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Difficulty: EasyFunctional Groups, Homologous Series, and IUPAC Nomenclature

In a homologous series of alkanoic acids, physical properties such as boiling point change systematically with increasing molecular size. Arrange the following straight-chain alkanoic acids in order of increasing boiling point, starting with the compound that has the lowest boiling point.

  1. 1Methanoic acid (HCOOH\text{HCOOH})
  2. 2Ethanoic acid (CH3COOH\text{CH}_3\text{COOH})
  3. 3Propanoic acid (CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH})
  4. 4Butanoic acid (CH3CH2CH2COOH\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH})

Answer

The correct order of straight-chain alkanoic acids from lowest to highest boiling point is: Methanoic acid (HCOOH\text{HCOOH}), Ethanoic acid (CH3COOH\text{CH}_3\text{COOH}), Propanoic acid (CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}), and Butanoic acid (CH3CH2CH2COOH\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}).
In any homologous series of organic compounds, physical properties such as boiling point increase with increasing relative molecular mass and carbon chain length. Methanoic acid has 1 carbon, Ethanoic acid has 2, Propanoic acid has 3, and Butanoic acid has 4. Therefore, the boiling point increases steadily from methanoic acid to butanoic acid.

Step-by-Step Solution

1
Identify the homologous series and structural difference among the compounds.
All four compounds belong to the alkanoic acid homologous series, differing consecutively by a −CH2−-\text{CH}_2- (methylene) unit.
Members of a homologous series share similar chemical properties but show a gradual gradation in physical properties.
2
Determine how molecular mass and chain length affect the boiling point.
As the number of carbon atoms in the chain increases, the relative molecular mass increases and the surface area for intermolecular contact expands.
Greater molecular mass and contact surface area lead to stronger London dispersion (van der Waals) forces, requiring more thermal energy to boil.
3
Sequence the compounds by increasing carbon chain length and molar mass.
Methanoic acid (1 C1\text{ C}) < Ethanoic acid (2 C2\text{ C}) < Propanoic acid (3 C3\text{ C}) < Butanoic acid (4 C4\text{ C}).
Methanoic acid has the lowest boiling point (~101∘C101^\circ\text{C}) and Butanoic acid has the highest (~163∘C163^\circ\text{C}).

Key Concept

Gradation of physical properties in a homologous series
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Topics

Functional Groups, Homologous Series, and IUPAC Nomenclature

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