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Difficulty: EasyFunctional Groups, Homologous Series, and IUPAC Nomenclature

Arrange the following straight-chain alkanes in order of increasing boiling point, starting with the alkane that has the lowest boiling point:

  1. 1Methane (CH4CH_4)
  2. 2Ethane (C2H6C_2H_6)
  3. 3Propane (C3H8C_3H_8)
  4. 4Butane (C4H10C_4H_{10})

Answer

The correct sequence from lowest to highest boiling point is Methane (CH4CH_4), Ethane (C2H6C_2H_6), Propane (C3H8C_3H_8), and Butane (C4H10C_4H_{10}).
In the unbranched alkane homologous series, boiling point increases progressively with increasing carbon chain length and relative molecular mass due to stronger intermolecular van der Waals forces. Consequently, Methane (CH4CH_4) has the lowest boiling point, followed by Ethane (C2H6C_2H_6), Propane (C3H8C_3H_8), and Butane (C4H10C_4H_{10}) with the highest boiling point.

Step-by-Step Solution

1
Identify the relationship between molecular size and boiling point in a homologous series
As the number of carbon atoms in straight-chain alkanes increases, the molecular mass increases.
Members of a homologous series share identical functional groups and exhibit a gradual gradation in physical properties.
2
Evaluate the strength of intermolecular forces
Van der Waals (dispersion) forces become stronger with increasing molecular surface area and electron count.
Greater intermolecular attraction requires more thermal energy to separate molecules into the gas phase.
3
Sequence the compounds from smallest to largest carbon chain
Methane (1C) < Ethane (2C) < Propane (3C) < Butane (4C)
Fewer carbon atoms correlate directly with a lower boiling point.

Key Concept

Physical Property Trends in Homologous Series
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Topics

Functional Groups, Homologous Series, and IUPAC Nomenclature

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