Hydrocarbons

Nomenclature and Preparation of Aromatic Hydrocarbons

Aromatic Hydrocarbon is an essential class of hydrocarbons and organic substances. However, what are aromatic hydrocarbons and what is the basis of naming the compounds in such a manner? Moreover, what characteristics make this class of organic compounds so unique? In this topic, we will tackle two of the above questions-nomenclature and preparation of aromatic hydrocarbons.

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Aromatic Hydrocarbons

There was a time when chemists use to smell the compound and sometimes even taste it to identify chemicals. Many people believe that Carl Scheele, Swedish Chemist, died because he tasted a poisonous chemical while experimenting in the lab. Another example is of the famous scientist “Robert Bunsen” who had developed the habit of smelling arsenic as he continuously was exposed to it. Furthermore, the chemical made his tongue black.

During this period of time, chemists and scientist started naming certain carbon-compounds having a distinct odour as an aromatic hydrocarbon. One of the major reasons for such a nomenclature method was because the compounds such as benzene and toluene were able to emit a sweet smell. This made the scientists name the compounds as aromatic hydrocarbons (the Greek word for pleasant smelling compounds).

Therefore, aromatic hydrocarbon consists primarily of carbon and hydrogen. They are stable and unsaturated cyclic organic compounds.  Aromatic hydrocarbons or arenes have a special smell to the compound, hence the name aromatic compounds. Mostly the compounds will contain as well as retain one or more benzene ring, even after undergoing different reactions.

However, there is another class of aromatic hydrocarbon which do not contain benzene ring but they have a highly unsaturated ring. Aromatic compounds with benzene ring refer to as benzenoids and the compounds which do not contain benzene ring refer to as non-benzenoids.

Definition of Aromatic Hydrocarbons

Aromatic Hydrocarbons are compounds having sigma bonds as well as delocalized pi electrons in between the carbon atoms present in the ring form. Refer to see the different examples of aromatic compounds containing a benzene ring. A hydrocarbon can be an aromatic compound if it follows the Huckel Rule. According to this rule, a compound can be aromatic if it contains the following distinct properties:

  • Planarity
  • Delocalization of the pi-electrons in the carbon ring entirely
  • A compound having (4n + 2) π electrons in its structure, where n is an integer.

aromatic hydrocarbons

Substituted Benzene

Replacement of one hydrogen atom from benzene and addition of another atom results in the substitution of benzene. The compounds are known as substituted benzenes. Depending on the number of the substituents, the compounds can either be monosubstituted benzene, disubstituted benzene, and trisubstituted benzene.

aromatic hydrocarbons

Isomerism of Aromatic Hydrocarbons

If we consider any “disubstituted benzene”, there is a possibility of the formation of three different position isomers on the basis of substituent’s position in relation to the other. Thus, we use ortho-position to indicate the position of two substituents (1,2-).

Similarly, meta-position will represent the relative position (1,3-) and para-position will represent the relative position (1,4-). Let’s take the example of xylene. Refer to the diagram below to observe the different isomers of xylene (dimethylbenzene) depending on the position.

aromatic hydrocarbons

Nomenclature of Aromatic Hydrocarbons

IUPAC System

  • Rule 1: As per IUPAC nomenclature system, it is important to place the substituent’s name before the name of the compound as a prefix in any substituted aromatic hydrocarbon. For example, nitrobenzene where the benzene ring is present along with a nitro group.

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  • Rule 2: You have to attach Greek numerical prefixes such as di, tri, and tetra to indicate similar substituents group in case of compounds with more than one substituent group present in the benzene ring. For instance, a benzene ring with two bromo groups present on the adjacent carbon atoms of the benzene ring refers to as 1,2-di-bromobenzene.

aromatic hydrocarbons

  • Rule 3: If different substituent groups are present in the aromatic compounds, it is important to assign number one position to the substituent of the base. Furthermore, the numbering direction for the rest of the compound is chosen in such a manner that the next substituent will have the lowest numbering position. Moreover, we have to use alphabetical order for the naming of the substituent. For example, if a benzene consists of chloro group as well as a nitro group, then we start with the chloro group and then the nitro groups on the basis of the alphabetical order.

aromatic hydrocarbons

  • Rule 4: In case of aromatic compounds with more than one substituents, it is necessary to use terms such as ortho, meta, and para as prefixes to represent the relative positions like 1,2-; 1,3-; 1,4-. For instance, we can rewrite 1,2 di-bromo-benzene as o-di-bromo-benzene.
  • Rule 5: If an organic compound consists of an alkane with a functional group and aromatic compound, then the aromatic compound will act as a substituent instead of the parent group. For instance, when there is a benzene ring joined with an alkane and a functional group, then the aromatic group is known as phenyl (Ph-).

Diagrams representing Nomenclature of Aromatic Hydrocarbons

aromatic hydrocarbons

Preparation of Aromatic Hydrocarbons

One of the important commercial preparation methods of benzene is by isolation of coal tar. However, the laboratory techniques for preparation of aromatic hydrocarbons are different.

Cyclic Polymerization of Alkynes

Alkynes undergo polymerization reaction similar to alkenes. It can undergo two types of polymerization reaction- linear and cyclic. However, only cyclic polymerization can yield ethyne.

Cyclic polymerization of ethyne results in the formation of aromatic hydrocarbons. It is one of the important chemical reactions in alkynes. Ethyne undergoes reaction by passing it from the red-hot iron tube at a very high temperature of 873K to form benzene. This reaction is cyclic polymerization of ethyne. Refer to the example below

aromatic hydrocarbons

Aromatic Hydrocarbons by Decarboxylation of Aromatic Acids

The sodium salt of benzoic acid and soda lime react under heating conditions to produce benzene.

aromatic hydrocarbons

Aromatic Hydrocarbon by Reduction of Phenol

Phenol vapours undergo reduction reaction by passing extremely heated zinc dust. This reaction results in the formation of benzene.

aromatic hydrocarbons

Solved Questions for You

Question 1: Mention the structure and IUPAC name for the tautomer of phenol. The structure of phenol is

aromatic hydrocarbons

Solution: The name of the tautomer of phenol is Cyclohexa-2,4-dien-1-one. The structure of the tautomer of phenol is

aromatic hydrocarbonsQuestion 2: Name the compound which undergoes polymerization reaction to produce the compound 1,3,5-trimethylbenzene or mesitylene.

Solution: Propyne undergoes polymerization reaction to produce mesitylene.

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