How to draw Skeletal Formulae of Organic Molecules
What are "skeletal formulae" or "skeletal structures (of organic compounds)" ?
In organic chemistry, skeletal formulae are the most abbreviated diagrammatic descriptions of molecules in common use. They look very bare because in skeletal formulae the hydrogen atoms (attached directly to carbons) are removed, leaving just a "carbon skeleton" with functional groups attached to it.
Don't be fooled: The hydrogen atoms are present in the molecules but their presence is assumed - rather than drawn or stated - in the case of skeletal formulae.
This type of representation may take some getting used-to and is not always taught at school-level chemisty (so you may not need to know about use it, check your syllabus to be sure).
Simple Examples of Skeletal Formulae:
As for organic molecules in general, the simplest examples of skeletal formulae of organic molecules are those of linear alkanes.
The above sets of examples are included because some people find it easier to understand skeletal formulae of organic molecules by looking at series of similar molecules (see also homologous series) and noticing trends through the series, as well as comparisons between drawings of the full displayed formula and the skeletal formula of individual molecules.
Why draw skeletal structures of organic molecules ?
Organic molecules can reach huge sizes. Sometimes it is necessary to describe the whole molecule, but not to draw every single atom and chemical bond in full.
Organic molecules are usually only represented using skeletal formulae (that is, by drawing their skeletal structures) in cases of molecules beyond a certain size and complexity. Therefore moderately advanced knowledge of organic chemistry is assumed.
The benefits of using the skeletal structures (which are sometimes referred to as "skeletal formulae") include:
- Such simpler (abbreviated) diagrams are generally quicker and easier to draw - by hand, or electronically.
- such simpler (abbreviated) diagrams often take-up less space.
- Important parts of the molecules are more obvious, i.e. have more prominence, in skeletal diagrams than in most fully displayed formulae.
- This type of diagram is commonly used in the chemical industry so it is useful for students to be familiar with it.
When and where are skeletal formulae used ?
This type of representation of organic molecules is most frequently used in more advanced texts, research papers, and specialist areas. It is generally the most practical way to draw large and very complicated organic molecules. Even at lower levels of complexity (e.g. High School Chemistry and UK A-Level), skeletal formulae may be used - especially to describe structures involving carbon rings, such as cycohexane and benzene - see below.
Skeletal Formulae of Organic Molecules that have a "ring", rather than a "linear" or "branched" structure:
Cyclohexane and Benzene
As can be seen from the example of ethylbenzene (above), units of alkane chains are represented by straight lines at alternating angles, as indicated with examples further up this page.
More Examples of Skeletal Formulae
Note that some sources such as books, websites, reports etc., emphasize the presence of atoms other than carbon and hydrogen using colours - as in the
following examples of skeletal formulae of predominately linear carbon-chain molecules. This representation (with use of colours) may be helpful but isn't strictly necessary and obviously cannot be used everywhere as some journals are printed in black ink only.
See also our main page about how to draw organic molecules.