Multiple Bonds


   
This website offers an interesting way to approach the subject of multiple bonds (moving from sigma structures to lewis structures). Since multiple bond theory was developed simply as a way to explain the uncharacteristically short bonds found in some molecules, using the data in this website, students can gain an accurate appreciation of multiple bonds by approaching the subject from the same angle: data first, theory second.  We also decided not to show double bonds in the molecules in the database, which is helpful for two reasons. 1. It does not complicate things for beginning chemistry students who haven’t learned multiple bonds yet and 2. Multiple bonds are theoretical interpretations of the experimental data. We prefer, for the purposes of this website, to include as few of our interpretations of the data as possible so that the students can come to their own conclusions, rather than be force-fed ours.

 

Suggested Activity #1

We suggest having students explore the database, taking special note of variation in bond length between molecules. The "bonds" search was developed especially for that purpose. Once the "bonds" search is selected, the red histogram on the left can be used to narrow down the search to show only molecules that contain bond lengths within a certain range. It is difficult to see the pattern if the whole database is searched, so we suggest using the “entry level” database and restricting the search to C-C bonds. When the results come up, you can sort by bond length, and then, as you scroll down you can see that triple bonds are the shortest, followed by double and then finally all the single bonds show up. Students should be making sigma structures of some molecules on the list, and then a classroom discussion could follow about why some of the bonds are so much shorter than others. At that point, the octet rule for the central atom should be introduced.

The same website that is helpful in learning sigma structures can be used for lewis structures.

http://www.stolaf.edu/depts/chemistry/courses/toolkits/121/js/lewis/

 

 

        sigma structure

        lewis structure

 

 

              sigma structure

               lewis structure

 

 

                                                                       

 

 

Suggested activity #2:

          Along with the concept of double bonds comes the concept of resonance: the idea that some bonds do not have complete single or multiple bond character. This website can also be used to demonstrate that such molecules have bonds that are intermediate in length, basically that they are measurably in between a single and a double bond. The “cp database” option is a good place to start looking for molecules to use as examples of this. (All hydrogens are shown in the structures. Remember that if there are only 3 bonds to a carbon atom, it is extremely likely that one of the there is a double bond somewhere. For instance, if there is a 6 member carbon ring with only one hydrogen on each carbon, there are 3 double bonds in the ring being shared equally by all the atoms.)

Some good molecules to look at are:

·        Benzene

·        carbonates