Substituent constant constancy

The substituent constant cr is defined at a given temperature. If 8rA// and 8rAS are independent of temperature, p should be linearly related to 1/7" if all other conditions are held constant. (The constancy of other conditions is not strictly possible, however, because the dielectric constant is a function of temperature.) Data are in reasonable agreement with this prediction. 

If a series of bases obeys a Hammett type of treatment toward a constant acid, the above discussion is applicable to this problem also. Now the transformed base parameter Eb is related to the substituent constant a and Ea is related to g. The appropriate transformation equations result by simply interchanging the subscripts B and A in all of the above equations. It should also be mentioned that the constancy of — R, the requirement for a Hammett-t5q)e equation, is a different requirement than the constancy of the C/E ratio for a one term equation. A limited set of data can obey a one term equation and not be amenable to a Hammett-type of approach. For example, the parameters of all of the alcohols (aliphatic and aromatic) undergoing a hydrogen bonding interaction have a fairly constant C/E ratio and give fair agreement with a one term treatment. 

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

The problem of testing the application of a linear free-energy treatment for substitution reactions was solved by reversing the usual Hammett procedure. In the conventional approach, log (kjkn) is plotted against a. It is evident that an alternative would be to examine the application of a linear correlation by maintaining the group constant and to plot log (k/ka) versus p for different reactions. A satisfactory linear relationship would indicate constancy of the electronic contribution of the substituent. The slope of the line would define a substituent constant for the substituent in question. 

Of the various geometric parameters associated with molecular shape, the one most nearly constant from molecule to molecule and most nearly independent of substituent effects is bond length. Bond lengths to carbon depend strongly on the hybridization of the carbon involved but are little influenced by other factors. Table 1.2 lists the interatomic distances for some of the most common bonds in organic molecules. The near constancy of bond lengths from molecule to molecule reflects the fact that the properties of individual bonds are, to a good approximation, independent of the remainder of the molecule. 

Thus crx can be regarded as a measure of the overall polar effect exerted by a substituent, X, on the reaction centre. Its sign indicates the direction (-ve = electron-donating +ve = electron-withdrawing), and its magnitude the extent, of the effect that X exerts—compared, of course, with the effect exerted by H. Indeed, the assumed constancy of a substituent s absolute polar effect of X always remains constant, but only that its effect relative to H remains constant. 

Hammett, after illustrating the existence of linear relationships among the data for a variety of side-chain reactions, defined the (7-constants to characterize the behavior of substituent groups. In the application of the Hammett equation the cr-parameters are assumed to be constant. The assessment of the validity of this same assumption for substituents in aromatic substitution reactions is the major problem which must be considered prior to the adoption of a simple two-parameter linear free-energy relationship for these reactions. Preliminary evaluations of linear relationships were undertaken through somewhat modified procedures as discussed in Section IV. Now, however, with many quantitative data available it is no longer necessary to rely on the less direct Selectivity Relationship. Rather, the more straightforward conventional Hammett approach is applicable. This procedure requires the adoption of the a1 -constants derived from the study of substituted phenyldimethylcarbinyl chlorides and the assumption of constancy of the values. This assumption is shown to be fully justified in subsequent tests of the relationship. 

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