1. Home
  2. Blog

Aromatic compounds – Who really we are?

by egpat      Posted on 20 Mar 2019

Aromatic compounds…..Are they produce fragrance always? Of course, very few of them produce aroma but not all. Even though the name is derived from identification of fragrance from these compounds, but not all these aromatic compounds produce fragrance. Then how can you define these aromatic compounds? Simply, by their chemical nature! Yes, these aromatic compounds stand unique in few properties like enhanced stability and their preference to substitution over addition.


It’s great, but how can we identify whether a compound is aromatic or not? Can we say it by just seeing the structure?

Definitely, we can assess a compound whether aromatic or not just by studying few of the criteria for aromaticity in its structure.

For a compound to be aromatic, it should be cyclic, planar and should have conjugated double bonds with odd number of pi bonds.”

Let’s take one example here.

example of non aromatic compound

In the above compounds, structure I is not cyclic while structure II is cyclic but doesn’t contain conjugated double bonds. So, both structure I and II  are not aromatic compounds.

Note: Conjugated double bonds are the alternate arrangement of double bond along with single bonds.

Now let’s take another example.

aromatic vs non-aromatic compound

Here structure I is cyclic and contains conjugated double bonds. But it is still not aromatic.

Why? Simply it doesn’t met one of the criteria – odd number of pi bonds.

It contains only two pi bonds hence not aromatic. On the other hand, structure II, well known as benzene, is cyclic, planar, with 3 pairs of conjugated pi bonds hence it is aromatic.
That’s it. Now by finding these criteria in the structure you can easily identify aromatic compounds.


So, let’s go with more examples and find aromatic compounds.

1. Annulenes

As you know that two important criteria for aromaticity are cyclic structure and conjugated double bonds. Annulenes are such type of cyclic compounds with conjugated double bonds with different ring sizes. So it’s good to start our discussion with them and let’s identify which anuulene is aromatic.

Let’s begin.

4-annulene

Above structure is a simple annulene with 4 carbons and 2 pi bonds. It is called as [4]-annulene. From the above discussion you can clearly observe that it is not aromatic as it contains even number of pi bonds.

Let’s go with higher annulenes.

6 and 8 annulenes

First structure is [6]-annulene which is nothing but benzene and undoubtedly it is aromatic. Second structure is [8]-annulene which again has even number of pi bonds, so not aromatic.

As the annulenes are always cyclic and conjugated, you have to check the total number of pi bonds whether odd or even to assess them to be aromatic or not.

Suppose we represent odd number by 2n+1, an annulene with 2n+1 pi bonds will be aromatic. It can also be stated in another way. As you know each pi bond contains 2 pi electrons, annulene with 2 x (2n+1)=4n+2 pi electrons is again aromatic.

general structure of annulene

This is somewhat simple as every annulene is prefixed with total number of pi electrons.

"Therefore, those annulenes with 4n+2 pi electrons are aromatic.”

Substituting various values for n=1,2,3,4…..and so on we will get 4n+2 values as 6,10,14,18…..and we can easily assess that they are aromatic.

4n+2 pi electrons in annulene

Wait, the game is not complete yet!!

Even most of these are aromatic but not all due to another condition required to be met, that is, structure should be planar.

Even [6]-annulene is flat and aromatic, 10-annulene is not aromatic as it a bent structure due to some ring strain in its conformation.

6 and 10 annulene

On the other hand, [14] and [18]-annulenes are flat and again aromatic.

14 and 18 annulene

 

2. Polycyclic rings

Till now we have seen monocyclic rings with conjugated double bonds. Now let’s go with polycyclic ring systems and let’s check whether they are aromatic or not.

naphathalene

Above structure is naphthalene which is cyclic and planar structure. it contains totally 10 pi electrons with conjugated double bonds hence it is perfectly aromatic.

Note: Even ten pi electrons are present in both structures naphthalene is aromatic while [10]-annulene is not as the later is not a flat structure.

Similarly the following rings are aromatic as they contain 4n+2 pi electrons.

polycyclic aromatic compounds

Now let’s see few other examples.

Is it aromatic?

The structure is cyclic, planar and contains conjugated double bonds but the total number of pi electrons is 16 which doesn’t fit with 4n+2 pi rule.

Surprisingly, it is still aromatic!!

Even it has 16 pi electrons only 14 pi electrons participate in the delocalization of electrons in pi cloud above and below the plane bringing aromaticity to the compound.

3. Heterocyclic compounds

Now let’s turn into heterocyclic rings. Are they aromatic?

Let’s take simple example, pyrrole.

pyrrole is aromatic compound

It has only 4 pi electrons, so again it doesn’t fulfil Huckel’s rule. Still the compound is aromatic.

How it’s possible?

It’s all due to lone pair of electrons that contribute to the pi cloud making the total number of electrons participating as 6.

Now let’s take pyridine, six membered heterocyclic ring.

pyridine is also aromatic

If you count only pi electrons, it contains totally 6 electrons and it obeys Huckel’s rule. But if you consider, lone pair of electrons total electrons are 6 +2=8.

So what happens?

Clearly, the lone pair of electrons will not participate into the pi cloud making effective electrons as 6 in the pi cloud hence aromatic.

In this way, most of the heterocyclic rings attain aromaticity either by including or excluding pi electrons according to the situation arise in the structure.

So, all the following heterocyclic rings are aromatic in nature.

heterocyclics as aromatic compounds

4. Cations and anions

One of the primary role of aromaticity is to delocalize the electrons thereby impart stability to the ring. So, why can’t we apply same logic to the cations or anions that behave as aromatic?

Yes, we will peep into this with simple examples.

Cyclopentadiene is not aromatic as it contains only 4 pi electrons and even it is not planar as one the of the carbon is sp3 hybridised.

cyclopentadiene

Similarly cyclopentadienyl cation is also not aromatic. Even it is a planar structure due to sp2 hybridization of carbocation, still it contains only 4 pi electrons hence not aromatic.

cyclopentadienyl cation

Now consider cyclopentadienyl anion.

cyclopentadienyl anion

It has extra lone pair of electrons on the negatively charged carbanion, which participates in delocalization and formation of stable pi cloud. Therefore it acts as aromatic compound.

Similarly one of the cation that exhibits extra stability due to aromaticity is tropylium ion.

tropylium ion

It is a seven membered ring with 6 pi electrons and one positively charged carbocation.  Again positive charge on the carbon makes it sp2 hybridised and brings planarity to the structure resulting in aromaticity.

Conclusion

Finally we can conclude that aromaticity is a special property of cyclic compounds with conjugated double bonds that imparts extra stability to ring systems. A cyclic compound, anion or cation always tends to delocalise the electrons and tries to acquire aromaticity. In that process it may include or omit lone pair of electrons to bring a stable aromatic compound. Hope you enjoyed learning about aromatic compounds by reading this article and please share this post with your friends.

Aromatic compounds…..Are they produce fragrance always? Of course, very few of them produce aroma but not all. Even thogh the name is derived from identification of fragrance from these compounds, but not all these aromatic compounds produce fragrance. Then how can you define these aromatic compounds? Simply, by their chemical nature! Yes, these aromatic compounds stand unique in few properties like enhanced stability and their preference to substitution over addition.


It’s great, but how can we identify whether a compound is aromatic or not? Can we say it by just seeing the structure?

Definitely, we can assess a compound whether aromatic or not just by studying few of the criteria for aromaticity in its structure.

For a compound to be aromatic, it should be cyclic, planar and should have conjugated double bonds with odd number of pi bonds.”

Let’s take one example here.

example of non aromatic compound

In the above compounds, structure I is not cyclic while structure II is cyclic but doesn’t contain conjugated double bonds. So, both structure I and II  are not aromatic compounds.

Note: Conjugated double bonds are the alternate arrangement of double bond along with single bonds.

Now let’s take another example.

aromatic vs non-aromatic compound

Here structure I is cyclic and contains conjugated double bonds. But it is still not aromatic.

Why? Simply it doesn’t met one of the criteria – odd number of pi bonds.

It contains only two pi bonds hence not aromatic. On the other hand, structure II, well known as benzene, is cyclic, planar, with 3 pairs of conjugated pi bonds hence it is aromatic.
That’s it. Now by finding these criteria in the structure you can easily identify aromatic compounds.


So, let’s go with more examples and find aromatic compounds.

1. Annulenes

As you know that two important criteria for aromaticity are cyclic structure and conjugated double bonds. Annulenes are such type of cyclic compounds with conjugated double bonds with different ring sizes. So it’s good to start our discussion with them and let’s identify which anuulene is aromatic.

Let’s begin.

4-annulene @**@

Above structure is a simple annulene with 4 carbons and 2 pi bonds. It is called as [4]-annulene. From the above discussion you can clearly observe that it is not aromatic as it contains even number of pi bonds.

Let’s go with higher annulenes.

6 and 8 annulenes

First structure is [6]-annulene which is nothing but benzene and undoubtedly it is aromatic. Second structure is [8]-annulene which again has even number of pi bonds, so not aromatic.

As the annulenes are always cyclic and conjugated, you have to check the total number of pi bonds whether odd or even to assess them to be aromatic or not.

Suppose we represent odd number by 2n+1, an annulene with 2n+1 pi bonds will be aromatic. It can also be stated in another way.As you know each pi bond contains 2 pi electrons, annulene with 2 x (2n+1)=4n+2 pi electrons is again aromatic.

general structure of annulene

This is somewhat simple as every annulene is prefixed with total number of pi electrons.

"Therefore, those annulenes with 4n+2 pi electrons are aromatic.”

Substituting various values for n=1,2,3,4…..and so on we will get 4n+2 values as 6,10,14,18…..and we can easily assess that they are aromatic.

4n+2 pi electrons in annulene

Wait, the game is not complete yet!!

Even most of these are aromatic but not all due to another condition required to be met, that is, structure should be planar.

Even [6]-annulene is flat and aromatic, 10-annulene is not aromatic as it a bent structure due to some ring strain in its conformation.

6 and 10 annulene

On the other hand, [14] and [18]-annulenes are flat and again aromatic.

14 and 18 annulene

 

2. Polycyclic rings

Till now we have seen monocyclic rings with conjugated double bonds. Now let’s go with polycyclic ring systems and let’s check whether they are aromatic or not.

naphathalene

Above structure is naphthalene which is cyclic and planar structure. it contains totally 10 pi electrons with conjugated double bonds hence it is perfectly aromatic.

Note: Even ten pi electrons are present in both structures  naphthalene is aromatic while [10]-annulene is not  as the later is not a flat structure.

Similarly the following rings are aromatic as they contain 4n+2 pi electrons.

polycyclic aromatic compounds

Now let’s see few other examples.

Is it aromatic?

The structure is cyclic, planar and contains conjugated double bonds but the total number of pi electrons is 16 which doesn’t fit with 4n+2 pi rule.

Surprisingly, it is still aromatic!!

Even it has 16 pi electrons only 14 pi electrons participate in the delocalization of electrons in pi cloud above and below the plane bringing aromaticity to the compound.

3. Heterocyclic compounds

Now let’s turn into heterocyclic rings. Are they aromatic ?

Let’s take simple example, pyrrole.

pyrrole is aromatic compound

It has only 4 pi electrons, so again it doesn’t fulfil Huckel’s rule. Still the compound is aromatic.

How it’s possible?

It’s all due to lone pair of electrons that contribute to the pi cloud making the total number of electrons participating as 6.

Now let’s take pyridine, six membered heterocyclic ring.

pyridine is also aromatic

If you count only pi electrons, it contains totally 6 electrons and it obeys Huckel’s rule. But if you consider, lone pair of electrons total electrons are 6 +2=8.

So what happens?

Clearly, the lone pair of electrons will not participate into the pi cloud making effective electrons as 6 in the pi cloud hence aromatic.

In this way, most of the heterocyclic rings attain aromaticity either by including or excluding pi electrons according to the situation arise in the structure.

So, all the following heterocyclic rings are aromatic in nature.

heterocyclics as aromatic compounds

4. Cations and anions

One of the primary role of aromaticity is to delocalize the electrons thereby impart stability to the ring. So, why can’t we apply same logic to the cations or anions that behave as aromatic?

Yes, we will peep into this with simple examples.

Cyclopentadiene is not aromatic as it contains only 4 pi electrons and even it is not planar as one the of the carbon is sp3 hybridised.

cyclopentadiene

Similarly cyclopentadienyl cation is also not aromatic. Even it is a planar structure due to sp2 hybridization of carbocation, still it contains only 4 pi electrons hence not aromatic.

cyclopentadienyl cation

Now consider cyclopentadienyl anion.

cyclopentadienyl anion

It has extra lone pair of electrons on the negatively charged carbanion, which participates in delocalization and formation of stable pi cloud. Therefore it acts as aromatic compound.

Similarly one of the cation that exhibits extra stability due to aromaticity is tropylium ion.

tropylium ion

It is a seven membered ring with 6 pi electrons and one positively charged carbocation.  Again positive charge on the carbon makes it sp2 hybridised and brings planarity to the structure resulting in aromaticity.

Conclusion

Finally we can conclude that aromaticity is a special property of cyclic compounds with conjugated double bonds that imparts extra stability to ring systems. A cyclic compound, anion or cation always tends to delocalise the electrons and tries to acquire aromaticity. In that process it may include or omit lone pair of electrons to bring a stable aromatic compound. Hope you enjoyed learning about aromatic compounds by reading this article and please share this post with your friends.

aromaticity How wonderful it will be when you able to write IUPAC name of a given compound without any ambiguity. You can learn new structures very easily when you know their names even you can remember and analyse well. How to write IUPAC name – practical examples How-to-write-iupac-name–practical-examples /images/Chemistry/ iupac-ex-fb-min.jpg /images/list/iupac-examples-list-min.jpg https://egpat.com/blog/4-criteria-for-aromaticity https://egpat.com/blog/4-criteria-for-aromaticity 18 Aug 2019@ 4-criteria-for-aromaticity- - compound- needs Aromaticity – 4 criteria every compound needs Proteins can be synthesised from mRNA by the help of few proteins in eukaryotes. Let’s see how they helping this process. Aromaticity, huckel rule, 4n+2 pi rule Four criteria for aromaticity /images/Chemistry/aromaticity-fb.jpg /images/list/aromaticity-list.jpg Aromaticity–4 criteria a compound needs We usually know that phenol is aromatic while alcohol is aliphatic. How could we say that? Even both have hydroxyl groups still one is aromatic while other is not. What makes the difference?

We usually know that phenol is aromatic while alcohol is aliphatic. How could we say that? Even both have hydroxyl group still one is aromatic while other is not. What makes the difference?

Yes, it’s all due to those little double bonds that join their hands with single bonds on both sides, yet arranged systematically throughout the ring bringing a unique property to it....that is Aromaticity !!


Here we will go with all the prerequisites required for a compound to be aromatic along with examples. Let’s start.

 

1. Conjugation

The unique pattern of alternate arrangement of double and single bonds is none other than conjugation that make the compound stand unique among the fellow organic compounds. For example, in the following compounds, the first structure has conjugated double bonds while second structure has no conjugation. Therefore later compound is not aromatic!

 

conjugated double bonds are required for aromaticity

 

2. Cyclic structure

Just we have seen that conjugation is an important prerequisite for a compound to be aromatic. But is that property what we call conjugation, sufficient to win the game? Certainly, it’s not! Let’s take another example. Consider 1,3,5-hexatriene. Is it aromatic?

 

aromatic compound should be cyclic

Undoubtedly it is not aromatic even it has conjugated double bonds. It can be easily visualised by comparing relative stability and chemical properties of these two compounds.


 So conjugation is not only the factor responsible for aromaticity but still another factor plays important role.  By this time, you can easily understand that another property required for aromaticity is the cyclic structure. 1,3,5-hexatriene is conjugated but it is not cyclic hence not aromatic while benzene is both cyclic and conjugated hence aromatic.

 

3. Number of pi electrons

It’s great that now we know two conditions that required for a compound to be aromatic. Is anything else?

Let’s take one example.

4n+2 pi rule

Here both the compounds are cyclic and conjugated still one is aromatic while other is not. The first structure contains totally six pi electrons and aromatic while the second structure has 8 pi electrons but not aromatic.

Is there any rule relating number of pi electrons in the ring and aromaticity?

Yes, of course, you may be already familiar with that rule, Huckel’s rule or 4n+2 pi rule. It simply states that,an aromatic ring should have 4n+2 pi electrons.

Let’s substitute different values for n such as n=0,1,2,3…..and so on.

If n=0, it will be 4(0)+2=2

If n=1, it will be 4(1)+2=6

If n=2, it will be 4(2)+2=10

In this way 2, 6,10,14,18…..and so on are the number of pi electrons that are only allowed for aromaticity. You can easily observe that all these numbers differ by 4.

So, in the previous example ring containing 6 pi electrons is only aromatic.

You can observe that all the numbers stated above are even. As each pi bond contains two pi electrons, its quiet natural that the number of pi electrons allowed in an aromatic compound is always an even number.

 

Stated in another way, the number of pi bonds allowed in an aromatic compound will be 1, 3,5,7 and so on.

Quite interestingly, all these are odd numbers!! So, we can state Huckel’s rule in another way.

“An aromatic ruing should have odd number of pi electrons”

 

Note: For an aromatic compound, the number of pi bonds should be odd but pi electrons always will be even.


4. Role of lone pairs

By the previous discussion, now we can clearly identify which is aromatic and which is not aromatic.

Let’s take one example.

odd number of pi bonds

In the above example, you can easily find that first structure is not aromatic as it contains 8 pi electrons as well as it is not completely conjugated. On the other hand, the second structure is aromatic as it is cyclic, conjugated with 10 pi electrons or 5 pi bonds, all the requirements met for a compound to be aromatic.

Now let’s turn into another example, still little bit interesting.

aromaticity of indole

Is that compound aronmatic? By the first look and counting pi electrons you may declare that is not aromatic as it contains 8 pi electrons. But still the compound is aromatic!!

Here lone pair of electrons comes into the play. These electrons participate in the pi cloud resulting in stable uninterrupted pi cloud above and below the molecule making total number of electrons included in the pi cloud as again 10 hence aromatic.

It’s great, now let’s go with another example.

aromaticity of pyridine

Is that aromatic? Now by using the same logic as above , total number of electrons that participate in the pi clouds is 6=2=8. So we may flag this compound as not aromatic. Surprisingly, this compound is also aromatic.

How it’s possible?

As the already attained 6 pi electrons through the pi bonds, there is no need of participation of lone pair of electrons in pi cloud, so they will be out of the ring.

In this way, lone pair of electrons play key role in stabilising the ring. Whenever deficiency is there they will supply the electrons otherwise not ultimately stabilising the ring towards aromaticity.

 

5. Planar structure

In an aromatic compound, a pi cloud is formed by pi electrons and sometimes lone pair of electrons all that bringing the total count as 4n+2.

But who forms this pi cloud?  Undoubtedly, the p orbitals which are perpendicular the ring can form pi cloud. In another words, the ring should be planar where these p orbitals can form a stable pi cloud without a strain.

So, planarity or flat structure is another essential feature for aromaticity. To be more clear, let’s take one example.

aromatic compound shoould be planar

The above structure is cyclic and contains 6 pi electrons in somewhat conjugation. But conjugation was lost at one carbon which is saturated hence sp3 hybridised. So this carbon will not be oriented within the same plane as other carbons in the structure do, therefore the ring is not completely flat and the compound is not aromatic.

 

You can also observe that the structure has three conjugated double bonds but they are not continuous enough! In another words, the pi cloud is interrupted by a carbon bearing no double bond.

So we can define that an uninterrupted pi cloud is essential for aromaticity.

 

Conclusion

Aromaticity is a unique property shown by few of the cyclic compounds having pi electrons. For a compound to be aromatic, it should be cyclic, planar, contain uninterrupted pi cloud above and below the plane with 4n+2 pi electrons. One of the quite interesting fact is that the lone pair of electrons can also be involved in the pi cloud to make a stable aromatic ring.

Hope you enjoyed reading the article, post your comments if any in the below section. Please don’t forget to share this post with your friends. Have a great day!!




Comments

Follow us

Join with us

Get the latest updates and posts

Follow us at



Recent articles