CHAPTER 5 Stereoisomers
5-1 Chiral Molecules
Sp3 carbons with four different substituents are centers of chirality. Molecules with one center of chirality are not superimposable on their mirror images. Non-superimposable mirror images are enantiomers of each other.
5-2 Optical Activity
Chiral molecules rotate the plane of polarized light. Enantiomers will have opposite signs of rotation. The value of the rotation is a linear function of path length, concentration, and a molecular parameter called specific rotation, [a]:
a = [a] x l x c
l is in decimeters and c is in g/cc. [a] varies with wavelength and temperature. [a] is also concentration dependent.
5-3 Absolute Configuration: R-S Sequence Rules
Cahn, Ingold, Prelog rules are used to specify the three-dimensional arrangment of groups about a center of chirality. Groups are rank ordered (prioritized) by the atomic number of the first attached atom, 1, 2, 3, 4. If there are duplicates, then the atoms attached to these first atoms are compared one-by-one in decreasing order of atomic number until a difference is found. Viewing the center of chirality with the lowest priority (4) at the back, a progression from 1 to 2 to 3 that goes clockwise is referred to as R (rectus) whereas that going counterclockwise is called S (sinister).
5-5 Molecules Incorporating Several Stereocenters:
Molecules with two (or more) centers of chirality can be related as either enantiomers or diastereomers.
5-6 Meso Compounds
Molecules with multiple centers of chirality will lead to 2n stereoisomers except when molecular symmetry is present where it is possible to have centers of chirality in an achiral molecule.
5-7 Stereochemistry in Chemical Reactions
Chiral molecules can be formed from achiral starting materials. Formation of a new chiral center in a molecule already containing one will produce diastereomers in unequal amounts.
5-8 Resolution: Separation of Enantiomers
Enantiomers can be separated by interaction with a single enantiomer of another chiral molecule. For example, a racemic mixture of amines can be combined with a chiral acid in an acid-base reaction. Two salts are formed that are diastereomers, which have different physical properties (for example, solubility).