What are Aromatic Compounds?

Aromatic organic compound or aromatic system is an important class of hydrocarbons under the branch of organic chemistry. Aromatic compounds are also called arenas. The definition of aromatic organic compounds can be stated as a class of organic compounds with a planar cyclic ring of atoms bonded through alternated single and double bonds. Due to these alternating bonds, a peculiar feature termed as ‘resonance’, that is, delocalization of pi electrons or the electrons is shared within the atoms of the ring which are present in the aromatic compounds. The word ‘aromatic’ includes a Greek word ‘aroma’ which means smell, indicating that they were named as such due to the sweet odor they produce. Many well-known flavors such as vanilla, cinnamon, and few scents from flowers are extracted from aromatic compounds. Benzene with chemical formula C₆H₆ is a very common aromatic compound that contains six atoms of carbon held in the cyclic ring of alternate double bonds. It is also to be noted that not all aromatic compounds contain benzene ring. This can be explained by Huckel’s rule. 

What is Huckel’s Rule? 

Huckel’s rule postulated by Erich Huckel plays an important role in estimating whether a given organic planar ring structure will show an aromatic property or not. A cyclic ring molecule is said to follow Huckel’s rule, if the л-electrons count in a molecule is [4n+2], where n stands for a positive numerical value. It is also named as [4n+2]rule. 

Thus, the aromatic compounds are expected to be much more stable than the respective alkenes or alkynes due to the delocalization of electrons which leads to resonance energy. Thus, for a molecule to be aromatic in nature, it should be cyclic, planar, and should have [4n+2] pi electrons in the conjugated system of p-orbitals. Thus, based on Huckel’s rule aromatic compounds are classified as Benzenoids and non-benzenoids. Benzenoids are aromatic compounds which contain at least one benzene ring within their structure and non-benzenoids are those which follow Huckel’s rule with cyclic and planar structure but do not contain benzene ring within their structure. 

Nomenclature of Benzene Derivatives 

Nomenclature of benzene derivative might be confusing as one compound might have multiple possible names such as systemic and common names. Thus, the simplest way to name a benzene derivative is to add the suffix of the substituent along with the position of substituted carbon to the word benzene. While naming the disubstituted benzene, without using the numbers to point out the substituted carbon atom, prefixes such as ortho (o-), meta (m-) and para (p) can be taken to indicate the position of the substituent. Ortho indicates 1,2 or 1,6 positions on the benzene ring, meta indicates 1,3 or 1,5 positions and para indicates 1,4 positions on the benzene ring. 

Synthesis of Benzene

Arene compounds are synthesized by aromatization of partially unsaturated or saturated cyclic precursors. Reactions such as cyclol addition reactions of alkyne i.e., Dötz reaction, Diels-Alder reaction of alkynes, Bergman cyclization of enyne and a hydrogen donor, hydrogenation of cycloalkanes etc., form various aromatic compounds. 

The preparation of benzene can be done in a variety of laboratories and industrial settings. Benzene is primarily derived commercially from coal tar. Some laboratory methods for the synthesis of benzene are given below;

• Preparations from alkynes
• Preparations from aromatic acids
• Preparations from phenol
• Preparations from sulphonic acids

Properties of Aromatic Compounds

Due to their high stability and unreactive nature, most of the aromatic compounds are used as solvents. Further aromatic compounds are nonpolar, immiscible in water and on heating are characterized by sooty yellow flame. The main sources of aromatic hydrocarbons (aromatic compounds with carbon and hydrogen alone) are petroleum and coal. Coal tar obtained on distillation while processing cooking of coal, act as major source for many aromatic compounds. Most of the aromatic compounds are known to be carcinogenic in nature. Amino acids are also aromatic compounds that act as precursors for nucleotide synthesis.            

Even though the aromatic compounds are said to be stable, they participate in various chemical reactions such as substitution, coupling, hydrogenation reactions, etc., due to the presence of delocalized pi-electrons. These compounds specifically do not undergo additional reactions like alkenes, because then they will lose their aromatic property. Most of the reactions taking place might be similar to that of alkenes due to the presence of double bonds but the energy required for the reactions will be obviously high due to their aromatic stability.  

Substitution Reactions

Aromatic compounds feasibly undergo both nucleophilic and electrophilic substitution reactions. In substitution reactions, one of the substituents on the benzene ring is displaced by the electrophile or nucleophile. Reactions such as Friedel-Craft alkylation, Friedel Craft acylation, nitration, halogenation, sulfonation are included under electrophilic substitution reactions. Electrophilic substitution reaction takes place through an intermediate named as ‘carbocation’, whose reactivity is high, and nucleophilic substitution reaction takes place via an electron-rich intermediate named as ‘Meisenheimer’ intermediate. 

Based on the position of the substituent, a generalized statement reveals that the presence of electron withdrawing substituent reduces the progress of the reaction while electron releasing substituent increases the progress of the reaction. Further electron donating substituent increases reactivity towards ortho and para position while electron withdrawing substituent does the same towards meta position.   

Nucleophilic Reactions of Aromatic Compounds

Nucleophilic aromatic reactions are a significant class of organic reactions in which nucleophilic aromatic substitution is important. In this type of reactions, the aromatic ring consists of poor electron. And the nucleophile attacks this electron deficient aromatic ring. Since, the attack of nucleophile disturbs the aromaticity, this acts as the rate determining step. The presence of electron withdrawing groups on the aromatic ring increases the rate of the reaction.

Hydrogenation reactions of aromatic compounds occur in extreme conditions. Hydrogenation of benzene occurs in presence of a strong catalyst at high temperature and pressure forming cyclohexane. 

Coupling between two radical fragments in presence of a metal catalyst resulting to form C-C bonds, C-N bonds, and C-O bonds are termed as coupling reactions. Benzene diazonium chloride readily undergoes coupling reactions. 

An unusual cycloaddition reaction named after Theodor Wagner-Jauregg as Wagner-Jauregg reaction describes the double Diels-Alder reaction which involves the starting materials named as maleic anhydride and 1,1-diarylethylene which leads to the formation of a naphthalene derivative with a phenyl substituent in it. 

DE aromatization reaction leads to the loss of aromaticity of the arene compounds permanently. Few methods such as Birch reduction or hydrogenation reaction, alkylative DE aromatization, photochemical, thermal, oxidative, and enzymatic DE aromatization reactions are very important as they play a prominent role in the organic synthesis of new building blocks.  

Uses of Aromatic Compounds or Aromatic Hydrocarbons  

Aromatic compounds are synthesized in a large scale as they are highly used in high-octane gasoline and in the manufacture of polymers, detergents, pesticides, dyes, etc. Few compounds have wide application in industries like toluene is used as a solvent for model glues and naphthalene is used as mothballs. Phenanthrene, an aromatic hydrocarbon is an intermediate in various synthetic processes of dyes, explosives, drugs, etc., TNT (2,4,6-trinitrotoluene) is an important explosive of aromatic compounds. 1,2-benzenediols, also called as pyrocatechol, is an important compound in photographic development.  

Practice Problem

Pick out the aromatic compounds from the following;

Solution:

Note: Consider the Huckel rule and the number of pi electrons to find out the answer.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for:

Bachelor of Science in Chemistry 

Masters of Science in Chemistry