Before we dive into the intricacies of isomerism, let’s take a look at what isomerism is. Notice your hands. They are both made up of four fingers and a thumb and they look identical on both the hands. Now put one hand on top of the other such that the palm of one hand is on the top of the other hand. Aren’t your thumbs pointing in opposite directions? They are made of the same constituents and yet they are different. Isomers are exactly this. Isomers are molecules with the same chemical formulae but with a different chemical structure. Isomers are molecules that have the same molecular formula but have a different arrangement of the atoms in space.
This means a different arrangement entirely and not just a changed orientation or the slight rotation of the constituents of the molecules. Isomers do not necessarily share similar properties unless they also have the same functional groups in the case of isomerism in organic chemistry. Isomerism is most widely observed in organic chemistry and has a huge application in chemistry and medicine. Let’s take a look at one specific group of isomerism; Stereoisomerism.
We know that isomerism is the characteristic exhibited by molecules that have the same constituents but a different arrangement. In structural isomerism, the constituents of the molecules either have a different attachment point or the entire carbon chain is made differently. Stereoisomerism instead refers to the varied spatial arrangement of molecules with identical constituents. Stereoisomers are joined up in the same order but still manage to have a different geometric and spatial arrangement. Let’s explore an important type of stereoisomerism; Geometric Isomerism.
Isomerism in organic molecules is extremely intriguing. For example, geometric isomerism arises due to the fact that some bonds between the carbon atoms in organic molecules have restricted rotation. Unrestricted rotation is only observed in the simplest of the carbon-carbon bonds; the single bond. Let’s see what we mean by restricted rotation. Take for example dichloromethane.
The green balls are the chlorine atoms, the black is the carbon atoms and the white are the hydrogen atoms respectively. If you have read structural isomerism, you will instantly identify the two molecules above as isomers. In contrast, the above models represent the very same molecules. You can obtain the second model by just twisting one carbon molecule in the chain. These molecules are not isomers.
This very same molecule when made with a carbon-carbon double bond ie, dichloroethene shows very different characteristics. The carbon molecules here are connected with a stronger double bond. If the bonds here were assumed to be sticks connecting the carbon balls, the rotation of a single bond is not at all a problem. If we connect two balls with two sticks representing the C-C double bond, then we need to break apart the entire assembly, twist the carbon atom and then put it all together. This is what is meant by restricted rotation.
From the model, you can see that there are two possible ways to put the molecules together. 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). In the other, the two chlorine atoms are locked on the same side of the double bond. This is known as the cis isomer. (cis: from Latin meaning “on this side”).
Naming Geometrical Isomers: Cis-Trans v/s E-Z
Geometric isomerism can also be exhibited when different functional groups are attached to the organic molecule. For example, if instead of two green dots representing the chlorine atoms, what if you had two different atoms attached, say –OH represented by blue and bromine represented by yellow? As you probably can see, cis and trans system does not make much sense here. This is where the E-Z system works.
In the E-Z system, the functional groups attached to the carbon molecules are assigned a priority. Firstly, the organic molecule is split sideways, the first carbon molecule is broken down the middle to give us a first carbon molecule and a second. If four different functional groups are attached to the simple organic molecules discussed before. If instead of two chlorine molecules, four different groups were attached to the carbon, then the molecule is split along the centre and the groups are assigned a priority on both the sides with one group having a higher priority than the other.
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 the compound. The symbol Z comes from a German word (zusammen) which means together. If the two groups with the higher priorities are on opposite sides of the double bond, then this is the (E)- isomer. E comes from the German (entgegen) which means the opposite.
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