Structural formulas have particular value in the study of organic chemistry. They show the arrangement of the atoms within the molecules as far as which atoms are bonded to which and whether single, double or triple bonds are used. Let's look at those particular aspects of structural formulas.
The fact that carbon can bond in different types of arrangements, even when the same number of carbon atoms is involved, leads to a situation where different organic molecules can have the same molecular formula and still be different compounds by having their atoms arranged in different ways, as you have just seen. Such different compounds which have the same molecular formulas are called isomers.
Structural isomers are compounds with the same number and type of atoms in their molecules, but have them bonded to one another in a different arrangement. It can also be said that structural isomers have the same molecular formula but different structural formulas. We will deal with structural isomers quite a bit in the next several lessons.
It is important to point out that all the diagrams that you see in textbooks and workbooks and computer monitors are drawn flat. They are drawn out on a two-dimensional piece of paper or computer screen. Usually, the drawings seem to indicate the bonds for carbon atoms go up, down, to the left, and to the right. This is not really the way that those bonds point in carbon compounds. The bonds from carbon to other atoms have a very important structural feature. Those bonds go out in four different directions in three dimensions rather than in the two dimensions that are drawn on the paper or monitor.
Another way to represent molecules on computer screens is with rotatable images using programs such as RasMol and Chime. For more information about those programs you can link to the RasMol Home Page maintained by Eric Martz of the University of Massachusetts at http://www.umass.edu/microbio/rasmol/index.html. Chime is a program from MDL Information Systems, Inc., that allows these images to be manipulated by the viewer. To use Chime you need to have a browser plug-in that can be downloaded from http://www.mdli.com/download/chime. To see some of these images I recommend that you download and install Chime and then look at the site called Molecules from Chemistry at Okanogan University College in British Columbia at http://www.sci.ouc.bc.ca/chem/molecule/molecule.html. Start with "alkanes" about half-way down the page, then click on methane (CH4). When you have the image, you can alter its display and orientation by right-clicking on the image and then selecting the options you want.
All of these diagrams and images are trying to represent the shape and arrangement of molecules. That can also be done using models, which we will consider in the next page.
E-mail instructor: Eden Francis
Clackamas Community College