Aromatic Compounds

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What are Aromatic Compounds? Recommended VideosAromatic Compounds ExamplesProperties of Aromatic CompoundsIUPAC Nomenclature of Aromatic CompoundsFrequently Asked Questions – FAQs

What are Aromatic Compounds?

Aromatic compounds are chemical compounds that consist of conjugated planar ring systems accompanied by delocalized pi-electron clouds in place of individual alternating double and single bonds.

They are also called aromatics or arenes. The best examples are toluene and benzene. Aromatics require satisfying Huckel’s rule. Plants and micro-organisms have an exclusive route to benzene-ring compounds. The great majority of aromatic compounds in nature, therefore, are produced by plants and micro-organisms, and animals are dependant upon plants for many aromatic compounds either directly or indirectly.

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Aromatic Compounds Examples

Aromatic hydrocarbon, are hydrocarbons containing sigma bonds and delocalized pi electrons between carbon atoms in a ring. For example, benzene. They are known as aromatic due to their pleasant smell.

Aromatic compounds are broadly divided into two categories: benzenoids (one containing benzene ring) and non-benzenoids (those not containing a benzene ring) for example, furan. Any hydrocarbon can be classified as an aromatic compound provided they follow the Huckel rule. According to Huckel rule, for a ring to be aromatic it should have the following properties:

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

Huckel’s Rule of Aromaticity

Huckel’s rule states that only planar, fully conjugated monocyclic polyenes having 4n + 2 π electrons, where n is an integer, that is, n = 0, 1, 2, 3, 4, etc., should possess aromatic stability. An aromatic compound must be planar and contain a cyclic cloud of π electrons below and above the plane of the molecule. It contains SP² hybridized carbon atoms and must obey the Huckel rule.

According to this rule, the ring system must have (4n+2) π electrons, where n is any whole number (0, 1, 2, 3, etc). On this basis the ring systems which have 2(n=0), 6(n=1), 10(n=2), 14(n=3) etc pi electrons are aromatic. Typical examples of aromatic compounds are benzene, naphthalene, and anthracene.

Properties of Aromatic Compounds

Arenes are mostly nonpolar and non-miscible in water. These compounds are usually unreactive and are used as solvents for various other nonpolar compounds. Their carbon to hydrogen ratio is high therefore, they are characterized by sooty yellow flame.

Classification of Aromatic Compounds

The classification of arenes is based on the position of the functional group. They are classified into two and we have discussed below:

1. Nuclear Substituted Compounds

In any aromatic compound whenever any substituent or functional group, is directly linked to the benzene ring, it is known as a nuclear-substituted compound. 

2. Side chain Substituted Compounds

In any aromatic compound if the functional group is available in the side chain of the ring then it is known as a side chain substituted compound. These compounds are named as the phenyl derivatives of the relative aliphatic compounds.

IUPAC Nomenclature of Aromatic Compounds

Earlier, most of the compounds with the same structural formula were known by different names depending on the regions where they were synthesized. This naming system was very trivial since it raised a lot of confusion. Finally, a common naming system enlisting standard rules was set up by IUPAC (International Union for Pure and Applied Chemistry) for the naming of compounds. This method of naming is IUPAC naming or IUPAC nomenclature.

IUPAC nomenclature of aromatic hydrocarbons is explained below:

1. According to IUPAC nomenclature of substituted aromatic compounds, the substituent name is placed as a prefix to the name of aromatic compounds. For example, a benzene ring attached to a one-nitro group is named as nitrobenzene.

2. When more than one similar substituent group is present in the ring, they are labelled with the Greek numerical prefixes such as di, tri, tetra to denote the number of similar substituent groups attached to the ring. If two bromo- groups are attached to the adjacent carbon atoms of the benzene ring, it is named 1,2-dibromobenzene.

3. When different substituted groups are attached to the aromatic compounds, the substituent of the base compound is assigned number one and then the direction of numbering is chosen such that the next substituent gets the lowest number. Substituents are named in alphabetical order. For example: when chloro and nitro groups are attached to the benzene ring, we first locate the chloro group then nitro groups.

4. In the case of multiple substituted aromatic compounds, sometimes terms like ortho (o), meta (m) and para (p) are also used as prefixes to indicate the relative positions 1,2-; 1,3- and 1,4- respectively. For example, 1,2-Di-bromo-benzene can be named as o-di-bromo-benzene.

5. When an alkane with a functional group is attached to an aromatic compound, the aromatic compound is considered as a substituent, instead of a parent. For example: when a benzene ring is attached to an alkane with a functional group, it is considered as a substituent named phenyl, denoted by Ph-.

For a detailed discussion on the nomenclature of alkanes, alkenes, and alkynes, download BYJU’S – the learning app.