Hydrogenation of unsaturated hydrocarbons is an exothermic reaction. Due to hyperconjugation and resonance the stability of unsaturated hydrocarbons increases and the increase in stability is more due to resonance. Compound with same number of `pi-`bonds and more stability has lower heat of hydrogenation.
Heat of formation is defined as the energy evolved when a molecule is formed from its atoms. For isomers the more stable compounds has higher heat of formation.
The correct heat of hydrogenation order is :
(p) 1,3-Pentadiene
(q) 1,3-Butadiene
(r) 2,3-Dimethyl-1,3-butadiene
(s) Propadiene

Options

A. p > q > r > s
B. s > q > p > r
C. q > s > p > r
D. s > p > q > r

Answer

To determine the correct heat of hydrogenation order for the given unsaturated hydrocarbons, we need to analyze their stability based on the number of substituents on the double bonds and the presence of resonance.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- (p) 1,3-Pentadiene
- (q) 1,3-Butadiene
- (r) 2,3-Dimethyl-1,3-butadiene
- (s) Propadiene

2. **Draw the Structures**:
- **1,3-Pentadiene**: CH2=CH-CH=CH-CH3 (two double bonds, both at positions 1 and 3)
- **1,3-Butadiene**: CH2=CH-CH=CH2 (two double bonds, both at positions 1 and 3)
- **2,3-Dimethyl-1,3-butadiene**: CH2=C(CH3)-C(CH3)=CH2 (two double bonds, with methyl groups at positions 2 and 3)
- **Propadiene**: CH2=C=CH2 (two double bonds, both adjacent)

3. **Assess Stability**:
- Stability of alkenes increases with the number of substituents on the double bond due to hyperconjugation and resonance.
- **1,3-Pentadiene**: Disubstituted (more stable than mono)
- **1,3-Butadiene**: Disubstituted (more stable than mono)
- **2,3-Dimethyl-1,3-butadiene**: Tetrasubstituted (most stable)
- **Propadiene**: Monosubstituted (least stable)

4. **Rank the Stability**:
- The stability order based on substitution is:
- 2,3-Dimethyl-1,3-butadiene (r) > 1,3-Pentadiene (p) = 1,3-Butadiene (q) > Propadiene (s)
- Thus, the stability order is: r > p = q > s

5. **Determine Heat of Hydrogenation**:
- The heat of hydrogenation is inversely related to the stability of the compound. More stable compounds have lower heat of hydrogenation.
- Therefore, the heat of hydrogenation order will be the reverse of the stability order:
- Highest heat of hydrogenation (least stable): s > p = q > r

6. **Final Order**:
- The correct order of heat of hydrogenation is:
- s > q = p > r
- This can be expressed as: **sqpr**

### Conclusion:
The correct heat of hydrogenation order for the compounds is **sqpr**.