Radicals Hammett reaction constants

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

As a rule, H-acid catalysis lowers stereospeciflcity of the Diels-Alder reactions. Besides, kinetic parameters of these reactions correlates to the Hammett substituent constants. In the case of the cation-radical mechanism, correlation of kinetic parameters to the Hammett-Brown substituent constants gives the best accordance. 

The above Hammett correlation shows a direct correlation between the number of carbon atoms and the rate constant for alcohols. The kinetic rate constant is observed to increase with the number of carbon atoms on the alcohol chain. The reaction rate between alcohols and hydroxyl radicals can be described as follows methanol < ethanol < propanol < butanol < pentanol < hexanol < heptanol. Heptanol has the fastest reaction constant among all the alcohols and is about 10 times faster than the reference compound of methanol. The reaction pathway for substituted alcohols is shown in Figure 5.21. 

Hammett correlations were developed from experimental data for substituted phenols studied under the UV/Ti02 process (D Oliviera et al., 1993). The mechanism for this reaction is understood to proceed via the hydroxyl radical. Experimental data from the study of dichlorophenols and trichlo-rophenols under UV/Ti02 were used for QSAR analysis (D Oliviera et al., 1993). Figure 9.13 demonstrates the QSAR model for substituted phenols formulated from experimental data. The QSAR model developed for substituted phenols shows a goodness of fit of 0.9766. A good correlation was also established for substituted phenols using Hammett s constant, o the correlation coefficient is 0.987 (D Oliviera et al., 1993). Similar correlation coefficients for the constants o and ores demonstrate that the descriptor ores can be used to accurately predict kinetic rate constants for substituted phenols. 

Oxidation of trisubstituted N, N, IV -1 r i p h on y I -1,3,5 - tr i am i n o ben zcn cs (2a-2e) showed one to three irreversible cyclic voltammetric peaks. Potentials of the first peak fulfill the Hammett equation (against the 3 cr+ values, according to the additivity rule for three para-substituents) giving the slopes, i.e. the reaction constants p+, equal to —1.53, —1.45 and — 1.43 V/(3a+ unit) in solutions of methylene chloride, ACN and propylene carbonate, respectively (solutions contained 0.1 M tetrabutylammonium perchlorate)15. An interpretation of the above reaction constants is rather difficult because of the irreversibility (radical cations formed by the first electron transfer evidently disappear in fast chemical steps). However, relatively small values of p+ may be related15 to a charge delocalization onto the outer aromatic ring of the radical cation. 

Since the bond is broken symmetrically and results in free radicals, the process is called either a radical or a homolytic reaction. The rate of a homolytic reaction is highly dependent on the stabilities of the radicals, and substituent constants for homolytic reactions should therefore take into account the effects of substitution on the resonance stabilisation of the radical transition state. It is therefore not surprising that Hammett a constants have enjoyed very little success in predicting the rates of radical reactions. 

Japanese workers [26, 27] proposed Equation (24), where Ef is the resonance contribution of the substituent to the transition state and y its reaction constant. When the resonance contribution is very small the expression reduces to Hammett s equation. The hydrogen abstraction reaction of nuclear substituted cumenes toward the polystyryl radical was taken as the model reaction and given the arbitrary value of 7= 1.0 and r (cumene) = 0.0. Cammarata, Yau, Collett and Martin [28] related to Hammett s a by equation (25). 

Something with ArX character seems to be involved in the reduction of tri-n-butylstannane by benzyl halides. Relative relativities have been shown by Blackburn and Tumer to follow Hammett relations in which a (as opposed to or a) substituent constants are used. The authors point out that this is a novelty for a free radical chain reaction, and suggest a transition state in which the organic halide has anionic character. Two mechanisms are suggested for chlorides and bromides, while iodides appear to react differently. Fluorides are very inert. 

In the luminol-H202-HRP chemiluminescence system, some of the aniline derivatives were functionalized as enhancers, while some were inhibitors of the fluorescence [26,27]. A variety of meta-, para-, and ortlzo-substituted aniline were studied. The structures of some of these aniline derivatives are listed in Scheme 5. The reaction constants with HRP-I and HRP-II were measured, and the Hammett constants (a) of their substituents were determined. The results indicated that the aniline substituents with a less than - 0.27 exhibited inhibition behavior. The aniline derivatives acted as enhancers when a was between - 0.27 and + 0.18. A slight enhancement or inhibition was observed if a is greater than -l- 0.18. The factors of enhancement were determined by the potential of reduction of its aniline radicals, the reaction rates of the aniline with HRP-I and HRP-II, and the maximum concentration of the aniline radicals allowed during the oxidation [28]. 

As in the case of radical stability, the usual method for measuring the polarity of a molecule or functional group is to study its effea on the kinetics or thermodynamics of a chemical reaction, carefully chosen so to be governed primarily by polar effeas. The most widely used polar descriptors are Hammett constants, as originally derived from fitting to pJfa values of substituted benzoic acids by Hammett. Hammett s basic eqn [3] relates the equilibrirrm constant (or rate constant) for a reaction of a species with substituent R to the same qrrantity for the same reaction but with R=H, where a is the substituent constant and p is the reaction constant. 

Waters61 have measured relative rates of p-toluenesulfonyl radical addition to substituted styrenes, deducing from the value of p + = — 0.50 in the Hammett plot that the sulfonyl radical has an electrophilic character (equation 21). Further indications that sulfonyl radicals are strongly electrophilic have been obtained by Takahara and coworkers62, who measured relative reactivities for the addition reactions of benzenesulfonyl radicals to various vinyl monomers and plotted rate constants versus Hammett s Alfrey-Price s e values these relative rates are spread over a wide range, for example, acrylonitrile (0.006), methyl methacrylate (0.08), styrene (1.00) and a-methylstyrene (3.21). The relative rates for the addition reaction of p-methylstyrene to styrene towards methane- and p-substituted benzenesulfonyl radicals are almost the same in accord with their type structure discussed earlier in this chapter. 

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