Stereoisomers are compounds which have the same structural formula but the atoms are arranged differently in 3d space.

There are two types of stereoisomers:

• optical isomers.
• geometric isomers such as cis/trans and E/Z isomers.

Cis/trans isomers

The diagram below shows two molecules of 1,2-dichloroethane (C₂H₄Cl₂). These two molecules appear to have a different shape from each other; so are they isomers? They have the same structural formula but the atoms are arranged differently in 3d space, they appear to fit the definition for a stereoisomer!

At first glance you may think that these two molecules are indeed isomers however they are NOT. The reason they are not isomers is simple enough; there is free rotation about the carbon-carbon bond.

If you can imagine that each of the carbon atoms and all the groups attached to them are in constant rotation; this means that these two apparent isomers are actually the same compound. It is possible to convert one of these apparent isomers into the other by simply rotating the carbon-carbon bond as shown below:

However if we could restrict or stop the rotation about the C-C bond then we would indeed have two stereoisomers. The molecules shown below are alkenes which contain the C=C functional group. The presence of the double covalent bond between the atoms of carbon stops any rotation:

How would we name these two different geometric isomers of 1,2-dichloroethene?
The traditional method of naming these two isomers is using the prefixes cis and trans. In the cis-isomer both the chlorine atoms are on the same side of the molecule and the hydrogen atoms are on the other, whereas in the trans-isomer the two chlorine atoms are on opposite sides of the molecule. This is shown below:

Example 2 - But-2-ene

But-2-ene exists as a pair of geometric isomers. The cis and trans isomers of but-2-ene are shown below. In the cis-isomer the two methyl groups (CH₃) attached to the carbon atoms in the C=C are on one side of the molecule whereas in the trans-isomer the two methyl groups (CH₃) are on opposite sides of the molecule:

Geometric isomers

Care is needed in deciding if a particular alkene will show geometric isomerism; that is have a cis and trans isomer. The main requirement for cis/trans isomerism is that the two groups bonded to each of the carbon atoms in the carbon-carbon double bond (C=C) are different. The cis/trans naming system is used to name disubstituted alkenes.

So ethene shown opposite will not have cis/trans isomers since each of the carbon atoms in the carbon-carbon double bond is bonded to identical hydrogen atoms.

In the example above but-2-ene can exist as a pair of geometric isomers however but-1-ene does not have any cis/trans isomers. The reason for this is the same reason why ethene has no geometric isomer, two of the atoms attached to one of the carbon atoms in the C=C are the same. This is shown below:

The structure of but-1-ene

E-Z stereoisomerism

In the examples used above to illustrate cis-trans isomerism we used alkene that were disubstituted with two different groups on each carbon atom in the C=C but the groups were the same on each of the carbon atoms in the C=C. However the cis-trans naming system while very useful has some limitations for example the two molecules shown below are stereoisomers (geometric isomers). It would prove impossible to identify which is the cis and which is the trans isomer based on the information above.

An improved naming system called the E-Z system can be used to describe these two geometric isomers. This system basically uses the atomic number (to be more precise it uses the (Cahn-Ingold-Prelog rules) of the atom attached to the carbon atom in the carbon-carbon double bond (C=C) to put the atoms in order of priority. So for the two geometric isomers mentioned above the numbers beside the atoms in the image below are the orders of priority of the atoms based on their atomic number. The higher the atomic number of the attached atom the higher is its priority rating.

If the two higher priority groups are on the same side of the molecule then the molecule is designated the Z-isomer ( z for zusammen, German for together) and if the two groups of higher priority are on opposite sides of the molecule then this isomer is designated the E-isomer (e for entgegen, German for opposite). This is outlined in the image below:

So although the cis-trans naming system does not work for all molecules it is still widely used in chemistry to name disubstituted alkenes and you should not dismiss it out of hand, you will meet lots of molecules which are described using this system however the E-Z system has the advantage over the cis-trans system in that it works for all molecules.

However it is clearly possible to use both systems to describe simple molecules; the image below is of the cis-trans isomers of but-2-ene but they are also identified using the E-Z system where the order of priority of the atoms/groups attached to the carbon atoms in the C=C are labelled.

Key Points

• Stereoisomers are compounds which have the same structural formula but the atoms are arranged differently in 3d space.
• Geometric isomers are a type of stereoisomer found in alkenes. Cis-trans and E-Z naming systems are used to identify geometric isomers.
• Using the E-Z system we have:
  
  • E-isomer : the higher priority groups are on opposite sides of the double bond.
  • Z-isomer: the higher priority groups are on the same side of the double bond.

Check your understanding - Questions on stereoisomers

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