Radical reactions initiation

Styrene is a colorless Hquid with an aromatic odor. Important physical properties of styrene are shown in Table 1 (1). Styrene is infinitely soluble in acetone, carbon tetrachloride, benzene, ether, / -heptane, and ethanol. Nearly all of the commercial styrene is consumed in polymerization and copolymerization processes. Common methods in plastics technology such as mass, suspension, solution, and emulsion polymerization can be used to manufacture polystyrene and styrene copolymers with different physical characteristics, but processes relating to the first two methods account for most of the styrene polymers currendy (ca 1996) being manufactured (2—8). Polymerization generally takes place by free-radical reactions initiated thermally or catalyticaHy. Polymerization occurs slowly even at ambient temperatures. It can be retarded by inhibitors. 

Thus, this first example of stereoselective radical reaction, initiated with the system based on Fe(CO)5, shows opportunities and prospects of using the metal complex initiators for obtaining the stereomerically pure adducts of bromine-containing compounds to vinyl monomers with chiral substituents. 

Examples of samarium-promoted radical procedures, which were combined with anionic processes, were detailed in Section 3.3. Here, we describe two-fold radical reactions initiated by Sml2. A combination of two samarium(II) iodide-promoted... 

Media separators, 14 496 Media suppliers, 11 33t Mediated radical reactions, initiators for, 14 297-299... 

Most of the reactions the inorganic chemist encounters in the laboratory involve ionic species such as the reactants and products in the reactions just discussed or those of coordination compounds (Chapter 13). However, in the atmosphere there are many free radical reactions initiated by sunlight. One of the most important and controversial sets of atmospheric reactions at present is that concerning stratospheric ozone. The importance of ozone and the effect of ultraviolet (UV) radiation on life has been much discussed. Here we note briefly that only a small portion of the sun s spectrum reaches the surface of the earth and that parts of the UV portion that are largely screened can cause various ill effects to living systems. 

Hydrosilylations of fluorine-containing alkenes are free radical reactions initiated by UV light or organic peroxides The direction of addition is the same as with fluonnated alkyl halides However, the reaction between hydrosilanes and fluorine-containing olefins catalyzed by platinum group metal complexes may result in bidirectional addition and/or formation of a vinylic silane, the latter by de hydrogenative silylation[/] The natures of both the silane and the catalyst affect the outcome of the reaction[7] A random selection of some typical new reactions of silanes are shown in Table 1 [1, 2, 3 4]... 

To explain their results, Daniel and Keulks proposed that allyl hydroperoxide (or peroxide) was the initiator of the gas-phase reaction since hydroperoxides are known to be good radical reaction initiators. In addition, it was felt that the decomposition of allyl hydroperoxide in the gas phase could result in the formation of propylene oxide and even acrolein. They proposed a modified reaction mechanism, which is presented in Fig. 3. 

These radical-neutral molecule reactions tend to be chain reactions, which is an important facet of a large percentage of radical mechanisms. As with all chain mechanisms, there are initiation, propagation and termination steps. The efficiency of the chain is defined by the number of propagation cycles (chain length) and, in many reactions, termination becomes unimportant so is not commonly considered, except for conventional radical polymerisations or reactions in viscous solvents. Rate constants for propagation help to determine the synthetic utility and mechanism of radical reactions. Initiation is covered in Section 10.2. 

Radical reactions initiated by dimethylzinc have been reviewed.101... 

The isomerization of the decalins is most easily explained as a free radical reaction initiated by hydrogen abstraction at one of the tertiary carbons. Inversion at the free radical center followed by hydrogen abstraction would yield the other isomer of decalin. Sufficient repetition of the process would bring about equilibrium between the cis and trans forms. 

Historically, the first color-forming reaction to be discovered which involves electron transfer is probably the photoinitiated reaction of diphenylamine with carbon tetrabromide, which forms blue colors [42]. In fact, the major path for color formation is due to radical reactions, initiated by photolysis of C—Br bonds to produce Br3C and bromine atoms. An alternative mechanistic path, possible when the light is absorbed by the diphenylamine, would involve electron transfer. MacLachlan has shown that such processes do occur durin> photolysis of aminotriarylmethanes in the presence of electron acceptors such as CBr4 and CC14 [43]. Other electron deficient species (quinones, nitroaromatics) were also demonstrated to be effective. 

However, Kupletskaya and co-workers (100) have shown that the products of photodecomposition of CpFe(CO)2R (R = Ph, 1-naphthyl, 1-azulenyl, or acenaphthyl) in air-saturated solution were those expected from the trapping of Ar radicals by oxygen. These results could be construed as evidence for photoinduced Fe—Ar bond homolysis, but it is more likely that these products arise from a free radical reaction initiated by the oxygen present in solution. A similar free radical-initiated homolysis of an Fe—(tj -CsHs) bond has been postulated for CpFe(CO)2(V CsH5) 101). 

Metal carbonyls like Mo(CO)s or Mn2(CO)io will catalyze the reduction of carbon tetrachloride to chloroform by isopropyl alcohol. Apparently these are radical reactions initiated by abstraction of a chloride from carbon tetrachloride by the metal complex. ... 

Predict the product of the following radical reaction (initiated by light). 

This reaction demonstrates that fluorinated alkenes are especially reactive towards nucleophilic radicals. Throughout the text, examples of free—radical reactions initiated by -rays are interspersed with peroxide—initiated processes and it is worth emphasising that the o -ray technique is an extremely useful experimental probe because it is, in effect, a universal initiator, i.e. the method can be applied over a wide range of temperature. Furthermore, it leaves essentially no fragments in the product, as do chemical initiators, and the technique may be used with metal apparatus. 

The quasi-steady hypothesis is used when short-lived intermediates are formed as part of a relatively slow overall reaction. The short-lived molecules are hypothesized to achieve an approximate steady state in which they are created at nearly the same rate that they are consumed. Their concentration in this quasi-steady state is necessarily small. A typical use of the quasi-steady hypothesis is in chain reactions propagated by free radicals. Free radicals are molecules or atoms having an unpaired electron. Many common organic reactions such as thermal cracking and vinyl polymerization occur by free-radical processes. There are three steps to a typical free-radical reaction initiation, propagation, and termination. 

Scheme 17. Preparation of branched-chain sugars via serial radical reactions initiated by tethered radical addition to unsaturated sugars...

Radical reactions. Initiation oi photochemical reaction in the present... 

The photoiodination of 105 in hexane has been studied [102], as has its thermal decomposition, a free-radical reaction initiated by homolysis of a C —I bond and leading, inter alia, to triiodoethylene, a product not detected in previous investigations [103]. 

Pyrolysis is a technique that uses heat to transform macromolecules into a series of lower molecular weight volatile products characteristic of the original sample. The initiation reactions involved in the pyrolysis of most organic materials are the result of free radical reactions initiated by bond breaking or by the unimolecular elimination of simple molecules such as water or carbon dioxide [172-175]. The products of pyrolysis are identified by gas chromatography, often in combination with mass spectrometry. 

Inference of a radical path can also be made on the basis of structure and reactivity arguments. For example, two-electron Sn2 processes generally show a maximum rate in halide displacements from alkyls of Me > Et > /-Pr > r-Bu. The reverse reactivity order is generally characteristic of radicals. R > RBr > RC reactivity is also expected for radical reactions. Initiation of polymerization of styrene or acrylonitrile is also an indicator of the presence of radicals. 

Detection of specific stable radicals in plants by EPR can be considered as a sensitive method of air pollution monitoring. The stable radicals are associated with products of free radical reactions initiated by various air pollutants, particularly nitrogen oxides. 

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