|Table of contents
|What are Aromatic Hydrocarbons?
|IUPAC Nomenclature of aromatic hydrocarbons:
|Properties of Aromatic Hydrocarbons
|Reactions of Aromatic Hydrocarbons
|Uses of Aromatic Hydrocarbons
|Polycyclic Aromatic Hydrocarbons
Aromatic Hydrocarbons are circularly structured organic compounds that contain sigma bonds along with delocalized pi electrons. They are also referred to as arenes or aryl hydrocarbons.
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.
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 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 the benzene ring, it is named as 1,2-dibromobenzene.
3. When different substituted groups are attached to the aromatic compounds, the substituent of 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-.
The first compound that was categorized as an aromatic hydrocarbon was benzene.
It is also the most complex aryl hydrocarbon. Each carbon atom belonging to the benzene ring has two carbon-carbon sigma bonds, one carbon-hydrogen sigma bond, and one double bond with a neighbouring carbon in which the pi electron is delocalized.
This delocalization of pi electrons in the benzene molecule is represented by a circle inside the hexagon. The bond order of all carbon-carbon bonds in this molecule is considered to be 1.5 and this equivalency can be explained with the help of the resonance structures of benzene.
Some general properties of aromatic hydrocarbons have been listed below:
Many organic chemical reactions involve the use of aromatic hydrocarbons as the primary reactant. Some such reactions are listed in this subsection along with a brief description of each of these reactions.
These reactions involve the replacement of one substituent on the ring of an aromatic hydrocarbon, commonly a hydrogen atom, by a different substituent group.
The common types of aromatic substitution reactions include:
An example of an aromatic substitution reaction is the electrophilic substitution observed in the nitration reaction of salicylic acid.
In these types of reactions, the coupling of two fragments that have a radical nature is achieved with the help of a metal catalyst. When aromatic hydrocarbons undergo coupling reactions, the following type of bonds can be formed.
An example of a coupling reaction involving aromatic hydrocarbons can be observed in the arylation of perfluorobenzenes, as illustrated below.
The catalyst used in this reaction is Palladium(II) acetate. It can also be noted that DMA is the abbreviation of Dimethylacetamide.
The use of aromatic hydrocarbons is common in both biological and synthetic processes.
|1. What is the IUPAC nomenclature for aromatic hydrocarbons?
|2. What are the properties of aromatic hydrocarbons?
|3. What are the reactions of aromatic hydrocarbons?
|4. What are the uses of aromatic hydrocarbons?
|5. What are polycyclic aromatic hydrocarbons (PAHs)?