Radical reactions, rate determination

Even though the rate of radical-radical reaction is determined by diffusion, this docs not mean there is no selectivity in the termination step. As with small radicals (Section 2.5), self-reaction may occur by combination or disproportionation. In some cases, there are multiple pathways for combination and disproportionation. Combination involves the coupling of two radicals (Scheme 5.1). The resulting polymer chain has a molecular weight equal to the sum of the molecular weights of the reactant species. If all chains are formed from initiator-derived radicals, then the combination product will have two initiator-derived ends. Disproportionation involves the transfer of a P-hydrogen from one propagating radical to the other. This results in the formation of two polymer molecules. Both chains have one initiator-derived end. One chain has an unsaturated end, the other has a saturated end (Scheme 5.1). 

Throughout this work, we will refer to the heats of reactions and the methods used for the kinetic determinations. We have collected background information for discussion in this section. A list of important bond dissociation energies is followed by brief descriptions of the more important kinetic methods used for determining radical reaction rate constants discussed in this review. 

Cyclizations of amidyl radicals have been studied both synthetically and kinetically. A detailed study on the rates of a variety of amidyl radical reactions was determined by both LFP and indirect competition methods (Table l) In addition, the rate constants for reactions with BusSnH and PhSH were also reported (thus giving a range of simple amidyl radical clocks). The results obtained will be useful in synthetic sequenceplanning involving amidyl radicals. 

Equation (211) is applicable to the reaction rate determined by stages 1 and 2a of mechanism (327). It is only necessary, as the adsorbed radical N2H2 is a product of stage 2a, to substitute for the partial pressure of this radical in the kinetic equation, its fugacity, />NzHz, determined by the equilibrium of stage 2b ... 

For the majority of gas-phase organic chemicals present in the troposphere, reaction with the OH radical is the dominant loss process (Atkinson, 1995). The tropospheric lifetime of a chemical is the most important factor in determining the relative importance of transport, to both remote regions of the globe and to the stratosphere, and in determining the possible buildup in its atmospheric concentration. Knowledge of the OH radical reaction rate constant for a gas-phase organic compound leads to an upper limit to its tropospheric lifetime. 

Bond Homolysis. A substantial number of gas-phase bond homolysis rate constants and free-radical enthalpies of formation have been determined (1,11) and a far greater number may be reliably estimated. However, two factors must be considered when applying gas-phase bond homolysis rate constants to condensed-phase systems. First, any selective solvation of product radicals will tend to increase (k /kg). However, solvation effects on free radical reaction rates are generally the... 

Dreisbach, R.R., Shrader, A.A.I. (1949) Vapor pressure-temperature data on some organic compounds. Ind. Eng. Chem. 41,2879-2880. Eadsforth, C.V. (1986) Application of reverse-phase HPLC for the determination of partition coefficients. Pest. Sci. 17(3), 311-325. Edney, E.O., Corse, E.W. (1986) Validation of OH Radical Reaction Rate Constant Test Protocol. NTIS PB86-166 758/as. U.S. Environmental Protection Agency, Washington, D.C. 

A review of chemiluminescent and bioluminescent methods in analytical chemistry has been given by Kricka and Thorpe. A two-phase flow cell for chemiluminescence and bioluminescencc has been designed by Mullin and Seitz. The chemiluminescence mechanisms of cyclic hydrazides, such as luminol, have been extensively analysed. " Fluorescence quantum yields of some phenyl and phenylethynyl aromatic compounds in peroxylate systems have been determined in benzene. Excited triplet states from dismutation of geminate alkoxyl radical pairs are involved in chemiluminescence from hyponitrite esters. Ruorophor-labelled compounds can be determined by a method based on peroxyoxalate-induced chemiluminescence. Fluorescence and phosphorescence spectra of firefly have been used to identify the multiplicity of the emitting species. " The chemiluminescence and e.s.r. of plasma-irradiated saccharides and the relationship between lyoluminescence and radical reaction rate constants have also been investigated. Electroluminescence from poly(vinylcarbazole) films has been reported in a series of four... 

In the atmosphere, the vapor-phase reaction of PCBs with hydroxyl radicals (photochemicaUy formed by sunlight) is the dominant transformation process (Brubaker and Hites 1998). The calculated tropospheric lifetime values for this reaction increases as the number of chlorine substitutions increases. The tropospheric lifetime values (determined using the calculated OH radical reaction rate constant and assuming an annual diurnally averaged OH radical concentration of 5x10 molecule/cm ) are 5-11 days for monochlorobiphenyls, 8-17 days for dichlorobiphenyls, 14—30 days for trichlorobiphenyls,... 

Absolute Rate Constants. Absolute rate constants for the hydroxyl radical reactions, as determined from the formation curves of the hydroxycyclohexadienyl radicals, are summarized in Table I. Detailed data for benzoate ion are shown in Table II. In all cases the rate curves fit closely to a first order rate law. A detailed examination of this case seems warranted not only as an example of the data, but because of the possible use of this reaction as a reference reaction in competition kinetics. 

Figure 6.17 Illustration of relative rate data used for determining hydroxyl radical reaction rates. [Reproduced with permission from W. R. Haag and C. C. D. Yao, Environ. Sci. Technol. 26, 1005 (1992). Copyright 1992, American Chemical Society.]...

Radical (or free radical) Radical anion Radical cation Rate-determining step Reducing agent Reduction reaction... 

For the experiments in which decay rates are used to determine individual NO3 radical reaction rate constants, N O, NO and the organics are quantitatively monitored by Fourier transform infrared technology (FT-IR) spectroscopy. For the relative rate constant studies, the reacting organics are analysed prior to and during these reactions by gas chromatography. 

An indirect method (Method I) was also enqiloyed to determine these hydroxyl radical reaction rate values. Since a relationship has been developed between the degradation rate of carbofuran and its initial concentration in the previous work. 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

The degree of polymerization is controlled by the rate of addition of the initiator. Reaction in the presence of an initiator proceeds in two steps. First, the rate-determining decomposition of initiator to free radicals. Secondly, the addition of a monomer unit to form a chain radical, the propagation step (Fig. 2) (9). Such regeneration of the radical is characteristic of chain reactions. Some of the mote common initiators and their half-life values are Hsted in Table 3 (10). 

The reaction rate of molecular oxygen with alkyl radicals to form peroxy radicals (eq. 5) is much higher than the reaction rate of peroxy radicals with a hydrogen atom of the substrate (eq. 6). The rate of the latter depends on the dissociation energies (Table 1) and the steric accessibiUty of the various carbon—hydrogen bonds it is an important factor in determining oxidative stabiUty. 

Chain reactions such as those described above, in which atomic species or radicals play a rate-determining part in a series of sequential reactions, are nearly always present in processes for the preparation of thin films by die decomposition of gaseous molecules. This may be achieved by thermal dissociation, by radiation decomposition (photochemical decomposition), or by electron bombardment, either by beams of elecuons or in plasmas. The molecules involved cover a wide range from simple diatomic molecules which dissociate to atoms, to organometallic species with complex dissociation patterns. The... 

In mutual termination the rate of reaction is determined by the concentration of growing radicals and since two radicals are involved in each termination the reaction is second order. 

For the acetoxy radical, the for decarboxylation is about 6.5 kcal/mol and the rate is about 10 s at 60°C and 10 s at —80°C. Thus, only very rapid reactions can compete with decarboxylation. As would be expected because of the lower stability of aryl radicals, the rates of decarboxylation of aroyloxy radicals are slower. The rate for p-methoxybenzoyloxy radical has been determined to be 3 x 10 s near room temperature. Hydrogen donation by very reactive hydrogen-atom donors such as triethylsilane can compete with decarboxylation at moderate temperatures. 

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