Induced chain reactions

Barr and Allen (6) measured the effect of 02 on the chain reaction induced by ionizing radiation to combine H2 and H202 to water. H202 can be reduced by both H and e aq to OH. 

The operation of the first atomic bomb hinged on the fission of uranium in a chain reaction induced by absorption of neutrons. The two most abundant isotopes of uranium are and whose natural relative abundances are... 

Free radicals produced by the thermolysis of typical radical Initiators as AIBN, benzoplnacole and phenylazotrlphenylmethane reduce the dlaryllodonlum salt generating aryl radicals and solvent derived radicals which In a chain reaction Induce the decomposition of more dlaryllodonlum salt. Through the selection of particular radical Initiators with specific decomposition rates, It Is possible to adjust the Initiation temperature of the cationic polymerization with considerable latitude. 

Decarbonylations can occur by either an ionic or a radical mechanism. The former mechanism is involved in reactions catalysed by acids, Lewis acids, or bases, and the latter in chain reactions induced thermally, photo-lytically, or by some other radical-forming process. 

Lipid Analyses. Verification of PUFA radical chain reactions induced by light-activated DPE molecules can be made by examining changes in a polar lipid fraction collected from injured tissues. Radical chain reactions can be terminated through cross-reactions of the endoperoxides formed (48). Therefore, either an appearance of lipid polymers or disappearance of certain phospholipids should occur. If only the PUFA moieties are involved in these reactions, a loss of PUFA in the DPE-treated tissue or an increase in the ratio of saturated to unsaturated fatty acids should occur. 

Shono T, Ohmizu H, Kawakami S, Nakano S, Kise N (1981) Electroorganic chemistry. 47. A novel chain reaction induced by cathodic reduction. Additirai of trichloromethyl anion to aldehydes or vinyl acetate. Tetrahedron Lett 22 871-874 Shono T, Kise N, Yamazaki A, Ohmizu H (1982) Electroorganic chemistry. 61. Novel selective synthesis of ot-chloromethyl, a, a-dichloromethyl, and a, a, a-trichloromethyl ketones from aldehyde utilizing electroreduction as key reaction. Tetrahedron Lett 23 1609-1612... 

Am undergo fission with thermal neutrons of these isotopes and Pu are the most important as they are most readily obtainable. Other heavy nuclei require fast neutrons to induce fission such neutrons are much more difficult to control into a self-sustaining chain-reaction. 

Azobisnittiles are efficient sources of free radicals for vinyl polymerizations and chain reactions, eg, chlorinations (see Initiators). These compounds decompose in a variety of solvents at nearly first-order rates to give free radicals with no evidence of induced chain decomposition. They can be used in bulk, solution, and suspension polymerizations, and because no oxygenated residues are produced, they are suitable for use in pigmented or dyed systems that may be susceptible to oxidative degradation. 

In early 1941, 0.5 )-lg of Pu was produced (eqs. 3 and 4) and subjected to neutron bombardment (9) demonstrating that plutonium undergoes thermal neutron-induced fission with a cross section greater than that of U. In 1942, a self-sustaining chain reaction was induced by fissioning 235u... 

In order to induce the free-radical chain reaction, a starter compound such as dibenzoyl diperoxide, azo-Zj -(isobutyronitrile) or tcrt-butyl hydroperoxide or UV-light is used. The commercially available, technical grade N-bromosuccinimide contains traces of bromine, and therefore is of slight red-brown color. Since a small amount of elemental bromine is necessary for the radical... 

The fission ofor Tu liberates, on average, two to three neutrons. One neutron is required to sustain the nuclear fission chain reaction. In a nuclear breeder reactor, the extra neutrons are used to induce nuclear reactions that lead to the production of Tu. The sequence begins by arranging for... 

The two nuclei on the right side are just two of the many possible products of the fission process. Since more than one neutron is released in each process, the fission reaction is a self-propagating, or chain reaction. Neutrons released by one fission event may induce other fissions. When fission reactions are run under controlled conditions in a nuclear reactor, the energy released by... 

Packer and Richardson (1975) and Packer et al. (1980) made use of the fact that electrons can be generated in water by y-radiation from a 60Co source (Scheme 8-29) to induce a free radical chain reaction between diazonium ions and alcohols, aldehydes, or formate ion. It has to be emphasized that the radiolytically formed solvated electron in Scheme 8-29 is only a part of the initiation steps (Scheme 8-30) by which an aryl radical is formed. The aryl radical initiates the propagation steps shown in Scheme 8-31. Here the alcohol, aldehyde, or formate ion (RH2) is the reducing agent (i.e., the electron donor) for the main reaction. The process is a hydro-de-diazoniation. 

Induced chain reactions. In acidic solution, oxalic acid reacts very slowly with chlorine, but iron(ll) reacts rapidly." The net reactions are these ... 

If the three product neutrons strike three other fissile nuclei, then after the next round of fission there will be nine neutrons, which can induce fission in nine more nuclei. In the language of Section 13.9, neutrons are carriers in a branched chain reaction (Fig. 17.24). 

FIGURE 17.24 A self-sustaining chain reaction, in which neutrons are the chain carriers, takes place when induced fission produces more than one neutron per fission event. These newly produced neutrons can stimulate fission in increasingly greater numbers of other nuclei. 

The reaction with sulfides occurs efficiently only when the resulting carbon-centered radicals are further stabilized by a a-heteroatom. Indeed, (TMSfsSiH can induce the efficient radical chain monoreduction of 1,3-dithiolane, 1,3-dithiane, 1,3-oxathiolane, 1,3-oxathiolanone, and 1,3-thiazolidine derivatives. Three examples are outlined in Reaction (12). The reaction of benzothiazole sulfenamide with (TMS)3SiH, initiated by the decomposition of AIBN at 76 °C, is an efficient chain process producing the corresponding dialkylamine quantitatively. However, the mechanism of this chain reaction is complex as it is also an example of a degenerate-branched chain process. 

In the early years of this century Luther and Schilow attempted to classify induced reactions. This classification was based on the specific nature of substances taking part in the induced reaction. Thus they distinguished two groups coupled and induced chain reactions (this term was adopted later). 

According to the authors mentioned above, induced chain reactions (Livingston ) or induced catalysis (Bray and Ramsey ) take place when the very slow reaction between the acceptor and actor is catalyzed by the inductor. However, since the chemical characters of the acceptor and the inductor are the same, the actor reacts with the inductor, too, thus a part of it will be excluded from the catalysis. The principal characteristics of reactions of this type according to Luther and Schilow are that the value of Fj largely exceeds 2 and that the plot of Fj rm 5([Ac]/[I])o rises exponentially. 

The situation is different when I autoxidation processes belonging to the category of induced chain reactions. 

Induced chain reactions occur when the very slow reaction between acceptor and actor is catalyzed by the inductor. Actually it means that primary reaction between actor and inductor induces a chain reaction between actor and acceptor. That is why this group of reactions is called induced chain reactions. 

The above scheme represents a closed-chain reaction showing that, at a high concentrations of Ac if the chain carrier reacts only with Ac j, F, never reaches a limiting value the plot of Fj versus ([Ac]/[I])o is exponential. If the concentration of Ac d is high enough, the competing reaction (14) is practically negligible thus, in the overall equation of the induced reaction, composed of steps (13), (15) and (16)... 

The principal characteristic of induced reactions of this type which have not been stressed so far, is that the extent of the induced change greatly decreases and in most cases reaction even ceases in the presence of chain-breaking substances. The induced reaction can be suppressed by any substances reacting with chain carriers at a higher rate than does the acceptor, and the product of the reaction of the suppressor can easily react with the inductor. Since the concentration of the chain carriers is generally low, the supressors of induced chain reactions exert considerable effect even in small quantity. The effect is particularly pronounced when the suppressor reacts reversibly. 

From the above it is obvious that merely the magnitude of the numerical value of Fj and the shape of the plot of Fj versus ([Ac]/[I])o do not make it possible to classify induced reactions correctly. It is necessary to learn more about the mechanism of induced reactions. The schemes presented show clearly that a genuine coupled reaction can be regarded as an open-chain, and an induced catalysis as a closed-chain reaction. However, these limiting types of reactions occur only rarely. 

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