Isomerism in Alkenes in Organic Chemistry

Isomers are the two individual compounds which have same molecular formula but they differ in the arrangement of atoms in the molecule.

Hi Friends, In this article we are going to learn about Isomerism in Alkenes in organic chemistry. .

Key words: Isomers, Structural isomers, Geometric isomers, Cis/trans isomers, E / Z naming system

So what are isomers?

The word isomer is derived from Greek word “isomeros” where, iso means “equal” and meros means “part”.

Alkene compounds are consisting of one or more double bonds. There are two types of isomers found in alkenes: Structural isomers and Geometric isomers.

Structural isomers

Structural isomers are differing in arrangement of atoms in the molecule.

For example; 1-butene and 2-butene, both have chemical formula C₄H₈. They are differing from each other by the position of carbon-carbon double bond. Hence they are termed as structural isomers of each other.  

Consider one more example; 2-pentene and 2-methyl-2-butene are having chemical formula C₅H₁₀. They are differing in position of carbon-carbon double bond as well as position of methyl (CH₃) group. 

Geometric isomers

These types of isomers have different arrangement of groups across the carbon-carbon double bonds. The geometric isomerism is found in organic and inorganic compounds where there is a restricted rotation across the carbon-carbon bond.

The alkene compounds have restricted rotation due to carbon-carbon double bonds. Consider following example of 2-butene, it has two different structures with same position of double bond. Here in one structure “methyl” groups are on same side of double bond and in other structure “methyl” groups apposite to each other. This type of isomers is known as Geometric isomers.

Now the question is how to differentiate Geometric isomers by systematic nomenclature?

To solve this problem; the compounds are assigned as cis and trans isomers. The compound which is having similar groups on same side of double bond is known as “cis isomer”. The prefix “cis” is a Latin word means “on the same side of”. In similar way an isomer in which identical groups are opposite to each other is known as “trans isomer”. The prefix “trans” is also a Latin word means “on the other side of”.

Let’s consider another example shown below; the structure A is having chlorine atoms on same side. Therefore it is known as “cis isomer”. And the structure B has chlorine atoms on opposite side of double bond so this is known as “trans isomer”.

Here important thing to notice is that the cis / trans naming system is restricted to symmetric alkenes where at least two similar groups are bonded to C=C bond. Hence there is need of another kind of naming system to assign geometric isomers of alkenes which is having different groups attached.

E / Z naming system

This is IUPAC preferred naming system where we can assign geometry of alkene in complex compounds. According to this naming system, all the groups attached to the double bond are given priority as “higher” or “lower” based on the electro-negativity of the first atom attached. Then position of the higher priority groups is compared. If the higher priority groups are on same side of the double bond then that isomer is known as “Z-isomer”. And if the higher priority groups are present opposite to each other then the isomer is named as “E-isomer”. For writing full name of the alkene compound; geometry of the double bond is written in bracket before name of compound. The geometry is shown by the letters E or Z.

The letter E comes from German word “entgegen” which means “opposite”. And the letter Z comes from German word “zusammen” means “together”.

Let’s take an example C where the alkene has four different groups attached.

To assign geometry of alkene; firstly draw a vertical line across the double bond which differentiate left and right half. Then compare groups in left half and give priorities higher or lower. Similarly do this for right half also. Then remove the vertical line and see whether higher priority groups are on same side or opposite side of the double bond. In this example higher priority groups are opposite to each other, therefore it is assigned as E-isomer.

Consider another example D where higher priority groups are on same side of double bond, so it is called as Z-isomer.

Now consider another example E, here in methyl (CH₃) and ethyl (-CH₂CH₃) first attachment site is carbon. In this situation we have to consider next higher electronegative atom attached. In this example methyl carbon is attached to three hydrogen atoms (H, H, H). And ethyl CH₂ is attached to one carbon and two hydrogen (C, H, H). Hence ethyl group gets higher priority. Bromine is more electronegative than carbon, so Bromine gets higher priority. Therefore in this compound higher priority groups are on same side and it is a Z-isomer.

Summary

To summarize this topic,

Alkene compounds shows two type of isomers : Structural isomers and Geometric isomer.

Structural isomers are differing in arrangement of atoms in the molecule.

Geometric isomers have different arrangement of groups across the carbon-carbon double bonds.

Symmetrical alkenes are named by cis / trans naming system.

Unsymmetrical alkenes are named by E / Z naming system.

That's all for this topic. If you have any questions please feel free to ask me. Also suggest me if any changes or additions are required. Thank you.....!