N Bonds in ethylene

Bearing in the mind that a real bond should be described by three classical VB structures, we return to the case of benzene. Across the whole history of resonance theory, Kekule structure has been treated as the hypothetical 1,3,5-cyclohexatiene whose double bonds are comparable to ethylene. However, it is clear from the previous paragraph that the n bond in ethylene should be expressed as a sum of three classical VB structures. Furthemore, there are three ji bonds in a Kekule structure. Consequently, from the mathematical point of view, the wave function for a Kekule structure should be expanded by 33=27 classical VB structures as follows ... 

Bond dissociation energies of the C-C bonds in ethylene (one o and one n bond) and acetylene (one c and two n bonds) can be used to estimate the strength of the second n bond of the triple bond. If we assume that the a bond and first n bond in acetylene are similar in strength to the G and n bonds in ethylene (88 and 64 kcaiymol, respectively), then the second n bond is worth 48 kcaiymol. 

FIGURE 18 Two methylene molecules forming a a and a n bond in ethylene. 

Consider, for example, the n bond in ethylene, which is formed by just such a two-center interaction of AOs (Section 1.13). The bond energy of... 

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