Question

Which of the following hydrocarbon pair have the highest boiling point and highest melting point respectively?

• A. Eicosane and Methane
• B. Eicosane and Decane
• C. Eicosane and 2,2-Dimethylpropane
• D. Eicosane and 2-Methylbutane
• E. 2,2-Dimethylpropane and Eicosane

Hint:

Boiling point usually increases with molecular size, but melting point can be unusually high for very symmetrical molecules because they pack better in the crystal.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The question asks to identify the hydrocarbons with the highest boiling point and the highest melting point from the given pairs. We need to understand the factors affecting boiling and melting points of alkanes.
Factors affecting boiling point:
1. Molar Mass: Boiling point increases with increasing molar mass (and number of carbon atoms) due to stronger van der Waals forces.
2. Branching: For isomers, branching decreases the surface area, which weakens the van der Waals forces, leading to a lower boiling point. Straight-chain alkanes have higher boiling points than their branched isomers.
Factors affecting melting point:
1. Molar Mass: Melting point generally increases with molar mass.
2. Molecular Symmetry and Packing: More symmetrical molecules can pack more efficiently into a crystal lattice. This leads to a stronger, more stable crystal structure that requires more energy to break, resulting in a higher melting point.
Step 2: Detailed Explanation:
Highest Boiling Point:
The boiling point of alkanes increases significantly with the number of carbon atoms. Let's look at the candidates for the highest boiling point: Eicosane (C\(_{20}\)H\(_{42}\)), Methane (CH\(_4\)), Decane (C\(_{10}\)H\(_{22}\)), 2,2-Dimethylpropane (C\(_5\)H\(_{12}\)), and 2-Methylbutane (C\(_5\)H\(_{12}\)).
Eicosane has 20 carbon atoms, which is far more than any other molecule listed. Therefore, Eicosane will have the strongest van der Waals forces and the highest boiling point among all the hydrocarbons mentioned.
Highest Melting Point:
Now we need to find the hydrocarbon with the highest melting point among the options given for the second part of the pair. The candidates are Methane, Decane, 2,2-Dimethylpropane, 2-Methylbutane.
- Generally, melting point increases with molar mass, so we might expect Decane to have the highest melting point.
- However, symmetry plays a crucial role. Let's compare the molecules:
- Methane (CH\(_4\)): Tetrahedral, very symmetrical. MP = -182.5 \(^\circ\)C.
- Decane (C\(_{10}\)H\(_{22}\)): Long chain. MP = -29.7 \(^\circ\)C.
- 2-Methylbutane (Isopentane, C\(_5\)H\(_{12}\)): Branched. MP = -159.9 \(^\circ\)C.
- 2,2-Dimethylpropane (Neopentane, C\(_5\)H\(_{12}\)): This molecule is highly branched and has a spherical, very symmetrical shape (tetrahedral arrangement of methyl groups around a central carbon). This high symmetry allows it to pack exceptionally well into a crystal lattice. MP = -16.6 \(^\circ\)C.
Comparing the melting points, 2,2-Dimethylpropane (-16.6 \(^\circ\)C) has a significantly higher melting point than Decane (-29.7 \(^\circ\)C), despite having half the number of carbon atoms. This is a classic example where molecular symmetry dominates over molar mass in determining the melting point.
Conclusion:
- The hydrocarbon with the highest boiling point is Eicosane.
- The hydrocarbon with the highest melting point among the choices is 2,2-Dimethylpropane.
The correct pair is therefore (Eicosane, 2,2-Dimethylpropane).
Step 3: Final Answer:
The pair with the highest boiling point and highest melting point respectively is Eicosane and 2,2-Dimethylpropane. This corresponds to option (C).