Structural Formulas
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Structural Formulas

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.

Molecular Formula

Electron Dot Diagram

Structural Formula

C2H6O

H   H   
    
H : C : C : O : H
    
H   H   

H   H    
|   |    
H - C - C - O - H
|   |    
H   H   

The molecular formula C2H6O does not really tell you which atoms are bonded to which other atoms. Even knowing how many valence electrons each atom has and needs, you would still be able to properly combine the atoms together in more than one way. Each way would be a different compound. The electron dot diagram for one of those arrangements is shown above. It shows how each atom has shared electrons to fill up its valence shell (or energy level) with eight electrons (two for hydrogen). The electron dot diagram also shows which atom is bonded to which other atoms and what kinds of bonds are used. The structural formula does the same thing, but with fewer strokes and less clutter.

 

Molecular Formula

Electron Dot Diagram

Structrual Formula

C2H6O

H       H
   
H : C : O : C : H
   
H       H

H      H
|       |
H - C - O - C - H
|       |
H       H

Here is the electron dot diagram and structural formula for another compound that has the same molecular formula, C2H6O. In each case notice how the structural formula gives you much more information about how the molecule is put together than does the molecular formula. Throughout the next few lessons you should use structural formulas instead of molecular formulas.

Structural Isomers

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.

Practice

Here are a few more structural formulas. (These are also shown in Exercise 3 in your workbook.) Again they show how the atoms are bonded to one another. They show which atoms are bonded to which other atoms. They also show whether the bonds are single, double or triple bonds. Take a look at these structural formulas to see how many bonding combinations you can find. For example, C-H single bond, C-C single bond, C=C double bond, etc. Check your observations below.

         H
         |
     H - C - C
C - H
         |
         H

         H   H   H   H
         |   |   |   |
     H - C = C - C = C - H
         H   H   O - H
         |   |   |
     H - C - C - C = O
         |   |
         H   N - H
             |
             H

 

Answers

         H
         |
     H - C - C
C - H
         |
         H
C-H single bonds
C-C single bonds
CC triple bond
         H   H   H   H
         |   |   |   |
     H - C = C - C = C - H
C-H single bonds
C-C single bonds
C=C double bonds
         H   H   O - H
         |   |   |
     H - C - C - C = O
         |   |
         H   N - H
             |
             H
C-H single bonds
O-H single bond
N-H single bonds
C-C single bonds
C-N single bond
C-O single bond
C=O double bond

 

Geometry

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.

Here is another flat drawing, but it is drawn with some perspective to give you some idea of the shape that these molecules have. Drawing of CH4 models.[61mod03.JPG]

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.

 

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E-mail instructor: Eden Francis

Clackamas Community College
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