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Difficulty: MediumAlkanes: Properties, Reactions, and Petroleum Refining

Three isomeric alkanes have the molecular formula C5H12C_5H_{12}: pentane, 2-methylbutane, and 2,2-dimethylpropane. Which of the following statements correctly accounts for the trend in their boiling points?

  1. Pentane has the highest boiling point because its straight-chain structure provides a larger surface area for intermolecular van der Waals forces.Answer
  2. B
    2,2-Dimethylpropane has the highest boiling point because its compact spherical shape maximizes molecular interactions.
  3. C
    2-Methylbutane has a higher boiling point than pentane because branching increases the strength of intermolecular forces.
  4. D
    All three isomers have identical boiling points because they possess the same molecular formula and molar mass.

Answer

Pentane has the highest boiling point because its straight-chain structure provides a larger surface area for intermolecular van der Waals forces.
Pentane possesses an unbranched, straight-chain hydrocarbon structure. This spatial arrangement allows adjacent molecules to lie close together with maximum surface contact. As a result, intermolecular van der Waals forces are strongest in pentane, requiring the highest temperature to transition from liquid to gas.

Step-by-Step Solution

1
Analyze the molecular structures of the three C5H12C_5H_{12} isomers.
Pentane is unbranched (straight-chain), 2-methylbutane is monobranched, and 2,2-dimethylpropane is highly branched (spherical).
Structural branching determines molecular shape and the overall contact area available between molecules.
2
Relate molecular shape to intermolecular forces.
Alkanes are non-polar and held together by weak London dispersion (van der Waals) forces, which scale with molecular surface contact area.
Greater surface contact area leads to stronger attractive forces that require more thermal energy to overcome.
3
Determine the boiling point trend based on surface area.
Pentane has the largest surface area of contact, giving it the strongest intermolecular forces and the highest boiling point, while 2,2-dimethylpropane has the lowest.
Increased branching compacts the molecule into a sphere, minimizing contact area and lowering the boiling point.

Key Concept

Effect of structural branching on alkane boiling points and intermolecular forces
Estimated Time:1m 0s
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Topics

Alkanes: Properties, Reactions, and Petroleum Refining

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