Which of the following pairs represents constitutional isomers ?

Options

A.
B. `CH_3CH = CH_2 and CH_3 = CHCH_3`
C.
D.

Answer

To determine which pair represents constitutional isomers, we need to analyze each option provided in the question. Constitutional isomers (or structural isomers) are compounds that have the same molecular formula but different structural arrangements of atoms.

### Step-by-Step Solution:

1. **Identify the Molecular Formulas**:
- For each pair of compounds, we will first identify their molecular formulas to see if they are the same.

2. **Analyze Each Option**:
- **Option A**:
- Compound 1: CH₃CH₂CH₃ (Propane)
- Molecular formula: C₃H₈
- Compound 2: Cyclopropane
- Molecular formula: C₃H₆
- **Conclusion**: Different molecular formulas (C₃H₈ vs C₃H₆), so they are not constitutional isomers.

- **Option B**:
- Compound 1: CH₃CH=CH₂ (Propene)
- Molecular formula: C₃H₆
- Compound 2: CH₂=CH-CH₃ (Also Propene)
- Molecular formula: C₃H₆
- **Conclusion**: Same molecular formula and same structure, so they are not constitutional isomers.

- **Option C**:
- Compound 1: Cis-1,2-dibromoethene (C₂H₂Br₂)
- Compound 2: Trans-1,2-dibromoethene (C₂H₂Br₂)
- **Conclusion**: Same molecular formula but different spatial arrangement (geometric isomers), so they are not constitutional isomers.

- **Option D**:
- Compound 1: 1-Bromopropene (C₃H₅Br)
- Compound 2: 3-Bromopropene (C₃H₅Br)
- **Conclusion**: Same molecular formula but different structural arrangement, so they are constitutional isomers.

3. **Final Answer**:
- The pair that represents constitutional isomers is **Option D**.