For free-radical reactions

Because many organic peroxides undergo thermolysis to form useful free radicals, they are used commercially as initiators for free-radical reactions. Many organic peroxides also undergo reactions in which free radicals are not involved, eg, heterolyses, hydrolyses, reductions, and rearrangements. Numerous reviews of the chemistry and appHcations of organic peroxides have been pubHshed (11,13—41). 

One of the common tests for free radical reactions seems to be violated in these experiments, namely that they should have nearly zero activation energy. Annealing reactions in the solid state are obviously temperature... 

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... 

In appearance, the PHIP phenomenon closely resembles those due to CIDNP [16], another phenomenon, which also gives rise to emission and enhanced absorption lines in NMR spectra. However, CIDNP is the consequence of the occurrence of free radicals, and previously has frequently been considered unequivocal proof for free radical reactions. 

In one of the earliest investigations of spin trapping, olefin polymerization was employed to demonstrate the utility of the method as a qualitative probe for free-radical reactions (Chalfont et al., 1968). The polymerization of styrene, initiated by t-butoxyl radicals, proved to be an excellent system with which to obtain spectra attributable to spin adducts with MNP (a) of the initiator radical... 

Huyser, E. S., Free Radical Chain Reactions, Chap. 10 and pp. 314—330, Wiley, New York, 1970. Ingold, K. U., Rate Constants for Free Radical Reactions in Solution, Chap. 2 in Free Radicals, Vol. I, J. K. Kochi, ed., Wiley, New York, 1973. 

Direct bromination of toluene and ethylbenzene form the corresponding benzyl bromides in high yield. The observed selectivity in SC-CO2 is similar to that observed in conventional organic solvents. Also, SC-CO2 is an effective alternative to carbon tetrachloride for use in the classical Ziegler bromination with N-bromosuccinimide. Reaction yields are high, side products are minimized, and bromine-atom selectivities are observed. Thus, SC-CO2 must be useful as a viable, environmentally benign substitute for many of the solvents typically used for free-radical reactions (Tanko and Blackert, 1994). 

For free radical reactions involving hydrocarbons, the bimolecular rate constants can be derived from measurements of their decay rates relative to those of a reference compound for which the rate constant has been well established ... 

Cumene Hydroperoxide is used as a catalyst for free radical reactions, such as polymerization as a vulcanization accelerator in rubber-reclaiming flotation and as a poly-... 

A detailed mechanistic analysis of the factors affecting the success of these tin hydride mediated addition reactions has been provided by Giese.3 This analysis, which is especially illustrative of how experimental conditions for free radical reactions are planned, is summarized in Scheme 27. Three intermediate radicals, (8), (9) and (10), are involved. As is characteristic of all radical reactions, these radicals are simultaneously exposed to the same reagent pool and each can potentially undergo an addition reaction or an atom transfer reaction. The required reaction of the tributyltin radical (8) is atom ab-... 

Radicals react with closed-shell (diamagnetic) molecules via two pathways addition to a multiple bond (eq. 4.1) and atom abstraction (eq. 4.2). Thus, a suitable solvent for a radical reaction must not possess either abstractable atoms or reactive multiple bonds. Unfortunately, most of the solvents suitable for free-radical reactions (e.g., benzene, CC14) are damaging to the environment or carcinogenic. 

Much of the research discussed in this chapter has demonstrated that sc C02 is a viable alternative solvent for free-radical reactions. Moreover, several of these studies demonstrate that the unique properties of a supercritical fluid, specifically the ability to change important solvent properties such as viscosity by varying pressure (and temperature), can be exploited to manipulate reaction yield and selectivity. Solvent viscosity is particularly important for reactions that are diffusion-controlled or those for which cage-effects are important. 

II/IV Rate constants for free radical reactions in solutions, including IVIRa (M = Si, Ge, or Sn) 107... 

Two-electron redox phenomena are rare for Cu, and therefore Cu is not a suitable catalyst for epoxidations, for example. However, Cu is useful for free radical reactions such as deep oxidation of organic molecules in waste streams and ring or side chain oxidation of aromatic compounds. 

Reactivity-selectivity relationships play an important part in free radical chemistry for the same reasons as in carbene chemistry and electrophilic substitution. Absolute rate constants for free radical reactions are not generally available (and when they are known they are often associated with large systematic errors), and the use of relative rate studies is an important technique in the study of free radical reactions. A comprehensive monograph dealing with various... 

As practice in using a systematic approach to proposing mechanisms for free-radical reactions, work Problem 4-24 by going through the four steps just outlined. 

Once you have decided which type of mechanism is most likely (acidic, basic, or free-radical), some general principles can guide you in proposing the mechanism. Some principles for free-radical reactions were discussed in Chapter 4. Now we consider reactions that involve either strong nucleophiles or strong electrophiles as intermediates. In later chapters, we will apply these principles to more complex mechanisms. 

Carmine TC, Evans P, Bruchelt G, Evans R, Handgretinger R, Niethammer D, HalliweU B. Presence of iron catalytic for free radical reactions in patients undergoing chemotherapy imphcations for therapeutic management. Cancer Lett 1995 94(2) 219-26. 

If the particular reaction studied is the unimolecular decomposition of a free radical, such as (3), then the use of a trap will enable the effective concentration of the radical to be measured. A radical trap will indicate the presence or absence of a free radical reaction and may sometimes provide evidence for a partly or entirely molecular reaction. Rate data for free radical reactions are derived assuming the occurrence of a steady state concentration of radicals. The time required to produce a steady state concentration of methyl radicals in the pyrolysis of AcH is shown for various temperatures in Fig. 1. Realistic values for rate coeflBcients may be obtained only if the time of product formation is long compared to the time to achieve the steady state concentrations of the radicals concerned. Thus deductions from the results from the bromination of isobutane , neopentane , and toluene have been criticised on the grounds that a steady state concentra-... 

In this book, single-headed arrows are often shown moving in one direction only for free-radical reactions other than bond homolysis. This convention is used in order to avoid cluttering the drawings with arrows. You may show the movement of electrons in all directions if it helps you to see the reactions better. In fact, most chemists show arrows moving in both directions. 

The solvents of choice for free-radical reactions (CH3OH, H2O, benzene) share the feature that their X-H bonds have very large BDEs, so that the solvents are unlikely to participate in the reaction. Free-radical reactions executed in ether, THF, toluene, CH2CI2, and CHCI3 are often complicated by atom abstraction from the solvent. Acetone is sometimes used as a solvent for free-radical reac-... 

The most important difference between a living ionic polymerization which has no termination or transfer mechanism and free-radical or ionic processes that do have termination or chain transfer steps is that the distributions of the degrees of polymerization are quite different. The distribution function can be derived by a kinetic approach due to Flory [6], which is analogous to that used earlier for free-radical reactions (see Problem 6.44). However, in the present case with no chain termination the simplifying steady-state approximation cannot be used. 

USE Blowing agent for elastomers and plastics. Initiator for free radical reactions Griesbaum el aL. J. Org Chem. 3D, 261 (1965). 

Chapter 11 deals with free radicals and their reactions. Fundamental structural concepts such as substituent effects on bond dissociation enthalpies (BDE) and radical stability are key to understanding the mechanisms of radical reactions. The patterns of stability and reactivity are illustrated by discussion of some of the absolute rate data that are available for free radical reactions. The reaction types that are discussed include halogenation and oxygenation, as well as addition reactions of hydrogen halides, carbon radicals, and thiols. Group transfer reactions, rearrangements, and fragmentations are also discussed. 

The second publication is a review article by Ingold [390] on rate coefficients for free radical reactions in solution which includes comprehensive coverage of radical—molecule reactions. Metathetical reactions are usually referred to as Sn 2 reactions, i.e. substitution, homolytic and bimolecular, by organic chemists. Most quantitative kinetic studies of this class of solution reactions involve H atom transfer but halogen-transfer reactions have also been studied. 

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