Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 328	Brief Organic Chemistry	Summer 2004
Lecture Notes: 9 June	© R. Paselk 2004
PREVIOUS		NEXT

Stereochemistry

D, L System

In the D, L system chiral centers are designated by comparison with a standard compound, glyceraldehyde, drawn below as Fischer structures:

Any other compound can then be determined to be D or L by aligning analogous side groups. For example, for an amino acid, the carboxylate group cooresponds to the aldehyded group, the amino group to the alcohol, the R- group to the methyl alcohol and hydrogen to hydrogen. Serine is shown as an example:

Note that for multicenter molecules, the penultimate chiral center determines whether the molecule is classified as D or L.

Acyclic Molecules with Multiple Chiral Centers

With a single chiral center (stereocenter) we have seen that we get a mirror image pair of enantiomers (nonsuperimposable mirror image sterioisomers).

What do we see with multiple centers? The classic example is the set of four carbon sugars, Erythrose and Threose, the stereoisomers of 2,3,4-trihydroxybutanal, HOCH₂CHOHCHOHCHO. Erythrose refers to the cis pair of enantiomers, while threose refers to the trans pair of enantiomers

So let's determine R and S and D and L for each of these stereoisomers and name them.

• For the erythrose isomer on the left,
  • For R,S:
    • Looking at C-2
      • OH will be highest priority, C-1 will be 2nd highest priority (two bonds to O), C-3 next and Hwill be lowest.
      • Next, we need to arrange the molecule so that we can look along the C-2 - H bond axis, with H away from us, and follow the substituents from highest to lowest. We see they are arranged in a clockwise (right-hand) manner, so C-2 is R
    • Looking at C-3 in a similar way we find that is also R.
  • This isomer is thus named: (2R,3R)-Erythrose or (2R,3R)-2,3,4-trihydroxybutanal.
  • For D,L remust go back and compare to glyceraldehyde, remembering we only look at the penultimate carbon to asign D or L. The left isomer mus therfore be D-Erythrose
• The erythrose isomer on the right must then be (2S,3S)-Erythrose or (2S,3S)-2,3,4-trihydroxybutanal or L-Erythrose, since it is the enantiomer.
• Similar reasoning will allow us to name the threose isomers as well (note that once we have assigned the carbons in the first isomer we can do ther other isomers by simple comparison.

Erythrose and Threose are Diastereomers: stereoisomers that are not mirror images, that is they are not enantiomers.

So what are some other examples of diastereomers? Remember stereoisomers are compounds where the atoms are connected in the same way, but which have different geometries. Thus cis , trans -isomers are diastereomers, as are configurational (optical) isomers, that is isomers differing only by the configurations around chiral centers but for which the entire molecules are not mirror images.

Note that the maximum number of stereoisomers for a given molecule = 2ⁿ where n = the number of chiral centers. If this is the maximum, how would we have fewer than the maximum? Let's look at another clssic example, tartaric acid, 2,3-Dihydroxybutanedioic acid, HOCOCHOHCHOHCOOH. Note that unlike our previous example, 2,3,4-trihydroxybutanal, this formula is symmetrical.

As a result, while the trans isomers of tartaric acid form a pair of enantiomers like threose,

the cis isomer has a single stereoisomer because it has an internal mirror plane and is thus its own mirror image. It is a Meso compound: Having two or more stereogenic centers, but also having a mirror plane of symmetry in the molecule, so molecule is not chiral.

Cyclic Molecules with Multiple Chiral Centers

Note illustrations and discussion in your text.

Schedule

C328 Home

Lecture Notes

Last modified 9 June 2004