SOME IMPORTANT CONCEPTS IN STEREOCHEMISTRY
1. RELATIONSHIP BETWEEN SYMMETRY AND CHIRALITY
Asymmetric objects are chiral
Symmetric objects are achiral
2. RELATIONSHIP BETWEEN OBJECTS AND THEIR MIRROR IMAGES
Symmetric objects are superposable with their mirror images. They are one and the same.
Asymmetric objects are nonsuperposable with their mirror images. They are different objects.
In the case of molecules, chiral molecules and their mirror images are different molecules.
Chiral molecules and their mirror images are a kind of stereoisomers called enantiomers.
Stereoisomers - Compounds that have the same molecular formula and the same connectivity,
but different arrangement of the atoms in 3-dimensional space.
Stereoisomers cannot be converted into each other without breaking bonds.
Enantiomers - Nonsuperposable mirror images, or chiral molecules which are mirror images.
Chiral, or asymmetric carbon - A tetrahedral carbon atom bearing four different substituents.
Chirality centers, or stereocenters - Asymmetrically substituted atoms in a molecular structure.
The most common type encountered in this course will be the chiral carbon described above.
Diastereomers - Stereoisomers which are not enantiomers (or mirror images).
Meso compounds, or meso forms - Symmetric, or achiral molecules that contain stereocenters.
Meso compounds and their mirror images are not stereoisomers, since they are identical.
Optical activity - The ability of chiral substances to rotate the plane of polarized light by a specific angle.
Dextrorotatory - Ability of chiral substances to rotate the plane of polarized light to the right.
Levorotatory - Ability of chiral substances to rotate the plane of polarized light to the left.
Specific rotation - The measured angle of rotation of polarized light by a pure chiral sample under specified standard conditions (refer to textbook for a description of these).
Racemic mixture, racemic modification, or racemate - A mixture consisting of equal amounts
of enantiomers. A racemic mixture exhibits no optical activity because the activities of the
individual enantiomers are equal and opposite in value, therby canceling each other out.
Optical purity - The difference in percent between two enantiomers present in a mixture in unequal amounts. For example, if a mixture contains 75% of one enantiomer and 25% of the other, the optical purity is 75-25 = 50%.
Absolute configuration - A description of the precise 3-dimensional topography of the molecule.
Relative configuration - A description of the 3-dimensional topography of the molecule relative
to an arbitrary standard. Absolute and relative configurations may or may not coincide.