Benzene Structure

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 them are explained in this article.

Based upon observable facts given above and the tetravalency of carbon, the following open chain structures were proposed for benzene.

The open chain structure of 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 mono - substituted 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 200°C, actually a cyclic compound cyclohexane is obtained.

These facts suggest a ring structure for benzene.

After taking into 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.

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's formula(1867)

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, benzene is represented as a hybrid structure of these two structures, i.e.,

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.

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.

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

These results indicate that each carbon atom in benzene molecule is sp² 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.

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 of 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.

The shape and size of benzene molecule.

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