Radicals, in chain reactions

Photochemically generated radicals in chain reactions are less familiar to synthetic chemists [8,21]. The above mentioned peroxides have been used in the presence of light to initiate radical chain reactions at room or lower temperatures. Azo compounds are also known to decompose photo-lytically to afford alkyl radicals. AIBN has rarely been used under such conditions. 

The method is applied for measuring the lifetime of active centers (atoms and radicals) in chain reactions with square chain termination. The method is based on the periodical initiation with light of formation of active centers and the change in the duration of light and dark periods from experiment to experiment. When the duration of the dark period is shorter than the lifetime of the active center (the sector is rapidly rotated), the reaction occurs rapidly, as under continuous initiation, with the rate (1 + r) , where r is the ratio of the dark to light periods, and the rate is f V/ (l + r) At a long duration of the dark period (the sector rotates slowly), the reaction occurs only in irradiation periods, and the average rate is Vj v/ (l + r)... 

The individual steps in chain reactions involving radicals are characteristically of small activation energy, between about 10 and 50kJmol and so these reactions should occur at an immeasurably high rate at temperatures above 500 K (see Table 2.1), which is a low temperature for a useful combustion process. The overall rate of the process will tlrerefore depend mainly on the concentrations of tire radicals. 

Atoms and free radicals are highly reactive intermediates in the reaction mechanism and therefore play active roles. They are highly reactive because of their incomplete electron shells and are often able to react with stable molecules at ordinary temperatures. They produce new atoms and radicals that result in other reactions. As a consequence of their high reactivity, atoms and free radicals are present in reaction systems only at very low concentrations. They are often involved in reactions known as chain reactions. The reaction mechanisms involving the conversion of reactants to products can be a sequence of elementary steps. The intermediate steps disappear and only stable product molecules remain once these sequences are completed. These types of reactions are refeiTcd to as open sequence reactions because an active center is not reproduced in any other step of the sequence. There are no closed reaction cycles where a product of one elementary reaction is fed back to react with another species. Reversible reactions of the type A -i- B C -i- D are known as open sequence mechanisms. The chain reactions are classified as a closed sequence in which an active center is reproduced so that a cyclic reaction pattern is set up. In chain reaction mechanisms, one of the reaction intermediates is regenerated during one step of the reaction. This is then fed back to an earlier stage to react with other species so that a closed loop or... 

The allylic position of olefins is subject to attack by free radicals with the consequent formation of stable allylic free radicals. This fact is utilized in many substitution reactions at the allylic position (cf. Chapter 6, Section III). The procedure given here employs f-butyl perbenzoate, which reacts with cuprous ion to liberate /-butoxy radical, the chain reaction initiator. The outcome of the reaction, which has general applicability, is the introduction of a benzoyloxy group in the allylic position. 

Termination Occasionally, two radicals might collide and combine to form a stable product. When that happens, the reaction cycle is broken and the chain is ended. Such termination steps occur infrequently, however, because the concentration of radicals in the reaction at any given moment is very small. Thus, the likelihood that two radicals will collide is also small. 

There is less information available in the scientific literature on the influence of forced oscillations in the control variables in polymerization reactions. A decade ago two independent theoretical studies appeared which considered the effect of periodic operation on a free radically initiated chain reaction in a well mixed isothermal reactor. Ray (11) examined a reaction mechanism with and without chain transfer to monomer. 

In chain reactions the addition of vinyl monomers to a free radical can occur in one of two ways (Reaction 2.8). 

At room temperature under photostimulation a-nitrosulfones react with a variety of nucleophiles via radical anion chain reactions interestingly, in none of the cases where the PhSOj group is involved in SrnI type of substitution does the O end of the ambident anion " play a role. This strong regioselectivity is reminiscent of the one reported for other ambident anions involved in these radical chain substitutions. ... 

Lipid peroxidation is a radical-mediated chain reaction resulting in the degradation of polyunsaturated fatty acids (PUFAs) that contain more than two covalent carbon-carbon double bonds (reviewed by Esterbauer et al., 1992). One of the major carriers of plasma lipids is LDL, a spherical molecule with a molecular weight of 2.5x10 . A single LDL particle contains 1300 PUFA molecules (2700 total fatty-acid molecules) and is... 

Catalysts that decrease reaction rates are usually referred to as inhibitors. They usually act by interfering with the free radical processes involved in chain reactions, and the mechanism differs from that involved in accelerating a reaction. The most familiar example of the use of inhibitors is the addition of tetraethyl lead to gasoline to improve its antiknock properties. 

Gas-phase radiolysis can sometimes result in chain reactions involving H atoms or other radicals. As in other cases with chain reactions, termination is due to either recombination or reaction with other radicals. Typical chain length is -1000 or more. Some specific examples will be considered in Sect. 5.2. 

A different situation in the oxidation of these two alcohols is seen. The hydroperoxyl radical is the main chain propagating species in oxidized 2-propanol the portion of alkylhydroxy-peroxyl radicals in this reaction is less than 2.5%. In oxidized cyclohexanol, on the contrary, the stationary state concentrations of both radicals are close and both of them take important part in chain propagation. 

We see that the participation of the inhibitor radical in chain propagation decreases the reaction order of the initiation rate, i.e., initiator diminishes the dependence of the reaction rate on the inhibitor concentration. The retardation of oxidation can be performed with the antioxidant concentration [InH] > 2k(,k 10[R H]/fk7kH. The kinetics of chain oxidation obeys the equation (for t[Pg.494]

Radicals are also formed in solution by the decomposition of other radicals, which are not always carbon free radicals, and by removal of hydrogen atoms from solvent molecules. Because radicals are usually uncharged, the rates and equilibria of radical reactions are usually less affected by changes in solvent than are those of polar reactions. If new radicals are being made from the solvent by hydrogen abstraction, and if the new radicals participate in chain reactions, this may not be true of course. But even in cases of non-chain radical reactions in which no radicals actually derived from the solvent take part in a rate-determining step, the indifference of the solvent has perhaps been overemphasized. This will be discussed more fully when radical and polar reactions are compared in Chapter XII. 

Reviewing now the last four sections, it is obvious that the major problem in radical chain reactions involving dienes or polyenes is the low reactivity of the diene (or polyene) adduct radicals. This allows for the occurrence of allyl radicals in intramolecular reactions but poses a major problem in intermolecular radical chain reactions. The obvious solution to this problem is to use methods in which radicals are produced stoichiometrically and not... 

In chain reactions, the inhibitor 7n interacts with free radical P and makes it unavailable for the propagation of chain, thus providing an extra termination step... 

In designing preparative radical ionic chain reactions, including the fragmentation approach to alkene radical cations, careful choice of the radical... 

In stepwise reactions, all functional groups take part in bond formation. Their reactivity can be considered independent of the size and shape of the molecules or substructures they are bound to (Flory principle). If such a dependence exists, it is mainly due to steric hindrance. In chain reactions only activated sites participate in bond formation if propagation is fast relative to initiation, transfer and termination, long multifunctional chains are already formed at the beginning of the reaction and they remain dissolved in the monomer. Free-radical copolymerization of mono- and polyunsaturated monomers can serve as an example. The primary chains can carry a number of pendant C=C double bonds... 

In chain reactions involving three termination steps (two uncrossed and one crossed) the quantity = 1c /(1c 1c )1/2 is frequently used to interrelate the cross-termination constant with the two uncrossed termination constants. For many different types of radical < is found to be about 1 (or alternatively, if the statistical factor of 2 favoring the crossed termination process is ignored in the definition of the rate constants, < 2). In the present reaction system —3-6, in agreement with the value obtained by Russell at 90°C. (26). The crossed termination constant itself is somewhat less than half the value found for kt. This seems reasonable since only one hydrogen atom will be available for transfer in the crossed termination, compared with the two that are available in the self-reaction of two tetralylperoxy radicals. In addition, steric hindrance to reaction should be greater for the crossed termination than for Reaction 8. The products are presumably cumyl alcohol, a-tetralone [3,4-dihydro-l(2H)naphthalenone], and oxygen (28). 

I. Additions. Radicals can react with anionic species to give radical anion adducts as shown for radical 11. Such addition reactions are steps in chain reaction processes described as SrnI (unimolecular radical nucleophilic substitution) reac-... 

Antioxidants are very effective in stabilizing products undergoing a free-radical mediated chain reaction. These products possess lower oxidation potentials than the active drug. Ideally, antioxidants are stable over a wide pH range and remain soluble in the oxidized form, colorless, and nontoxic. A listing of commonly used antioxidants can be found in Table 3. 

The development of mass spectrometric ionization methods at atmospheric pressures (API), such as the atmospheric pressure chemical ionization (APCI)99 and the electrospray ionization mass spectrometry (ESI-MS)100 has made it possible to study liquid-phase solutions by mass spectrometry. Electrospray ionization mass spectrometry coupled to a micro-reactor was used to investigate radical cation chain reaction is solution101. The tris (p-bromophenyl)aminium hexachloro antimonate mediated [2 + 2] cycloaddition of trans-anethole to give l,2-bis(4-methoxyphenyl)-3,4-dimethylcyclobutane was investigated and the transient intermediates 9 + and 10 + were detected and characterized directly in the reacting solution. However, steady state conditions are necessary for the detection of reactive intermediates and therefore it is crucial that the reaction must not be complete at the moment of electrospray ionization to be able to detect the intermediates. 

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