the inductive effect of the adjacent alkyl groups.

6. **Second Step - Leaving Group Departure**:
- The next step involves the departure of the leaving group (F). This is the rate-determining step of the E1CB mechanism.
- The structure changes to:
\[ CH_3-CH_2-CH=CH_2 \]
- This results in the formation of but-1-ene.

7. **Final Product**:
- The major product of the reaction is but-1-ene.

### Conclusion:
- The assertion that 2-fluorobutane reacts with KNH2 to give but-1-ene as the major product is correct.
- The reason that it follows the E1CB mechanism is also correct, as it involves a poor leaving group and a strong base.