Stereoisomeric Conformations, Different Perspectives, Achiral Stereoisomers and Meso Compounds

Read this page after “The Basics” section.

Most of what I explain on this page can actually be understood by thoroughly studying the relevant terminology. Use of a model kit will also be essential. Nonetheless I see many students missing some of these relatively subtle points:

1) There is a distinction between stereoisomeric molecules and stereoisomeric conformations of the same molecule arising through bond rotation.  Chair conformations and gauche/anti conformations are examples of stereoisomeric conformations. Again, two conformations can be either enantiomeric or diastereomeric. Did I mention how great model kits are?

OCS-Chirality-Confusions-Conformations

 Stereoisomeric conformations, AKA conformers: A) Diastereomeric chair conformations of a trans substituted cyclohexane B) Enantiomeric chair conformations of a cis subsituted cyclohexane C) Diastereomeric conformations of an alkyl halide

Generally, however, when we refer to stereoisomers we are referring to two different stereoisomeric molecules that do not interconvert at room temperature. In contrast, conformational stereoisomers are often simply referred to as conformers.

2) Note the difference between simply drawing the exact same thing from a different perspective and drawing actual conformers. The graphic below shows different ways to draw the exact same conformation of the exact same molecule. Note how the various ways to draw chair conformations make these representations especially confusing! Model kits are helpful.

OCS-3-Ways-To-Draw-Same-Thing

The structures in the boxes above are actually the same conformation of the same molecule, just drawn from different perspectives. (5 membered rings aren’t actually flat, we just draw them in that conformation)

3) Chirality is NOT a pre-requisite for diastereomers to exist. There are many examples of achiral molecules that have stereoisomers that are also achiral. If you think about the last sentence enough, you’ll realize that these types of molecules must have diastereomeric relationships. Sometimes it might even be helpful to play with a model kit.

OCS-Achiral-Molecules-With-Stereoisomers

Examples of achiral molecules with achiral diastereomers. There are no possible stereoisomers of the molecules above that are chiral!

4) There are also examples of achiral molecules that have chiral diastereomers. Achiral molecules that have chiral diastereomers are called meso compounds. Another common definition of a meso compound is an achiral compound that has chiral centers. Humans are (roughly) meso. Although our hands and feet are like chiral centers, both of our “chiral centers” have mirror images that cause us to be, overall, essentially achiral. People are not truly achiral because technically no person is symmetrical. Not even Ryan Gosling or Angelina Jolie. Model kit, model kit, model kit, model kit?

OCS-Meso-Compounds

 Meso compounds (A, D) and their chiral diastereomers (B, C, E, F). B and C, as well as E and F are pairs of enantiomers. A and D do not have enantiomers because they are achiral.

Time to go do 700 practice problems……….