What is syn-anti isomerism?

Syn-anti isomerism in organic chemistry is a type of geometrical isomerism. The prefixes “syn” or “anti” are used to denote geometrical isomers. 

We have discussed geometrical isomerism in previous articles.1 Preferably, syn /anti notations are used for nomenclature of oximes that is aldoxime and ketoxime. Meaning of the word syn is “together” or “with”.  And meaning of the word anti is “opposite”. 

Hi Friends, in this article we will learn about Syn/ anti isomerism is a type of geometrical isomerism and it is found in organic compounds such as;

• Oximes
• Diazo-compounds
• Cycloalkanes

Now we will discuss each type of compound in detail.

Oximes

The oximes are derivative of aldehyde or ketones. The compound which are originated from aldehyde are known as “aldoximes” whereas oximes come form ketones are called as “keoximes”.

 To identify isomers of aldoxime, we should see spatial relationship of hydroxyl group and H.

If OH and H are on same side then it is called as syn isomer

If OH and H are on opposite side then is called as anti-isomer

For example; methyl aldoxime

In the case of ketoxime, the hydrogen is replaced by another group.

Then how to assign the correct geometry of the compound?

So here first thing we should do is that write appropriate name of that molecule. And see the relationship of first group and OH.

If the first group and OH are on same side then assign it as syn isomer.

Or if the first group and OH are on opposite side then this will be anti-isomer

For example; ethyl methyl ketoxime here “ethyl” and “methyl” are written in the name in alphabetical order. Therefore, first group will be “ethyl” and we need to see relationship of “ethyl” and OH groups. In this way we can find out syn/anti isomers.

Diazo-compounds

The organic compounds which have diazo group bonded to alkyl / aromatic moiety are known as diazo-compounds.

To assign geometry of the molecule we need to see the bonded groups are on same side or opposite. If the groups are on same side then call it as syn isomer or if the groups are opposite to each other, then assign the molecule as anti-isomer.

For example; 1,2-diphenyldiazene

Here the phenyl groups can be present on same / opposite side.

Cycloalkanes

We have leaned nomenclature of cycloalkanes in previous article.2 To understand syn/anti isomerism in cycloalkanes. First thing we need to do is that write the molecule in more stable chair form and identify the relationship of corresponding groups. Here two groups should be same.

For example, cyclohexane-1.2-diol.

To assign the isomer, As stated before write the molecule in chair form. Then see that both OH are on “UP” side. That is same side therefore it is referred as syn isomer.

Let’s see another isomer, here one OH is “UP” and other OH is “DOWN” so they are opposite to each other. Therefore, it will be called as anti-isomer.

In the same way the cyclohexane molecules need to write in chair form and see the relationship of corresponding groups to assign syn/anti nomenclature.

For example; cyclohexane-1.3-diol.

Summary

In summary, we have seen syn / anti isomerism in oximes, diazo compounds and cycloalkanes. In case of oxide relationship of OH group and H / first group is seen to identify the isomers. The diazo compounds are named based on the relationship of substituents on nitrogen atoms. Finally, in case of cycloalkanes the isomers are named based on presence of same group on same / opposite side. To do this we need to write the molecule in its chair form.

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Thank you. See you in the next blog.