2.2 Stereoisomers

LEARNING GOALS

After   Chapter 2.2,    you will    be  able    to:

Differentiate   between conformational  and configurational isomers

Distinguish enantiomers from    diastereomers

Identify    enantiomers,    diastereomers,  and meso    compounds

Convert between Newman  and 3D  molecular   projections:    

Like structural isomers—and all isomers, for that matter—stereoisomers have the same chemical
formula. Unlike structural isomers, however, stereoisomers also share the same atomic
connectivity. In other words, they have the same structural backbone. Stereoisomers differ in how
these atoms are arranged in space (their wedge-and-dash pattern), and all isomers that are not
structural isomers fall under this category. The largest distinction within this class is between
conformational and configurational isomers. Conformational isomers differ in rotation around
single (σ) bonds; configurational isomers can be interconverted only by breaking bonds.

CONFORMATIONAL ISOMERS

Of all of the isomers, conformational isomers or conformers are the most similar. Conformational
isomers are, in fact, the same molecule, only at different points in their natural rotation around
single (σ) bonds.

While double bonds hold molecules in a specific position (as explained with cis–trans isomers later),
single bonds are free to rotate. Conformational isomers arise from the fact that varying degrees of
rotation around single bonds can create different levels of strain. These conformations are easy to
see when the molecule is depicted in a Newman projection, in which the molecule is visualized