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Aromatic Compounds

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Aromatic Hydrocarbons are alkyl, alkenyl, and alkynyl derivatives of cyclic hydrocarbons which include one or more benzene rings fused or isolated in their molecules and cyclic hydrocarbons are those hydrocarbons in which carbon atoms are connected to form a complete cycle or closed ring structure. Benzene ring is an extremely unsaturated example of aromatic hydrocarbons and this unsaturation of the benzene ring is preserved in most of the aromatic hydrocarbons. Those Aromatics hydrocarbons which contain benzene rings are known as benzenoids and those which lack the benzene ring are called non-benzenoids. 

In this article, we will try to grasp the concept of aromatic hydrocarbons and why they are necessary for the daily lives of the common man.

Cyclic Hydrocarbons

A compound in which the atoms are linked together to form a ring or loop-like structure is termed a cyclic hydrocarbons. They can be fully saturated or unsaturated i.e. they may contain only sigma bonds or both sigma and pi bonds. 

Cyclic compounds are further of two types:

  • Carbocyclic Hydrocarbons
  • Heterocyclic Hydrocarbons

Carbocyclic

Those cyclic hydrocarbons in which all the atoms connected in the chain form are carbon atoms are called Carbocylic. Some examples of these compounds are cyclopropane, cyclobutene, cyclopentane, etc.

Heterocyclic

Those cyclic hydrocarbons in which all the atoms connected in the closed chain can be both Carbon and non Carbon atoms are called Heterocyclic. Some examples of these hydrocarbons are Ethylene oxide, Thiirane, Ozetane, 1,2-dioxane THF (Tetrahydrofuran), Piperidine, etc.

Cyclic Hydrocarbons

 

What are Aromatic Hydrocarbons?

Those Cyclic hydrocarbons that have planer rings with all the pi electrons completly delocalized, are called aromatic hydrocarbons. These compounds are also known as arenes. As these compounds have very unique fragrance and aroma, hence the name aromatic hydrocarbons.

Aromatic Hydrocarbons Examples

In nature, there exist many many examples of aromatic hydrocarbons such as spices(cinnamon), coal derivatives, some unique smelling food items, etc.

Aromatic Hydrocarbons Examples

 

Properties of Aromatic Hydrocarbons

Aromatic Hydrocarbons are organic compounds that have a special aroma and hence the name aromatic hydrocarbon. The first aromatic carbon produced in the lab was Benzene and all aromatic hydrocarbons are cyclic in nature. They have very distinct properties from the other hydrocarbons. Some of the common properties of the Aromatic Hydrocarbons are,

  • They are non-polar in nature
  • They are immiscible in water due to their non-polar nature
  • They burn with a sooty yellow flame due to the high Carbon- Hydrogen ratio
  • They are used as solvents for non-polar compounds and are usually unreactive.
  • They undergo electrophilic substitutions and nucleophilic aromatic substitution reactions.

It is very difficult to identify an aromatic compound just by looking at its structure. Thus we make use of the Huckel rule. So the definition of aromatic compounds can be extended as compounds that follow the Huckel rule are known as aromatic compounds and the property is known as aromaticity.

Huckel’s Rule

Huckel’s Rule states that a planer ring molecules will have aromatic properties if they satisfy the following criteria:

  • Planarity: The compound must be planar and all the Carbon atoms in the compounds must be sp2 hybridized.
  • Delocalized π electrons: The compound must have a completely delocalized π electron cloud both above and below the ring.
  • 4n+2 π electrons: This is the most important condition of Huckel’s rule. According to it, an aromatic compound must have 4n+2 π electrons where n is a positive integer i.e., n = 0, 1, 2, 3, 4, . . .

Let us look at an example to verify Huckel’s rule. Consider the compound shown below i.e. benzene

Example of Huckel Rule

 

The above compound is planar in nature as all the Carbon atoms are sp2 hybridized. The compound also has completely delocalized pi-electrons. Now, the no. of \pi electrons in benzene is 6 which is equal to 4(1)+2 for n = 1. Thus it satisfies all the conditions of Huckel’s rule and is, therefore, an aromatic compound.

Anti Aromatic Hydrocarbons

Apart from aromatic compounds, there are antiaromatic compounds also. Anti-aromatic compounds follow the first two conditions of Huckel’s rule but differ in the third one. They have (4n\π) electrons instead of (4n+2) \ π electrons. These compounds are very reactive in nature and highly unstable.

e.g.: Pentalene with 8π e.

Pentalene with 8π e−

 

Reactions of Aromatic Hydrocarbons

Aromatic Hydrocarbons reacts with various compounds forming all sort of products. Some of the common reactions involving aromatic compounds are,

  • Aromatic Substitution Reactions
  • Coupling Reactions
  • Hydrogenation Reactions
  • Dearomatization Reactions

Aromatic Substitution Reactions

Aromatic substitution takes place when one substituent (commonly hydrogen) is replaced by another substituent on the arena ring. There are two types of substitution, the first is electrophilic (where the adjoining substituent is electrophile) and the second is nucleophilic ( where the joining substituent is nucleophile). Some examples of substitution reactions are given as follows:

Examples of Aromatic Substitution

 

Coupling Reactions

In a coupling reaction, two radical aromatic hydrocarbons are coupled together using the metal catalyst. For example, the reaction of Benzene diazonium chloride with aniline in the presence of dilute hydrochloric acid gives p-amino azobenzene as a product which is a yellow dye.

reaction of Benzene diazonium chloride with aniline

 

And, if the same BDC (Benzene diazonium chloride) reacts with phenol in the presence of sodium hydroxide, p-hydroxy azobenzene is a product.

BDC (Benzene diazonium chloride) reacts with phenol

 

Hydrogenation Reactions

Hydrogenation is the reaction in which saturated hydrocarbons are formed from unsaturated carbons. In the case of aromatic hydrocarbons, arenes are converted into cyclic alkanes. 

For, examples hydrogenation of benzene, results in cyclohexane.

hydrogenation of benzene

 

And, hydrogenation of 1-naphthol to form decalin-ol.

hydrogenation of 1-naphthol to form decalin-ol.

 

Dearomatization Reactions

Deaomatization is the reaction in organic chemistry that is used to create non-aromatic compounds from the arenes i.e., products of this reaction lost their aromaticity permanently. Hydrogenation(Birch Reduction) is one example of this reaction, other than hydrogenation, there are many dearomatization reactions such as photochemical dearomatization, Enzymatic dearomatization are some examples of this reaction.

Polycyclic Aromatic Hydrocarbons(PAHs)

Polycyclic aromatic hydrocarbons (PAHs) are those aromatic hydrocarbons where two or more aromatic rings are fused. They are most commonly produced due to the incomplete combustion of organic matter; due to this fact, they can be found in many food substances which are cooked on coal or fire such as barbeque, toast, meat, etc. Other than this PAHs can be found in air, water, soil, and sediment. Some examples of PAHs include naphthalene, anthracene, and pyrene.

Polycyclic Aromatic Hydrocarbons

 

Toxicity of PAHs

Polycyclic aromatic hydrocarbons (PAHs) have some potential health hazards and some undesirable environmental effects. Some PAHs are considered carcinogenic in nature, i.e., they may cause cancer. Also, PAHs are toxic to aquatic life and another organism as well. Some countries came forward and established some rules for the regulations of the number of PAHs in air, water, and soil. It’s an ongoing field of study where scientists are still trying to find its extreme impacts on human health and the environment.

Uses of Aromatic Hydrocarbons

Aromatic hydrocarbons have many applications in industries, some of which are listed below:

  • The nucleic acids and amino acids which are found in the human body are aromatic in nature.
  • Chlorophyll, an essential compound for plant survival as plants use chlorophyll for food production, is also an aromatic compound.
  • Naphthalene, an aromatic compound, is used to make mothballs for protecting clothes. Other than mothballs, naphthalene is used in many industries to make paint, resins, and dyes.
  • Trinitrotoluene (TNT) is used in the manufacture of explosives, is also an aromatic hydrocarbon.
  • Aromatic hydrocarbons are used to produce drugs and dyes.
  • They are extensively used in the plastics and petrochemical industries.

Read More,

Sample Questions on Aromatic Hydrocarbons

Question 1:What is Huckel’s rule?

Answer:

According to Huckel’s rule, the aromatic compounds may possess the following properties or satisfy the following criteria:

  1. Planarity: The compound must be planar i.e. it should not have any Carbon-Carbon single bonds. All the Carbon atoms in the compounds must be sp^2 hybridized.
  2. Delocalized pi electrons: The compound must have a completely delocalized \pi electron cloud both above and below the ring.
  3. 4n+2 \pi electrons: This is the most important condition of the Huckel’s rule. According to it, an aromatic compound must have 4n+2 \pi electrons where n is an integer and n = 0, 1, 2, 3, 4,………

Question 2: State two uses of aromatic compounds.

Answer:

Following are the two uses of aromatic compounds are:

  • Naphthalene which is an aromatic compound is used to make mothballs to protect clothes.
  • Tri Nitro Toluene, an aromatic compound, is used to make explosives.

Question 3: State any two properties of aromatic compounds.

Answer:

Following are the two properties of aromatic compounds are:

  • They are immiscible in water due to their non polar nature.
  • They burn with a sooty yellow flame due to high Carbon-Hydrogen ratio.

Question 4: Verify if the given compound is aromatic or not.

given compound is aromatic or not

 

Answer:

The compound shown in above image is planar in nature as all the Carbon atoms are sp2 hybridized but it has 4 pi electrons which violates Huckel’s rule. Hence it is not aromatic but anti aromatic.

SP2 hybridisation of cyclo but-2-diene

 

FAQs on Aromatic Hydrocarbons

Q1: Define Aromatic Compounds.

Answer:

Cyclic Compounds that have planar rings with completely delocalized pi electrons are called aromatic compounds. They are also known as arenes. Example: Benzene, Naphthalene

Q2: What are differences between Aromatic and Nonaromatic compounds?

Answer:

The difference between aromatic and nonaromatic compounds are as follows:

Aromatic

Nonaromatic

They have 4n+2 π electrons They have 4n π electrons
They are very stable and unreactive They are highly reactive in nature

Q3: Why Aromatic Compounds burn with sooty flame?

Answer:

Aromatic compound burn with sooty flame because they have a ring structure of carbon atom. This causes incomplete combustion of the carbon chain. Thus they give sooty flame.

Q4: What are Some Common Examples of Aromatic Hydrocarbons? 

Answer:

There are various aromatic hydrocarbons around us and some of the examples include

  • Benzene
  • Toluene
  • Naphthalene
  • Anthracene
  • Pyrene

Q5: What are the Applications of Aromatic Hydrocarbons? 

Answer:

There are a lot of applications of aromatic hydrocarbons in industries, some of which include production of plastics, dyes, drugs, explosives, and resins. These hydrocarbons are also found in many consumer products, including paints, solvents, and cleaning agents.

Q6: What are Polycyclic Aromatic Hydrocarbons(PAHs)?

Answer:

A polycyclic aromatic hydrocarbon is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings

Q7: Are Aromatic Hydrocarbons Saturated or Unsaturated?

Answer:

Aromatic hydrocarbons are a special class of unsaturated hydrocarbon based on a six carbon ring moiety called benzene.

Q8: What are the main sources of Aromatic Hydrocarbons?

Answer:

Simple aromatic hydrocarbons come from two main sources: Coal and petroleum. Coal is a complex mixture of a large number of compounds, most of which are long-chain compounds.


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Last Updated : 14 Jun, 2023
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