Representing Organic Compounds



  • Organic Chemistry is the scientific study of the structure, properties, and reactions of organic compounds. Organic compounds are those which contain carbon
  • For conventional reasons metal carbonates, carbon dioxide and carbon monoxide are not included in organic compounds


Definition of a Hydrocarbon

  • A compound that contains only hydrogen and carbon atoms


Representing Organic Molecules

  • Organic compounds can be represented in a number of ways:
    • Empirical Formulae
    • Molecular Formulae
    • General Formulae
    • Structural Formulae
    • Condensed Structural Formulae



  • The empirical formula shows the simplest possible ratio of the atoms in a molecule
    • For example: Hydrogen peroxide is H2O2 but the empirical formula is HO


  • The molecular formula shows the actual number of atoms in a molecule
    • For example:



  • The general formula shows a ratio of atoms in a family of compounds in terms of 'n' where n is a varying whole number
    • For example, the general formula of a molecule that belong to the alkane family is CnH2n+2




  • The displayed formula shows the spatial arrangement of all the atoms and bonds in a molecule
  • This is also known as the graphical formula.
    • For example:




  • In a structural formulae enough information is shown to make the structure clear, but most of the actual covalent bonds are omitted
  • Only important bonds are always shown, such as double and triple bonds
  • Identical groups can be bracketed together
  • Side groups are also shown using brackets
  • Straight chain alkanes are shown as follows:

Exam Tip

For defining a hydrocarbon, you must specify that they are compounds which contain hydrogen and carbon atoms only, no other element is present.You may not be asked to name branched chain organic compounds but you will come across them. It is useful to know that the numbers in the names of these compounds refer to the position of the side chains with respect to the main chain.

Organic Terminology


  • Three important terms to know in this topic are homologous series, functional group and isomerism


Homologous Series

  • This is a series or family of organic compounds that have similar features and chemical properties due to them having the same functional group
  • All members of a homologous series have:
    • The same general formula
    • Same functional group
    • Similar chemical properties
    • Gradation in their physical properties
    • The difference in the molecular formula between one member and the next is CH2





Functional Group

  • Functional group: A group of atoms bonded in a specific arrangement that influences the properties of the homologous series
  • Some examples are shown here


Structures and Names of Common Functional Groups



  • Isomers are compounds that have the same molecular formula but different displayed formulae
    • Eg. propene and cyclopropane



Isomers of C3Hshow the same molecular formula but different structures. Isomers can show similar physical and chemical properties or if they have different functional groups, the properties can be different.