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# Benzene

Organic compounds which are composed of carbon and hydrogen are called as hydrocarbons. They can be two types, open chain and cyclic hydrocarbon.

Cyclic hydrocarbon contains at least one cyclic ring which Cyclic hydrocarbons can be further classified as alicyclic and aromatic compounds. Alicyclic hydrocarbons are cyclic compounds while aromatic compounds contain double bond in conjugated manner which provides stability to molecules. They are planer ring compounds with conjugated pi system in which single and double bond present in alternate manner. Aromatic term purposed by August Wilhelm Hofmann in 1855. Later August Kekulé, J. J. Thomson and Sir Robert Robinson played an important role in explanation of structure and bonding of aromatic compounds. The first aromatic compounds are considered as benzene and all other aromatic compound explained as derivatives of benzene, as at least one benzene ring must be present in an aromatic compound.

Benzene first discovered by Faraday and then named in 1834. The cyclic structure of benzene was first deduced by chemist Fredric Auguet Kekulé in 1865 and confirmed by the crystallographer Kathleen Lonsdale. Almost all aromatic compounds including benzene have characteristics aroma and highly flammable liquid. They are mainly used as a precursor for the synthesis of heavy chemicals and an important component of gasoline.

Aromatic compounds or arenes can be mainly three types.

1. Monocyclic arenes: Involves mainly benzene and its derivatives.
2. Polycyclic isolated arenes: They are composed of more than one benzene ring which is isolated from each other. For example; Biphenyl.
3. Polycyclic condensed arenes: They have fused benzene rings for example; naphthalene.

## What is Benzene?

1. Benzene is a simplest aromatic hydrocarbon with six carbon atom bonded in a hexagonal manner. It was first isolated from an oily film which was deposited from the gas used for lighting in 1825 by Michael Faraday.
2. The molecular formula for benzene is C6H6, in which all the six carbon atoms and hydrogen arranged in same plane and show planer geometry. There are three pi bonds arranged in alternate manner in hexagonal ring.
3. Although the presence of pi bonds makes the molecule unsaturated which are more reactive for additional reactions like alkene and alkyne, yet benzene is not considered as unsaturated compounds in organic chemistry and generally shows substitution reaction instead of additional reaction.
4. Hence benzene can easily give substitution reaction with sulfuric acid and bromine to form benzene sulphonic acid and bromobenzene respectively.
5. However alkenes like Cyclohexene can easily form additional product with bromine and sulfuric acid. Similarly Cyclohexene oxidized to adipic acid while benzene shows no reaction with potassium permanganate solution.

1. This proves the extra stability of benzene compare to Cyclohexene. The thermodynamic
2. stability of benzene can be proving by using heat of hydrogenation which released during the hydrogenation reaction of molecule due to the presence of double bond.
3. The hydrogenation of Cyclohexene, 1,3-cyclohexadiene and benzene form common product that is cyclohexane. All these three reactant have 1,2 and 3 pi bonds respectively.
4. Hence the heat of hydrogenation must be differ for all these reactant and increases from Cyclohexene to benzene as the number of pi bond increases.

The hydrogenation of Cyclohexene released 28.6 kcal per mole to form cyclohexane. Since there is one pi bond which release 28.6 kcal/mole heat , therefore for 1, 3-cyclohexadien it must be twice 28.6 x 2 = 57.2 kcal/mol and 28.6 x 3 = 85.8 kcal/mol for benzene. But the experimental heat of hydrogenation for benzene is 49.8 kcal/mol, hence benzene is 36 kcal/mole more stable due to resonance. This is called as resonance energy.

## Benzene Formula

The molecular formula of benzene is C6H6, where each carbon atom is sp2 hybridized and arrange in trigonal planer geometry with the bond angle of 120Â°. Three sp2 hybridized orbitals on each carbon atom get bonded with two carbon atoms and one hydrogen atom. There is one unhybridized p-orbital on each carbon atom, which involves in side-to-side overlap with next carbon atom and form pi bond. The pi electron density equally distributed between the carbon atoms and exists as two clouds above and below the plane of ring.

Since six pi electrons are not confined on any specific carbon atoms, therefore said to be delocalized over whole ring and provide stability to molecule. That is the reason the structural formula of benzene represents as a hexagon with a circle in the center which represents the delocalized electrons.

## Benzene Ring

Benzene (C6H6) is a well known carcinogenic compound with pleasant aroma and highly flammable. It boils at 80.1Â°C and the melting point of benzene is 5.5Â°C. The heat of vaporization of benzene is 44.3 kJ/mol and a heat of fusion is 9.84 kJ/mol. The molecular formula that C6H6 proves that there must be some un saturation in molecule.

In 1867, Claus first explained the ring structure of benzene with three diagonal pi bonds. Later in 1867 and 1869 Dewar and Ladenburg gave a new ring form to explain the properties of benzene. In 1887 Armstrong purposed a prismane form for benzene. All these ring structure failed to explain the stability and chemical properties of benzene.

In 1899, Thiele purposed one new ring form in which three pi bonds represents by a dotted circle and finally in 1865 Kekul gave the cyclic structure of benzene with three alternate pi bonds which is used in currently in organic structure. It exists in a cyclic ring form with three pi bonds which are arranged in alternate manner and show conjugation.

So there are three Carbon-carbon single bonds with three carbon-carbon double bonds. The bond length of carbon-carbon single bond is 0.1536 nm and C=C bind length is 0.130 nm. The same bond length must be there in benzene molecule. But in benzene molecule all carbon-carbon bonds are of same bond length that is 0.1399 nm which is intermediate to single and double bond length.

The intermediate value of bond length proves the delocalization of pi bonds in molecule. Since all carbon atoms are sp2 hybridized in molecule, therefore the bond angle is around 120Â° and ring has planer hexagonal geometry.

## Molecular Structure of Benzene

The molecular structure of benzene can be explained by using valence bond theory which based on resonance and hybridization. Benzene is a type of hydrocarbon composed of carbon (1s22s22p12py1) and hydrogen (1s1). In cyclic structure of benzene, each carbon is bonded with two carbon atoms and one hydrogen atom through sigma bond. Hence each carbon required minimum three unpaired electrons to form three sigma bonds and for that one of the 2s2 electron has to promote to empty 2pz orbital.

This excitation of electron makes four unpaired electrons in each carbon atom. Out of these four electrons, only three electrons involve in hybridization to form three sigma bonds and give sp2 hybridization.

Three sp2 hybrid orbitals get arranged in trigonal planer geometry at 120Â° bond angle and overlap with two carbon atoms and one hydrogen atom to form three sigma bonds. The unhybridized p-orbital oriented at right angle to hybridized orbital and involve in pi bond formation.

The side way overlapping of these unhybridized p-orbitals form pi bonds which are delocalized over all carbon atom of ring, as there is equal probability of each p-orbital to get overlap with any one of the neighbor p-orbital.

Overall benzene molecule is a planer hexagon with three pi bonds arranged in alternate manner with the carbon- carbon bond length 1.39 Ã…. The pi electron clouds distributed above and below the plane of ring and perpendicular to plane of ring.

## Structure of Benzene

Although the stability factor explained by valence bond theory, yet resonance concept was not enough to explain the structure and stability of benzene. Later molecular orbital theory was used to explain the extra stability and complete structure of benzene.

According to molecular orbital theory atomic orbitals combined together to form same number of molecular orbitals which can be classified further three types.

• Bonding molecular orbitals: These molecular orbitals involve in bond formation and at energy level compare to atomic orbitals, hence more stable.
• Non-bonding molecular orbitals: These molecular orbitals are at same energy level as atomic orbital and not involve in bond formation.
• Anti bonding molecular orbitals: The energy level of anti-bonding energy level is high compare to atomic orbitals, hence they are less stable and the presence of electron in anti-bonding orbital makes the molecule less stable.

In case of benzene molecule, the sigma bond formation takes place in same way as given in valence bond theory, but the explanation of pi bond formation is different in molecular orbital theory. There are six atomic orbital one on each carbon atom, which involves in pi bond formation in benzene. These six p-atomic orbital create a three dimensional cyclic system of six molecular orbitals.

1. Out of these six molecular orbitals, three are bonding molecular orbitals and three are anti-bonding molecular orbitals with total six pi electrons. These molecular orbitals are arranged according to their energy levels. The $\Pi$1 is the lowest lying orbital with minimum energy.
2. Next molecular orbitals $\Pi$2 and $\Pi$3 are degenerate orbitals as they are at same energy level.
Both of these orbitals are comprise of four bonding interactions and two anti-bonding interactions with one nodal plane.
3. In $\Pi$2 bonding occurs between atoms C2 -C3 and between C5 -C6 while $\Pi$3 occurs between bonding interaction of C1-C2 and C1-C6 and C4- C3 and C4- C5 with anti-bonding interactions between C2 - C3 and between C5 -C6.
4. Next two molecular orbitals are $\Pi$4 and $\Pi$5 which are also degenerate orbitals like $\Pi$2 and $\Pi$3 with overall a net two anti-bonding interaction. Out of them, $\Pi$4 has two non-bonding interactions with two anti-bonding interactions and $\Pi$5 has two bonding and four anti-bonding interactions.
5. Both of these anti-bonding orbitals are with two nodal planes. The last anti-bonding orbital is $\Pi$6 with highest energy and contains six anti-bonding interactions with three nodal planes.

Since there are six electrons in benzene molecules, they always try to arrange at lowest energy levels, therefore six electrons get pair up in three bonding orbitals with no electron in any one of the anti bonding energy level which provide molecule extra stability.

Resonance Structure of Benzene

1. A benzene molecule is a regular hexagonal structure with three pi bonds arranged in conjugated manner.
2. All the carbon-carbon bonds in benzene molecule are of equal bond length around 1.39 pm which is an intermediate value of carbon-carbon singe bond (1.46 pm) and carbon -carbon double bond (1.34 pm) in hydrocarbons.
3. The molecule is a planer hexagonal geometry with the bond angle of 120Â°. Sometime one structure of molecule cannot explain all the chemical properties of molecule, in such case more than one structure can be drawn for same molecule.
4. These structures are differs from each other in the arrangement of bonds but all the atoms remains at same position.
5. The overall structure is a combination of all structures and called as hybrid structure which is more stable compare to all other structures. The phenomenon is known as resonance.
6. The difference between drawn structure and the resonance hybrid structure is known as resonance energy.
7. For benzene, there are two possible resonance structures given by Kekul and also called as Kekul structures.
8. Both resonating structure contains alternate double and single bonds and resonance hybrid shows with a ring in hexagonal ring.

Lewis Structure of Benzene

Lewis structures of benzene show all the carbon-carbon and carbon-hydrogen bonds by lines with two possible positions which are resonating structures of benzene.

Kekule Structure of Benzene

In 1865, Friedrich August Kekul explained many facts about benzene molecule which was a recurring problem in 19th century. Kekul purposed the benzene molecule structure as a hexagonal ring which consists of six carbon atoms with alternate carbon-carbon single and carbon-carbon double bond.

In a hexagonal ring, there are two possibilities of arrangement of double and single bonds. Therefore Kekul purposed both structures for benzene and considered them in equilibrium.

Kekul structure was good enough compare to Dewar model and other purposed models of benzene. But there are some limitations with Kekule's structure.
1. These structures could not explain the extra stability of benzene compare to alkenes. For example, alkenes can easily undergo additional reaction, while benzene cannot give additional reactions like alkenes.
2. Kekule structure of benzene was also a hexagonal planer ring which must have Carbon-carbon single bond around 0.154 nm and carbon-carbon double bond 0.134 nm.
3. Hence there must be long single and short double bonds in alternate manner. But experiments prove that the bond length of carbon - carbon bond in benzene is an intermediate of single and double bond that is 0.139 nm.
4. In real benzene is much more reactive than Kekul structures and there was no explanation for that.
5. If benzene is a molecule which can be exist in two possible forms which are in equilibrium with each other , it means there is an equal possibilities for both structure and there must be two ortho-disubstituted products; 1,2 and 1,6.

### Explanation for Kekul structure drawbacks

Kekul purposed the oscillation in double bonds of benzene ring. Due to this oscillation both Kekul structures are rapidly inter-converted into each other by movement of bonds.

## Chemical Structure of Benzene

Kekul structures are most popular form of benzene ring which can be easily explained by using resonance concept. Since double bind and single bond present in alternate manner, therefore they can show conjugation and stabilized by resonance.

Both resonating structures are readily inter convertible into each other and can be representing by a resonance hybrid in which all the three bonds shown by a circle. Due to resonance, all the carbon-carbon bonds are of equal bond length and intermediate to single and double carbon-carbon bond.