overlap of p orbitals of C –C , C – C , C – C or C – C , C – C , C – C respectively as shown in the following figures. Fig. . (a) Fig.
. (b) Structures shown in Fig. . (a) and (b) correspond to two Kekulé’s structure with localised π bonds.
The internuclear distance between all the carbon atoms in the ring has been determined by the X-ray diffraction to be the same; there is equal probability for the p orbital of each carbon atom to overlap with the p orbitals of adjacent carbon atoms [Fig. . (c)]. This can be represented in the form of two doughtnuts (rings) of electron clouds [Fig.
. (d)], one above and one below the plane of the hexagonal ring as shown below: Fig. . (c) Fig.
. (d) The six π electrons are thus delocalised and can move freely about the six carbon nuclei, instead of any two as shown in Fig. . (a) or (b).
The delocalised π electron cloud is attracted more strongly by the nuclei of the carbon atoms than the electron cloud localised between two carbon atoms. Therefore, presence of delocalised π electrons in benzene makes it more stable than the hypothetical cyclohexatriene. X-Ray diffraction data reveals that benzene is a planar molecule. Had any one of the above structures of benzene (A or B) been correct, two types of C—C bond lengths were expected.
However, X-ray data indicates that all the six C—C bond lengths are of the same order ( pm) which is intermediate between C— C single bond ( pm) and C—C double bond ( pm). Thus the absence of pure double bond in benzene accounts for the reluctance of benzene to show addition reactions under normal conditions, thus explaining the unusual behaviour of benzene. . .
Aromaticity Benzene was considered as parent ‘aromatic’ compound. Now, the name is applied to all the ring systems whether or not having benzene ring, possessing following characteristics. (i) Planarity (ii) Complete delocalisation