Which of the following statements are correct?

Options

A. (a) `DeltaH_c^@` of cis-pent-2-ene is greater than trans-pent-2-ene
B. (b) `DeltaH_c^@` of hex-1-ene is greater than trans-hex-2-ene
C. (c) `DeltaH_c^@` of 2,5-dimethyl hexane is greater than octane
D. (d) `DeltaH_c^@` of 2-methyl-pent-2-ene is greater than trans-hex-2-ene
`DeltaH_c^@=`Heat of combusion

Explanation

To solve the question regarding the correctness of the statements about the heat of combustion of various alkenes and alkanes, we will analyze each statement step by step.

### Step-by-Step Solution:

**Step 1: Analyze Statement A**
- **Statement**: The heat of combustion (ΔHc) of cis-2-pentene is greater than that of trans-2-pentene.
- **Analysis**:
- Cis-2-pentene has bulky groups on the same side, leading to steric hindrance, making it less stable than trans-2-pentene, where bulky groups are on opposite sides.
- Less stable alkenes have higher heats of combustion.
- Therefore, this statement is **correct**.

**Step 2: Analyze Statement B**
- **Statement**: The heat of combustion (ΔHc) for compound X1 (cis-2-pentene) is greater than that of compound X2 (trans-2-pentene).
- **Analysis**:
- We already established that cis-2-pentene is less stable than trans-2-pentene due to steric hindrance.
- Hence, ΔHc for cis-2-pentene will be greater than that for trans-2-pentene.
- Therefore, this statement is **false**.

**Step 3: Analyze Statement C**
- **Statement**: The heat of combustion (ΔHc) of 2,5-dimethylhexane is greater than that of octane.
- **Analysis**:
- 2,5-dimethylhexane is branched, and branching increases the stability of alkanes, leading to a lower heat of combustion.
- Therefore, ΔHc for 2,5-dimethylhexane will be less than that of octane.
- Thus, this statement is **false**.

**Step 4: Analyze Statement D**
- **Statement**: The heat of combustion (ΔHc) of 2,2-methyl-pent-2-ene is greater than that of trans-hex-2-ene.
- **Analysis**:
- 2,2-methyl-pent-2-ene has more alpha hydrogens (8) compared to trans-hex-2-ene (5), indicating that 2,2-methyl-pent-2-ene is more stable.
- More stable alkenes have lower heats of combustion.
- Therefore, this statement is **false**.

### Conclusion:
The only correct statement is **A**.