Aromatic Hydrocarbons and Types | Organic Chemistry PDF Download

Aromatic Hydrocarbons

The aromatic hydrocarbons are “unsaturated hydrocarbons which have one or more planar six-carbon rings called benzene rings, to which hydrogen atoms are attached”. Many aromatic hydrocarbons contain a benzene ring (also referred to as an aromatic ring). The benzene ring is stabilized by resonance and the pi electrons are delocalized in the ring structure.

What are Aromatic Hydrocarbons?

Aromatic Hydrocarbons are circularly structured organic compounds that contain sigma bonds along with delocalized pi electrons. They are also referred to as arenes or aryl hydrocarbons.

A few examples of aromatic hydrocarbons are provided below. It can be observed that all these compounds contain a benzene ring.

The aromatic hydrocarbons which do not contain a benzene ring are commonly referred to as heteroarenes. All of these heteroarenes obey Huckel’s rule (total number of pi electrons in a monocyclic ring = 4n + 2 where n is any positive integer or zero).

In these types of compounds, a minimum of one carbon is replaced by either nitrogen, oxygen, or sulphur. Common examples of heteroarenes include furan (contains oxygen) and pyridine (contains nitrogen).

Properties of Aromatic Hydrocarbons

“The first compound that was categorized as an aromatic hydrocarbon was benzene”. It is also the most complex aryl hydrocarbon. Each carbon atom belonging to the benzene ring has two carbon-carbon sigma bonds, one carbon-hydrogen sigma bond, and one double bond with a neighbouring carbon in which the pi electron is delocalized.
This delocalization of pi electrons in the benzene molecule is represented by a circle inside the hexagon. The bond order of all carbon-carbon bonds in this molecule is considered to be 1.5 and this equivalency can be explained with the help of the resonance structures of benzene. Some general properties of aromatic hydrocarbons have been listed below.

• These compounds exhibit aromaticity (additional stability granted by resonance)
• The ratio of carbon atoms to hydrogen atoms is relatively high in these types of molecules.
• When burnt, the aromatic hydrocarbons display a strong and sooty flame which is yellow.
• These compounds generally undergo electrophilic substitutions and nucleophilic aromatic substitution reactions.

It can be noted that these compounds can be either monocyclic or polycyclic.

Reactions of Aromatic Hydrocarbons

Many organic chemical reactions involve the use of aromatic hydrocarbons as the primary reactant. Some such reactions are listed in this subsection along with a brief description of each of these reactions.

1. Aromatic Substitution Reactions

These reactions involve the replacement of one substituent on the ring of an aromatic hydrocarbon, commonly a hydrogen atom, by a different substituent group.
The common types of aromatic substitution reactions include:

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

An example of an aromatic substitution reaction is the electrophilic substitution observed in the nitration reaction of salicylic acid.

2. Coupling Reactions In these types of reactions, the coupling of two fragments which have a radical nature is achieved with the help of a metal catalyst. When aromatic hydrocarbons undergo coupling reactions, the following type of bonds can be formed.

• Carbon-carbon bonds can be formed from the coupling reactions of arenes and products such as vinyl arenes, alkyl arenes, etc. are formed.
• The formation of carbon-oxygen bonds can occur in these reactions, forming aryloxy compounds.
• Carbon-nitrogen bonds can form in coupling reactions, giving products such as aniline.

An example of a coupling reaction involving aromatic hydrocarbons can be observed in the arylation of perfluorobenzenes, as illustrated below.

The catalyst used in this reaction is Palladium(II) acetate. It can also be noted that DMA is the abbreviation of Dimethylacetamide.

3. Hydrogenation Reactions 
The hydrogenation reactions involving arenes generally lead to the formation of saturated rings. An example of such a reaction is the reduction of 1-naphthol into a mixture containing different isomers of decalin-ol. Another example of such a reaction is the hydrogenation reaction of resorcinol with the help of spongy nickel (also referred to as Raney nickel) and aqueous NaOH. This reaction proceeds via the formation of an enolate, and the successive alkylation of this enolate (with methyl iodide) to yield 2-methyl-1,3-cyclohexanedione.

Uses of Aromatic Hydrocarbons

The use of aromatic hydrocarbons is common in both biological and synthetic processes. Some numerous uses of aromatic hydrocarbons are listed below.

• The green pigment found in plants, more commonly known as chlorophyll, consists of aromatic hydrocarbons and is very important in the process of food production in plants.
• The nucleic acids and amino acids in the human body also consist of these aromatic hydrocarbons.
• Methylbenzene which is an aromatic hydrocarbon is used as a solvent in model glues
• Naphthalene is an important item in the production of mothballs
• For the synthesis of drugs, dyes, and explosives, an aryl hydrocarbon known as Phenanthrene is used
• Trinitrotoluene or TNT is a very important aromatic hydrocarbon which is widely used for explosive purposes.
• Plastic industry and petrochemical industries make use of aromatic hydrocarbons extensively.

Polycyclic Aromatic Hydrocarbons

• These are the hydrocarbons which comprise aromatic rings in fused form. These are found in coal, tar, oil and some cooked foods such as smoked fish, burnt toast, etc.
• One common example of these polycyclic hydrocarbons is naphthalene. These compounds are said to be pollutants.
• Some examples of aromatic hydrocarbons are Methylbenzene, Naphthalene, Phenanthrene, Trinitrotoluene, and o-dihydroxybenzene.

What is Benzene?

Benzene is one of the most important organic compounds with the chemical formula C₆H₆. Benzene is the parent compound of the various aromatic compound.

Benzene is the simplest organic, aromatic hydrocarbon. Benzene is one of the elementary petrochemicals and a natural constituent of crude oil. It has a gasoline-like odour and is a colourless liquid. Benzene is highly toxic and carcinogenic in nature. It is primarily used in the production of polystyrene.
Benzene is a naturally occurring substance produced by volcanoes and forest fires and present in many plants and animals, but benzene is also a major industrial chemical made from coal and oil. As a pure chemical, benzene is a clear, colourless liquid. In industry benzene is used to make other chemicals as well as some types of plastics, detergents, and pesticides. It is also a component of gasoline.

Discovery of Benzene

The word benzene derives historically from gum benzoin, sometimes called ‘Benjamin’. Gum benzoin was known as an aromatic resin. Michael Faraday, an English scientist first discovered Benzene in illuminating gas. The name benzene was given by German Chemist Mitscherich in 1833. The cyclic structure of benzene remained a mystery until 1865 when German professor August Kekule elucidated it when he dreamt of a snake biting its own tail.
However, Kekule did not discover the presence of interactions between the double bonds. Americal professor Linus Pauling proposed that benzene exhibited a hybrid structure composed of delocalized electrons. This was the refinement of Kekule’s discovery. Benzene has a somewhat pleasant, sweet smell, however it is carcinogenic.

Benzene Characteristics

Benzene was discovered in 1825 by the English physicist Michael Faraday, and was made available in large quantities in 1842 after it was found to contain benzene. Large amounts of benzene are now extracted from petroleum. Benzene is a colorless liquid with a characteristic odor of formula C₆H₆.
Benzene is a closed ring of six carbon atoms linked by bonds that alternate between single and double bonds. Each carbon atom is bound by a single hydrogen atom. Benzene melts at a temperature of 5.5 ° C, boils at 80.1°C. Benzene and its derivatives are part of an essential chemical community known as aromatic compounds. Benzene is a precursor in the production of medicines, plastics, oil, synthetic rubber and dyes.
As a result, the hydrogenation of benzene happens much more slowly than the hydrogenation of other organic compounds containing carbon-carbon double bonds, and benzene is much more difficult to oxidize than alkenes. Most of the reactions of benzene belong to a class called electrophilic aromatic substitution, which leaves the ring intact but replaces one of the hydrogens attached to it. These reactions are versatile and commonly used for the preparation of benzene derivatives.

Structure of Benzene

The structure of benzene has been of interest since its discovery. Benzene is a cyclic hydrocarbon (chemical formula: C₆H₆), i.e., each carbon atom in benzene is arranged in a six-membered ring and is bonded to only one hydrogen atom. According to molecular orbital theory, benzene ring involves the formation of three delocalized π – orbitals spanning all six carbon atoms, while the valence bond theory describes two stable resonance structures for the ring.

Properties of Benzene

The various properties of benzene are mentioned below:

• Benzene is immiscible in water but soluble in organic solvents.
• It is a colourless liquid and has an aromatic odour.
• It has a density of 0.87g cm-3. It is lighter than water.
• Benzene has a moderate boiling point and a high melting point. (Boiling point: 80.5°C, Melting point: 5.5°C)
• Benzene shows resonance.
• It is highly inflammable and burns with a sooty flame.

Resonance of Benzene
The oscillating double bonds in the benzene ring are explained with the help of resonance structures as per valence bond theory. All the carbon atoms in the benzene ring are sp² hybridized. One of the two sp² hybridized orbitals of one atom overlaps with the sp² orbital of adjacent carbon atom forming six C-C sigma bonds. Other left sp2 hybridized orbitals combine with s orbital of hydrogen to form six C-H sigma bonds. Remaining unhybridized p orbitals of carbon atoms form π bonds with adjacent carbon atoms by lateral overlap. 
This explains an equal possibility for the formation of C1 –C2, C3 – C4, C5 – C6 π bonds or C2 – C3, C4 – C5, C6-C1 π bonds. The hybrid structure is represented by inserting a circle in the ring as shown below in the figure. Hence, it explains the formation of two resonance structures proposed by Kekule. 

Aromaticity of benzene

Benzene is an aromatic compound, as the C-C bonds formed in the ring are not exactly single or double, rather they are of intermediate length. Aromatic compounds are divided into two categories: benzenoids (one containing benzene ring) and non-benzenoids (those not containing benzene ring), provided they follow Huckel rule. According to Huckel rule, for a ring to be aromatic it should have the following property:

• Planarity
• Complete delocalization of the π electrons in the ring
• Presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . . .)

Uses of Benzene

Benzene is used in various industrial processes such as in the manufacture of lubricants, plastics, rubbers, dyes, synthetic fibres, etc. However, it has non-industrial uses too which are limited due to the reason benzene is toxic and carcinogenic. The different uses of Benzene are mentioned below.

• Benzene is used in the preparation of phenol. It is also used to prepare aniline used in dyes and in dodecylbenzene used for the detergents.
• In early times, benzene was used in degreasing of metal.
• It is used for manufacturing of nylon fibres.
• The main use of benzene is that it is used in the manufacture of other chemicals such as ethylbenzene, cyclohexane, cumene, nitrobenzene, alkylbenzene, etc.