Stereochemistry addresses the characteristics of the chiral carbon, the
concept of polarimetry, the assignment of configurations, the nature
of racemic mixtures, molecules with multiple chiral carbons, resolution
of a racemic modification, and reactions generating asymmetric centers.
The stereochemistry unit begins by having the computer construct
a simple molecule and its mirror image to demonstrate the concept
of enantiomers. The mirror imagery, but non-identical character
and non-superimposability, are graphically demonstrated using animated
molecular structures. A small change is shown to produce identical
structures that subsequently demonstrate superimposition. A method
of identifying the chiral center or centers on cyclic structures
Brief mention is made of Jean Baptiste Biot and and the serendipitous
work of Louis Pasteur in development of optical activity concepts
and the use of the polarimeter to differentiate among stereoisomers.
Dextrorotatory and levorotatory molecules are introduced, as is
the manner in which specific rotation may be calculated.
The Cahn-Ingold-Prelog rules are presented so that students may
understand assignment of absolute configuration and the way in which
handedness differs from the way in which a molecule rotates a plane
of polarized light. The concept of rotating the lowest priority
group behind the chiral carbon so subsequent determination of handedness
(or absolute configuration) may be made is readily demonstrated
using animated structures and molecules. Demonstrations show that
switching atoms changes absolute configuration. How to deal with
double bonds in determining group priorities, as in carbonyl groups,
is demonstrated. Calculations of degree of optical activity in racemic
modifications or mixtures are also treated.
The situation of multiple chiral carbons is used to open discussion
of diastereomers and the way in which their stereochemistry differs
from that of the previously mentioned enantiomers. Characteristics
of the meso structure are addressed and digital photography is used
to further clarify the concept. Digital photography is also used
to clearly show the manner in which the allene structure can generate
enantiomers, even though a single, asymmetric carbon is not present.
Resolution of racemic mixtures both in the laboratory and in nature
are discussed, particularly in light of pharmaceuticals. The physical
and chemical differences between enantiomers and diastereomers are
stressed throughout the study of stereochemistry.
Reactions to both generate and destroy an asymmetric center are
introduced and the student is given opportunity to predict the stereochemical
products expected from simple halogenation of both hydrocarbons
and alkyl halides. The components of each fraction generated are
used to further understanding of the structurally different compounds
produced in these reactions.