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Benzene is a hybrid of various resonating structures The hybrid structure is represented by inserting a circle or a dotted circle in the hexagon as shown in (C) representing the delocalization of the six electrons between the six carbon atoms of the benzene ring.
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๐๐๐๐๐๐๐๐๐ ๐๐๐๐๐ ๐๐๐๐๐๐๐ ๐๐๐๐๐๐๐ ๐๐๐๐๐๐๐:
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All the six carbon atoms in benzene are `color{red}(sp^2)` hybridized.
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Two `color{red}(sp^2)` hybrid orbitals of each carbon atom overlap with `color{red}(sp^2)` hybrid orbitals of adjacent carbon atoms to form six `color{red}(CโC)` sigma bonds which are in the hexagonal plane.
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The remaining `color{red}(sp^2)` hybrid orbital of each carbon atom overlaps with `color{red}(s)` orbital of a hydrogen atom to form six `color{red}(CโH)` sigma bonds.
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Each carbon atom is now left with one unhybridised `p` orbital perpendicular to the plane of the ring as shown below:
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The unhybridised `color{red}(p)` orbital of carbon atoms are close enough to form a `ฯ` bond by lateral overlap. There are two equal possibilities of forming three `color{red}(ฯ)` bonds by overlap of `color{red}(p)` orbitals of `color{red}(C_1 โC_2, C_3 โ C_4, C_5 โ C_6)` or `color{red}(C_2 โ C_3, C_4 โ C_5, C_6 โ C_1)` respectively as shown in the following figures.
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Structures shown in Fig. 13.7(a) and (b) correspond to two Kekulรฉโs structure with localised `color{red}(ฯ)` bonds.
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X-ray diffraction technique is used for the determination of internuclear distance between all the carbon atoms in the ring and it was found to be the same; there is equal probability for the `color{red}(p)` orbital of each carbon atom to overlap with the `color{red}(p)` orbitals of adjacent carbon atoms [Fig. 13.7 (c)]. This can be represented in the form of two doughtnuts (rings) of electron clouds [Fig. 13.7 (d)], one above and one below the plane of the hexagonal ring as shown below:
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The six `color{red}(ฯ)` electrons are thus delocalised and can move freely about the six carbon nuclei, instead of any two as shown in Fig. 13.6 (a) or (b). The delocalised `color{red}(ฯ)` 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 `color{red}(ฯ)` electrons in benzene makes it more stable than the hypothetical cyclohexatriene.
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X-Ray diffraction data reveals that benzene is a planar molecule. However, X-ray data indicates that all the six `color{red}(CโC) ` bond lengths are of the same order (139 pm) which is intermediate between `color{red}(Cโ C)` single bond (154 pm) and `color{red}(CโC)` double bond (133 pm).
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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.
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Benzene is a hybrid of various resonating structures The hybrid structure is represented by inserting a circle or a dotted circle in the hexagon as shown in (C) representing the delocalization of the six electrons between the six carbon atoms of the benzene ring.
โ
๐๐๐๐๐๐๐๐๐๐๐ ๐๐
๐๐๐๐๐๐๐๐๐ ๐๐๐๐๐ ๐๐๐๐๐๐๐ ๐๐๐๐๐๐๐ ๐๐๐๐๐๐๐:
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All the six carbon atoms in benzene are `color{red}(sp^2)` hybridized.
โ
Two `color{red}(sp^2)` hybrid orbitals of each carbon atom overlap with `color{red}(sp^2)` hybrid orbitals of adjacent carbon atoms to form six `color{red}(CโC)` sigma bonds which are in the hexagonal plane.
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The remaining `color{red}(sp^2)` hybrid orbital of each carbon atom overlaps with `color{red}(s)` orbital of a hydrogen atom to form six `color{red}(CโH)` sigma bonds.
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Each carbon atom is now left with one unhybridised `p` orbital perpendicular to the plane of the ring as shown below:
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The unhybridised `color{red}(p)` orbital of carbon atoms are close enough to form a `ฯ` bond by lateral overlap. There are two equal possibilities of forming three `color{red}(ฯ)` bonds by overlap of `color{red}(p)` orbitals of `color{red}(C_1 โC_2, C_3 โ C_4, C_5 โ C_6)` or `color{red}(C_2 โ C_3, C_4 โ C_5, C_6 โ C_1)` respectively as shown in the following figures.
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Structures shown in Fig. 13.7(a) and (b) correspond to two Kekulรฉโs structure with localised `color{red}(ฯ)` bonds.
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X-ray diffraction technique is used for the determination of internuclear distance between all the carbon atoms in the ring and it was found to be the same; there is equal probability for the `color{red}(p)` orbital of each carbon atom to overlap with the `color{red}(p)` orbitals of adjacent carbon atoms [Fig. 13.7 (c)]. This can be represented in the form of two doughtnuts (rings) of electron clouds [Fig. 13.7 (d)], one above and one below the plane of the hexagonal ring as shown below:
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The six `color{red}(ฯ)` electrons are thus delocalised and can move freely about the six carbon nuclei, instead of any two as shown in Fig. 13.6 (a) or (b). The delocalised `color{red}(ฯ)` 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 `color{red}(ฯ)` electrons in benzene makes it more stable than the hypothetical cyclohexatriene.
โ
X-Ray diffraction data reveals that benzene is a planar molecule. However, X-ray data indicates that all the six `color{red}(CโC) ` bond lengths are of the same order (139 pm) which is intermediate between `color{red}(Cโ C)` single bond (154 pm) and `color{red}(CโC)` double bond (133 pm).
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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.
