Graphical and Textual Representations of Stereochemistry

Where the absolute configuration is known, structures can be represented either as Fischer type diagrams or as perspective diagrams. Fischer diagrams follow the convention that the principal chain occupies the vertical position, with the head of the chain uppermost. 

In addition to Fischer and perspective diagrams, physical organic chemists use Newman and sawhorse diagrams to show conformations as well as configurations of two-centre compounds. 

In CAS presentation (9C1 period), the labels R, S are used not only where the absolute configuration is unknown, but also where it is known. R is allocated to the centre of highest sequence priority, e.g., in the above example, position 3 (since Br Cl). The general descriptor (R, S ) for this diastereoisomer is then modified where the absolute configuration is known, and the citation refers to the optically active material. Thus, the isomer illustrated above is [R-(R, S )], and its racemate, when specifically referred to, is [(R, S )-( )]. 

These CAS rules, which had been in use since the beginning of the 9th Collective Index period (1972), have now been thoroughly revised to give a simplified and more intuitive description. The need for a single expression to describe the total stereochemistry of a molecule has been eliminated. Stereochemical terms are now placed within the parts of a chemical name to which the stereochemical information applies. The following diagram shows the now superseded 9C1 descriptors alongside the revised equivalents, which are closer to now current CAS practice. 

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