Free radicals reaction rates

Free-radical reaction rates of maleic anhydride and its derivatives depend on polar and steric factors. Substituents added to maleic anhydride that decrease planarity of the transition state decrease the reaction rate. The reactivity decreases in the order maleic anhydride > fumarate ester > maleate ester. 

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... 

Measurements of Free Radicals Reaction Rate Constants.120... 

Studies show that the measured composition of the product mixture at constant temperature depended on the water density (Fig. 7.7). This was taken as an indication that these products could be formed by competing ionic and free-radical reaction pathways. Usually in gas-phase kinetics the product composition changes with temperature because of the different activation energies and, to a minor extent with pressure, mainly because of the concentration effect on bimolecular elementary reaction steps. In water, the drastic dependence on pressure is likely a consequence of the competition between reactions with different polarity. Free radical reaction rates (involving large free radicals beyond the RRKM high-pressure limit, see, for example, [25]) should decrease with pressure as a result... 

Free radicals are studied by generating them using radiolysis. These studies have produced extensive tabulations of rate constants for free radical reaction rates (Bielski et al, 1985 Buxton et al, 1988 Buxton et al, 1995 Neta et al., 1988). 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

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 "...

Radiolytic ethylene destruction occurs with a yield of ca. 20 molecules consumed/100 e.v. (36, 48). Products containing up to six carbons account for ca. 60% of that amount, and can be ascribed to free radical reactions, molecular detachments, and low order ion-molecule reactions (32). This leaves only eight molecules/100 e.v. which may have formed ethylene polymer, corresponding to a chain length of only 2.1 molecules/ ion. Even if we assumed that ethylene destruction were entirely the result of ionic polymerization, only about five ethylene molecules would be involved per ion pair. The absence of ionic polymerization can also be demonstrated by the results of the gamma ray initiated polymerization of ethylene, whose kinetics can be completely explained on the basis of conventional free radical reactions and known rate constants for these processes (32). An increase above the expected rates occurs only at pressures in excess of ca. 20 atmospheres (10). The virtual absence of ionic polymerization can be regarded as one of the most surprising aspects of the radiation chemistry of ethylene. 

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. 

Initiation occurs as a molecule of intiator decomposes to free radicals (Reaction 2.9), where is the rate constant for the reaction. These newly generated free radicals can then interact with monomer molecules preserving the free radical centre (Reaction 2.10). These two processes may be considered to form part of the initiation stage, the first of them being rate determining. 

Our treatment of chain reactions has been confined to relatively simple situations where the number of participating species and their possible reactions have been sharply bounded. Most free-radical reactions of industrial importance involve many more species. The set of possible reactions is unbounded in polymerizations, and it is perhaps bounded but very large in processes such as naptha cracking and combustion. Perhaps the elementary reactions can be postulated, but the rate constants are generally unknown. The quasi-steady hypothesis provides a functional form for the rate equations that can be used to fit experimental data. 

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. 

The study of the detailed mechanism of free radical initiation (rate constant k ) and ozone decay (rate constant d) by the reaction with cyclohexane, cumene, and aldehydes gave the following results (7 = 298 K) ... 

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

Rate Constants of Free Radical Reactions with Ozone (Experimental Data)... 

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],... 

Several empirical correlations are known for rate constants and activation energies of bimo-lecular radical reactions [1 4], Evans and Polyany [5] were the first to derive the linear correlation between the activation energy and the enthalpy of reaction of R,X with Na. Later Semenov [1] generalized this empirical equation for different free radical reactions in the following form ... 

When a molecule consists of a few similar fragments n, the rate constant of the reactant reaction with this molecule can be expressed as the product of the partial rate constants ky k = nx kj. This was proved many times for free radical reactions for groups of reactants where both reactants or one of them are nonpolar. For example, the rate constants of peroxyl radical reactions with nonbranched aliphatic hydrocarbons Me(CH2) Me can be presented in... 

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