Arenes or Aromatic Hydrocarbons: Hydrocarbons are organic compounds composed entirely of two types of atoms: carbon and hydrogen. Hydrocarbons are typically colourless gases with very weak odours. Hydrocarbons can have simple or complex structures and are classified into four categories: alkanes, alkenes, alkynes, and aromatic hydrocarbons.

The study of hydrocarbons can provide insight into the chemical properties and preparation of other functional groups. Benzene, one of the most important aromatic hydrocarbons, is used as a starting material in the synthesis of many synthetic drugs. In this article, we will study one of the types of hydrocarbons- Aromatic hydrocarbons which are also known as Arenes.

What are Aromatic Hydrocarbons?

Aromatic compounds are chemical compounds (usually organic) that have one or more rings with pi electrons delocalized all the way around them. They are also referred to as arenes or aryl hydrocarbons. In contrast to aromatic compounds, aliphatic compounds lack this complete delocalization. The term “aromatic” was thought up before the physical mechanism determining aromaticity was discovered. It simply referred to the fact that many such compounds have a sweet or pleasant odour. However, not all aromatic compounds have a sweet odour, and not all compounds with a sweet odour are aromatic. Thus,

Aromatic hydrocarbons are defined as “unsaturated hydrocarbons with one or more planar six-carbon rings known as benzene rings to which hydrogen atoms are attached.” A benzene ring can be found in many aromatic hydrocarbons (also referred to as an aromatic ring). The benzene ring is stabilised by resonance, and the pi electrons in the ring structure are delocalized.

Few examples of aromatic hydrocarbons are as follows-

Aromaticity is not limited to benzene-based compounds; aromaticity can also be found in heteroarenes, which follow Hückel’s rule (for monocyclic rings: when the number of pi-electrons equals \(\left({4{\text{n}} + 2} \right),\) where \({\text{n}} = 0,1,2,3,….)\). At least one carbon atom is replaced by one of these compounds’ heteroatoms, oxygen, nitrogen, or sulphur. Furan, a heterocyclic compound with a five-membered ring containing a single oxygen atom, and pyridine, a heterocyclic compound with a six-membered ring containing one nitrogen atom, are two examples of non-benzene compounds with aromatic properties.

Physical Properties

Aromatic compounds are nonpolar and insoluble in water. Because they are frequently unreactive, they can be used as solvents for other nonpolar compounds. Aromatic compounds have a sooty yellow flame due to their high carbon-to-hydrogen ratio.

Reactivity of Aromatic Compounds

The double bonds in aromatic compounds are less likely to participate in addition reactions than those found in typical alkenes. Instead, cyclic aromatic compounds undergo electrophilic substitution reactions (reactions in which the ring acts as a nucleophile to a suitable electrophile). When benzene participates in these substitution reactions, the product retains the aromatic \({\rm{\pi }}\) electron system’s stability. Because in the electrophilic addition reactions, the product is not aromatic, so this stability is lost.

Sources of Aromatic Compounds

Aromatic compounds are made from a variety of materials, including petroleum and coal tar. Poly-aromatic hydrocarbons are known carcinogens and are components of atmospheric pollution. Aromatic compounds are also intriguing due to their presumed role in the origin of life as nucleotide and amino acid precursors.

Reactions of Aromatic Hydrocarbons

Aromatic hydrocarbons are used as a primary reactant in many organic chemical reactions. We will discuss few reactions in detail-

1. Aromatic Substitution

In aromatic substitution, one of the arene ring’s substituents, usually hydrogen atoms, are replaced by another substituent. Electrophilic aromatic substitution occurs when the active reagent is an electrophile, and nucleophilic aromatic substitution occurs when the reagent is a nucleophile. The active reagent in radical-nucleophilic aromatic substitution is radical. The nitration of salicylic acid is an example of electrophilic aromatic substitution.

2. Coupling Reactions

A metal catalyses the coupling of two formal radical fragments in coupling reactions. Common coupling reactions with arenes produce new carbon-carbon bonds, such as alkyl arenes, vinyl arenes, biaryls, new carbon-nitrogen bonds (anilines), or new carbon-oxygen bonds (aryloxy compounds). The direct arylation of pentafluorobenzene is one example.

3. Hydrogenation

When arenes are hydrogenated, they form saturated rings. \(1\)-naphthol is completely reduced to a mixture of decalin-\(1 – {\text{ol}}\) isomers.
Another example is when resorcinol is hydrogenated with Raney nickel in the presence of aqueous sodium hydroxide, an enolate is formed, which is then alkylated with methyl iodide to form \(2\)-methyl-\(1,3\)-cyclohexanedione:

Uses of Aromatic Hydrocarbons

Aromatic hydrocarbons find their uses in both biological and synthetic processes. A few of the uses are given below
i. The green pigment found in plants, more commonly known as chlorophyll, is made up of aromatic hydrocarbons and is crucial in the process of plant food production.
ii. The aromatic hydrocarbons are also found in nucleic acids and amino acids in the human body.
iii. In model glues, methylbenzene, an aromatic hydrocarbon, is used as a solvent.
iv. Naphthalene is a key ingredient in the manufacture of mothballs.
v. Phenanthrene, an aryl hydrocarbon, is used in the synthesis of drugs, dyes, and explosives.
vi. TNT, or trinitrotoluene, is a very important aromatic hydrocarbon that is widely used in explosives.
vii. Aromatic hydrocarbons are used in the plastic and petrochemical industries.

Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are aromatic hydrocarbons made up of fused aromatic rings that lack heteroatoms and carry substituents. The most basic example of a PAH is naphthalene. PAHs are found in the oil, coal, and tar deposits and are emitted as a byproduct of fuel combustion (whether fossil fuel or biomass). They are a concern as pollutants because some compounds have been identified as carcinogenic, mutagenic, and teratogenic. PAHs can be found in cooked foods as well. High PAH levels have been found in meat cooked at high temperatures, such as grilling or barbecuing, and smoked fish.

Summary

We can conclude that:

1. An aromatic hydrocarbon, also known as an arene, is a hydrocarbon with alternating double and single bonds between carbon atoms that form rings. Arene examples include benzene, toluene, phenol, and others.
2. Aromatic compounds or arenes go through substitution reactions in which the aromatic hydrogen is replaced with an electrophile, resulting in electrophilic substitution.
3. Arenes, like alkenes, have double bonds, but they do not undergo electrophilic addition because it would result in ring aromaticity loss.
4. The nature of the substituents present in the aromatic ring governs the order of substitution on aromatic compounds.
5. A carbocation is formed in electrophilic aromatic substitution reactions, whereas a carbanion is formed in nucleophilic aromatic substitution reactions.
6. Aromatic compounds are converted into saturated compounds during hydrogenation reactions.

Frequently Asked Questions

Q.1. What are aromatic hydrocarbons?
Ans: Aromatic hydrocarbons, also known as arenes, are aromatic organic molecules that are entirely composed of carbon and hydrogen. In aromatic compounds, a “benzene ring,” named after the simple aromatic chemical benzene, or a phenyl group when part of a larger structure, is the configuration of six carbon atoms.

Q.2. Why are arenes also known as aromatic hydrocarbons?
Ans: Arenes are also known as aromatic hydrocarbons because these compounds have a sweet or pleasant smell. However, not all aromatic compounds have a sweet odour, and not all compounds with a sweet odour are aromatic.

Q.3. How do aromatic compounds react?
Ans: Aromatic compounds, also known as arenes, undergo substitution reactions in which an electrophile replaces the aromatic ring’s hydrogen, resulting in electrophilic substitution. They have double bonds, just like alkenes, but they do not undergo electrophilic addition because it would result in ring aromaticity loss. Metal cross-coupling, such as the Suzuki reaction, enables the formation of carbon-carbon bonds between two or more aromatic compounds.

Q.4. What is the difference between aromatic hydrocarbons and aliphatic hydrocarbons?
Ans: Aliphatic and aromatic hydrocarbons are organic compounds composed solely of carbon and hydrogen atoms. The primary distinction between aliphatic and aromatic hydrocarbons is that aliphatic hydrocarbons have a lower carbon-to-hydrogen ratio, while aromatic hydrocarbons have a high carbon-to-hydrogen ratio.

Q.5. What are aromatic substitution reactions?
Ans: Electrophilic aromatic substitution reactions occur in organic processes when an electrophile replaces an atom connected to an aromatic ring. In these reactions, a hydrogen atom from a benzene ring is generally substituted with an electrophile.