Substituent constant electrophilic

Reactions that occur with the development of an electron deficiency, such as aromatic electrophilic substitutions, are best correlated by substituent constants based on a more appropriate defining reaction than the ionization of benzoic acids. Brown and Okamoto adopted the rates of solvolysis of substituted phenyldimeth-ylcarbinyl chlorides (r-cumyl chlorides) in 90% aqueous acetone at 25°C to define electrophilic substituent constants symbolized o-. Their procedure was to establish a conventional Hammett plot of log (.k/k°) against (t for 16 /wcra-substituted r-cumyl chlorides, because meta substituents cannot undergo significant direct resonance interaction with the reaction site. The resulting p value of —4.54 was then used in a modified Hammett equation. 

The rate constants for OH radical addition to aromatic rings are calculated using the correlation between the OH radical addition rate constant and the sum of the electrophilic substituent constants Xo (Zetzsch, 1982 Atkinson, 1986). As Zetzsch (1982) discussed, Xo is calculated by assuming that steric hindrance can be neglected Xo+ is the sum of all of the... 

The electrophilic substituent constants for a -C(CH3)3 substituent group are o+0 p = -0.256 and o+m = -0.059, and for an -OH group o+G p = -0.92 (Brown and Okamoto, 1958). For 2,6-di-ferf-butylphenol, the most negative value of Xo+ = -1.038 is obtained for initial OH radical addition at the 4-position. Use of this value of Xo+ together with Equation (6) then leads to a calculated value of k(OH radical addition to the aromatic ring) = 4.8 x 10-11 cm3 molecule-1 s-1. The rate constant for H-atom abstraction from the two -C(CH3)3 groups is given by ... 

Zetzsch, C. (1982) Predicting the rate of OH-addition to aromatics using o+-electrophilic substituent constants for mono- and polysubstituted benzene. 15th Informal Conference on Photochemistry, June 27-July 1, 1982, Stanford, CA. 

With respect to a-substituents bearing p- or 7r-electrons which are directly attached to the C—Cl bond (Table 6, Z = CH2=CH to CH3CH20), these may delocalize their electrons through resonance or mesomeric effects with the positively charged carbon atom in the transition state. Because of this, they were not plotted in the Taft figure for a-substituted ethyl chlorides. Furthermore, the rates for these substituents also could not be correlated with the electrophilic substituent constants a+. The o+ parameters have been defined for substituents on the benzene ring which are far from the reaction site. Even though steric effects may interfere with the coplanarity and hence with delocalization, the effect of these substituents was believed to be polar in nature. 

Atkinson77 and Kwok et al.95 have proposed and discussed methods for the estimation of OH radical reaction rate constants for the PCBs, PCDDs and PCDFs, based on the correlation of the rate constants for OH radical addition to aromatic rings, kOH, with the sum of the electrophilic substituent constants, Ecr+.77,100 Based on a review of the literature rate constants for OH radical addition to a wide range of aromatic compounds, Atkinson77 derived the correlation,... 

The electrophilic substituent constants, given in Table 1, were defined by a set of apparent substituent constants, i.e. (1/p) log(A /Aro), derived from the solvolysis rates of a,a-dimethylbenzyl(a-cumyl) chlorides [2] (Scheme 1) in 90% aqueous acetone at 25°C. For the definition, the reaction constant p = -4.54, based exclusively on meta and ir-electron withdrawing (tt-EW) para substituents, was applied. 

Table 4. Hammett and electrophilic substituent constants of some common...

The electrophilic substituent constant measures the electronic effects for electron-releasing substituents (e.g. -OMe, -Me, -OH, -NH2) and for a strong electron-acceptor active site, while the nucleophilic substituent constant cr measures the electronic effects for electron-acceptor substituents (e.g. -NO2, -CN, -CO2H) and strong electron-releasing active site. 

Brown, H.C., Okamoto, Y. and Inukai, T. (1958b). Rates of Solvolysis of the m- and p-Phenyl-, m- and p-Methylthio-, and m- and p-Trimethylsilylphenyldimethylcarbinyl Chlorides. Steric Inhibition of Resonance as a Factor in Electrophilic Substituent Constants. J.Am.Chem.Soc., 80,4964 968. 

Okamoto, Y, Inukai, T. and Brown, H.C. (1958b) Rates of solvolysis of phenyldimethylcarbinyl chlorides in methyl, ethyl and isopropyl alcohols. Influence of the solvent on the value of the electrophilic substituent constant. /. Am. Chem. Soc., 80, 4972-A976. 

For monoalkyl benzenes, there is evidence of a second mechanism for hydrocarbon decomposition. Since substantial amounts of benzalde-hyde (representing a 5-25% yield) and traces (< 1% yield) of peroxy-benzoyl nitrate have been identified (10) in smog chamber experiments with aromatic hydrocarbons having at least two benzylic hydrogens, hydrocarbon oxidation apparently may occur at the a-carbon atom. Application of the usual Hammett treatment and electrophilic substituent constants (23) to any of our rate measurements for toluene, fert-butyl-benzene, and benzene indicates no linear Hammett relationship toluene... 

Attempts to relate the efficiency of inhibition with the Hammett— Taft equation were soon abandoned in favour of more accurate correlations with Brown s electrophilic substituent constants, o+, as given by [43]... 

H.C. Brown and Y. Okamoto, Electrophilic substituent constants, /. Amer. Chem. 

Azzaro, M., Gal, J.F. and Geribaldi, S. (1984) Basicity of the carbonyl group. Part 11. A reexamination of the electrophilic substituent constants for amino substituents from thermodynamic measurements. J. Chem. Soc., Perkin Trans. 2,111-11 A. 

In contrast, neither electrophilic substituent constants nor constants provide a suitable basis for correlating the relative rates of solvolysis of l-(2-substituted 4-thiazolyl)ethyl chlorides, recalling similar poor correlations in several other heterocyclic systems. An alternative working model, based on a comparison of appropriately substituted pyridines and thiazoles, provides an adequate treatment of the substituent effects in these instances. ... 

The rate of solvolysis of l-(2-phenyl-5-thiazolyl)ethyl chloride is consistent with the substituent having an electrophilic substituent constant value of cTp = -0.34. This result is accounted for satisfactorily in terms of the coplanarity of the triazole and phenyl rings in this structure. ... 

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