When we say that Br and Cl are trans to each other, we can also say with an equal degree of confidence that I and Cl are cis to each other. It is, thus, difficult to name such a substance as the cis or the trans isomer.
To eliminate this confusion, a more general and easy system for designating configuration about a double bond has been adopted. This method, which is called the E and Z system, is based on a priority system originally developed by Cahn, Ingold and Prelog for use with optically active substances. In olefins, we assign the first (1) and a second (2) priority to the atoms or groups attached to each carbon of the double bond. Priority is then compared with one carbon relative to the other. If the higher-priority atoms or groups are on opposite side of the double bond, the configuration is designated as E (German: entgegen meaning across). If the higher-priority atoms or groups are on the same side of the double bond, the configuration is designated as Z (German: zusammen meaning together).
Rules for fixing the priorities of atoms or groups attached to doubly bonded carbon atoms are the same as discussed earlier. Some examples are as follows:
If there are two or more double bonds, prefixes E and Z are used for designating configuration around each double bond. For example,
The number of geometrical isomers of compounds containing two or more double bonds with non-equivalent termini:
Dienes in which the two termini are different (XHC=CH-CH=CHY) exist as four geometrical isomers.
Thus, it can be concluded that for a given molecule the number of geometrical isomers is 2n , where n is the number of double bonds. It may be noted here that this formula applies only to those molecules in which the two termini are different. However, in identical termini (XHC=CH-CH=CHX) the number of geometrical isomers is given by the following expressions:
(i) If n is even, the number of geometric isomers=2(n-1)+2(n/2-1)
(ii) If n is odd, the number of geometric isomers=2(n-1)+2(n-1/2)
Geometrical (cis-trans) Isomerism in Alicyclic Compounds:
In addition to the compounds containing a double bond polysubstituted alicyclic compounds such as disubstituted derivatives of cyclopropane, cyclopentane, cyclobutane and cyclohexane can also show cis-trans isomerism. In such cases, cis-trans isomerism arises due to restricted rotation about a single bond.
To study geometric isomerism in cyclic compounds, it is necessary to understand the relationship between the substituents in terms of cis and trans orientation. Although rings and substituents can be represented by using planar bonds, flying wedge projections are often used for a better understanding.
Some representative examples are: