Which of the following is the least volatile?

Options

A. `CH_(3) - CH_(2) - CH_(2) - F`
B. `CH_(3) - CH_(2) - CH_(2) -Cl`
C. `CH_(2) - underset(Br)underset(|)(CH) -CH_(3)`
D. `CH_(3) - CH_(2)-CH_(2)-Br`

Answer

To determine which of the given compounds is the least volatile, we need to analyze their boiling points, as a higher boiling point indicates lower volatility.

### Step-by-Step Solution:

1. **Understanding Volatility**:
- Volatility refers to the tendency of a substance to vaporize. The less volatile a substance is, the higher its boiling point will be.

2. **Identifying the Compounds**:
- We have four compounds to consider:
- Option A: CH3CH2CH2F (1-fluoropropane)
- Option B: CH3CH2CH2Cl (1-chloropropane)
- Option C: CH2CHBrCH3 (1-bromopropane)
- Option D: CH3CH2CH2Br (1-bromopropane)

3. **Analyzing the Halogens**:
- The volatility of these compounds is influenced by the type of halogen present. The order of molecular weight for halogens is:
- Fluorine (F) < Chlorine (Cl) < Bromine (Br)
- Higher molecular weight generally leads to higher boiling points.

4. **Comparing Molecular Weights**:
- Since the carbon and hydrogen atoms are the same in all options, we focus on the halogen:
- A: F (lowest molecular weight)
- B: Cl (medium molecular weight)
- C: Br (higher molecular weight)
- D: Br (same as C but with a different structure)
- Bromine has the highest molecular weight, indicating that options C and D will have higher boiling points than A and B.

5. **Considering Branching**:
- Branching in a molecule can affect boiling points. More branching typically leads to a lower boiling point due to decreased surface area and weaker intermolecular forces.
- Option C has a branched structure (CH2CHBrCH3), while option D (CH3CH2CH2Br) is straight-chain.

6. **Conclusion**:
- Between options C and D, option D has no branching and thus will have a higher boiling point than option C.
- Therefore, the compound with the highest boiling point (and thus the least volatile) is option D: CH3CH2CH2Br.

### Final Answer:
**Option D: CH3CH2CH2Br is the least volatile compound.**