Aromatic Hydrocarbons: Structure, Properties, Reactions and Uses

Aromatic Hydrocarbons are circular structured unsaturated compounds consisting of a sigma bond and delocalized pi electrons between carbon atoms. Aromatic hydrocarbons are also called aryl hydrocarbons or arenes. The general chemical formula of aromatic hydrocarbons is CnHn. Some aromatic hydrocarbons consist of benzene rings termed aromatic rings. This benzene ring is stabilized through resonance, while the pi electrons get delocalized in the ring.

Key Terms: Aromatic hydrocarbons, aromatic compounds, carbon, benzene, pi electrons, Huckel's rule, aromaticity, hydrocarbons. 

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Explanation of Aromatic Hydrocarbons

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Aromatic Hydrocarbons are defined as unsaturated hydrocarbons which contain one or more than one six-carbon cycle (ring) called benzene, through which hydrogens are attached. This benzene ring is stabilized through resonance, while the pi electrons get delocalized in the ring. Aromatic compounds that contain benzene rings are called Benzenoids and those not containing them are called non-benzenoids. In contrast, those aromatic hydrocarbons that do not contain benzene rings are called heteroarenes. In heteroarenes, the total number of pi electrons is 4n + 2 (Huckel's Rule), and ‘n’ is +ve integer or zero. Pyridine and Furan are common examples of heteroarenes. 

Some examples of aromatic hydrocarbons are given below. 

[Examples of Aromatic Hydrocarbons]

From the examples given, it is observed that each compound has a benzene ring in its structure.

Some other examples of aromatic hydrocarbons are 

1. Phenol (C₆H₅OH)
2. Aniline (C₆H₅NH₂)
3. Benzaldehyde (C₆H₅CHO)
4. Benzoic Acid (C₆H₅COOH)
5. Acetophenone (C₆H₅COCH₃)
6. Benzonitrile (C₆H₅CN)

All aromatic hydrocarbons follow the Huckel Rule. That is, the total number of pi electrons is (4n+2)π where ‘n’ can be a positive integer or zero.

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Structure of Aromatic Hydrocarbons

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Aromatic compounds consist of C-C and C-H bonds. Benzene is a ring containing 6 sp² carbons in the same plane and 6π electrons delocalized in the cyclic structure. As a result, optical and rotational isomerism is not possible in aromatic hydrocarbons. Also, since aromatic hydrocarbons are cyclic structures, therefore, cis/trans isomers are not possible. 

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Properties of Aromatic Hydrocarbons

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Benzene was the first compound that was categorized as an aromatic hydrocarbon. Benzene is also considered the complex aryl hydrocarbon. The carbon atom in the benzene has 2 carbon-carbon sigma bonds, one carbon-hydrogen sigma bond, and a double bond. 

In the benzene molecule, the delocalization of pi electrons by a solid or dashed circle inside a ring. All the carbon-carbon bonds have a bond order of 1.5. The equivalency of the bond can be observed with the study of the resonance structure of benzene. 

Aromatic hydrocarbons exhibit some properties that are given below.

1. Aromatic hydrocarbons show Aromaticity (stability property granted by resonance).
2. The carbon to hydrogen atoms ratio is relatively high in these compounds.
3. Aromatic hydrocarbons undergo nucleophilic aromatic substitution and electrophilic substitution reactions.
4. On burning, aromatic hydrocarbons yield sooty and strong yellow flame.

Note: Aromatic hydrocarbons can either be monocyclic or polycyclic compounds. 

Also Read: Carbocation Stability

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Reactions of Aromatic Hydrocarbons

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In many chemical reactions, aromatic hydrocarbon is used as a primary reactant. Aromatic hydrocarbons exhibit three kinds of reactions. These types of reactions are 

1. Aromatic Substitution Reaction
2. Coupling Reactions
3. Hydrogenation Reactions

Aromatic Substitution Reaction

In an aromatic substitution reaction, one substituent (nucleophile, electrophile, radical) present on the ring is replaced by another substituent concerned with the hydrogen atom as a common atom between two different substituents.

Types of aromatic substitution reactions are given below:-

• Electrophilic aromatic substitution reactions
• Nucleophilic aromatic substitution reactions
• Radical nucleophilic aromatic substitution reactions

The nitration of salicylic acid (C₇H₆O₃) is an example of an aromatic substitution reaction. In this reaction electrophilic substitution takes place. The reaction is shown below.

Nitration of salicylic acid

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Coupling Reactions

In coupling reactions, the coupling of two fragments having radical nature takes place using a metal catalyst. A list of bonds formed during the coupling reactions is given below:-

• Carbon-Carbon bonds: These bonds are formed by the coupling reactions between aromatic hydrocarbons and arenes such as alkyl arenes, vinyl arenes, etc.
• Carbon-Oxygen bond: This bond formation occurs in a coupling reaction resulting in the formation of aryloxy compounds. 
• Carbon-Nitrogen bonds: This bond results in the formation of products like aniline.

The arylation of perfluorobenzene (C₆F₆) is an example of a coupling reaction involving aromatic hydrocarbons. The coupled reaction is shown below.

Arylation of perfluorobenzene

• Palladium (II) acetate is used as a catalyst in the above reaction.

Hydrogenation Reactions

Hydrogenation reactions involve arenes that form the saturated ring. An example of a hydrogenation reaction is the reduction of 1-naphthol (C10H8O) into a mixture containing different isomers of decalin-ol. 

Reduction of 1-naphthol

Another example of a hydrogenation reaction is the hydrogenation of resorcinol (C₆H₆O₂) using spongy nickel and aqueous NaOH. This reaction involves the formation of an enolate and alkylation of this enolate with methyl iodide gives 2-methyl-1,3-cyclohexanedione. 

Hydrogenation of resorcinol

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Uses of Aromatic Hydrocarbons 

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Aromatic hydrocarbons are used in both biological and synthetic processes. Some uses are given below.

1. The amino and nucleic acids present in the human body contain aromatic hydrocarbons.
2. The green pigment called chlorophyll found in plants contains aromatic hydrocarbons that are required for food production in plants.
3. Methylbenzene (aromatic hydrocarbon) is used as a solvent in model guess.
4. Phenanthrene (an aryl hydrocarbon) is used for the synthesis of drugs, dyes, and explosives. 
5. Naphthalene is an important material used in the production of mothballs. 
6. Trinitrotoluene (TNT), is a famous aromatic hydrocarbon used for explosive activities.
7. Aromatic hydrocarbons have extensive use in Petrochemical and Plastic Industries.

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Polycyclic Aromatic Hydrocarbons

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Polycyclic aromatic hydrocarbons are the hydrocarbons in which an aromatic ring is present in the fused form. These hydrocarbons are found in coal, oil, tar, and some baked foods like burnt toast, smoked fish, etc. These compounds are considered pollutants.

Naphthalene is a common example of polycyclic aromatic hydrocarbon. Some other examples of polycyclic aromatic hydrocarbons are Methylbenzene, Phenanthrene, o-dihydroxybenzene, and Trinitrotoluene (TNT). 

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