Free radicals reaction rates, table

Oxidation of SOp to sulfate Homogeneous, gas-phase reactions. Free radical reactions (see Table V). Rate constants for SO2-OH and SO2-HO2 measured but still somewhat uncertain. Rate constants for SO2-RO and SO2-RO2 reactions inknown. Mechanism for the SO2-OH reaction unknown product HOSO2 assximed to be hydrated to HpS0. ... 

The two possible initiations for the free-radical reaction are step lb or the combination of steps la and 2a from Table 1. The role of the initiation step lb in the reaction scheme is an important consideration in minimising the concentration of atomic fluorine (27). As indicated in Table 1, this process is spontaneous at room temperature [AG25 = —24.4 kJ/mol (—5.84 kcal/mol) ] although the enthalpy is slightly positive. The validity of this step has not yet been conclusively estabUshed by spectroscopic methods which makes it an unsolved problem of prime importance. Furthermore, the fact that fluorine reacts at a significant rate with some hydrocarbons in the dark at temperatures below —78° C indicates that step lb is important and may have Httie or no activation energy at RT. At extremely low temperatures (ca 10 K) there is no reaction between gaseous fluorine and CH or 2 6... 

Recent development of techniques for measuring the rates of very fast reactions has permitted absolute rates to be measured for some fundamental types of free-radical reactions. Some examples of absolute rates and values are given in Table 12.2. 

Table 12.2. Absolute Rates of Some Free-Radical Reactions "...

Ionic Reactions in TD/D2 Methane Mixtures. Previous investigation of the radiolysis of D2 containing small quantities of CH4 demonstrated that at low conversions all products anticipated from the H atom abstraction sequence except CH3D are absent from 125° to —196°C. and that the temperature coefficient of the rate of CH3D formation between 25° and 125 °C. is much too small for a purely atomic and free-radical reaction sequence (8). These observations are confirmed by new data presented in Table I. The new data also demonstrate the initial value of G(CH3D) is independent of temperature at 25°C. and below. 

There is a good deal of information available about the absolute rates of free radical reactions. A selection from these data is given in Table 11.3 of Part A. If the steps in a projected reaction sequence correspond to reactions for which absolute rates are known, this information can allow evaluation of the kinetic feasibility of the reaction sequence. 

For the values of rate constants of the free radical reactions in the gas and the liquid phases, see Table 3.9. 

Due to these reactions, hydrogen peroxide is an intermediate product of radiolysis of aerated water. Rate constants of free radical reactions with dioxygen and hydrogen peroxide are collected in Table 3.19. For the characteristics of solvated electron and information about its reactions, see monographs [219-223],... 

The observed rate constant is kobs = kkn(k + vD)-1. For the fast reactions with k vD the rate constant is kobs = kI). In the case of a slow reaction with k vD the rate constant is k0bs = kx KAb, where KAB = k y vn is the equilibrium constant of formation of cage pairs A and B in the solvent or solid polymer matrix. The equilibrium constant KAB should not depend on the molecular mobility. According to this scheme, the rate constant of a slow bimolecular reaction kobs = kKAB(kobs kD) should be the same in a hydrocarbon solution and the nonpolar polymer matrix. However, it was found experimentally that several slow free radical reactions occur more slowly in the polymer matrix than in the solvent. A few examples are given in Table 19.1. 

Besides diffusion, there is another underlying reason for leveling of reactivity of reactants in polymer media. The phenomena of leveling of reactivity of slowly reacting reactants was observed in the study of the reactions of peroxyl radicals with phenols and amines in the solid PS [23]. Later, this peculiarity was detected for different free radical reactions in the polymer matrix. All these reactions occur with rate constants much lower than kV) in polymer and cannot be limited by translational diffusion (see Table 19.6). 

We have seen that the polarization forces cause very large collision cross sections between ions and molecules, and it is experimentally observed that a chemical reaction occurs on almost every collision. Some observed ion molecule reactions and their observed rate constants and cross sections are tabulated in Table IV. In addition, several free radical reactions are shown for comparison purposes. Let us look at the reactions at the top of Table IV and try to understand why the reaction occurs. 

A study of the overall rate of addition of radicals to olefins shows us that polar forces, familiar to chemists working with ionic species in solution, are apparent in free radical reactions, i.e. in reactions involving uncharged species in the gas phase. The effects are smaller than in solution but are none the less clearly apparent. The results also show that polarity is not the whole story and that some of the trends observed in Tables 1 to 7 may be partly due to steric... 

Table 11.3. Absolute Rates for Some Free Radical Reactions ...

Rate constants for free radical propagation increase with decreasing polymer free radical resonance stabilization (Table 20-2). The activation energies, however, are more or less independent of the constitution. Consequently the rate constants are predominantly determined by the preexponential factors of the Arrhenius equation. In addition, they also depend on the viscosity of the reaction medium to a slight extent. 

From Table 17 in Ingold, K. U., Rate Constants for Free Radical Reactions in Solution ch. 2 in Kochi, J. K.. Free Radicals , Volume 1, Wiley-Interscience. New York, N.Y. 1973. 

Table 6.5 Some Free Radical Combination Reactions Which Yield n-mers and Their Rate Laws...

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). 

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