Arrange the following in decreasing order of their boiling points
(A) 2-Methylbutane
(B) 2,2-Dimethylpropane
(C) Pentane
(D) Hexane
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For boiling points of alkanes:
1. Higher molar mass means higher boiling point.
2. For isomers (same molar mass), more branching means a more compact shape, less surface area, weaker van der Waals forces, and thus a lower boiling point. Straight-chain isomers always have the highest boiling point among isomers.
The boiling point of non-polar alkanes depends on the strength of the intermolecular van der Waals forces. These forces increase with:
1. Increasing molecular mass (more electrons).
2. Increasing surface area (for isomers).
Let's analyze the given compounds.
(A) 2-Methylbutane: Formula C$_5$H$_{12}$. It is a branched isomer of pentane.
(B) 2,2-Dimethylpropane: Formula C$_5$H$_{12}$. It is another, more branched isomer of pentane.
(C) Pentane: Formula C$_5$H$_{12}$. It is a straight-chain alkane.
(D) Hexane: Formula C$_6$H$_{14}$. It is a straight-chain alkane.
Step 1: Compare based on molecular mass.
Hexane (D) has the highest molecular mass (C$_6$H$_{14}$). Therefore, it will have the strongest van der Waals forces and the highest boiling point. So, D is first.
Step 2: Compare the isomers of pentane (A, B, C).
All three have the same molecular mass (C$_5$H$_{12}$). Their boiling points will depend on their surface area.
- Pentane (C) is a straight-chain molecule, giving it the largest surface area for intermolecular contact.
- 2-Methylbutane (A) has one branch, making it more compact than pentane.
- 2,2-Dimethylpropane (B) has two branches on the same carbon, making it the most compact and spherical of the three.
As branching increases, the molecule becomes more spherical, the surface area decreases, and the van der Waals forces become weaker. This leads to a lower boiling point.
Therefore, the order of boiling points for the isomers is:
Pentane (C)>2-Methylbutane (A)>2,2-Dimethylpropane (B).
Step 3: Combine the results.
Hexane (D) has the highest boiling point, followed by the isomers of pentane in the order C>A>B.
The complete decreasing order of boiling points is D>C>A>B.
