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The Three-Dimensional Structure of Molecules - Isomerism

Drawing of Structural Formulas

Structural formulas are intended to show as many details as necessary. But they are also intended not to show too many details so the illustration does not become confusing. As a result, different ways to graphically illustrate the structure of molecules have been introduced.

Here, the structure of propanol is taken for illustration:


The carbon-hydrogen bonds are not usually shown specially. A methylene group, for example, is illustrated by CH2. Further simplified formulas show only the heteroatoms and the hydrogen atoms which are not connected to carbon atoms. In these formulas the carbon chain is illustrated as an angled line, in which every tip represents a carbon atom.

Also, cyclic carbon compounds are normally illustrated by a ring-shaped angled line without specially showing hydrogen atoms, where the tips of the line represent the carbon atoms.


The methods of illustrating organic molecules introduced so far serve to describe the constitution of the molecules, i.e. the connectivity of the atoms. Often, additional information about the configuration, that is the geometrical arrangement of the atoms, are needed.

In this case:

  • Bonds which are located in the plane of the two-dimensional illustration are shown as straight lines.
  • Bonds which extend behind the illustration plane are shown as crosswise broken wedges or crosswise broken lines.
  • Bonds which extend out of the illustration plane towards the viewer are shown as filled wedges or as wide lines.

Look at the structural formula of methane, for example. The bonds from carbon to the blue- and magenta-colored hydrogen atoms are located in the illustration plane. The green hydrogen atom is behind and the red hydrogen atom in front of the illustration plane. The sawhorse formulas and the Newman projection, for example, are special drawing methods, which can be used to describe the spatial arrangemant of the substituents at two adjacent tetrahedral atoms.


Exercise 1

Exercise 2

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