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Structure of Benzene
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 Structure
Based 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 200°C, 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.
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
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.,
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.
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 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.
The shape and size of benzene molecule.
Thus, each bond in benzene has a character intermediate between a single and a double bond.
orientation of benzene derivatives
The substituent already present on the benzene ring directs the incoming substituent to occupy ortho (2 or 6), meta (3 or 5) or para (4) position. This direction depends on the nature of the first substituent and is calleddirective or the orientation effect.
The substituent already present can increase or decrease the rate of further substitution, i.e., it either activates or deactivates the benzene ring towards further substitution. These effects are called activity effects.
There are two types of substituents which produce directive effect are,
(i) Those which direct the incoming group to ortho- and para-positions simultaneously (Neglecting meta all together).
(ii) Those which direct the incoming group to meta-position only (Neglecting ortho- and para-positions all together).
ortho para directors
Strong activating NH2, NHR, NR2, OH, O:-
moderately activating NHCOCH3, NHCOR, OCH3, OR
weakly activating CH3, C2H5, R, C6H5
Strong deactivating CN, SO3H, COOH, COOR, CHO, COR
Moderately deactivating NO2, NR3, CF3, CCl3
Weakly deactivating F, Cl, Br, I