Initiation of chain reactions

Taking into account that ROS produced by irradiated fullerenes C60 may act only in the radius of their short diffusion existence, one may suppose that cytotoxic effect is determined by the interaction of fullerene C60 with the surface of cells and initiation of chain reactions of free radical peroxidation in membranes. That is why the influence of photoexcited fullerene C60 on the course of LPO process was studied and evaluated by the content of generated primary (diene conjugates) and final (Schiff bases) products. The content of diene conjugates in thymocytes was 17.7 4.2, inEAC cells was 21.1 1.3, andinL1210 was 12.8 3.1 nM/mg protein, and Schiff bases -56 7.9,46.5 4.5, and 36.6 4.6 rel. units/mg protein, respectively, and did not alter during 1 h incubation of the cells. 

Initiators such as BPO are used not only for the initiation of chain reaction polymerization, but also for the curing of polyesters and ethylene-propylene copolymers, and for the grafting of styrene on elastomeric polymer chains. 

Both these mechanisms involve free radicals which should cause the initiation of chain reactions.56-59 Livingston3 has discussed type I direct and indirect photooxygenation processes in detail. In the latter case benzoyl peroxide or azoisobutyronitrile are photolyzed into free radicals. These induce chain reactions, the quantum yields of which are generally greater than unity and inversely proportional to the square root of the light intensity.3... 

According to Semenov [11] at thermolysis of chain process initiators the energy of the weakest bond break in molecules of almost all substances fits the range of 209-419 kJ. In this regard, the chain initiation by initial reagents themselves may progress at a low rate. Therefore, substances promoting initiation of chain reactions are often used. For example,... 

The free radical, single-electron forms of FAD are very reactive, and FADH can lose its electron through exposure to water or the initiation of chain reactions. As a consequence, FAD must remain very tightly, sometimes covalently, attached to its enzyme while it accepts and transfers electrons to another group bound on the enzyme (Fig 20.6). Because FAD interacts with many functional groups on amino acid side chains in the active site, the E° for enzyme-bound FAD varies greatly and can be greater or much less than that of NAD. In contrast, NAD and NADH are more like substrate and product than coenzymes. 

Conversion of superoxide anion to hydrogen peroxide and O2 (dismntation) by superoxide dismutase (SOD) is often called the primary defense against oxidative stress because superoxide is such a strong initiator of chain reactions (Fig 24.14). SOD exists as three isoenzyme forms, a Cn -Zn form present in the cytosol, a Mn form present in mitochondria, and a Cn -Zn form found extracellnlarly. The activity of Cu -Zn SOD is increased by chemicals or conditions (snch as hyperbaric oxygen) that increase the prodnction of snperoxide. 

Cage effects tend to decrease the efficiency of utilization of free radicals produced for the initiation of chain reactions. A quantitative treatment appears in sight but will not be discussed here. 

The initiation of chain reactions is often effected by the thermal decomposition of alkyl hydroperoxides (e.g. t-butyl hydroperoxide) into free radicals ... 

The concept on multicentered reaction system of chain conversion [9] (Section 5.1) enables to consider the conditions of both the selective inhibition and selective initiation of chain reactions from the more general viewpoint. 

The following inequality meets the condition of selective initiation of chain reaction by free radicals of R type... 

The field began in the 1920s when Cario and Franck discovered how these so-called collisions of the second kind could be put to practical use, e.g., the Hg -sensitized activation of paraffins. This is due to initiation of chain reactions by radical formation,... 

Initiator of chain reaction. If the reaction proceeds via the chain mechanism, the introduction of a substance, which decomposes to radicals under experimental conditions, accelerates this reaction. Very often the chain process, for example, polymerization, does not occur without an initiator. The yield of the reaction products per decomposed initiator is always higher than unity. 

Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

One characteristic of chain reactions is that frequentiy some initiating process is required. In hydrocarbon oxidations radicals must be introduced and to be self-sustained, some source of radicals must be produced in a chain-branching step. Moreover, new radicals must be suppHed at a rate sufficient to replace those lost by chain termination. In hydrocarbon oxidation, this usually involves the hydroperoxide cycle (eqs. 1—5). 

The degree of polymerization is controlled by the rate of addition of the initiator. Reaction in the presence of an initiator proceeds in two steps. First, the rate-determining decomposition of initiator to free radicals. Secondly, the addition of a monomer unit to form a chain radical, the propagation step (Fig. 2) (9). Such regeneration of the radical is characteristic of chain reactions. Some of the mote common initiators and their half-life values are Hsted in Table 3 (10). 

In this work the development of mathematical model is done assuming simplifications of physico-chemical model of peroxide oxidation of the model system with the chemiluminesce intensity as the analytical signal. The mathematical model allows to describe basic stages of chemiluminescence process in vitro, namely spontaneous luminescence, slow and fast flashes due to initiating by chemical substances e.g. Fe +ions, chemiluminescent reaction at different stages of chain reactions evolution. 

The left-hand end of the activated monomer is sealed off by the OH terminator, but the right-hand end (with the star) is aggressively reactive and now attacks another ethylene molecule, as we illustrated earlier in Fig. 22.1. The process continues, forming a longer and longer molecule by a sort of chain reaction. The —OH used to start a chain will, of course, terminate one just as effectively, so excess initiator leads to short chains. As the monomer is exhausted the reaction slows down and finally stops. The DP depends not only on the amount of initiator, but on the pressure and temperature as well. 

The overall rate of a chain process is determined by the rates of initiation, propagation, and termination reactions. Analysis of the kinetics of chain reactions normally depends on application of the steady-state approximation (see Section 4.2) to the radical intermediates. Such intermediates are highly reactive, and their concentrations are low and nearly constant throughout the course of the reaction ... 

The chain addition of formamide to alkenes is a closely related reaction. It results in the formation of primary amides. The reaction is carried out with irradiation in acetone. The photoexcited acetone initiates the chain reaction by abstracting hydrogen from formamide ... 

Free radical chain reactions depend on an easily generated free radical to initiate the chain. One way to generate this radical is to irradiate halogens, such as Ch and Brj. Another way is to add a small amount of an initiator molecule to the reaction mixture, such as AIBN. This molecule, when heated, decomposes into free radicals that react with other molecules to initiate a chain reaction. 

As mentioned in an earlier section (cf. Chapter 1, Section III), allylic positions are subject to attack by free radicals resulting in the formation of stable allyl radicals. A-Bromosuccinimide (NBS) in the presence of free-radical initiators liberates bromine radicals and initiates a chain reaction bromination sequence by the abstraction of allylic or benzylic hydrogens. Since NBS is also conveniently handled, and since it is unreactive toward a variety of other functional groups, it is usually the reagent of choice for allylic or benzylic brominations (7). 

The initial products of beta-scission are an olefin and a new carbenium ion (Equation 4-9). The newly-formed carbenium ion will then continue a series of chain reactions. Small ions (four-carbon or five-carbon) can transfer the positive charge to a big molecule, and the big molecule can crack. Cracking does not eliminate the positive charge it stays until two ions collide. The smaller ions are more stable and will not crack, They survive until they transfer their charge to a big molecule,... 

In a nuclear weapon, the fissile material is initially subcritical. The challenge is to produce a supercritical mass so rapidly that the chain reaction takes place uniformly throughout the metal. Supercriticality can be achieved by shooting two subcritical blocks toward each other (as was done in the bomb that fell on Hiroshima) or by implosion of a single subcritical mass (the technique used in the bomb that destroyed Nagasaki). A strong neutron emitter, typically polonium, helps to initiate the chain reaction. 

The rate of copolymerization in a binary system depends not only on the rates of the four propagation steps but also on the rates of initiation and termination reactions. To simplify matters the rate of initiation may be made independent of the monomer composition by choosing an initiator which releases primary radicals that combine efficiently with either monomer. The spontaneous decomposition rate of the initiator should be substantially independent of the reaction medium, as otherwise the rate of initiation may vary with the monomer composition. 2-Azo-bis-isobutyronitrile meets these requirements satisfactorily. The rate Ri of initiation of chain radicals of both types Ml and M2 is then fixed and equal to 2//Cd[7], or twice the rate of decomposition of the initiator I if the efficiency / is equal to unity (see Chap. IV). The relative proportion of the two types of chain radicals created at the initiation step is of no real importance, for they wall be converted one into the other by the two cross-propagation reactions of the set (1). Melville, Noble, and Watson presented the first complete theory of copolymerization suitable for handling the problem of the rate. The theory was reduced to a more concise form by Walling, whose procedure is followed here. 

Each molecule of chlorine, on photochemical fission, will give rise to two chlorine atoms, i.e. radicals, each of which is capable of initiating a continuing reaction chain. That each quantum of energy absorbed does indeed lead to the initiation of two reaction chains is confirmed by the observation that ... 

For the copper-induced decomposition of diazodiphenylmethane in acetonitrile, a fundamental difference in the catalytic action of Cu C104 and Cu ClO was detected. Whilst with CuC104, intermediary copper carbenoids are believed to be responsible for the mainly formed benzophenone azine402, CufClO initiates a chain reaction, promoted by radical cations and yielding mainly tetraphenylethene... 

The temperature increases the rate of the one-stage simple reaction. The situation is different in the case of chain reaction. The change in temperature and other conditions not only change the reaction rate but can change the mechanism of the reaction and composition of the formed products. This can be illustrated by analysis of the mechanism of the hydrocarbon oxidation at different temperatures, concentration of the reactants, and the rates of initiation [288]. Varying the conditions of oxidation, the mechanism and products of the reaction can be changed. 

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