IB DP Chemistry: SL复习笔记10.1.1 Homologous Series

Homologous Series

  • Organic chemistry is the chemistry of carbon compounds
  • Carbon forms a vast number of compounds because it can form strong covalent bonds with itself
  • This enables it to form long chains of carbon atoms, and hence an almost infinite variety of carbon compounds are known
  • The tendency of identical atoms to form covalent bonds with each other and hence form chains is known as catenation

10.1.1-Catenation-in-Carbon-1

Catenation in carbon allows an almost infinite variety of chains, branches and rings

  • Carbon always forms four covalent bonds which can be single, double or triple bonds
  • functional group is a specific atom or group of atoms which confer certain physical and chemical properties onto the molecule
  • Organic molecules are classified by the dominant functional group on the molecule
  • Organic compounds with the same functional group, but a different number of carbon atoms, are said to belong to the same homologous series
  • Every time a carbon atom is added to the chain, two hydrogen atoms are also added

Homologous Series of Alkanes Table

10.1.1-Homologous-Series-of-Alkanes-2

  • Things we can say about a homologous series:
    • each member has the same functional group
    • each member has the same general formula
    • each member has similar chemical properties
    • each member differs by  -CH-
    • members have gradually changing physical properties, for example, boiling point, melting point and density
  • As a homologous series is ascended, the size of the molecule increases
  • This has an effect on the physical properties, such as boiling point and density

Boiling Point Trends

  • A graph of boiling point for the first eight alkanes looks like this:

10.1.1-Alkanes-Boiling-Point-Graph

  • The broad trend is that boiling point increases with increased molecular size
  • Each additional -CH- (called the homologous increment ) adds 8 more electrons to the molecule
  • This increases the strength of the London Dispersion Forces
  • Stronger LDF leads to a higher boiling point
  • These trends are followed in other homologous series

 

 

 

 

 

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