3.2 E, Z nomenclature
3.2 E, Z nomenclature
The cis-trans nomenclature leaves some vagueness as is shown in the examples as Q3.1 and Q3.4. Cis-trans nomenclature is originally a trivial system, and useful when simple compounds are treated. It is not competent, however, as a general nomenclature. There was proposed a general nomenclature of geometrical isomers based on the sequence rule. The procedure is as follows:
- 1) To determine the priority of two ligands (including lone pair) bonded to the each atom forming the double bond based on the sequence rule.
- 2) To name the isomer as below when the ligands in upper rank are located on:
- the same side of the double bond: Z (abbreviation of zusammen) or seqcis
- the opposite side of the double bond: E (abbreviation of entgegen) or seqtrans
Prefixes such as seqcis and seqtrans are equal to cis and trans, respectively, in the sequence rule (SEQuence rule). Symbols Z and E are, however, exclusively used today.