The Tau Bond Model

A number of physical methods have found support in molecular orbital theory, or have provided evidence that the deductions of molecular orbital theory have some experimental basis. Electron affinities correlate moderately well with the calculated energies of the LUMO, ionisation potentials correlate moderately well with the calculated energies of the HOMO, and spectroscopic methods reveal features that support molecular orbital theory. 

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The tau bond model appears to provide a quick and easy explanation. An anti interaction between each of the breaking bonds and the lower tau bond leads overall to a syn selective reaction for each diastereoisomer. 

The tau bond model is an intriguing, but evidently defective approach to understanding the stereochemistry of elimination reactions. The problem therefore remains—there is no simple and satisfying way to explain the stereochemistry beyond the simple (3 elimination. 

The tau bond model is an intriguing, but evidently defective approach to understanding the stereochemistry of elimination reactions. The problem therefore remains—there is no simple and satisfying way to explain the stereochemistry beyond the simple /3-elimination. We shall return to the problem later, when we come to discuss how cr bonds adjacent to a re bond influence the stereochemistry of attack on the n bond, but first we must discuss the angle of attack on a re bond, and the stereochemistry of their addition and substitution reactions. 

Predicting the stereochemistry of an SE2" reaction is even less convincing. Both alternating pyramidalisa-tion, greatly exaggerated in 5.236, and the tau bond model 5.237 predict that electrophilic attack at C-5 should be from above, syn to the donor substituent. 

Bent bonds are also known as banana bonds or t (tau) bonds. For a discussion of the utility of this model in explaining molecular conformation and reactivity, see Wintner, C. E. /. Chem. Educ. 1987, 64, 587 and references therein. 

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