Organic and Inorganic Chemistry Lesson of the Day – Cis/Trans Isomers Are Diastereomers
August 28, 2014 Leave a comment
Recall that the definition of diastereomers is simply 2 molecules that are NOT enantiomers. Diastereomers often have at least 2 stereogenic centres, and my previous lesson showed an example of how such diastereomers can arise.
However, while an enantiomer must have at least 1 stereogenic centre, there is nothing in the definition of a diastereomer that requires it to have any stereogenic centres. In fact, a diastereomer does not have to be chiral. A pair of cis/trans isomers are also diastereomers. Recall the example of trans-1,2-dibromoethylene and cis-1,2-dibromoethylene:
These 2 molecules are stereoisomers – they have the same atoms and sequence/connectivity of bonds, but they differ in their spatial orientations. They are NOT mirror images of each other, let alone non-superimposable mirror images. Thus, by definition, they are diastereomers, even though they are not chiral.