A : But-2-ene is more stable than propene.
R : Heat of hydrogenation of but-2-ene is lesser than that of propene.

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 stability of but-2-ene compared to propene and the relationship with heat of hydrogenation, we can break it down into clear steps:

### Step-by-Step Solution

1. **Understanding the Structures**:
- But-2-ene (C4H8) has the structure: CH3-CH=CH-CH3.
- Propene (C3H6) has the structure: CH3-CH=CH2.

2. **Analyzing Stability**:
- Stability of alkenes is influenced by hyperconjugation and the number of alkyl groups attached to the double bond.
- But-2-ene has more alkyl substituents (2 methyl groups) compared to propene (1 methyl group).
- More alkyl groups lead to greater hyperconjugation, which stabilizes the alkene.

3. **Hyperconjugation**:
- But-2-ene has 6 possible hyperconjugative structures due to the presence of 2 methyl groups.
- Propene has 3 possible hyperconjugative structures due to the presence of only 1 methyl group.
- Therefore, but-2-ene is more stable than propene due to increased hyperconjugation.

4. **Heat of Hydrogenation**:
- The heat of hydrogenation is the energy released when an alkene is converted to an alkane.
- The more stable the alkene, the lower the heat of hydrogenation because it is already at a lower energy state.
- Since but-2-ene is more stable than propene, it will have a lower heat of hydrogenation.

5. **Conclusion**:
- Assertion (A): But-2-ene is more stable than propene. This is true.
- Reason (R): The heat of hydrogenation of but-2-ene is lesser than that of propene. This is also true.
- Both statements are correct, but the reason explains the assertion.

### Final Answer
Both the assertion and reason are true, and the reason correctly explains the assertion.