The Bones of Organic Molecules: The Hydrocarbons

Comparing Relationships between

Stereoisomers and Meso Compounds

Stereoisomers are molecules that have the same atom connectivity but differ in the configuration of their chiral centers. Just as the relationship of a right hand to a left hand is that of

mirror images, a molecule with a chiral center of R configuration is the mirror image of the

same molecule with the S configuration. Chemists call molecules that are mirror images of

each other enantiomers, so in orgo-speak, your right hand is the enantiomer of your left.

(its enantiomer) is an arm with two right hands attached. Similarly, the enantiomer of a molecule with two R chiral centers is a molecule with two S chiral centers. The bottom line is that

the enantiomer of a molecule is a molecule with all the configurations switched (all R configurations go to S, and all S configurations go to R).

When you have more than one chiral center on a molecule, getting stereoisomers that aren’t

mirror images is possible. If you were to have an arm with two right hands and another arm

with one right hand and one left hand, the two arms would have a different relationship than

that of mirror images. Stereoisomers that aren’t mirror images of each other are in a diastereomeric relationship. One example of a pair of diastereomers is one molecule that has two R

chiral centers and a molecule with one R and one S chiral center. In other words, a diastereomer is a stereoisomer that doesn’t have each and every chiral center switched and is not a

mirror image.

With one exception, any molecule that has a chiral center is a chiral molecule. A chiral molecule is a molecule that has a non-superimposable mirror image. What that means is that you

can’t superimpose the mirror image of the molecule onto the original molecule. A molecule

with an R chiral center isn’t superimposable on its mirror image, the enantiomer with an S

chiral center — the atoms simply don’t align, much like the fingers of your right hand don’t

align with the fingers on your left hand.

Molecules that have chiral centers are chiral molecules — with one exception. Meso compounds are molecules that have chiral centers but are achiral (not chiral) as a result of having

a plane of symmetry in the molecule. A plane of symmetry is an imaginary line that you can

draw in a molecule for which both halves are mirror images of each other. Unlike chiral molecules, meso compounds have mirror images that are superimposable on the original molecule (that is, the mirror images are identical to the original molecule). Thus, being able to

spot planes of symmetry in molecules that have chiral centers is important for determining

whether the molecule is chiral or achiral.

Watch for rotations around single bonds. Anytime you can rotate around a single bond to

give a structure that has a plane of symmetry — in other words, when any conformation has Here you have two chiral centers. By convention, chiral centers in Fischer projections are

aligned vertically, so you first rotate the molecule so that the bond connecting the two chiral

centers is vertical. Then perform two quarter turns to get the molecule into the right orientation for the Fischer projection.

a plane of symmetry — the molecule is meso and achiral Even for people with excellent visualization skills, this is a difficult process. Molecules with

more than one chiral center are often easier to convert to the Fischer projection by bypassing

the quarter turn and 3-D visualization business: Assign the R or S configurations to the chiral

centers in the 3-D projection and then add the substituents to a Fischer projection to make sure

that the configurations remain unchanged.