The correct order of heat of combustions of the following hydrocarbon is :
`underset(P)Pent-1-ene " " underset(Q)Pentane " " underset(R)neopentane" "underset(S)isopentane`

Options

A. `PgtQgtRgtS`
B. `QgtSgtRgtP`
C. `PgtQgtSgtR`
D. `SgtRgtRgtQgtP`

Explanation

To determine the correct order of heat of combustion for the given hydrocarbons (Pent-1-ene, Pentane, Neopentane, and Isopentane), we need to analyze the stability of each compound based on their structure and branching. 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 Compounds**:
- P: Pent-1-ene (an alkene)
- Q: Pentane (an alkane)
- R: Neopentane (a branched alkane)
- S: Isopentane (a branched alkane)

2. **Understand Stability and Heat of Combustion**:
- The heat of combustion is inversely proportional to the stability of the compound. More stable compounds will have lower heat of combustion.
- Stability increases with branching in alkanes. The more branched the alkane, the more stable it is.

3. **Rank the Compounds by Stability**:
- **P (Pent-1-ene)**: As an alkene, it is less stable than alkanes.
- **Q (Pentane)**: A straight-chain alkane, more stable than alkenes but less stable than branched alkanes.
- **R (Neopentane)**: Highly branched alkane, very stable.
- **S (Isopentane)**: Moderately branched alkane, more stable than straight-chain but less stable than neopentane.

4. **Determine the Order of Heat of Combustion**:
- Since heat of combustion is inversely proportional to stability:
- **P (Pent-1-ene)** will have the highest heat of combustion (least stable).
- **Q (Pentane)** will have a lower heat of combustion than P.
- **S (Isopentane)** will have a lower heat of combustion than Q.
- **R (Neopentane)** will have the lowest heat of combustion (most stable).

5. **Final Order**:
- The correct order of heat of combustion from highest to lowest is:
- P (Pent-1-ene) > Q (Pentane) > S (Isopentane) > R (Neopentane)

### Conclusion:
Thus, the correct order of heat of combustion for the given hydrocarbons is:
**P > Q > S > R**