In the following compunds the decreasing order of B.P. is
(i) `CH_(3)-CH_(2)-CH_(2)-CH_(2)-CH_(2)-CH_(3)`
(ii) `CH_(3)-overset(CH_(3))overset(|)CH-CH_(2)-CH_(2)-CH_(3)`
(iii) `CH_(3)-overset(CH_(3))overset(|)underset(CH_(3))underset(|)C-CH_(2)-CH_(3)`

Options

A. (i)gt(ii)gt(iii)
B. (i)gt(iii)gt(ii)
C. (ii)gt(iii)gt(ii)
D. (iii)gt(ii)gt(i)

Answer

To determine the decreasing order of boiling points (B.P.) for the given compounds, we need to consider two main factors that influence boiling points: molecular weight and branching.

1. **Identify the Compounds:**
- (i) `CH3-CH2-CH2-CH2-CH2-CH3` (n-hexane)
- (ii) `CH3-CH(CH3)-CH2-CH2-CH3` (2-methylpentane)
- (iii) `CH3-C(CH3)2-CH2-CH3` (2,2-dimethylbutane)

2. **Molecular Weight:**
- All three compounds have the same number of carbon (C) and hydrogen (H) atoms, which means they have the same molecular weight. Therefore, we can disregard molecular weight as a factor for differentiating their boiling points.

3. **Branching:**
- The boiling point is inversely proportional to the degree of branching. More branching generally leads to a lower boiling point due to a decrease in surface area and weaker van der Waals forces.
- **Compound (i)**: n-hexane has no branching, which means it has the highest boiling point.
- **Compound (ii)**: 2-methylpentane has one branching point (the methyl group), which will lower its boiling point compared to n-hexane but is higher than that of 2,2-dimethylbutane.
- **Compound (iii)**: 2,2-dimethylbutane has two branching points (two methyl groups), leading to the lowest boiling point among the three.

4. **Conclusion:**
- Based on the analysis of branching:
- **Highest B.P.**: (i) n-hexane
- **Middle B.P.**: (ii) 2-methylpentane
- **Lowest B.P.**: (iii) 2,2-dimethylbutane

Therefore, the decreasing order of boiling points is:
**(i) > (ii) > (iii)**