E, Z system

It becomes difficult to decide whether, for example 1 is cis or trans isomer No. two groups are the same and the same is true for the compound II. The Chemical Abstracts Service has proposed an unambiguous system that has been adopted by IUPAC and is based on the preferences of groups. It is also called the E-Z system and applies to alkene diastereomers of all types. 

To simplify this situation, the E/Z system is used for naming geometrical isomers. Z stands for German zusammen, which means the same side, and E for German entgegen, meaning on the opposite side. 

This w-ay of naming double bonds is far superior to cis/trans nomenclature because you can use this E/Z system for any double bond, even if all four groups are different. Cis/trans nomenclature requires two groups to be the same. 

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. 

Some of the following examples can show geometric isomerism, and some cannot. For the ones that can, draw all the geometric isomers, and assign complete names using the E-Z system. 

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. 

We have already seen in Chapter 3, Section 3.5B, the configuration of geometric isomers can be expressed using the E,Z system. If the two high priority groups are on the same side of the double bond, E is assigned if they are on opposite sides, the configuration is Z. 

An alternative convention is also often used in which the position of the first double bond is numbered from the end of the molecule bearing the functional group, the co symbol being replaced by A. Thus vaccenic acid, which is most commonly found as the trans isomer, can be represented as frans-18 1 co7 or frans-18 1A11. The alternative E/Z system for labelling cis/trans isomers (see Box 2.3) is not commonly applied to fatty acids. 

I shall use the configurational descriptors ci5 and trans with the 0" or group always being the point of reference. In the E-Z system, depending on the nature of R, either OM (0 ) or R may be fiducial, which is confusing. 

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.)... 

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