Aromatic compound

An aromatic compound, or arene,Mechanisms of activation of the aryl hydrocarbon receptor by Maria Backlund, Institute of Environmental Medicine, Karolinska Institutet is a chemical compound that has a specific ring-like structure that gives it increased chemical stability. These compounds are called "aromatic" because many of them have sweet or pleasant smells. Benzene is the simplest aromatic compound.

The aromatic ring is often depicted as alternating double and single bonds between carbon atoms; this is a simplification. Formally, the electrons in the ring are shared across all the carbon atoms: this is called delocalization, and is why aromatic rings are so stable. Aromatic rings are also depicted as a circle drawn inside a ring of single bonds.

Hückel's rule predicts whether or not a ring will be aromatic by counting the number of electrons and atoms in the ring.

Aromatic rings are not limited to carbon; in heteroarenes, at least one carbon atom is replaced by another atom such as oxygen, nitrogen, or sulfur.HighBeam Encyclopedia: aromatic compound Compounds with more than one aromatic ring are called polycyclic arenes.

 Benzene ring model 
100px|right|thumb| Benzene 

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 1925James Wilkins Armit and Robert Robinson (1925) "Polynuclear heterocyclic aromatic types.  Part II.  Some anhydronium bases," Journal of the Chemical Society, Transactions, 127: 1604-1618. 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.Jensen, William B. 2009. "The circle symbol for aromaticity," Journal of Chemical Education, 86(4): 423-424.

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, furan 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