Aromatic hydrocarbon

An aromatic hydrocarbon, or arene, is a hydrocarbon ring compound. It has alternating double and single bonds between carbon atoms forming rings. Many of the compounds have a sweet scent, hence the term 'aromatic'. The ring of six carbon atoms in aromatic compounds is known as a benzene ring, after the simplest possible hydrocarbon ring, benzene. Aromatic hydrocarbons can be monocyclic (MAH) or polycyclic (PAH).

Some non-benzene-based compounds called heteroarenes, which follow Hückel's rule, are also aromatic compounds. In these compounds, at least one carbon atom is replaced by one of oxygen, nitrogen, or sulfur.

Benzene ring model 

Benzene, C6H6, is the simplest aromatic hydrocarbon and was recognized as the first aromatic hydrocarbon, with the nature of its bonding first being recognized by Kekulé in the 19th century.  
Each carbon atom in the hexagonal cycle has four electrons to share. One goes to the hydrogen atom, and one each to the two neighboring carbons. This leaves one to share with one of its two neighboring carbon atoms, which is why the benzene molecule is drawn with alternating single and double bonds around the hexagon.

The structure is also illustrated as a circle around the inside of the ring to show  six electrons floating around in delocalized molecular orbitals the size of the ring itself. This also represents the equivalent nature of the six carbon-carbon bonds each equivalent to ~1.5. The electrons are visualized as floating above and below the ring with the electromagnetic fields they generate acting to keep the ring flat.

General properties:
 Display aromaticity.
 The carbon-hydrogen ratio is high.
 They burn with a sooty yellow flame because of the high carbon-hydrogen ratio.
 They undergo substitution reactions.

The circle symbol for aromaticity was introduced by Sir Robert Robinson and his student James Armit in 1925 and popularized starting in 1959 by the Morrison & Boyd textbook on organic chemistry. The proper use of the symbol is debated; it is used to describe any cyclic pi system in some publications, or only those pi systems that obey Hückel's rule on others.

Arene synthesis
A reaction that forms an arene compound from an unsaturated or partially unsaturated cyclic precursor is simply called an aromatization. Many laboratory methods exist for the organic synthesis of arenes from non-arene precursors.

Importance of aromatic compounds 
Aromatic compounds play key roles in the biochemistry of all living things.  The four aromatic amino acids histidine, phenylalanine, tryptophan, and tyrosine each serve as one of the 20 basic building blocks of proteins.  Further, all 5 nucleotides (adenine, thymine, cytosine, guanine, and uracil) that make up the sequence of the genetic code in DNA and RNA are aromatic purines or pyrimidines.  As well as that, the molecule heme contains an aromatic system with 22 π electrons. Chlorophyll also has a similar aromatic system.

Aromatic compounds are important in industry.  Key aromatic hydrocarbons of commercial interest are benzene, toluene, ortho-xylene and para-xylene.  About 35 million tonnes are produced worldwide every year.  They are extracted from complex mixtures obtained by the refining of oil or by distillation of coal tar. They are used to produce a range of important chemicals and polymers, including styrene, phenol, aniline, polyester and nylon.

Types of aromatic compounds 
Almost all aromatic compounds are compounds of carbon, but they need not be hydrocarbons.

Heterocyclics 
In heterocyclic aromatics one or more of the atoms in the aromatic ring is of an element other than carbon. Examples include pyridine, pyrazine, imidazole, pyrazole, oxazole, and thiophene.

Polycyclics 

Polycyclic aromatic hydrocarbons are molecules with two or more simple aromatic rings fused together by sharing two neighboring carbon atoms. Examples are naphthalene, anthracene and phenanthrene.

Substituted aromatics 
Many chemical compounds are aromatic rings with other things attached.  Examples include trinitrotoluene (TNT), acetylsalicylic acid (aspirin), paracetamol, and the nucleotides of DNA.

References 

Hydrocarbons