[Chemistry Class Notes] on Aromatic Compounds Pdf for Exam

Aromatic compounds have played an indispensable role in 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 used for ages are aromatic compounds. Not only this, the drug used for the treatment of malaria, chloroquine is also an aromatic hetropolycyclic compound that is also being studied to treat novel Coronavirus (COVID – 19) as well. The 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 that are stable in nature. 

 

Arene and Aryl Groups

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, the 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. 

 

Aromaticity

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 a distinct odor so, the compounds were named 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. Compounds must fulfill the following four conditions to be an aromatic compound –

Structures are given below – (image)

• 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 – (image)

• 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.

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

 

Examples of Aromatic Compounds 

All aromatic compounds are hydrocarbons. A few examples of aromatic hydrocarbons are listed below –

• Benzene

• Aniline

• Aspirin

• Paracetamol

• Benzyl Alcohol

 

Benzene

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 were 1st modeled by Huckel in 1931. 

 

Benzene is a colourless, clear, highly flammable, and volatile, liquid aromatic hydrocarbon. Benzene is found in crude oils, as a by-product of oil-refining processes, as a solvent, and in the synthesis of numerous chemicals. It is formed from both natural processes and human activities. Natural sources of benzene are forest fires, volcanos, crude oil, gasoline, and cigarette smoke. Benzene is also used to make other chemicals that are used to produce nylon, synthetic fibers, plastics, and resins. It is also used to make some lubricants, dyes, detergents, types of rubbers, drugs, and pesticides. 

 

Aniline

It is also an aromatic compound. Most of the aromatic compounds are benzene derivatives. Although not all aromatic compounds are benzene based. With an amine attached to a benzene ring, aniline is the prototypical aromatic amine. It was first obtained by the destructive distillation of indigo in the year 1826. It’s named after the indigo-yielding plant Indigofera anil. 

 

Aniline is prepared commercially by the catalytic hydrogenation of nitrobenzene or by the action of ammonia on chlorobenzene and possesses somewhat of an unpleasant smell, like that of a rotten fish. It is used in manufacturing precursors to polyurethane. It is also used to make antioxidants, dyes, rubber accelerators, photographic chemicals, pharmaceuticals, urethane foams, petroleum refining, explosives, fungicides, and herbicides, among others.

 

Aspirin

Aspirin, also known as acetylsalicylic acid, is a drug used to reduce pain, fever, and inflammation. It was discovered by chemist Charles Frederic Gerhardt in 1853. Phenol, an aromatic compound, is one of the raw materials used in making the drug.

 

Paracetamol

It is also a drug that 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.

 

Benzyl alcohol

It is an aromatic alcohol. It is a clear colorless liquid with a pleasant odor, and is slightly denser than water. Like all aromatic alcohols, benzyl alcohol is also a compound in which the -OH group is not directly attached to the benzene ring. Its role is that of a solvent, an antioxidant, a metabolite, and a fragrance. It is used as an anti-parasite medication. It is mainly used as a medicine to treat head lice in people. 

 

Properties of Aromatic Compounds

• 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 addition 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 the two are listed below –

 

 

This ends our coverage of 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 recognize aromatic compounds through their structures. If you are looking for solutions to NCERT Textbook problems based on this topic, then log in to the website or download 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.