Question

Why is it not possible to assign R or S to a compound if it is drawn such that the lowest-priority substituent is in the plane of the paper? What are some ways to handle this situation?

Answer 1

• In the CIP system of nomenclature, each chiral center in a molecule is assigned a prefix (R or S), according to whether its configuration is right- or left-handed. The symbol R comes from the Latin rectus for right, and S from the Latin sinister for left. The assignment of these prefixes depends on the application of two rules:   The Sequence Rule and The Viewing Rule. The viewing rule states that when the lowest priority substituent is oriented behind the triangular face defined by the three higher priority substituents, a clockwise sequential arrangement of these substituents is defined as R, and a counter-clockwise sequence as S.
  If the lowest priority group is in the plane of the paper, it will be difficult to assign the priorities of the groups attached to the tetrahedral carbon in either clockwise or anticlockwise manner.
• Now a tetrahedral structure may be viewed from any of the four triangular faces, and the symmetry of the system is such that a correct R/S assignment is made if the remote out-of plane group has an even number sequence priority (2 or 4), whereas the wrong assignment results when the out-of plane group has an odd priority (1 or 3). Once one recognizes this relationship, the viewing options are increased and a configurational assignment is more easily achieved.