Polar substituent constant

It was found (32) that in the acid range (pH 4—6) the alkyl group does not influence the rate of decomposition, which is similar for all xanthates. In the alkaline range the rates are markedly influenced by the substitutional group, and the rates could be correlated with the Taft polar substituent constants estabhshed for the various groups. 

A more ambitious goal is to separate completely resonance effects from polar effects. This involves using separate substituent constants to account for resonance and polar effects. The modified equation, called a dual-substituent-parameter equation, takes... 

In general, the dissection of substituertt effects need not be limited to resonance and polar components, vdiich are of special prominence in reactions of aromatic compounds.. ny type of substituent interaction with a reaction center could be characterized by a substituent constant characteristic of the particular type of interaction and a reaction parameter indicating the sensitivity of the reaction series to that particular type of interactioa For example, it has been suggested that electronegativity and polarizability can be treated as substituent effects separate from polar and resonance effects. This gives rise to the equation... 

The constants oi were taken equal to a scaled value of the aliphatic polar substituent constants a (which are defined in Section 7.3), and a was set at 3 (or a = in for substituents capable of through resonance). The resulting plots of Eq. (7-32) gave good LEER, which was interpreted to justify the approach. Refinements, - of this treatment showed that a depends upon the reaction, although most values fell ... 

Research on the nature of substituent constants continues, with results that can bewilder the nonspecialist. The dominant approach is a statistical one, and the main goal is to dissect substituent effects into separate electronic causes. This has led to a proliferation of terms, symbols, and conclusions. A central issue is (here we change terminology somewhat from our earlier usage) to determine the balance of field and inductive effects contributing to the observed polar electronic effect. In... 

Table 7-9. Polar Substituent Constants, a, for Aliphatic Substrates X-R...

In Section 8.4 we will encounter many empirical measures of solvent polarity. These are empirical in the sense that they are model dependent that is, they are defined in terms of a particular standard reaction or process. Thus, these empirical measures play a role in the study of solvent effects exactly analogous to that of the substituent constants in Chapter 7.)... 

The symbol k or K is the rate or equilibrium constant, respectively, for a side-chain reaction of a meta- or para-substituted benzene derivative, and k° or K° denotes the statistical quantity (intercept term) approximating to k or K for the parent or unsubstituted compound. The substituent constant a measures the polar (electronic) effect of replacing H by a given substituent (in the meta- or para-position) and is, in principle, independent of the nature of the reaction. The reaction constant p depends on the nature of... 

Fig. 13. Dependence of half-wave potentials for the reduction of substituted benzenes on the total polar substituent constant, ax- Examples shown benzophenones [pH 0], benzophenone-oximes [pH 0], thiobenzophenones [pH 0], nitrobenzenes [pH 2-0], azo derivatives [pH 2-6]. (Taken from Zuman, 1969.)...

Finally, in this account of multiparameter extensions of the Hammett equation, we comment briefly on the origins of the a, scale. This had its beginning around 1956 in the a scale of Roberts and Moreland for substituents X in the reactions of 4-X-bicyclo[2.2.2]octane-l derivatives. However, at that time few values of o were available. A more practical basis for a scale of inductive substituent constants lay in the o values for XCHj groups derived from Taft s analysis of the reactivities of aliphatic esters into polar, steric and resonance effects . For the few o values available it was shown that o for X was related to o for XCHj by the equation o = 0.45 <7. Thereafter the factor 0.45 was used to calculate c, values of X from o values of XCH2 . ... 

Taft has also introduced the steric substituent constant E, which is used separately in an expression of the type shown in Eq. (3) or together with polar (T constants in a four-parameter equation. 

We have discussed at some length correlation of the chemical properties of X-substituted molecules with trx—the polar substituent constant for X—and it is pertinent to enquire whether similar correlations can also be established between ax and their physical properties, among which spectroscopic data constitute a readily accessible example. 

M. Bols, X. Liang, and H. H. Jensen, Equatorial contra axial polar substituents. The relation of a chemical reaction to stereochemical substituent constants, J. Org. Chem., 67 (2002) 8970-8974. 

Fig. 1. Correlation of potentials Ej and Ej of 1 with the sum of the polar substituent constants a. ----- a from the literature --- o this work )...

Table 3. Linear interdependences between lipophilic and steric substituent constants (13 polar substituents)...

Chemical shifts of 4-methyne protons in condensed 4//-pyran derivative 630 correlate well with Hammett a constants for substituents X polar substituent effects operating in molecules 630 were discussed.462 The spectra of some 13C-IabeIed 2//-pyrans were also measured.309... 

Sterigmatocystin, 48 Stevens rearrangements, 293 Stobbe reaction, 228 Substituent constant, a, 362 constancy of, 368, 372 determination of, 363 physical significance of, 364 pKa and, 362 polar effects and, 366 sign of, 364... 

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