Aromatic Chemistry – An Introduction

By Author: Anil Kumar
Total Articles: 56
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The term aroma generally denotes odour which helps in distinguishing different compounds. However, in chemistry, it is a branch of organic chemistry that studies compounds having particular structural, electronic, or chemical properties – typically characterized by a cyclic structure called an aromatic ring.

What are aromatics?
Aromatics are hydrocarbons consisting of one or more conjugated planar ring systems. These are accompanied by delocalized pi-electron clouds instead of individual alternating single and double bonds. These organic unsaturated compounds are also known as aromatics or arenes and are stable.
The exceptional stability of these compounds is termed aromaticity, a property in organic chemistry. A particular bonding arrangement causes certain pi-electrons within a molecule by firmly holding it which results in aromaticity.

Aromatic compounds and their importance:
Aromatic compounds play an indispensable role in improving life quality. These compounds have a huge contribution to the development of mankind and still have high potential. The major aromatics are benzene, ortho-xylenes, ...
... and toluene. Phenol, styrene, aniline, acetophenone, benzoic acid, benzaldehyde, and benzonitrile are some more aromatic hydrocarbons. Aromatic compounds play crucial roles in biochemistry and are used as starting materials for a wide range of products.

Aromacity properties are exhibited by aromatic compounds. Further, the aromatic products are categorized into two broad categories based on the presence or absence of benzene rings as benzenoids and non-benzenoids respectively. When compared with similar non-aromatic molecules, the aromatic molecules display enhanced chemical stability. The molecules that can be aromatic will tend to change towards aromaticity. The added stability changes the molecule’s chemistry. Further, the aromatic compounds undergo nucleophilic and electrophilic aromatic substitution reactions. The aromatic molecules can interact with each other in so-called π–π stacking and also in an ‘edge-to-face’ orientation.

Aromatic compounds and their properties:
Arenes are non-miscible and nonpolar in water. These compounds are unreactive and are usually used as solvents for several other nonpolar compounds. The carbons to hydrogen ratio in arenes are high due to which they get characterized by a sooty yellow flame.

Classification:
Arenes are classified into two categories and the classification is based on the position of the functional group – nuclear-substituted compounds and sidechain-substituted compounds.

Compounds need to fulfill certain conditions to be aromatic compounds:

• The molecule must be cyclic.
• The atoms in the cyclic ring must be conjugated.
• Hydrocarbons can be classified as aromatic compounds if they follow the Huckel rule. According to this rule, the planar ring must have (4n+2) π electrons, in which n is any whole number. Based on this, the ring systems having 2(n=0), 6(n=1), 10(n=2), 14(n=3), etc. pi electrons are aromatic.
• Molecules should be planar or flat.

Aromatic Chemistry and its history:
Benzene and toluene are many of the earliest-known examples of aromatic compounds. These compounds have distinctive smells. Such property guided to the term ‘aromatic’ under which these types of compounds get classified. With the eventual discovery of electronic property, the term aromaticity got invented.

Chemists in the 19th century found that benzene was unreactive towards addition reactions. This puzzled the chemists considering the presumed high degree of unsaturation. August Kekule, a German organic chemist, first proposed the cyclohexatriene structure for benzene in 1865. A maximum number of chemists accepted the structure as it reckoned for most of the familiar isomeric relations of aromatic chemistry. Also, Huckel was the first to model the quantum mechanical origins of aromaticity in 1931.

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