Exothermic radical reaction

Vinyl chloride polymerization occurs via an exothermic radical reaction. In fact, the reaction is approximately 25% more exothermic than polyethylene polymerization. The highly exothermic nature of the reaction and the strong molecular weight dependence on temperature make heat transfer, and its control, critical to the manufacture of polyvinyl chloride. 

The bulk polymerization cannot be scaled-up. The process consists of an exothermal radical reaction that can be controlled only when the amount of polymerization mixture does not exceed a few hundredths of milliliters. After that, heat dissipation becomes difficult and dangerous overheating of the sample can occur, with significant risks for operators. 

Nitrations are highly exothermic, ie, ca 126 kj/mol (30 kcal/mol). However, the heat of reaction varies with the hydrocarbon that is nitrated. The mechanism of a nitration depends on the reactants and the operating conditions. The reactions usually are either ionic or free-radical. Ionic nitrations are commonly used for aromatics many heterocycHcs hydroxyl compounds, eg, simple alcohols, glycols, glycerol, and cellulose and amines. Nitration of paraffins, cycloparaffins, and olefins frequentiy involves a free-radical reaction. Aromatic compounds and other hydrocarbons sometimes can be nitrated by free-radical reactions, but generally such reactions are less successful. 

The highly exothermic chlorination reaction produces approximately 95 KJ/mol of HCI. The first step is the breaking of the Cl-Cl bond (bond energy = -1- 584.2 KJ), which forms two chlorine free radicals (Cl atoms) ... 

The last comprehensive review of reactions between carbon-centered radicals appeared in 1973.142 Rate constants for radical-radical reactions in the liquid phase have been tabulated by Griller.14 The area has also been reviewed by Alfassi114 and Moad and Solomon.145 Radical-radical reactions arc, in general, very exothermic and activation barriers are extremely small even for highly resonance-stabilized radicals. As a consequence, reaction rate constants often approach the diffusion-controlled limit (typically -109 M 1 s"1). 

The addition of a 2-methyl-2-penten-4-yl radical to the QDI (based on p-phenylene diamines [PPDs] thus producing the corresponding PPD radical) is highly exothermic. The reaction not only stabilizes the relatively unstable alkenyl radical, but also results in the aromatization of the diimino-cylcohexadienyl ring. The enthalpy of reaction for this reaction is calculated (using MOPAC/AMl Hamiltonian ) to be about —40 kcal/mol. 

The fundamental mechanisms of free radical reactions were considered in Chapter 11 of Part A. Several mechanistic issues are crucial in development of free radical reactions for synthetic applications.285 Free radical reactions are usually chain processes, and the lifetimes of the intermediate radicals are very short. To meet the synthetic requirements of high selectivity and efficiency, all steps in a desired sequence must be fast in comparison with competing reactions. Owing to the requirement that all the steps be fast, only steps that are exothermic or very slightly endothermic can participate in chain processes. Comparison between addition of a radical to a carbon-carbon double bond and addition to a carbonyl group can illustrate this point. 

All these reactions are exothermic, and the AH values are negative. All these reactions should seemingly occur equally rapidly. The question to how easily the aminyl radicals react with the H—O and H—C bonds of the peroxyl radicals can be answered by analyzing these reactions in terms of the IPM model of free radical reaction (see Chapter 6). This model gives a tool to perform the calculation of the activation energy for a thermally neutral reaction of each class. Analysis of experimental data has shown (see Chapter 15) that, when aminyl... 

The radical reactions (57) and (58) are both highly exothermic and should have small or negligible activation energies. Any significant temperature dependence of 0 must, therefore, arise from the temperature dependence of k56. On this basis values of 0 were calculated for the conditions employed by Modica and Hornig and used to derive the true values of k% from their data by an iterative procedure. The temperature dependence of k% over the range 1440-1975 °K was found to obey the expression... 

Polanyi and co-workers used the technique, which involved the experiments in bulk. The reactants are put into desired vibrational and rotational states by producing them in an exothermic pre-reaction. For example, OH radical in vibrational excited states can be formed by the reactions... 

The addition of radicals to alkenes is used to assess the performance of various levels of theory in the prediction of radical reaction enthalpies. Results for the addition of methyl radical to ethylene (Table 6.24) [41] show that the higher-level methods perform well in predicting the reaction enthalpy values range from -105.6 to -111.5 kJ/mol compared with the corrected experimental value of -113.1 kJ/mol. The AMI method greatly overestimates the exothermicity while the UB3LYP/6-311+G(3df,2p) level of theory, which performs well for the reaction barrier, significantly underestimates the exothermicity. The RB3LYP values... 

The exothermicity of reaction (8.87) is sufficient to fragment the formyl radical and could be written as... 

The light energy stored in the radical ion pair is utilized to split water or to produce other high-energy chemicals. While impressive progress has been made in increasing the efficiency of this charge separation reaction (Fox Chanon, 1988 Norris Meisel, 1989), inhibition of the exothermic back-reaction to neutral species,... 

There are two possible initiation steps for the free radical reaction step lb and the combination of steps la and 2a. The role of step lb in the reaction scheme is an important consideration in minimizing the population of atomic fluorine. This step was first postulated by Miller et al. (9) on the basis of reaction products. As indicated in Table I, this process is exothermic at room temperature (AC2 9g° k = -5.84 k cal mole 1) although the enthalpy is slightly positive. The validity of this step has not yet been conclusively established by spectroscopic methods, and this remains an unsolved problem of prime importance in fluorine chemistry. The fact that fluorine reacts at a significant rate with hydrocarbons in the dark at temperatures lower than —78°C is an indication that step lb is a significant step that may have very little or no activation energy at room temperature. At very low temperatures there is no reaction between molecular F2 and CH4 or C2H6 when isolated in 10°K matrices (10). 

That the carbon—metal or carbon—metalloid bonds are preserved at all in these reactions is quite surprising. With tetramethylgermanes, for example, this free radical reaction must be a 24 step process. The success in preserving carbon-germanium bonds must arise from very rapid molecular vibrational, rotational, and translational relaxation processes occurring on the cryogenically cooled surfaces such that the energy from the extremely exothermic reaction is smoothly dissipated. 

The mechanism of titanocene (TiCp2Cl2)-mediated 3-exo cyclization of vinyl epoxides containing carbonyl, phenyl, and vinyl groups has been determined by DFT calculations and experimental studies (Scheme 52).86 The reaction is exothermic and reversible, in contradiction to an earlier mechanistic proposal. The energies of activation for ring closure are among the lowest known for radical reactions. The crucial step for a successful completion of the reaction is rapid trapping of the cyclopropylcarbinyl... 

The time to tQ is the time for the wood-monomer mass to reach oven or curing temperature at T5. During the period of constant temperature, the induction period, the inhibitor is being removed by reaction with the free radicals. Once the inhibitor is eliminated from the monomer and wood, the temperature rises to a maximum which corresponds to the peak of the exothermic polymerization reaction. Polymerization continues to completion although at a decreased rate and the temperature returns to that of the curing chamber. The time to the peak temperature depends upon the amount of catalyst present, the type of monomer, the type of crosslinker, and the ratio of the mass of monomer to that of the wood. The wood mass acts as a heat sink. Figure 4 illustrates the effect of increased Vazo catalyst on the decrease in time to the peak temperature, and the increase in the peak temperature(10)... 

At first sight, radical reactions seem to be ideally adapted to frontier orbital analysis. They are usually exothermic, so their transition states resemble the reagents. Therefore, the starting material frontier orbitals should provide us with a very good approximation. Furthermore, radicals are soft reagents, whose reaction partners are often neutral molecules. Frontier control is dominant in such reactions. 

The direct chlorination of methane is carried out as a radical reaction in the gas phase and the highly exothermal reaction produces a mixture of chlorinated methanes (Table 1, entry 6). Higher chlorination is achieved by recycling lower... 

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