Group substituent factor

Table 16.3 Group Rate Constants (A prim, k, ktert, kOH) and Group Substituent Factors, F(X), at 298 K for H Abstraction at C-H or O-H bonds a...

Table 16.5 Group Substituent Factors, C(X), at 298 K for HO Addition to Carbon-Carbon Double and Triple Bonds ...

This approach has recently been extended to keto tautomers (Bassetti et al., 1988). Fourier-transform nmr spectroscopy is needed to record the signals of the small amounts of the keto isomers that are present. The carbonyl carbons of the keto form are to lower field of the corresponding atoms in the enol by ca. 10 ppm. The data were analysed into substituent factors relative to pentane-2,4-dione. Replacing the methyl groups of this compound with 2-thienyl, phenyl and t-butyl groups caused upfield shifts to the a-carbon of —3.14, —4.4 and —6.5 ppm, respectively, when the substituents were introduced at this site. 

Each of these four second-order rate constants can be estimated from the structure of the compound of interest using group rate constants and substituent factors that have been derived from a large set of experimental data. This method has proven to be quite successful for prediction of k n0. for compounds that are well represented in this database (i.e., predictions within a factor of 2 to 3). Larger deviations are found,... 

In summary, when applied to the type of compounds from which the group rate constants and substituent factors given in Tables 16.3 to 16.7 have been derived, the estimation of k H0. from structure yields quite satisfactory results. As is demonstrated by the examples given in Illustrative Example 16.3, in many cases only one or two structural moieties dominate the overall k HQ. value. Therefore, calculation of k HQ. can often be simplified by taking into account only a few terms of the overall estimation equation (Eq. 16-25). 

Estimate the ho. and /I/2jHo- (for [HO ]ss = 106 molecule - cm 3) values at 25°C of the following compounds using the group rate constants and substituent factors and constants given in Section 16.3 and in Tables 16.3 to 16.6. Compare the estimated values of knQ. with the experimental values given in parentheses. Try to explain any larger discrepancies (i.e., > factor 3). 

The discussion thus far has emphasized sensitivity of the frequency of C02 s v3 mode to local stress, sensitivity of its absorption intensity to IR polarization, and sensitivity of both properties to resonant coupling of dimers. For the type of crystals under consideration, which consist mostly of simple hydrocarbon groups, these factors probably dominate in determining the IR spectral characteristics. Other factors can be involved, however, and although they can make simple interpretation of the spectra more problematic, they can also provide additional information about the environment of the C02 probe molecule. The following examples illustrate how one can track the motion of C02 over distances of 1-15 A by observing its proximity to free radical centers or to halogen or deuterium substituents in the crystal lattice. This information complements the previously discussed structural studies, which related to structure within the dimer rather than to the location of the C02 in the crystal matrix. 

As for thioaldehydes, the stability of thioketenes is largely influenced by the nature of the substituents and bulky groups tend to stabilize this functional group. Electronic factors such as those originating in silicon, phosphorus or trifluoromethyl substituents lead to a similar result. In general, however, the synthesis of monomeric thioketenes is difficult and requires the use of special techniques such as FVT, matrix isolation at low temperatures or generation under conditions which allow trapping in situ of the transient species. The... 

Electrophilic substituent factors for the -C6H5 XC1X and -OC6H5 XC1X groups, needed to calculate the OH radical addition rate constants for polychlorinated biphenyls (PCBs), polychlorodibenzo-p-dioxins (PCDDs), and polychlorodibenzofurans (PCDFs), appear in Atkinson (1996), with discussion of an approach to calculating the rate constants for the PCDDs and PCDFs. The room temperature rate constants for the reactions of the OH radical with phenanthrene and anthracene, recently measured by Kwok et al. (1994,1997), are lower by factors of 2.5-8 than the previous recommendations and rate data (Biermann et al., 1985 Atkinson, 1989), casting doubt on the previously proposed correlation between the OH radical addition rate constant and ionization potential (Biermann et al., 1985). 

Note that these group rate constants and substituent group factors should be used only for homologs of the compounds from which these factors were derived (Kwok and Atkinson, 1995 Koch et al., 1996). For example, the group rate constants k, N, k>NH and k N,l2 and the substituent factors F(-NH2), F(>NH) and F(>N-) are appropriate only for alkyl-substituted amines and not for amines containing halogen or other hetero-atoms (in particular, see Koch et al., 1996). For the phosphorothioamidates (with ->P(=S)N< structures) and... 

For example, the method Atkinson (1986) proposed requires a reliable rate constant database for the various classes of organic compounds in order to derive the various group rate constants and substituent factors. As may be expected, this estimation technique is reasonably reliable when used within its derivation database (Kwok and Atkinson, 1995). Thus, Kwok and Atkinson (1995) observed that the 298 K rate constants were predicted to within a factor of 2 of the experimental values for 90% of the 485 organic compounds that were considered and for which experimental data were available [and rate constants for alkyl nitrates are still predicted to within a factor of 2 after re-evaluation to take into account the recent kinetic and mechanistic data of Talukdar et al. (1997)]. 

In order to explain the experimental behavior found of X for PVP in the different mixtures, the polarizability was taken into account because of the methyl groups substituents of the aromatic ring. It is possible to And changes in the nature of the interactions between the polar solute, 2 - propanol, and the aromatic component in the binary mixtures and that these changes affect the X values. The importance of dipole - induced dipole interactions and steric factors in the formation of a molecular complex between a polar component and a non - polar aromatic solvent has been emphasized on the basis of NMR studies [111, 112], The molecular interactions in binary liquid mixtures have also been studied on the basis of viscosity measurements. The viscosity data have also been used by Yadava et al. [113,114] to obtain a value for the interchange energy (Wvisc) [115] This parameter can be estimated by the equation ... 

Here A(X)a is the substituent factor attributable to group X when calculating the chemical shift of the o carbon, 6Ca. For calculating the chemical shifts of the two /3 carbons <5C x and dCpy it is necessary to include a substituent factor for the group on the /3 carbon in question, i.e. the near group (n), and one for the group on the distant /3 carbon (d). 

For the 13C nmr spectra, three equations are necessary to define the chemical shifts since three different carbon atoms are involved. Consequently, three substituent factors are required Aa values to calculate the chemical shift of the a-carbon, An and Ad values for calculating the p-carbons, the former referring to the nearer group, the latter to the more distant P-substituent. The respective equations are (17) and (18). 

The s-R4 term indicates an unfavorable steric effect for R4 substituents in the 6-position of the pyridyl ring pointing out the detrimental effect of the o-group. Electronic factors were not found to play any definite role. 

Remarkably, the rate of propyne insertion via the MMA pathway in absolute terms is not significantly affected by the presence of medium-sized substituents, such as a methyl group (Figure 2) at the 6-position of the pyridyl group, whereas the insertion rate to methyl crotonate is very seriously suppressed (e. g., by a factor 20, with a methyl group substituent). 

Si and dij are substituent factors corresponding to the direct effect of substituent i and to the interaction of the pair of groups i and 7, respectively,... 

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