Question

Explain the method of writing E-Z configurations for geometrical isomers taking CHCL=CFBR as your example

Answer 1

E-Z NOTATION FOR GEOMETRIC ISOMERISM

This page explains the E-Z system for naming geometric isomers.

Important!  If you have come straight here via a search engine, you should be aware that this page follows on from anintroductory page about geometric isomerism. Unless you are already confident about how geometric isomers arise, and the cis-trans system for naming them, you should follow this link first. You will find links back to this current page at suitable points on that page.

The E-Z system

The problem with the cis-trans system for naming geometric isomers

Consider a simple case of geometric isomerism which we've already discussed on the previous page.



You can tell which is the cis and which the trans form just by looking at them. All you really have to remember is that trans means "across" (as in transatlantic or transcontinental) and that cis is the opposite. It is a simple and visual way of telling the two isomers apart. So why do we need another system?

There are problems as compounds get more complicated. For example, could you name these two isomers using cis and trans?



Because everything attached to the carbon-carbon double bond is different, there aren't any obvious things which you can think of as being "cis" or "trans" to each other. The E-Z system gets around this problem completely - but unfortunately makes things slightly more difficult for the simple examples you usually meet in introductory courses.

How the E-Z system works

We'll use the last two compounds as an example to explain how the system works.

You look at what is attached to each end of the double bond in turn, and give the two groups a "priority" according to a set of rules which we'll explore in a minute.

In the example above, at the left-hand end of the bond, it turns out that bromine has a higher priority than fluorine. And on the right-hand end, it turns out that chlorine has a higher priority than hydrogen.



If the two groups with the higher priorities are on the same side of the double bond, that is described as the (Z)- isomer. So you would write it as (Z)-name of compound. The symbol Z comes from a German word (zusammen) which means together.

Answer 2

Answer : The geometrical isomers are shown below.

Explanation :

The configuration of the geometrical isomers is designated by two system which are, Cis-trans system and E-Z system.

The rules for E-Z system are :

The atoms or groups attached to each olefinic carbon are given priority as per the sequence rule.

If the higher priority groups are present on same sides across the double bond, the geometrical isomer is said to have Z-configuration or 'cis'.

If the higher priority groups are present on opposite sides across the double bond, the geometrical isomer is said to have E-configuration or 'trans'.

In the given compound, higher priority groups are $Cl$ and $Br$.

When $Cl$ and $Br$ are present on same sides across the double bond, the geometrical isomer is said to have Z-configuration or 'cis'.

When $Cl$ and $Br$ are present on opposite sides across the double bond, the geometrical isomer is said to have E-configuration or 'trans'.

Hence, the geometrical isomers are shown below.