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Benzene Structure

All known chemical compounds can be classified as organic and inorganic compounds. Inorganic compounds are mainly originated from minerals like sulphuric acid, nitric acid etc. The complete study of inorganic compounds can be done in inorganic chemistry. Organic compounds are mainly composed of carbon. Today we know a vast number of organic compounds. This is because of tetravalency and catenation properties of carbon atom which allow them to bond with same or different atoms to form a variety of molecules. Hydrocarbons are simple example of organic compounds. They can be classified as aliphatic and aromatic compounds. In general hydrocarbons are composed carbon and hydrogen atoms. 

Classification of Hydrocarbons

Aliphatic hydrocarbons can be further classified as alicyclic and cyclic hydrocarbons. Alicyclic compounds or open chain compounds can be two types; saturated and unsaturated compounds. Saturated compounds consist of only single covalent bonds whereas un-saturated compounds have multiple bonds like alkene and alkyne. Saturated hydrocarbons are commonly called as alkanes. Cycloalkane, cycloalkene are good examples of cyclic alphatic hydrocarbons. Aromatic compounds can better identify for their unique aroma and extra stability which is due to delocalization of pi-electrons in the molecule. Aromatic compounds can further classified as benzanoid and non-benzanoid compounds. Benzene is one of the simplest aromatic compounds. The aromatic compounds with benzene molecule are known as benzanoid compounds such as phenol, aniline, naphthalene etc. Non-benzanoid compounds are aromatic in nature but do not have benzene ring such as Azulenes, Oxaazulanones, Tropones, Tropolones, Pentafulvene and Tropylium ion. 


Benzene Structure Definition

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Benzene is an aromatic compound with C6H6 molecular formula. The molecular formula indicates the unsaturation in the molecule as the number of H atoms is very less. So there must be some double or triple bonds in the molecule or the structure can be cyclic in nature. Today we have various concepts and theory that proves the cyclic structure of benzene. 

Kekulé Benzene Structure

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The molecular formula and structure was confirmed by X-diffraction method. The molecular formula of benzene proves that there must be some multiple bonds in the molecule. But presence of multiple bonds makes the molecule unstable. Unlike this, benzene is very stable molecule. First Kekulé suggested a sensible structure for benzene that is known as Kekulé structures of benzene. In this structure, he arranged six carbon atoms of molecule in a hexagon with alternate double and single bonds between them.  That satisfy 3 valency of each carbon atom and remaining one valency is bonded with one H atom.  
Kekule Structure of Benezene

Objection on Kekulé Structure of Benzene

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No doubt the Kekulé structure of benzene helped to predict the structure of benzene yet there are many limitations with Kekulé structure. Since Kekulé structure contains three double bonds in the molecule so it must be an unsaturated molecule. So it must be very reactive and unstable also. But it is not correct as benzene does not undergo addition reactions like alkene.  Benzene undergoes electrophilic substitution reactions which involves substitution of H by coming electrophile to form substituted product. According to Kekulé structure, benzene is a planer molecule with two types of C-C bonds. 

 c-c bond   0.154 mm 
 c=c bond  0.134mm

So one bond is shorter and another is longer one. But the exact bond length of C-C bond in benzene is intermediate value around 0.139 nm with hexagonal geometry. In other words, all the Carbon-Carbon bonds have same bond length. There was no clarification was given by Kekulé structure of benzene. Another limitation of Kekulé structure was 1, 2 position of benzene. If we follow Kekulé structure, the 1, 2 and 1, 6 di-substituted products of benzene must be different. In other words, there must be two ortho-positions in benzene molecule. Later Kekulé purposed a resonance structure which was combination of both Kekulé structures. 
Resonance Structure of Benzene
Kekulé structures of benzene could not explain the extra stability of benzene which was later confirmed by resonance energy of it.

Inorganic Benzene Structure

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Inorganic benzene is not an aromatic compound. The term benzene can confuse you but this name is due to structural similarity of it with benzene. The chemical name of it is 1, 3, 5, 2, 4, 6-Triazatriborinane and also known as Borazine. The chemical formula of inorganic benzene is (BH)3(NH)3. It is iso-electronic and iso-structural with benzene molecule. 

Inorganic Benzene
There are three BH and three NH groups. All B and N are bonded with single covalent bonds with bond length 144 pm. Each B and N atom also attach with single covalent bond.  It is a hexagonal structure like benzene but there is no pi-bond in the molecule. 

Molecular Orbital Structure of Benzene

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Valence bond theory purposed the hexagonal structure of benzene with alternate double bond. There are three pi bonds in the molecule which remain delocalized on all the 6 carbon atoms of the molecule. Let’s discuss the molecular orbital structure of benzene. Each carbon atom of benzene is $sp^{2}$ hybridized and have three $sp^{2}$ hybrid orbitals. Out of these three $sp^{2}$ hybrid orbitals, two hybrid orbitals form sigma bonds with neighbor carbon atoms and one hybrid orbital form sigma bond with H atoms.  Now each Carbon atom has a un-hybrid orbital which orientated perpendicular to sigma bond.  Each un-hybrid orbital involves in the formation of pi-bond with neighbor orbital through side way overlapping.  The pi-electron cloud is located above and below the plane of ring and delocalized over all the six carbon atoms of molecule. 
Molecular Orbital Structure of Benzene
The delocalization of pi-electrons makes the molecule extra stable. The extra stability of benzene can also explain with the help of Molecular orbital theory. The molecular orbital diagram of benzene is given below. 

Molecular Orbital Theory of Benzene
In the molecule, 6 atomic orbitals combine to form six molecular orbitals. Out of six molecular orbitals, 3 are bonding molecular orbitals and 3 are non-bonding molecular orbitals. Six electrons are placed in bonding molecular orbitals and non-bonding molecular orbitals remain empty. The absence of electrons in anti-bonding orbitals makes the molecule stable.
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