Out of the following compounds .
`I.` Pent `-1-`ene,
`II.` Pent `-2-`ene
`III.` `2-`Methyl but `-1-`ene
`IV.` `2-`Methyl bu`-2-`ene.
Which pair of the lowest and the highest heats of combustion, respectively ?

Options

A. `(IV)` and `(I)`, respectively
B. `(I)` and `(IV)`, respectively
C. `(II)` and `(III)`, respectively
D. `(III) and `(II)`, respectively

Explanation

To determine the pair of compounds with the lowest and highest heats of combustion among the given alkenes, we need to analyze their stability based on their structure. The heat of combustion is inversely related to the stability of the compound; more stable compounds release less heat upon combustion.

### Step-by-Step Solution:

1. **Identify the Structures**:
- **Pent-1-ene**: CH2=CH-CH2-CH2-CH3 (monosubstituted)
- **Pent-2-ene**: CH3-CH=CH-CH2-CH3 (disubstituted)
- **2-Methylbut-1-ene**: CH2=C(CH3)-CH2-CH3 (disubstituted)
- **2-Methylbut-2-ene**: CH3-C(CH3)=CH-CH3 (trisubstituted)

2. **Determine Substitution and Stability**:
- **Pent-1-ene**: Monosubstituted, least stable.
- **Pent-2-ene**: Disubstituted, more stable than pent-1-ene.
- **2-Methylbut-1-ene**: Disubstituted, but less stable than pent-2-ene due to steric hindrance.
- **2-Methylbut-2-ene**: Trisubstituted, most stable.

3. **Rank the Stability**:
- **Most Stable**: 2-Methylbut-2-ene (trisubstituted)
- **Next**: Pent-2-ene (disubstituted)
- **Next**: 2-Methylbut-1-ene (disubstituted, but less stable due to steric hindrance)
- **Least Stable**: Pent-1-ene (monosubstituted)

4. **Determine the Heat of Combustion**:
- Since stability is inversely proportional to heat of combustion:
- **Lowest Heat of Combustion**: 2-Methylbut-2-ene (most stable)
- **Highest Heat of Combustion**: Pent-1-ene (least stable)

5. **Conclusion**:
- The pair of compounds with the lowest and highest heats of combustion, respectively, is:
- **Lowest Heat of Combustion**: 2-Methylbut-2-ene
- **Highest Heat of Combustion**: Pent-1-ene

### Final Answer:
- The correct pair is **(IV) 2-Methylbut-2-ene (lowest heat of combustion)** and **(I) Pent-1-ene (highest heat of combustion)**.