Substituent effects values

The inclusion of is to deal with the possibility that consideration of electronegativity may be helpful in understanding substituent effects. Values of come from ab initio calculations. On this scale H is taken as a standard at = 0.00. and the values range from —0.15 for SMe to +0.70 for F. However, except at very short range, electronegativity effects of substituents are found not to be important, and the PSP equation may be simplified to equation (19) ... 

Fig. 9.2 shows some of the more likely structures occurring in the vulcanization of the natural rubber. The approximate chemical shifts for carbons in these structures have been calculated by using chemical shift substituent effects. The initial substituent effect values used are taken from Wehrli and Wirthlin [13] but were slightly modified based on model compound studies. The values are monosulfidic, O = +18 ppm, = +6 ppm, y = —A ppm di- and polysulfide, a = +25 ppm, yS = +6 ppm, / = —4 ppm. 

The term p is a reaction constant and is mathematically evaluated for a particular reaction by plotting log kjkQ against a. The slope of the straight lines is p, and reflects the sensitivity of the reaction under study to effects of substituents. The value of p is obviously affected by temperature, solvent changes, etc. 

So far the four metal ions have been compared with respect to their effect on (1) the equilibrium constant for complexation to 2.4c, (2) the rate constant of the Diels-Alder reaction of the complexes with 2.5 and (3) the substituent effect on processes (1) and (2). We have tried to correlate these data with some physical parameters of the respective metal-ions. The second ionisation potential of the metal should, in principle, reflect its Lewis acidity. Furthermore the values for Iq i might be strongly influenced by the Lewis-acidity of the metal. A quantitative correlation between these two parameters... 

The applicability of the two-parameter equation and the constants devised by Brown to electrophilic aromatic substitutions was tested by plotting values of the partial rate factors for a reaction against the appropriate substituent constants. It was maintained that such comparisons yielded satisfactory linear correlations for the results of many electrophilic substitutions, the slopes of the correlations giving the values of the reaction constants. If the existence of linear free energy relationships in electrophilic aromatic substitutions were not in dispute, the above procedure would suffice, and the precision of the correlation would measure the usefulness of the p+cr+ equation. However, a point at issue was whether the effect of a substituent could be represented by a constant, or whether its nature depended on the specific reaction. To investigate the effect of a particular substituent in different reactions, the values for the various reactions of the logarithms of the partial rate factors for the substituent were plotted against the p+ values of the reactions. This procedure should show more readily whether the effect of a substituent depends on the reaction, in which case deviations from a hnear relationship would occur. It was concluded that any variation in substituent effects was random, and not a function of electron demand by the electrophile. ... 

Here again it is possible to find a linear relationship between the log (k/feo) (ko = methyl) values of 2-alkyl- and 2,4-dialkylthiazoles and between the latter value and Tafts Eg parameter (256). The value of 5 for 2,4-dialkylthiazoles is 1.472 with a correlation coefficient of 0.9994. Thus the sensitivity to substituent effects is more marked than in the case of a single substituent in the 2-position. Furthermore, the 4-position is again more sensitive than the 2-position. 

Table 19 3 lists the ionization constants of some substituted benzoic acids The largest effects are observed when strongly electron withdrawing substituents are ortho to the carboxyl group An o nitro substituent for example increases the acidity of benzoic acid 100 fold Substituent effects are small at positions meta and para to the carboxyl group In those cases the values are clustered m the range 3 5-4 5... 

Since substituent effects in aliphatic systems and in meta positions in aromatic systems are essentially inductive in character, cr and cr values are often related by the expression cr = 0.217cr — 0.106. Substituent effects fall off with increasing distance from the reaction center generally a factor of 0.36 corresponds to the interposition of a —CHj— group, which enables cr values to be estimated for R—CHj— groups not otherwise available. 

Although UV spectra have been measured for a large number of substituted azoles, there has been no systematic attempt to explain substituent effects on such spectral maxima. Readily available data are summarized in Table 25, and some major trends are apparent. However, detailed interpretation is hindered by the fact that different solvents have been used and that in aqueous media it is not always clear whether a neutral, cationic or anionic species is being measured. Furthermore, values below 220 nm are of doubtful quantitative significance. 

A publication by Elguero et al. 66BSF3744) discusses UV spectra of 170 pyrazoles determined in 95% ethanol. Rigorously, the UV substituent effects must be discussed using wavenumbers, since only wavenumbers (in cm ) are proportional to transition energies, AE. However, in order to have more familiar values, data on l-(2,4-dinitrophenyl)pyrazoles <66BSF3744) have been transformed from wavenumbers to wavelengths (in nm) (Table 20). 

Adding these substituent effects gives the following calculated shift values (as compared with the observed values in parentheses) for C-1 to C-4 of the pura-disubstituted benzene ring in A and B ... 

Substituent effects calculated for structure B lead to values which are not perfect but which agree more closely than for A with the measured C shifts of the benzene ring carbon atoms. The dia-stereotopism of the NC//2 protons in the H NMR spectrum also points to B as the Newman projection C along the C/fj-ammonium-N bond shows ... 

Another example of enhanced sensitivity to substituent effects in the gas phase can be seen in a comparison of the gas-phase basicity for a series of substituted acetophenones and methyl benzoates. It was foimd that scnsitivtiy of the free energy to substituent changes was about four times that in solution, as measured by the comparison of A( for each substituent. The gas-phase data for both series were correlated by the Yukawa-Tsuno equation. For both series, the p value was about 12. However, the parameter r" ", which reflects the contribution of extra resonance effects, was greater in the acetophenone series than in the methyl benzoate series. This can be attributed to the substantial resonance stabilization provided by the methoxy group in the esters, which diminishes the extent of conjugation with the substituents. 

How do you interpret the values of p and r in these equations Which system is more sensitive to the aryl substituent How would you explain this difference in sensitivity Sketch the resonance, field and hyperconjugative interactions which you believe would contribute to these substituent effects. What, if any, geometric constraints would these interactions place on the ions ... 

This would disperse the positive charge over several atoms and diminish the sensitivity of the reaction to substituent effects. The p values that are observed are consistent with this interpretation. Whereas p is —3.25 for acetal hydrolysis, it is only —1.9 for hemiacetal hydrolysis. ... 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

Next we turn to the magnitudes of the p constants. Evidently if p = 0, there is no substituent effect on reactivity. Moreover because p = -I-1.000 by definition for the aqueous ionization of benzoic acids, we have a scale calibration of sorts. Wiberg gives examples of p as a measure of the extent of charge development in the transition state. McLennan" has pointed out that p values must first be adjusted for the transmission factor before they can be taken as measures of charge devel-... 

Charton has recently examined substituent effects in the ortho position in benzene derivatives and in the a-position in pyridines, quinolines, and isoquinolines. He concludes that, in benzene derivatives, the effects in the ortho position are proportional to the effects in the para position op). However, he finds that effects of a-sub-stituents on reactions involving the sp lone pair of the nitrogen atoms in pyridine, quinoline, and isoquinoline are approximately proportional to CT -values, or possibly to inductive effects (Taft s a ). He also notes that the effects of substituents on proton-deuterium exchange in the ortho position of substituted benzenes are comparable to the effects of the same substituents in the a-position of the heterocycles. 

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