Representing Formulas of Organic Compounds (DP IB Chemistry): Revision Note

Representing formulas of organic compounds

• Organic compounds can be represented using a variety of different formulae:

  • Empirical

  • Molecular

  • Structural

  • Condensed Structural

  • Skeletal

  • Stereochemical

Empirical formula

• What is empirical formula?

  • The empirical formula shows the simplest whole-number ratio of the atoms in a molecule

• The empirical formula of hydrogen peroxide is:

  • Hydrogen peroxide is H₂O₂

  • This shows that there are two hydrogen atoms and two oxygen atoms, but this is not the simplest whole-number ratio

  • Since there is a factor of 2, the empirical formula is HO

Molecular formula

• What is molecular formula?

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

The molecular formulae of butane and butene

Structural formula

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

  • This is also known as the displayed formula or graphical formula

The structural formula of 2-methylbutane

• In a condensed structural formula, 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 as shown below

Representing the condensed structural formula of straight-chain alkanes

Representing the condensed structural formula of branched chain alkanes

• Specific bonds such as double (or triple) bonds are not always shown

  • It can be expected for you to deduce if there is a double (or triple) bond within the structure from the number of hydrogens attached to the carbon atoms

Representing the condensed structural formula of alkenes

Skeletal formula

• A skeletal formula is a simplified way of representing organic molecules where:

  • Carbon–carbon bonds are shown as straight lines

    • Each end or junction of a line represents a carbon atom

  • Hydrogen atoms bonded to carbon are omitted unless they are part of a functional group (e.g. –OH)

  • Only functional groups and non-carbon atoms are shown explicitly

For more information about the different functional groups, see our revision note on Functional Groups

The skeletal formula of propane, but-2-ene and ethanoic acid

• Methane (CH₄) does not have a skeletal formula

  • This is because skeletal formulas rely on carbon–carbon bonds, and methane contains only one carbon atom

  • Some students mistakenly draw it as a dot, but this could be confused with a radical or lone atom

  • Others try to draw it as a full structure (with four C–H bonds), but this is a structural formula, not skeletal

Stereochemical formula

• A stereochemical formula shows the three-dimensional arrangement of atoms or groups around a chiral carbon. It helps visualise the spatial orientation of bonds and distinguish between stereoisomers

  • For more information about chiral carbons, see our revision note on Enantiomers

• Stereochemical formulae follow a standard convention:

  • Solid line: bond lies in the plane of the paper

  • Solid wedge: bond comes out of the plane (towards the viewer)

  • Dashed wedge: bond goes behind the plane (away from the viewer)

Diagram of the different bonds in a stereochemical formula

• A chiral carbon is bonded to four different atoms or groups, creating a tetrahedral shape with bond angles of approximately 109.5^o 

• This geometry results from the four electron domains repelling each other equally in three dimensions

  • For more information about the shapes and bond angles of molecules, see our revision note on Shapes of Molecules