Sometimes, when you grade papers, you have to laugh to keep from crying. I’ve been teaching chemistry to college sutdents for a number of years now, and I keep a file of some of the strangest student answers I’ve ever received. I didn’t keep what I dubbed the “e.e. cummings lab report” – written without any capitalization or puctuation – but I do have a few of what I call “Classics of Student Literature”. Here is one of the classics.
Once upon a time, I asked some of my students on an exam to use the valence bond model to explain why there were two distinct forms of the dichloroethene molecule: cis-dichloroethene and trans-dichloroethene.
![[cis-dichloroethene]](http://whenchemistsattack.com/blogfiles/cisdce.png)
cis-dichloroethene (Chlorine atoms on the same side)
![[trans-dichloroethene]](http://whenchemistsattack.com/blogfiles/transdce.png)
trans-dichloroethene (Chlorine atoms on opposite sides)
I also told the students in the question that the cis form has a larger dipole moment and a higher boiling point than the trans form. The students were merely asked to explain why there are two forms of the molecule in the first place. Since they’d just studied sigma and pi bonding, I was expecting an answer that mentioned that since the double bond contained an off-axis pi bond, that rotation of the molecule around the double bond (changing the cis form into the trans form or vice versa) would not be easy. The pi bond would have to be broken for the molecule to rotate.
Whether you remember enough freshman chemistry to make sense of the above paragraph or not, you might appreciate one of the answers I was given.
In cis molecule that is polar with a higher boiling point is different than the trans molecule. It has a double bond between C atoms. The shape of the two are the same but each one is different in other areas. Because of the structure + make of the cis and trans molecules is the reason it is able to exist. Both have the double bond between C atoms. But because one is polar and the other is nonpolar due to the charges it produces makes them able to exist. They also have different BP which makes them 2 different atoms. Because one is polar and one is nonpolar they have different properties which makes them different atoms that can exist.
Say what?