Substituent constants, estimated, validity

Methods have been presented, with examples, for obtaining quantitative structure-property relationships for alternating conjugated and cross-conjugated dienes and polyenes, and for adjacent dienes and polyenes. The examples include chemical reactivities, chemical properties and physical properties. A method of estimating electrical effect substituent constants for dienyl and polyenyl substituents has been described. The nature of these substituents has been discussed, but unfortunately the discussion is very largely based on estimated values. A full understanding of structural effects on dienyl and polyenyl systems awaits much further experimental study. It would be particularly useful to have more chemical reactivity studies on their substituent effects, and it would be especially helpful if chemical reactivity studies on the transmission of electrical effects in adjacent multiply doubly bonded systems were available. Only further experimental work will show how valid our estimates and predictions are. 

As usual, h is introduced in case the observed property of the unsubstituted molecule, (Qfi), is not equal to zero. The steric parameter ry was estimated from van der Waals radii in various ways. For ortho substituents it was found that the contributions from inductive and resonance terms outweigh by far the steric contribution, indicating the relative unimportance of the latter in the ortho effect. By contrast in aliphatic systems the contribution from r has been found to dominate all the others. The validity of this substituent constant has to be evaluated, but the correspondence between the ry from aliphatic compounds and Eg was shown by the good correlation between them (98, 99). Since the van der Waals radii do not contain directional information, the correlation between r and Eg indicates that the Eg constants reflect the size and bulk of the substituent and lack the important property of directionality. 

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