Geometrical isomers.

The RS system also gives an unambiguous designation of geometrical isomers containing a double bond. At each end of the bond, select the group of highest priority. If these two groups lie on the same side of the double bond the configuration is Z (from the German zusammen,“together”); if on opposite sides E (entgegen,“opposite”).



Configurations of amide or ester linkages may also be specified in this manner. This is possible because the C–N bond of an amide has partial double-bond character, as to a lesser extent does the C–O bond to the bridge oxygen in an ester. In this case, assign the lowest priority to the unshared electron pair on the ester bridge oxygen.



An amide of the Z configuration is ordinarily referred to as trans in protein chemistry because the main chain atoms are trans.