Stereochemistry

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Overview:

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 is introduced.

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.

 

 

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