The molecular formula of benzene is C6H6. It contains eight hydrogen atoms less than the corresponding parent hydrocarbon, i.e., hexane (C6H14). It took several years to assign a structural formula to benzene because of its unusual stability and peculiar properties.
KEKULE STRUCTURE OF BENZENS
In 1865, Kekule proposed the first acceptable ring structure for benzene. In this structure there is a hexagonal ring of carbon atoms distributed in a symmetrical manner, with each carbon atom carrying one hydrogen atom. The fourth valence of carbon atoms is fulfilled by the presence of alternate system of single and double bonds as shown:
The above formula had many drawbacks· as described below:
1. The presence of three double bonds should make the molecule highly reactive towards addition reactions. But contrary to this, benzene behaves like saturated hydrocarbons.
2. Moreover, two isomers should result in a ‘ 1, 2 disubstituted benzene as shown in Fig. 43.1. In one of the isomers, the bond between the substituted carbon atoms is single bond while in the other it is a double bond. Actually only one 1, 2-disubstituted (or ortho) isomer is formed.
While Kekule formula could not explain the difference in properties between benzene and alkenes based on his structure, he explained the lack of isomers as in Fig. 43.1 by postulating a rapid interchange in the position of the double bonds as follows:
This structure came to be known as Kekule’s dynamic formula.
RESONANCE STRUCTURE OF BENZENE
X-ray studies indicate that all the carbon-carbon bonds in benzene are equivalent and have bond length 140 pm which is intermediate between C-C single bond (154 pm) and C=Cbond (134 pm). This shows that double bonds in benzene differ from those of alkenes.
Structure of benzene can be explained on the basis of resonance. Benzene (C6H6) may be assigned following two structures A and B. Structures A and B have same arrangement of atoms and differ only in electronic arrangement. Any of these structures alone cannot explain all the properties of benzene.
Structures A and B are known as resonating or canonical structures of benzene. The actual structure of benzene is different from both A and B, and cannot be represented by conventional formulae. The actual structure of benzene lies somewhere in between A and B and may be represented as C, referred to as resonance hybrid. To indicate two structures which are resonance forms of the same compound, a double headed arrow is used as shown in Fig. 43.2.
The resonance hybrid is more stable than any of the contributing (or canonical) structures. The difference between the energy of the most stable contributing structure and the energy of the resonance hybrid is known as resonance energy.
Since the contributing structures (A) and (B) are of exactly same energy they make equal contribution to the resonance hybrid and also stabilisation due to resonance should be large.
Resonance energy of benzene has been found to be 152 kJ/mole. The value of resonance energy has been determined by studying the enthalpy of hydrogenation and enthalpy of combustion of benzene.
THE UNUSUAL STABILITY OF BENZENE
Reluctance of benzene to undergo alkene type reactions indicates that it must be unusually stable. The evidence for stability of benzene is obtained by comparing experimental and calculated values of enthalpies of hydrogenation of benzene. Benzene, cyclohexadiene and cyclohexene yield cyclohexane on hydrogenation.
Enthalpy of hydrogenation of cyclohexene is – 120 kJ mol-1
Enthalpy of hydrogenation of 1 ,4-cyclohexadiene is – 240 kJ mol-1
Thus, the calculated or expected value of enthalpy of hydrogenation of 1, 3, 5-cyclohexatriene is -360 kJ mol-1
Thus, the expected enthalpy of hydrogenation for benzene if it were to be represented hypothetically as 1, 3, 5- cyclohexatriene is- 360 kJ mol-1 The experimental value of enthalpy of hydrogenation of benzene has been found be – 208 kJ mol-1 Thus, 152 kJ mol-1 less energy is produced during hydrogenation of benzene than the expected for hypothetical 1, 3, 5-cyclohexatriene. In other words benzene molecule is more stable by 152 kJ mol-1 than 1, 3,
5-cyclohexatriene (Kekule benzene). This is the resonance energy of benzene. The unusual stability of benzene makes it resistant to the usual addition reactions of alkenes. It is this stabilisation due to resonance which is responsible for the aromatic character of benzene.
ORBITAL STRUCTUR.E OF BENZENE
According to orbital structure, each carbon atom in benzene assumes sp2-hybrid state. Each carbon has three sp2-hybrid orbitals lying in one plane and oriented at an angle of 120°. There is one unhybridised p-orbital having two lobes lying perpendicular to the plane of hybrid orbitals. Each carbon atom uses two hybrid orbitals for axial overlap with similar orbitals of two adjacent carbon atoms on either side to form C-C sigma bonds. The remaining one sp2-hybrid
orbital on each carbon atom overlaps axially with 1s orbital of hydrogen atom to form C-H sigma bond. The axial overlapping of hybrid orbitals to form C-C and C-H bonds has been shown in Fig. 43.3. As is clear, the framework of carbon and hydrogen atoms is coplanar with H-C-C or C-C-C bond angle as 120°.
The unhybridised p-orbital on each carbon atom can overlap to a small but equal extent with the p-orbitals of the two adjacent carbon atoms on either side to constitute n bonds as shown in Fig. 43.4.
Fig. 43.4. Sidewise overlapping of orbitals.
The molecular orbital containing n electrons spreads uniformly over the entire carbon skeleton and embraces all the six carbons as shown in Fig. 43.5.
This spreading of 1t electrons in the form of ring of n-electrons above and below the plane of carbon atoms is called delocalisation of n-electrons. This delocalisation of 1t-electrons, results, in the decrease in energy, and hence, accounts for the stability of benzene molecule.
C-C bond length in benzene is 140 pm and C-H bond length is 109 pm.
The delocalized structure of benzene also accounts for the X-ray data (all C-C bond lengths equal) and the absence of the type of isomerism shown in Fig. 43.1. Furthermore, molecular orbital theory predicts that those cyclic molecules which have alternate single and double bonds with 4n + 2 (n = 0, 1, 2, 3 etc.) electrons in the delocalized n-cloud are particularly stable and have chemical properties different from other unsaturated hydrocarbons.
AROMATICITY OR AROMATIC CHARACTER
Benzene undergoes substitution reactions in spite of the high degree of unsaturation. This behaviour of benzene is referred to as aromaticity or aromatic character. Aromatic character of benzene can be explained on the basis of resonance structure of benzene or on the basis of orbital structure of benzene. In terms of resonance structure, benzene prefers to undergo substitution reactions because during addition reactions the resonance stabilised benzene ring would be destroyed. On the other hand, during substitution ring structure remains intact.