Aromatic Hydrocarbons

What are Aromatic Hydrocarbons?

Aromatic hydrocarbon is “an unsaturated hydrocarbon with sigma bonds, and the ‘pi’ electrons are delocalized between carbon atoms forming a circle.” And, in contrast, aliphatic hydrocarbons lack this delocalization. The aromatic hydrogen atoms are attached with the general chemical formula CnHn. Plenty of aromatic hydrocarbons contain a benzene ring (also called an aromatic ring). The benzene ring is stabilized by resonance, and the pi electrons in the ring structure are delocalized.


Aromatic Hydrocarbons are the organic compounds structured circularly that contain sigma bonds in addition to delocalized pi electrons. Also, they are referred to as aryl hydrocarbons or arenes. Generally, the aromatics meaning is just that, a plant or a substance that emits a distinct and pleasant smell as well.


Few aromatic hydrocarbons examples of the hydrocarbons are given below. It also observed that all these compounds (which are aromatic hydrocarbons examples) consist of a benzene ring.


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


In such compounds, a minimum of one carbon is replaced by oxygen, nitrogen, or sulfur. Some common aromatic examples of heteroarenes include pyridine (contains nitrogen) and furan (contains oxygen).

Properties of Aromatic Hydrocarbons

“The compound that was categorized firstly as an aromatic hydrocarbon was benzene.” Benzene is also the most complex aryl hydrocarbon. Every carbon atom belonging to the benzene ring has two carbon-carbon sigma bonds, one double bond and one carbon-hydrogen sigma bond with a neighbouring carbon where the pi-electron is delocalized.


This delocalization process of pi electrons is represented by a circle in the benzene molecule inside the hexagon. All the carbon-carbon bond order in this molecule is considered to be 1.5, and this equivalency can be explained using the resonance structures of benzene.


Few of the general properties of aromatic hydrocarbons have been given below.

  • These properties of aromatic hydrocarbons exhibit aromaticity (means, additional stability - granted by resonance)

  • The ratio of hydrogen atoms to the carbon atoms is relatively high in these molecule types.

  • When burnt, the aromatic hydrocarbons exhibit a strong and sooty flame resulting as yellow.

  • In general, these compounds undergo electrophilic substitutions and nucleophilic aromatic substitution reactions.

It is to make a note that these compounds can either be monocyclic or polycyclic.

Reactions of Aromatic Hydrocarbons

Many organic chemical reactions participate in the use of aromatic hydrocarbons as a primary reactant. A few such reactions of aromatic hydrocarbons are listed in this subsection, along with a brief description of every reaction.

  1. Aromatic Substitution Reactions

These reactions of aromatic hydrocarbons involve the “replacing of one substituent on the ring of an aromatic hydrocarbon,” concerned as a common hydrogen atom, by a different substituent group.

Some common types of aromatic compounds substitution reactions are listed below.

  • Nucleophilic aromatic substitution reactions

  • Electrophilic aromatic substitution reactions

  • Radical nucleophilic aromatic substitution reactions

The example of an aromatic substitution reaction is an electrophilic substitution noticed 

in the nitration reaction of salicylic acid. This is a kind of aromatic example.

  1. Coupling Reactions

On these reactions, the coupling of two fragments that have a radical nature is achieved using a metal catalyst. When aromatic hydrocarbons undergo coupling reactions, the bonds that are formed are listed below.

  • Carbon-carbon bonds are formed from the coupling reactions of aromatic hydrocarbon products and arenes like alkyl arenes, vinyl arenes, and more are created.

  • A carbon-oxygen bond formation can occur in these reactions, forming the aryloxy compounds.

  • Carbon-nitrogen bonds are formed in coupling reactions, resulting in products such as aniline.

The example of a coupling reaction involving aromatic hydrocarbons can be observed in the arylation of perfluorobenzene, as explained below.


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The catalyst used in the above reaction is Palladium (II) acetate. Also, it is to be noted that DMA is the abbreviation of Dimethylacetamide.

  1. Hydrogenation Reactions

Generally, the hydrogenation reactions involving arenes lead to form the saturated rings. An example of such a reaction is the reduction of 1-naphthol into a mixture containing various isomers of decalin-ol.

Another such reaction is hydrogenation reaction of resorcinol with the help of spongy nickel (also referred to as Raney nickel), and aqueous NaOH. This reaction continues 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 quite common in both synthetic and biological processes. Some numerous uses of aromatic hydrocarbons are given below.

  • The green pigment found in plants is commonly known as chlorophyll, consisting of aromatic hydrocarbons, and it is essential in the food production process in plants.

  • The nucleic and amino acids present in the human body also include these aromatic hydrocarbons which is one of the uses of aromatic hydrocarbons.

  • Methylbenzene, an aromatic hydrocarbon is used as a solvent in model glues

  • Naphthalene is an essential item in the production of mothballs

  • For the synthesis of dyes, drugs, and explosives, an aryl hydrocarbon called Phenanthrene is used

  • Trinitrotoluene or TNT is an important aromatic hydrocarbon and is widely used for explosive purposes.

  • Petrochemical industries and Plastic industries also make use of aromatic hydrocarbons extensively.

FAQ (Frequently Asked Questions)

1. What are Polycyclic Aromatic Hydrocarbons?

Structurally, Polycyclic Aromatic Hydrocarbons (PAH) are hydrocarbons, composed of multiple aromatic rings infused form and found in tar oil, coal, and some of the cooked foods like smoked fish, burnt toast. Such simplest aromatic compound members are naphthalene, having two aromatic rings, and the three-ring compounds phenanthrene and anthracene. These compounds form under incomplete combustion conditions or organic material (For example, forest fires, chimney soot, and so on).


These compounds biologically tend to be carcinogenic. Smaller members like anthracene or naphthalene with 2-3 rings are not too bad. Whereas, the benzo[a]pyrene (BAP), with five rings, is acknowledged to be a carcinogen. Also, historically, BAP is the first pure compound to be shown to cause cancer probably and is considered to be responsible for the research area, which is chemical carcinogenesis.

2. What makes an unsaturated cyclic hydrocarbon aromatic?

All unsaturated cyclic hydrocarbons are not aromatic. For the compound to be aromatic, unsaturation and cyclic nature are essential, but we can’t say the compound be aromatic only by these two features.


Any compound is aromatic only if they follow the below criteria.

  • Compounds must be cyclic and SP2 hybridized

  • A compound must contain a vacant ‘P’ orbital

  • A compound must contain delocalized pi orbital

  • Compounds need to follow the Huckel rule.

Huckel rule is an essential condition for any compound aromaticity.


Huckel Rule

In order to be aromatic, a molecule must have a specific number of pi electrons (lone pairs within p orbitals or electrons with pi bonds) within a closed loop of adjacent parallel ‘p’ orbitals. The pi-electron count is determined by the series of numbers generated from 4n+2, where n = zero, or any positive integer (i..e, n = 0, 1, 2,...).


Example

In the above structure, three pi bonds are present, meaning six pi electrons.


4n+2=6

4n=4

Therefore, n=1, and so, the benzene is aromatic.


Note - If a ring contains an odd number of the pi bond, then the compound is aromatic.

If the compound has an even number of the pi bond, then the compound is antiaromatic.