OK it is time to think about alkenes, these are things which pi clouds which can do a series of reactions. But first we need to get some things clear about the names. We need to know about cis and trans. The cis isomer is the isomer where two hydrogens are on one side and the two other things are on the other side. In general cis isomers tend to be higher in energy because of steric effects. Below I have a picture of the cis isomer of 1,2-difluoroethylene.
While here is the trans isomer note where the pink flourine atoms are now.
Reblogged this on Organic Reactions in Medchem.