E,Z system of nomenclature

To deal with this problem, we use the E-Z system of nomenclature (pun intended) for cis-trans isomers, which is patterned after the Cahn-Ingold-Prelog convention for asymmetric carbon atoms (Section 5-3). It assigns a unique configuration of either E or Z to any double bond capable of geometric isomerism. 

For example, alkenes A and B are two different compounds that are both called 3-methyl-2-pentene. In A the two CH3 groups are cis, whereas in B the CH3 and CH2CH3 groups are cis. The E,Z system of nomenclature has been devised to unambiguously name these kinds of alkenes. 

E,Z System of nomenclature (Section I0.3B) A system for unambiguously naming alkene stereoisomers by assigning priorities to the two groups on each carbon of the double bond. The E isomer has the two higher priority groups on opposite sides of the double bond, and the Z isomer has them on the same side. 

The E,Z system of nomenclature for substituted alkenes, The higher-priority groups on each carbon are on the same side of the double bond in the 2 Isomer, but are on opposite sides In the isomer. 

Rank the groups (or atoms) bonded to the asymmetric carbon in order of priority. The atomic numbers of the atoms directly attached to the asymmetric carbon determine the relative priorities. The higher the atomic number, the higher the priority. (This should remind you of the way relative priorities are determined for the E,Z system of nomenclature because the system of priorities was originally devised for the R,S system of nomenclature and was later borrowed for the E,Z system. You may want to revisit Section 3.5 to review how relative priorities are determined before you proceed with the R,S system. 

Imines can exist as stereoisomers. The isomers are named by the E, Z system of nomenclature. (The lone pair has the lowest priority.)... 

The E,Z system of nomenclature was devised for alkenes that do not have a hydrogen attached to each of the sp carbons. ... 

Note that, as in R, S nomenclature, the E or Z is placed in parentheses. The E, Z system of nomenclature can be used for all alkene stereoisomers consequently, the lUPAC recommends that this system be used exclusively. However, many chemists continue to use the cis and trans designations for simple alkenes. 

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