Aromatic Compounds

Aromatic compounds have played an indispensable role for improving the quality of our lives in the past and continue to play the same in present. Computer parts, DVDs and linchpin components of automotive parts are made up of aromatic compounds. Drugs such as Aspirin and paracetamol which we are using since ages are aromatic compounds. Not only this, the drug used for treatment of malaria, chloroquine is also an aromatic heteropolycyclic compound which is also being studied to treat novel Coronavirus (COVID - 19) as well. Contribution of aromatic compounds is huge in the development of mankind and they still have more potential to do the same. So, in this scenario it becomes a must for all of us to have at least a basic understanding of aromatic compounds. 

What are Aromatic Compounds? 

The chemical compounds that contain conjugated planar ring systems with delocalized pi electron clouds instead of discrete alternating single and double bonds are called aromatic compounds. They are also known as aromatics or arenes. The most common example of aromatic compounds is benzene. These are unsaturated compounds which are stable in nature. 

Students generally get confused between arene and aryl groups. Let us clear this doubt here only. Arene is a compound containing one or more benzene rings. While when we remove a hydrogen atom from an arene, aryl group is formed. It means benzene is an arene, but phenyl is an aryl group. All arenes are aromatic compounds but it's not necessarily that all aromatic compounds are arenes. 

All aromatic compounds show aromaticity. The term aromaticity is used to describe a property of a cyclic, planar molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms. The word aromaticity comes from the word ‘aroma’ which means fragrance or odor. Since most of the aromatic compounds are derivatives of benzene and benzene gives distinct odor so, the compounds were named as aromatic compounds. Although presently many non – benzene aromatic compounds have been discovered which do not have any odor. The term aromatic was first used by August Wilhelm Hofmann in 1855. 

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Compounds must fulfil the following four conditions to be an aromatic compound –

  • The molecule must be cyclic. 

Example – Benzene and pyrrole are aromatic in nature while acyclic compound C4H5NH2 is a non-aromatic compound. Structures are given below –

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  • Every atom in the cyclic ring must be conjugated. As it will provide the cyclic ring delocalized pi-electron system. Thus, we can say every atom in the cyclic ring must have an empty p orbital and must be capable of participating in resonance. Examples are given below –

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  • Huckel’s Rule of Aromaticity - All compounds must obey Huckel’s Rule i.e. molecule must have [4n+2] pi-electrons where n is an integer (i.e. n= 0, 1, 2, 3, 4…etc.). For example,  Benzene has 6 pi-electrons and [(4×1)+2] = 6, thus it obeys Huckel’s Rule while cyclooctatetraene has 8 pi-electrons [4n+2]8, thus it does not follow Huckel’s Rule. So, benzene is aromatic and cyclooctatetraene is a non-aromatic compound. 

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  • The molecule should be planar or flat. Those compounds which follow the above 4 rules of aromaticity, they are generally flat as in that condition they possess large enough potential energy. 

Examples of Aromatic Compounds 

All aromatic compounds are hydrocarbons. Few examples of aromatic hydrocarbons are listed below –

  • Benzene – It is the best example for representing aromatic compounds. Scientists struggled a lot to determine the structure of benzene. Because benzene is unreactive towards addition reactions being a highly unsaturated compound. Thus, it shows exceptional stability. The cyclohexatriene structure for benzene was 1st proposed by August Kekule in 1865. After Kekule many other scientists also proposed the structure of benzene. The quantum mechanical origins of stability or aromaticity was 1st modelled by Huckel in 1931. 

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  • Aniline – It is also an aromatic compound. Most of the aromatic compounds are benzene derivatives. Although not all aromatic compounds are benzene based. 

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  • Aspirin – It is a drug used to reduce pain, fever and inflammation. It was discovered by chemist Charles Frederic Gerhardt in 1853. 


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  • Paracetamol – It is also a drug which is used to treat fever and pain. It was 1st made in 1877. Since then we are using this medicine for pain relief. It appears to be safe during pregnancy and breastfeeding.

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  • Benzyl alcohol – It is an aromatic alcohol. Aromatic alcohols are those compounds in which the -OH group is not directly attached to the benzene ring. 

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Properties of Aromatic Compounds

aromatic compounds possess following properties –

  • Aromatic compounds have extremely high resonance energy.

  • These are stable unsaturated compounds.

  • They are generally non – polar and immiscible with water.

  • They give a sooty yellow flame due to the high ratio of carbon to hydrogen.

  • Aromatic compounds undergo substitution reactions rather than additional reactions.

  • These have delocalized pi-electrons. 

  • They show a coplanar structure.

  • They are used as a solvent for non - polar compounds.

  • They show electrophilic aromatic substitution and nucleophilic aromatic substitution reactions. 

Difference Between Aromatic Hydrocarbons and Aliphatic Hydrocarbons 

Both aromatic hydrocarbons and aliphatic hydrocarbons are organic compounds composed of hydrogen and carbon but still they are different from each other and show different properties. Some of the differences between two are listed below –

S. No. 

Aromatic Hydrocarbons

Aliphatic Hydrocarbons

1.

These hydrocarbons contain carbon and hydrogen attached in a ring system with delocalized pi – electrons.

These hydrocarbons contain carbon and hydrogen attached in straight chains, branched chains or in non – aromatic ring forms. 

2.

They show aromaticity.

They do not show aromaticity.

3.

All aromatic compounds follow Huckel’s rule. 

It is not necessary that all aliphatic hydrocarbons will follow Huckel’s rule. 

4.

Most of the aromatic compounds have a pleasant odor. 

They do not have a pleasant odor.

5.

In these compounds, the carbon to hydrogen ratio is low. 

In these compounds, the carbon to hydrogen ratio is high. 

6.

These compounds generally burn with sooty flame (yellow).

These compounds burn with non – sooty flame. 

7.

These all are unsaturated compounds. 

Aliphatic hydrocarbons can be saturated and unsaturated both. 

8.

They have delocalized pi – electrons. 

They do not have delocalized pi – electrons. 

9.

These are conjugated compounds. 

The majority of these compounds are not conjugated. 

10.

Benzene, toluene, naphthalene etc. are examples of these compounds. 

Methane, ethane, propene etc. are examples of these compounds. 


This ends our coverage on Aromatic Compounds. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to recognise aromatic compounds through their structures. If you are looking for solutions to NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.