Homolytic substituent constant

Modified Hammett substituent constants (100) were used by Garrett et al. to describe the bacterostatic activities of a series of sulfanilamides (101). Hansch also used the homolytic substituent constants (ER) of Yamamoto and Otsu (102) in analyzing the activity of selected chloramphenicol derivatives (103). The resulting correlations led to the hypothesis of a free-radical mechanism of chloramphenicol action. Substituent measures of it electron charge density distributions (07 and ov)... 

LINEAR FREE ENERGY RELATIONSHIPS Hammett substituent constants Taft substituent constants Inductive and resonance substituent constants Steric substituent constants Homolytic substituent constants Field and resonance components Aqueous solubility The parachor... 

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. 

Alfrey and Price [24, 25] devised a new set of parameters, analogous to Hammett s, but specific to homolytic reactions. Their substituent constant Q was based on the 16 possible radical polymerisations involving styrene, methyl methacrylate, acrylonitrile and vinylidine chloride. Styrene was taken as the... 

Electronic and Steric Constants - In addition to hydrophobic parameters, homolytic and steric constants were successfully used in the structure-activity studies of 1,3-benzodioxole synergists of carbaryl in flies. Although the hydrophobic character of the synergists is quite important (log P = 4), electronic substituent effects paralleled those for homolytic arylation and improved the correlation. Use of Hammett s electronic constant (o) and Taft s steric parameter (E ) gave a better correlation when used with the hydrophobic substituent constant. Observations in this study have suggested a possible mechanism of action of the synergists on the molecular level. 

The rapid formation of the (Z)-diazoate is followed by the slower (Z/J )-isomeri-zation of the diazoate (see Scheme 5-14, reaction 5). Some representative examples are given in Table 5-2. Both reactions are first-order with regard to the diazonium ion, and the first reaction is also first-order in [OH-], i.e., second-order overall. So as to make the rate constants k and k5 directly comparable, we calculated half-lives for reactions with [ArNj ]0 = 0.01 m carried out at pH = 9.00 and 25 °C. The isomerization rate of the unsubstituted benzenediazonium ion cannot be measured at room temperature due to the predominance of decomposition (homolytic dediazoniations) even at low temperature. Nevertheless, it can be concluded that the half-lives for (Z/ )-isomerizations are at least five powers of ten greater than those for the formation of the (Z)-diazohydroxide (reaction 1) for unsubstituted and most substituted benzenediazonium ions (see bottom row of Table 5-2). Only for diazonium ions with strong -M type substituents (e.g., N02, CN) in the 2- or 4-position is the ratio r1/2 (5)/t1/2 (1) in the range 6 x 104 to 250 x 104 (Table 5-2). 

Apparent exceptions are the constants k2 for diazonium salts with the electron-withdrawing substituents 4-C1 and 3-CN. The values of k2 for these compounds are more than a factor of 10 larger than expected on the basis of Hammett relationships. Product analyses rationalize this observation whereas in all other cases products are likely to be formed by heterolytic dediazoniation, the products from the 4-chloro-and 3-cyanobenzenediazonium ions include chlorobenzene and benzonitrile, typical compounds obtained in homolytic dediazoniations. This result corresponds to the reaction products observed by Moss et al. (1982) in micellar dediazoniation, compared with the nonmicellar reaction (see Sec. 8.3). 

Having a weak O—O bond, peroxides split easily into free radicals. In addition to homolytic reactions, peroxides can participate in heterolytic reactions also, for example, they can undergo hydrolysis under the catalytic action of acids. Both homolytic and heterolytic reactions can occur simultaneously. For example, perbenzoates decompose into free radicals and simultaneously isomerize to ester [11]. The para-substituent slightly influences the rate constants of homolytic splitting of perester. The rate constant of heterolytic isomerization, by contrast, strongly depends on the nature of the para-substituent. Polar solvent accelerates the heterolytic isomerization. Isomerization reaction was proposed to proceed through the cyclic transition state [11]. 

By studying a series of complexes in which the various substituents on the alpha-carbon atom are varied, we can look at the change in the magnitude of the rate constant for homolytic dissociation as a function of these substituents. The values... 

The propagation step, Eq. (4), is much slower than Eq. (3) as an example, its rate constant kp is 0.18 M 1 sec-1 for cumene at 303K. Values of kp can vary considerably for different substrates, as shown by the oxidation rates of substituted toluenes (8). With respect to toluene, taken as 1.0, the reactivity of 4-nitrotoluene toward ROO is 0.33 and that of / -xylene is 1.6. A homolytic process like the fission of the C-H bond should be essentially apolar, but data for substituted toluenes correctly suggest that the hydrogen radical abstraction is favored by electron-donor substituents and that in the transition state the carbon atom involved has a partial positive charge. The difference in kp between different molecules or different groups of the same molecule is the reason of the selectivity of autoxidation. 

Several directly measured values of AH° for homolytic dissociation of a metal-metal-bonded carbonyl in solution have been obtained (9). This was for the complexes [(n3-C3H5)Fe(CO)2 )2 where L = CO or a number of different P-donor ligands. The low value AH = 56.5 kJ mol-1 when L = CO was not unexpected for such a sterically crowded molecule. The P-donor substituents increased the steric crowding and displaced the equilibria in favor of the monomers but the effect seemed to be controlled more by AS° than AH°. In general metal-metal bond energies, however they may have been estimated, are too large to allow for direct measurement of equilibrium constants in solution in this way. 

The energies of Si-X bond in the (CH3)3SiX compounds (kJ-mole ) calculated by Sanderson method [141] inductive homolytic dissociation of HX compound (Djj x) [143] and Si-X bond calculated by Luo and Benson... 

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