8/9/2019 The Molecular Formula of Benzene Has Been Found From Analytical Data

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The molecular formula of benzene has been found from analytical data, to be C6H6. Relatively higher proportion of

carbon and addition of chlorine to benzene molecule indicate it to be an unsaturated compound. Depending on the

various facts available to scientists from time to time, many structures for benzene had been proposed. Some are

described below.

Open Chain StructureBased upon observable facts given above and the tetravalency of carbon, the following open chain structures were

proposed for benzene.

Drawbacks of open chain structure

The open chain structure for benzene was rejected due to the following reasons:

Addition reactions usually given by alkenes and alkynes are not given by benzene.

Benzene forms only one kind of mono- substituted product. An open chain structure however, can form more

than one kind of monosubstituted product as shown below:

The open chain compounds do, not give reactions such as Friedel-Craft reaction, nitration, sulphonation.

On reduction with hydrogen in the presence of Ni at 200C, actually a cyclic compound cyclohexane is

obtained.

These facts suggest a ring structure for benzene.

Ring structure of benzene

After taking into account account the above observed facts, Kekule (1865) suggested a ring structure for benzene.

According to him, six carbon atoms occupied six corners of a regular hexagon in benzene and each carbon carried

one hydrogen atom. To satisfy the tetravalency of carbon, the system consisted of alternate single and double bonds.

Kekule's formula is shown below.

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Defects in Kekule's formula

While Kekule's formula explained most of theory served facts for benzene, it could still not explain the following facts,

The saturated nature of benzene.

In actuality only one kind of ortho derivatives are known, but according to Kekule's formula, there can be two

ortho positions.

The defect of having two ortho positions was explained by proposing that the positions of the double bonds in

benzene are not fixed. Instead, the double bonds in the benzene molecule keep changing their positions and thus all

positions in benzene molecule become identical.

Chemists generally used the Kekule's structure as late as 1945. Many ring structures for benzene have been

proposed after Kekule's structure. Some of them are,

Claus diagonal Dewar'sformula(1867) formula(1867)

Resonance hybrid structure of benzene

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The currently accepted structure was developed by the application of the theory of resonance proposed in 1933. This

theory states that benzene is a resonance hybrid of the following canonical forms.

Since, the forms I and II are the most contributing, hence benzene is represented as a hybrid structure of these two

structures, i.e.,

Resonance hybrid

Evidences which support resonance structure of benzene

The following facts support the resonance structure of benzene:

The carbon-carbon bond length in benzene is identical at 139 pm, for all bonds. This value is intermediate

between the bond lengths for C-C bond (154 pm) and C = C (134 pm).

A regular hexagonal structure for benzene is obtained by X-ray diffraction, which gives a C-C bond length of

139 pm.

Large resonance energyDue to resonance, the p-electron charge in benzene gets distributed over greater area, i.e., gets delocalized.

Delocalization results in the energy of the resonance hybrid decreasing relative to the contributing structures, by

about 150 kJ mol-1. This decrease in energy is called resonance energy. The unusual stability of benzene is due to

this resonance stabilization.

Orbital structure of benzene

X-ray studies show that a

Benzene molecule is a flat (planar) molecule. All carbon and hydrogen atoms lie in the same plane.

It has a regular hexagon structure with all six carbon atoms lying at the corners; each carbon atom is bondedto three other atoms.

All carbon-carbon bond lengths are equal at 139 pm.

All CC angles (or CH angles) are equal at 120.

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These results indicate that each carbon atom in benzene molecule is sp 2 hybridized. All sp2 hybrid orbitals lie in the

same plane (the plane of the carbon atoms) and are oriented towards the corners of an equilateral triangle. Thus,

each carbon in benzene has three sp2 hybrid orbitals lying in the same plane and one -unhybridized 'p' orbital.

(a) Formation of a planar hexagonal structure due to overlapping of the sp2 hybrid orbital of each carbon atom with its

neighboring carbon atoms and hydrogen atoms.

(b) A unhybridized 2p orbital on each carbon lies perpendicular to the carbon-carbon plane.

Out of the three hybrid orbitals, two overlap axially with the orbitals of the neighboring carbon atoms on either sides to

form C-C 's' bonds. The third, sp2 hybridized orbital of the carbon atom overlaps with the half-filled '1s' orbital of the

hydrogen atom forming a 's' C-H bond.

A planar hexagonal structure is formed when six carbons are placed in a hexagonal geometry, the orbital overlapping

leads to the structure (a).

In (b), each carbon is left with one unused '2p' orbital at right angle, to the hexagon. These unused '2p' orbitals of

carbon atoms overlap each other sideways, and form carbon-carbon p-bonds. As the system is completely

symmetrical, the '2p' orbitals can overlap sideways equally well with either of the neighboring carbon atoms. Hence

sideways overlapping of '2p' orbitals of carbon atoms can form two sets of

p-bonds as shown.

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Sideways overlap of 2p orbitals leading to formation of two sets of p-bonds.

All the 'p' orbitals on the six C atoms in benzene molecule are equidistant from each other. Thus 'p' orbitals of any

one carbon atom are able to overlap equally well with the similar orbitals of both the carbon atoms on either sides. A

continuous ring of electron cloud covering all the six carbon atoms results because of such overlap. Since a 'p' orbital

consists of two equal lobes, one lying above and the other below the plane of the ring, the sideways overlapping of

the p orbitals in benzene molecule leads to a molecular orbital consisting to two continuous rings, one lying above,

and the other below the plane of the ring as shown.

The continuous rings of the p molecular orbital of benzene. One lying above and the other below the plane of the ring.

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The shape and size of benzene molecule.

Thus, each bond in benzene has a character intermediate between a single and a double bond.

Drawbacks of open chain structure

The open chain structure for benzene was rejected due to the following reasons:

Addition reactions usually given by alkenes and alkynes are not given by benzene.

Benzene forms only one kind of mono- substituted product. An open chain structure however, can form more

than one kind of monosubstituted product as shown below:

The open chain compounds do, not give reactions such as Friedel-Craft reaction, nitration, sulphonation.

On reduction with hydrogen in the presence of Ni at 200C, actually a cyclic compound cyclohexane is

obtained.

These facts suggest a ring structure for benzene.

Ring structure of benzene

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After taking into account account the above observed facts, Kekule (1865) suggested a ring structure for benzene.

According to him, six carbon atoms occupied six corners of a regular hexagon in benzene and each carbon carried

one hydrogen atom. To satisfy the tetravalency of carbon, the system consisted of alternate single and double bonds.

Kekule's formula is shown below.

Defects in Kekule's formula

While Kekule's formula explained most of theory served facts for benzene, it could still not explain the following facts,

The saturated nature of benzene.

In actuality only one kind of ortho derivatives are known, but according to Kekule's formula, there can be two

ortho positions.

The defect of having two ortho positions was explained by proposing that the positions of the double bonds in

benzene are not fixed. Instead, the double bonds in the benzene molecule keep changing their positions and thus all

positions in benzene molecule become identical.

Chemists generally used the Kekule's structure as late as 1945. Many ring structures for benzene have beenproposed after Kekule's structure. Some of them are,

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Claus diagonal Dewar'sformula(1867) formula(1867)

Resonance hybrid structure of benzene

The currently accepted structure was developed by the application of the theory of resonance proposed in 1933. This

theory states that benzene is a resonance hybrid of the following canonical forms.

Since, the forms I and II are the most contributing, hence benzene is represented as a hybrid structure of these two

structures, i.e.,

Resonance hybrid

Evidences which support resonance structure of benzene

The following facts support the resonance structure of benzene:

The carbon-carbon bond length in benzene is identical at 139 pm, for all bonds. This value is intermediate

between the bond lengths for C-C bond (154 pm) and C = C (134 pm).

A regular hexagonal structure for benzene is obtained by X-ray diffraction, which gives a C-C bond length of

139 pm.

Large resonance energy

Due to resonance, the p-electron charge in benzene gets distributed over greater area, i.e., gets delocalized.

Delocalization results in the energy of the resonance hybrid decreasing relative to the contributing structures, by

about 150 kJ mol-1. This decrease in energy is called resonance energy. The unusual stability of benzene is due to

this resonance stabilization.

Orbital structure of benzene

X-ray studies show that a

Benzene molecule is a flat (planar) molecule. All carbon and hydrogen atoms lie in the same plane.

It has a regular hexagon structure with all six carbon atoms lying at the corners; each carbon atom is bonded

to three other atoms.

All carbon-carbon bond lengths are equal at 139 pm.

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All CC angles (or CH angles) are equal at 120.

These results indicate that each carbon atom in benzene molecule is sp 2 hybridized. All sp2 hybrid orbitals lie in the

same plane (the plane of the carbon atoms) and are oriented towards the corners of an equilateral triangle. Thus,

each carbon in benzene has three sp2 hybrid orbitals lying in the same plane and one -unhybridized 'p' orbital.

(a) Formation of a planar hexagonal structure due to overlapping of the sp2 hybrid orbital of each carbon atom with its

neighboring carbon atoms and hydrogen atoms.

(b) A unhybridized 2p orbital on each carbon lies perpendicular to the carbon-carbon plane.

Out of the three hybrid orbitals, two overlap axially with the orbitals of the neighboring carbon atoms on either sides to

form C-C 's' bonds. The third, sp2 hybridized orbital of the carbon atom overlaps with the half-filled '1s' orbital of the

hydrogen atom forming a 's' C-H bond.

A planar hexagonal structure is formed when six carbons are placed in a hexagonal geometry, the orbital overlapping

leads to the structure (a).

In (b), each carbon is left with one unused '2p' orbital at right angle, to the hexagon. These unused '2p' orbitals of

carbon atoms overlap each other sideways, and form carbon-carbon p-bonds. As the system is completely

symmetrical, the '2p' orbitals can overlap sideways equally well with either of the neighboring carbon atoms. Hence

sideways overlapping of '2p' orbitals of carbon atoms can form two sets of

p-bonds as shown.

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Sideways overlap of 2p orbitals leading to formation of two sets of p-bonds.

All the 'p' orbitals on the six C atoms in benzene molecule are equidistant from each other. Thus 'p' orbitals of any

one carbon atom are able to overlap equally well with the similar orbitals of both the carbon atoms on either sides. A

continuous ring of electron cloud covering all the six carbon atoms results because of such overlap. Since a 'p' orbital

consists of two equal lobes, one lying above and the other below the plane of the ring, the sideways overlapping of

the p orbitals in benzene molecule leads to a molecular orbital consisting to two continuous rings, one lying above,

and the other below the plane of the ring as shown.

The continuous rings of the p molecular orbital of benzene. One lying above and the other below the plane of the ring.

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The shape and size of benzene molecule.

Thus, each bond in benzene has a character intermediate between a single and a double bond.