Stereoisomerism

From WikidChem

Slide 2:

Of the four "C"s, composition, constitution, configuration, and conformation, the last three represent isomers. The last two, configuration and conformation, describe stereoisomers. The difference between the two is that to go from one configurational isomer to another, you would have to break bonds (or flatten the molecule, but that's also unfavorable and would require a lot of energy -ADK) . This is the case when the isomers involve asymmetric carbons. Going from one conformational isomer to another requires rotation around a single bond, as Paterno described when trying to explain the supposed three isomers of dibromoethane. It is usually easier to change between two different conformational isomers than two configurational isomers.

It is also important to note that isomers are energy minima. We do not consider every slightly different vibration a different isomer just as we do not consider every slightly different bond length a different kind of bond.

Slide 3:

The words we use to describe isomers refer to the relation between two molecules, not to the molecules themselves. If two molecules have the same constitution, they can be either identical (which could be referred to as "homomers," but that term is never used), completely different (referred to as "diastereomers"), or nonsuperimposable mirror images ("enantiomers").

   Enantiomers are mirror images, so they may smell differently and rotate light in equal and opposite directions.....However, properties like boiling points, density, etc, would likely be the same. -ADK

KK-3:54pm 11/30/06