Aromatic hydrocarbon, commonly known as arenes are hydrocarbons containing sigma bonds and delocalized pi electrons between the 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 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, . . .)
IUPAC Nomenclature of aromatic hydrocarbons:
Earlier, most of the compounds with same structural formula were known by different names depending on the regions where they were synthesised. 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: benzene ring attached to one -nitro group is named as nitrobenzene.
2. When more than one similar substituent group are present in the ring, they are labeled 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 benzene ring, it is named as 1,2-dibromobenzene.
3. When different substituted groups are attached to the aromatic compounds, 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, aromatic compound is considered as substituent, instead of a parent. For example: when benzene ring is attached to an alkane with a functional group, it is considered as substituent named as phenyl, denoted by Ph-.
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