Representing Formulas of Organic Compounds (DP IB Chemistry)

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

• For example, if you were asked "What is the empirical formula of hydrogen peroxide?"

  • 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

• For example:

The molecular formulae of butane and butene

The molecular formula of butane is C₄H₁₀ because it contains four carbon and ten hydrogen atoms, while butene is C₄H₈ because it contains four carbon and eight hydrogen atoms  

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

• For example:

The structural formula of 2-methylbutane

The structural formula shows all of the bonds between all atoms 

• 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:

Representing the condensed structural formula of straight-chain alkanes

The full structural formula shows all bonds, while the condensed structural formula indicates the structure of the compound 

• Side groups are shown using brackets:

Representing the condensed structural formula of branched chain alkanes

The full structural formula shows all bonds, while the condensed structural formula includes functional groups in brackets to indicate the structure of the compound 

• 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

The condensed structural formula of an alkene can be shown with or without the carbon-carbon double bond

Skeletal formula

• A skeletal formula is a simplified displayed formula with:

  • All of the carbon-carbon bonds are represented by lines  

    • The end of each line and the point where two lines meet is a carbon atom

  • Most of the hydrogen atoms are removed except hydrogen atoms that are part of a functional group, e.g. OH 

  • For more information about the different functional groups, see our revision not on Functional Groups

• For example:

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

Skeletal formulae do not show carbon atoms and only show hydrogen atoms that are contained within a functional group

• What is the skeletal formula of methane?

  • There is no skeletal formula for methane 

  • This is because carbon-carbon bonds are replaced with lines, which means that two carbon atoms are required

  • Some answers suggest the skeletal formula of methane is a dot, but this is unlikely as it could easily be mistaken or confused with the symbol for a free radical

  • Other answers incorrectly suggest that the skeletal formula of methane is a carbon atom showing all four carbon-hydrogen bonds, but this is a structural formula

Stereochemical formula

• A stereochemical formula is a formula that attempts to show the relative positions and three-dimensional geometry of atoms and groups of atoms around a chiral carbon 

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

• Stereochemical formulae follow a standard convention:

  • Bonds in the plane of the paper are drawn as solid lines

  • Bonds coming forward out of the plane (towards you) are drawn as a solid wedge

  • Bonds going backward out of the plane (away from you) are drawn as a dashed wedge

Diagram of the different bonds in a stereochemical formula

Stereochemical formulae use solid lines, solid wedges and dashed wedges to illustrate if the bonds are in the plane of the paper, forwards from the paper or backwards from the paper 

• Since the central, chiral carbon has four bonds / electron domains to different atoms or groups of atoms, the shape is tetrahedral with bond angles of 109.5^o 

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