What are aromatic compounds?
Aromatic compounds are chemical compounds which 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 to satisfy Huckel’s rule.
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:
- 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, . . .)
Properties of Aromatic compounds
Arenes are mostly nonpolar and non-miscible in water. These compounds usually are 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. Sidechain 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 sidechain substituted compound. These compounds are named as the phenyl derivatives of the relative aliphatic compounds.
IUPAC Nomenclature of aromatic hydrocarbons:
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 are 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 as 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 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 as phenyl, denoted by Ph-.
For the detailed discussion on the nomenclature of alkanes, alkenes, and alkynes, download BYJU’S – the learning app.