Substituent constant selected values

The chemical information available through LFER is primarily the reaction constant p. but this value depends upon the substituent constants selected for the construction of the LFER. The o values available are cr, cr, , a" or ct , and Oi ... 

The chemical information available through LFER is primarily the reaction constant p, but this value depends upon the substituent constants selected for the construction of the LFER. The u values available are ct, ct", ct" or and Ui, these quantities are listed, for many substituents, in Tables 7-1, 7-3, 7-4, 7-6, and 7-7. A reasonable approach is to plot log k against the substituent constant defined by a standard reaction that is expected to be most like the reaction under study. It is also reasonable to plot log k against several of the ct quantities, seeking the best correlation. [In choosing between two types of substituent constants, it is necessary to make use of substituents for which the two scales (say ct and rr, for example) are not themselves correlated, for otherwise both LFERs will be acceptable. ] The ct or o constants should be applicable to reactions that do not combine reaction sites and para substituents of the + and — type (push-pull systems capable of through resonance) for example, one would not expect ct" or o to provide good correlations for reactions of phenols or anilines substituted with nitro or cyano or for reactions of benzoic acids substituted with amino or methoxy. 

The special substituent constants for + R para-substituents are denoted by a, and those for — R para-substituents are denoted by a+ 54. They are based respectively on the reaction series discussed above. Selected values are given in Table 1. Characteristic a or a+ values are sometimes distinguished for meta-substituents also, but only for a minority of substituents which show very marked + R or — R effects do these differ significantly from ordinary a values. The range of applicability of the Hammett equation is greatly extended by means of a and cr+, notably to nucleophilic (by a ) and to electrophilic (by cr+) aromatic substitution. 

However, the duality of substituent constants and the attempt to deal with crossconjugation by selecting cr+, a or a in any given case is somewhat artificial. The contribution of the resonance effect of a substituent relative to its inductive effect must in principle vary continuously as the electron-demanding quality of the reaction center is varied, i.e. whether it is electron-rich or electron-poor. A sliding scale of substituent constants would be expected for each substituent having a resonance effect and not just a pair of discrete values a and a for — R, or o and a for + R substituents55. 

The am and ap values for R3Si groups have been measured in a variety of ways for a variety of reactions. Hansch, Leo and Taft11 have surveyed Hammett substituent constants, resonance and field parameters and a selection of values of am and ap for R3Si groups are presented in Table 1. 

To reveal the contributions of mechanisms (2) and (3) to the process of triplet state quenching of selected dyes and the effect of meso-substituent on the values of triplet state rate constants, we determined kq for dyes Kl, K2, and K4 in solutions in the absence of DNA. 

If we take a series of compounds which are related by structural substitutions, such as benzene derivatives, then we can assign the effects of the structural changes to the ratio KBJKB. if we select this property as a standard. Such a system of correlation was first proposed by Hammett and revised and extended by Jaff " to account for the effects of meta and para substituents on the reactivity of benzene derivatives. For convenience, the ionization constant of benzoic acia queous solution at 25°C was chosen as standard and for each meta or para substituent a, a value of ionization constant for benzoic acid and Ka the ionization constant of the corresponding, substituted benzoic acid. 

The independence of tt on the molecular system endows it with the characteristic of a substituent constant. This property is a direct consequence of the first approximation, namely that it reflects only the different solvation of substituent X compared to a reference substituent H. The validity of this approximation can be tested by obtaining tt from different molecular systems. It is well known (112) that irx values derived from aromatic systems differ from those derived from aliphatic systems tt The following table presents a few selected examples (112j>T ... 

The numerical values of the terms ct and p are defined by selection of the reference reaction, the ionization of benzoic acids. This reaction is assigned the reaction constant p = 1. The substituent constant, ct, can then be determined for a series of substituent groups by measurement of the acid dissociation constant of the substituted benzoic acids. The ct values are then used in the correlation of other reaction series, and the p values of the reactions are thereby determined. The relationship between Equations (3.44) and (3.45) is evident when the Hammett equation is expressed in terms of free energy. For the standard reaction log[/f/ fol = crp ... 

Those for reactions at the a position are numerous enough for testing the applicability of the extended selectivity treatment (Fig. 4). The points nicely fit a straight line with the theoretical origin (r = 0.995, s = 0.79). From the slope a substituent constant relative to the structural modification caused by the substitution of an oxygen atom for a CH=CH in the benzene ring may be calculated. The value obtained —0.93, is noticeably more negative than for thiophene, —0.79. 

In an empirical approach, Topsom and Katritzky and their associates have related the intensities of selected infrared absorptions to the substituent constant. They estimate that a% for the trifluoromethyl group is 0.11 on the basis of the infrared intensities for monosubstituted benzenes (36). In more recent work, this group has examined the infrared intensities of para-disubstituted compounds (34). They have found that there is a mutual interaction between the trifluoromethyl group and other substituents in the benzene nucleus such that a single value cannot adequately describe the influence of the group on... 

The effect of substituents on the rate of addition of carbanions to nitroarenes and the rate of p-elimination of HL from the o adducts have also been studied [8, 30, 31]. The former effect is an important parameter, because it is, in fact, a measure of influence of substituents on electrophilic activity of nitroaromatic rings. The effect of substituents on rate of the S Ar reactions of o- and p-halonitrobenzenes has been thoroughly studied [2, 32]. However, since the S Ar of halogen is a secondary process, the obtained data cannot be used as a real measure of electrophilicity of halonitroarenes. We have determined the effects of substituents and the ring structure on the rate of the VNS reactimi of nitroarenes with the carbanion of chloromethyl phenyl sulfone by using competitive experiments under the conditions, which assure a fast p-eUmination of HL from the o adducts [30, 31]. The values of VNS rates obtained under such conditions proved to correlate with those of the addition step. Selected values of the relative rate constants in relation to nitrobenzene as the standard are shown in Fig. 1. 

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