Organic Formulae (Cambridge (CIE) O Level Chemistry): Revision Note

Displayed Formulae

• 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

• Many of the structures you will be drawing are hydrocarbons

• A hydrocarbon is a compound that contains only hydrogen and carbon atoms

• Organic compounds can be represented in a number of ways:

  • Displayed Formulae

  • General Formulae

  • Structural Formulae

The displayed formula shows the spatial arrangement of all the atoms and bonds in a molecule

• For example:

  

• This displayed formula tells us several things about the compound

  • It has 5 carbon atoms

  • It has 12 hydrogen atoms

  • It has only single bonds 

Structural Formulae

• In 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:

Structural Isomers

• Structural isomers are compounds that have the same molecular formula but different structural formulae

• The molecular formula is the actual number of atoms of each element in a compound

• Compounds with the same molecular formula can have different structural formulae due to the different arrangement of their atoms in space

• Two examples of structural isomers are shown below 

Table showing Structural Isomerism in C₄H₁₀

Table showing Structural Isomerism in C₄H₈