Substituents electrical structural effects

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. 

On the condition that two subsets have a structural feature whose parameterization is essentially the same, they can be combined into a single data set. As an example consider set 0X13, anfi-Ak—iyw-X—C=NOH, where Ak is either Me or Et. The electrical effects of these groups for Me and Et, respectively, are a , —.01, —,Q ad, —.14, —.12 ae, —.030, —. 036 the values of the steric parameter v are. 52 and. 56. A significant difference is found only in the polarizability parameter a, where the values for Me and Et are. 046 and. 093, respectively. Combination of oxime pXa values for Ak = Me or Et results in set 0X13 the best correlation was with the LDR equation. As only three substituent types are present in this data set and is 0.33, this data set cannot be considered as proof of anything. The only acceptable conclusion is that it is in accord with the combination of the two subsets. 

A major method of modeling the effect of structural variation on chemical reactivity, physical properties or biological activity of a set of substrates is the use of correlation analysis. In this method it is assumed that the effect of structural variation of a substituent X upon some chemical, physical or biological property of interest is a linear function of parameters which constitute a measure of electrical, steric, and transport effects. 

Positive Cotton effects (c.d.) are reported for both 3a- and 3/8-trimethylstannyl-5a-cholestanes, at 203 nm and 210 nm, respectively.35 The compounds were studied in connection with an evaluation of the effects of /3 -trimethylstannyl substituents in cyclohexanone analogues, which provide evidence of through-bond coupling to augment that already recognized for electronegative substituents.36 The circular dichroism associated with the enone systems of cholest-4-en-3-one and 3/3-acetoxycholest-5-en-7-one has been recorded for samples oriented by an electrical field in a nematic phase composed of cholesteryl chloride and cholesteryl laurate.37 New rules are proposed for the correlation of D-Iine molecular rotations with structures of steroid derivatives.38 This work extends an earlier analysis39 and in the present case relates mainly to data for substituents at C-3, C-5, and C-6. 

It is very often necessary to estimate values of electrical-effect parameters for groups for which no measured values are available. This is of particular importance with regard to substituents in which arsenic, antimony or bismuth is the central atom. It has long been known the electrical-effect parameters of substituents X whose structure can be written as MZ are a function of the electrical effect of the Z when M is held constant In later work it was shown that when Z is held constant and M is allowed to vary, the substituent constant is a function of the Allred-Rochow electronegativity of M, Xm and the number of Z groups, n. It has been shown that when both Z and M vary, values for all groups of the type X = MZ Z Z are given by an equation of the form ... 

Examples of the various practical applications of dipole moments include, but are not limited to differentiation between isomers cis and trans, o, m, and p, tautomers, etc ), conformational analysis, studies of molecular geometry, supporting evidence for resonance hybrids, information about the polar character of molecules (important for solubility in different solvents and permeability through membranes), information about electrical effects of substituents (inductive, resonance), studies of hydrogen bonding, and studies of donor-acceptor interactions (e g., charge transfer complexes). Practical cases describing the use of dipole moments for different types of structural studies mentioned above can be found in numerous publications mentioned in this chapter. 

---