Homologous Series
Section: 11. Organic Chemistry | Syllabus: Cambridge AS Level Physics 9702
Homologous Series Homologous Series A family of organic compounds with the same general formula and similar chemical properties. Each member differs from the next by a CH₂ unit. Characteristics of Homologous Series Same general formula: All members can be represented by the same algebraic formula Similar chemical properties: Members react in similar ways due to the same functional group Gradation in physical properties: Properties like boiling point and melting point change gradually Successive members differ by CH₂: Each member has one more carbon and two more hydrogens than the previous member Major Homologous Series The four main homologous series you need to know are: Series General Formula Functional Group First Three Members Alkanes C n H 2n+2 None (single C-C bonds) CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane) Alkenes C n H 2n C=C (double bond) C₂H₄ (ethene), C₃H₆ (propene), C₄H₈ (butene) Alcohols C n H 2n+1 OH -OH (hydroxyl) CH₃OH (methanol), C₂H₅OH (ethanol), C₃H₇OH (propanol) Carboxylic Acids C n H 2n+1 COOH -COOH (carboxyl) HCOOH (methanoic acid), CH₃COOH (ethanoic acid), C₂H₅COOH (propanoic acid) Alkanes Series in Detail Alkanes are saturated hydrocarbons with only single C-C bonds.
[Diagram: Molecular structures of first four alkanes (methane, ethane, propane, butane) showing increasing carbon chain length] Name Molecular Formula Structural Formula Number of Carbons Methane CH₄ CH₄ 1 Ethane C₂H₆ CH₃CH₃ 2 Propane C₃H₈ CH₃CH₂CH₃ 3 Butane C₄H₁₀ CH₃CH₂CH₂CH₃ 4 Pentane C₅H₁₂ CH₃CH₂CH₂CH₂CH₃ 5 Alkenes Series in Detail Alkenes are unsaturated hydrocarbons with at least one C=C double bond.
[Diagram: Molecular structures of first four alkenes (ethene, propene, butene, pentene) showing C=C double bond position] Name Molecular Formula Structural Formula Number of Carbons Ethene C₂H₄ CH₂=CH₂ 2 Propene C₃H₆ CH₃CH=CH₂ 3 Butene C₄H₈ CH₃CH₂CH=CH₂ 4 Pentene C₅H₁₀ CH₃CH₂CH₂CH=CH₂ 5 Alcohols Series in Detail Alcohols contain the hydroxyl (-OH) functional group.
[Diagram: Molecular structures of first four alcohols (methanol, ethanol, propanol, butanol) showing -OH group attached to carbon chain] Name Molecular Formula Structural Formula Number of Carbons Methanol CH₃OH CH₃OH 1 Ethanol C₂H₅OH CH₃CH₂OH 2 Propanol C₃H₇OH CH₃CH₂CH₂OH 3 Butanol C₄H₉OH CH₃CH₂CH₂CH₂OH 4 Carboxylic Acids Series in Detail Carboxylic acids contain the carboxyl (-COOH) functional group.
[Diagram: Molecular structures of first four carboxylic acids (methanoic, ethanoic, propanoic, butanoic) showing -COOH group] Name Molecular Formula Structural Formula Number of Carbons (total) Methanoic acid HCOOH HCOOH 1 Ethanoic acid CH₃COOH CH₃COOH 2 Propanoic acid C₂H₅COOH CH₃CH₂COOH 3 Butanoic acid C₃H₇COOH CH₃CH₂CH₂COOH 4 Trends in Physical Properties As the carbon chain length increases in a homologous series: Boiling point increases: Larger molecules have stronger intermolecular forces (van der Waals forces) Melting point increases: Generally increases with molecular size Viscosity increases: Liquids become thicker and flow more slowly Flammability decreases: Smaller molecules are more volatile and burn more easily [Graph: Line graph showing boiling point vs.
number of carbon atoms for alkanes, demonstrating gradual increase] Why Chemical Properties Are Similar Members of the same homologous series have similar chemical properties because they have the same functional group.
For example: All alkenes undergo addition reactions due to the C=C double bond All alcohols react with sodium to produce hydrogen gas due to the -OH group All carboxylic acids react with bases to form salts due to the -COOH group Definition: Family of compounds with same general formula, differing by CH₂ Same functional group: Gives similar chemical properties Gradual trends: Physical properties change predictably with chain length Four main series: Alkanes (C n H 2n+2 ), Alkenes (C n H 2n ), Alcohols (C n H 2n+1 OH), Carboxylic Acids (C n H 2n+1 COOH) Naming pattern: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C)
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