GEOMETRICAL ISOMERISM

Geometric Isomerism in Organic Molecules

Geometric (cis / trans) isomerism

These isomers occur where you have restricted rotation somewhere in a molecule. At an introductory level in organic chemistry, examples usually just involve the carbon-carbon double bond - and that's what this page will concentrate on. Think about what happens in molecules where there is unrestricted rotation about carbon bonds - in other words where the carbon-carbon bonds are all single. The next diagram shows two possible configurations of 1,2-dichloroethane.

These two models represent exactly the same molecule. You can get from one to the other just by twisting around the carbon-carbon single bond. These molecules are not isomers.

If you draw a structural formula instead of using models, you have to bear in mind the possibility of this free rotation about single bonds. You must accept that these two structures represent the same molecule:

But what happens if you have a carbon-carbon double bond - as in 1,2-dichloroethene?

These two molecules are not the same. The carbon-carbon double bond won't rotate and so you would have to take the models to pieces in order to convert one structure into the other one. That is a simple test for isomers. If you have to take a model to pieces to convert it into another one, then you've got isomers. If you merely have to twist it a bit, then you haven't!

Drawing structural formulae for the last pair of models gives two possible isomers:

1. In one, the two chlorine atoms are locked on opposite sides of the double bond. This is known as the trans isomer. (trans : from latin meaning "across" - as in transatlantic).
2. In the other, the two chlorine atoms are locked on the same side of the double bond. This is know as the cis isomer. (cis : from latin meaning "on this side")

The most likely example of geometric isomerism you will meet at an introductory level is but-2-ene. In one case, the CH3 groups are on opposite sides of the double bond, and in the other case they are on the same side.

How to recognize the possibility of geometric isomerism

You obviously need to have restricted rotation somewhere in the molecule. Compounds containing a carbon-carbon double bond have this restricted rotation. (Other sorts of compounds may have restricted rotation as well, but we are concentrating on the case you are most likely to meet when you first come across geometric isomers.) If you have a carbon-carbon double bond, you need to think carefully about the possibility of geometric isomers.

What needs to be attached to the carbon-carbon double bond?

Think about this case:

Although we've swapped the right-hand groups around, these are still the same molecule. To get from one to the other, all you would have to do is to turn the whole model over. You won't have geometric isomers if there are two groups the same on one end of the bond - in this case, the two pink groups on the left-hand end. So there must be two different groups on the left-hand carbon and two different groups on the right-hand one. The cases we've been exploring earlier are like this:

But you could make things even more different and still have geometric isomers:

Here, the red and green groups are either on the same side of the bond or the opposite side.on the opposite , You still get geometric isomers, but by now the words cis and trans are meaningless. This is where the more sophisticated E-Z notation comes in.

Definition of cis—trans. Atoms or groups are

termed cis or trans to one another when they lie respectively on the same or on opposite sides of a reference plane identifiable as common among stereoisomers. The compounds in which such relations occur are termed cis—

trans -isomers. For compounds containing only doubly

bonded atoms the reference plane contains the doublybonded atoms and is erpendicular to the plane containingthese atoms and those directly attached to them. 

For cyclic compounds the reference plane is that in which the ring skeleton lies or to which it approximates.

To get geometric isomers you must have:

• restricted rotation (often involving a carbon-carbon double bond for introductory purposes);
• two different groups on the left-hand end of the bond and two different groups on the right-hand end. It doesn't matter whether the left-hand groups are the same as the right-hand ones or not.

The simple convention of denoting the geometrical isomers by cis/trans descriptors is not sufficient when there are more than two different substituents on a double bond. To differentiate the stereochemistry in them, a new system of nomenclature known as the E-Z notation method is to be adopted.

According to this method, if the groups with higher priorities are present on the opposite sides of the double bond, that isomer is denoted by E.

Where E = Entgegen  ( the German word for 'opposite')

However, if the groups with higher priorities are on the same side of the double bond, that isomer is denoted by Z.

Where Z = Zusammen (the German word for 'together')

The letters E and Z are represented within parentheses and are separated from the rest of the name with a hyphen.

Determination of E and Z isomers
for alkenes

1. The priority of a groups depends upopn the atomic number of the atom attached to the double bond. The higher the atomic number, the higher the priority of the group is.
2. If two atoms with the same priority are bound to the carbon atom of the double bond, a search is made along each group, moving away from the double bond until a difference is found, and priority is assinged on the basis of that difference.
3. The double bond is assigned a Z (for zusammen, German for together) configuration when the two groups with the higher priority at each end of the doube bond are on the same side of the molecule. If the two groups of higher priority are on opposite sides of the double bond, the molecule is given an E (for entgegen, German for opposite) configuration.