4.1.2 Functional Groups

• 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

An almost infinite variety of chains, branches and rings is possible 

Homologous series

• A homologous series is a group of organic compounds that have the same functional group, but each successive member differs by CH₂

Ethanol and propanol belong to the same homologous series

Homologous Series of Alkanes

• 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 successive member differs by  -CH₂ –
  • 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

• A functional group is a group of atoms that are responsible for the characteristic reactions of a compound
  • They determine the physical and chemical properties of molecules
• Organic compounds can be classified into one of three categories:
  1. Aliphatic - a compound containing carbon and hydrogen joined together in straight chains, branched chains or non-aromatic rings
  2. Alicyclic - this is an aliphatic compound arranged in non-aromatic rings with or without side chains
  3. Aromatic - a compound containing a benzene ring
• The table below shows a summary of common functional groups found in compounds
• R is any other atom or group of atoms (except for hydrogen)

Functional Groups found in Compounds Table

• As previously discussed, a homologous series is a group of organic compounds that have the same functional group, but each successive member differs by CH₂ 
  • The homologous series of hydrocarbons called the alkanes has the general formula C_nH_2n+2 
  • The homologous series of hydrocarbons called the alkenes has the general formula C_nH_2n 
• These general formulae can be used to predict the chemical formula of any member of the same homologous series

Answers

Answer 1: Eicosane is an alkane so has the general formula C_nH_2n+2 
There are 20 carbons so n is 20, which means that there are 2n+2 hydrogens or (2 x 20) + 2 = 42
Therefore, the formula of eicosane is C₂₀H₄₂ 

Answer 2: n-nonapentacontane is an alkane so has the general formula C_nH_2n+2 
There are 59 carbons so n is 59, which means that there are 2n+2 hydrogens or (2 x 59) + 2 = 120
Therefore, the formula of n-nonapentacontane is C₅₉H₁₂₀ 

Answer 3: Oct-3-ene is an alkene so has the general formula C_nH_2n
There are 8 carbons in oct-3-ene so n is 8, which means that there are 2n hydrogens or (2 x 8) = 16
Therefore, the formula of oct-3-ene is C₈H₁₆ 

• The process shown in the worked examples can be applied to any homologous series providing that the general formula and the number of at least one of the component elements are known