Organic Formulae (Cambridge International IGCSE Chemistry)

Displayed formulae

Spec point — 11.1.1 · Draw and interpret the displayed formula of a molecule to show all the atoms and all the bonds

• 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

Spec point — 11.1.1 · State that a structural formula is an unambiguous description of the way the atoms in a molecule are arranged, including CH2=CH2, CH3CH2OH, CH3COOCH3 Define structural isomers as compounds with the same molecular formula, but different structural formulae, including C4H10 as CH3CH2CH2CH3 and CH3CH(CH3)CH3 and C4H8 as CH3CH2CH=CH2 and CH3CH=CHCH3

Structural formulae

Extended tier only

• 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

Exam Hint Remember: Only double and triple bonds are shown in structural formulae.