ORGANIC CHEMISTRY - CHAPTER 5: STEREOISOMERS

Important Concepts

1. Isomers have the same molecular formula but are different compounds. Constitutional (structural) isomers differ in the order in which the individual atoms are connected. Stereoisomers have the same connectivity but differ in the three-dimensional arrangement of the atoms. Mirror-image stereoisomers are related to each other as object and mirror image.

2. An object that is not superimposable on its mirror image is chiral.

3. A carbon atom bearing four different substituents (asymmetric carbon)is an example of a stereocenter.

4. Two stereoisomers that are related to each other as image – nonsuperimposable mirror image are called enantiomers.

5. A compound containing one stereocenter is chiral and exists as a pair of enantiomers. A 1:1 mixture of enantiomers is a racemate (racemic mixture).

6. Chiral molecules cannot have a plane of symmetry (mirror plane). If a molecule has a mirror plane,then it is achiral.

7. Diastereomers are stereoisomers that are not related to each other as object to mirror image. Cis and trans isomers of cyclic compounds are examples of diastereomers.

8. Two stereocenters in a molecule result in as many as four stereoisomers — two diastereomerically related pairs of enantiomers. The maximum number of stereoisomers that a compound with n stereocenters can have is 2ⁿ. This number is reduced when equivalently substituted stereocenters give rise to a plane of symmetry. A molecule containing stereocenters anda mirror plane is identical with its mirror image (achiral) and is called a meso compound. The presence of a mirror plane in any energetically accessible conformation of a molecule is sufficient to make it achiral.

9. Most of the physical properties of enantiomers are the same. A major exception is their interaction with plane-polarized light: One enantiomer will rotate the polarization plane clockwise (dextrorotatory),the other counterclockwise (levorotatory).This phenomenon is called optical activity.The extent of the rotation is measured in degrees and is expressed by the specific rotation,  [α]. Racemates and meso compounds show zero rotation. The enantiomer excessor optical purity of an unequal mixture of enantiomers is given by
                           % optical purity = ([α]_observed/[α]) x 100

10. The “handedness” of a stereocenter (its absolute configuration) is revealed by X-ray diffraction and can be assigned as R or S by using the sequence rules of Cahn, Ingold, and Prelog.
11. Fischer projections provide stencils for the quick drawing of molecules with stereocenters.
12. Chirality can be introduced into an achiral compound by radical halogenation. When the transition states are enantiomeric (related as object and mirror image), the result is a racemate because the faces of the planar radical react at equal rates.
13. Radical halogenation of a chiral molecule containing one stereocenter will give a racemate if the reaction takes place at the stereocenter. When reaction elsewhere leads to two diastereomers, they will be formed in unequal amounts.
14. The preference for the formation of one stereoisomer, when several are possible, is called
stereoselectivity.
15.The separation of enantiomers is called resolution. It is achieved by the reaction of the racemate with the pure enantiomer of a chiral compound to yield separable diastereomers. Chemical removal of the chiral reagent frees both enantiomers of the original racemate. Another way of separating enantiomers is by chiral chromatography on an optically active support.

From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE