Of free-radical reactions

When a substance is heated, the kinetic energy of atoms and molecules increases. E.g., if methane is heated, the kinetic energy of translation, vibration and rotation of methane molecules increases, as discussed in section 1.2. As heat is applied, higher vibrational states are increasingly populated. In higher vibrational quantum states, the average C-H bond distance increases until finally the C-H bond breaks. The result is the formation of a methyl radical and a hydrogen atom. 

The enthalpy of this reaction 25 °C) amounts to about 435 Umol .  

The energy of 435 kJ moC is rather high and reveals that the C-H bond in methane is quite strong. A temperature of about KXX) °C is needed to achieve a significant rate. The enthalpy of the C-H bond in ethane is somewhat less and amounts to 418.4 kJmoC for the C-C bond breakage, A// amounts to only 368 kJmol Thus, when ethane is heated, the C-C bond rupture is somewhat faster and the reaction is usually carried out at 700 °C. 

It can be generally taken that in pyrolysis the bond rupture takes place on the weakest bond. Free radicals thus formed contain an odd number of electrons. At room temperature, alkyl radicals can exist only in small concentrations nevertheless, they have been identified by different spectroscopic methods. 

Free radicals are very important primarily in organic reactions, but also in inorganic reactions. 

Organic chemists are interested in the reactions of compounds, which yield products that may be isolated and purified easily. Free radical intermediates are generally involved in reactions carried out at high temperatures, under the influence of Ught, or in the presence of free-radical generators. Even at room temperature many reactions proceed by a free-radical mechanism, particularly in nonpolar solvents in the presence of a radical producer. A radical has a choice, it can find another radical and combine to form a spin-paired molecule or it can react with a spin-paired molecule to form a new radical both are possible. A third alternative is 

Till now, we have seen that radicals can be considered to be either nucleophilic (reacting fastest with electron-poor alkenes) or electrophilic (reacting fastest with electron-rich alkenes). These tendencies can be nicely rationahzed in terms of frontier molecular orbital (FMO) theory. Recall the key ideas of this theory  

1) The reactions are similar whether they take place in the vapor or liquid phase, though solvation of free radicals in solution does cause some differences. 

2) Acids, bases, and polar solvents have almost no effect on radical reactions. 

3) Free-radical initiators are necessary. The most frequentiy used radical initiators are AIBN and dibenzoyl peroxide. 

The reactivity of a free radical can be defined by the rate constants of its reactions with other molecules and other free radicals. In general this reactivity depends on the extent of localization of the unpaired electron. When it is highly localized on a single atom, as in the methyl radical for example, this site is highly reactive. However, delocalization of the unpaired electron over aromatic rings reduces the reactivity to the point where some free radicals can be kept virtually for ever in the form of stable, unreactive samples. The triphenylmethyl radical is the best known example. 

The formation and reactions of guanine radicals in DNA and their reactions in the presence of carbon-centered radicals derived from lipid molecules provide instructive examples of free radical reactions in solutions involving these biologically relevant species. The reactivities of free radicals derived from biomolecules depend on their structures. The carbon-centered radicals produced by either by hydrogen atom abstraction or the addition of oxyl radicals to double bonds of polyunsaturated fatty acids (PUFAs) are primary intermediates of lipid peroxidation 

The C-centered radicals of nucleic acid bases generated via hydrogen atom abstraction and addition of hydroxyl or other oxyl radicals to the double bonds also decay rapidly via bimolecular reactions with molecular oxygen. For instance, the rate constant for the reaction of 02 with the C-centered 5-(2 -deoxyuridinyl) methyl radical (U- CH2) derived by hydrogen atom abstraction from the methyl group of thymidine was estimated as 2.2 x 109 M 1 s 1 [61]. The radicals produced by the addition of OH radicals to the C8 position of dG (8-1 IO-dC) also rapidly 

A free radical, or univalent atom, is a chemical system like any other x and ri can be found for it, and Table 3.11 shows a listing of such data for a number of important radicals. The acid-base character of free radicals has been recognized for some time. It is common to speak of electrophilic radicals, such as Cl, and nucleophilic radicals, such as (CH3)C. Table 3.11 is a quantitative ordering of these descriptions. The alkali metal atoms could also be added to the list. These would be the most nucleophilic, or best electron donors. 

The calculated values of AA are also listed and are nearly in the same order. 

H and OH react somewhat faster than expected. The more electrophilic radicals 

Gas-phase reactions at 437 K. Data as given in Reference 27, except for NCO, Reference 31. 

We can reverse the order of reactivity of the radicals by going to a reactant which is more electronegative than C2H4. A good example is O2, to which most radicals add, with x 6.3 eV 

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Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

SECTION 12.7. REARRANGEMENT AND FRAGMENTATION REACTIONS OF FREE RADICALS... 

Rearrangement and Fragmentation Reactions of Free Radicals 12.7.1. Rearrangement Reactions... 

The kinetics of reaction of free radical chain reactions are complicated compared to the second-order kinetics of epoxy and urethane adhesives. Many of these complications offer practical advantages to the process of using acrylic adhesives. 

Lapin [33] suggested that photoinitiated cationic polymerization can proceed through reactions of free radicals, as shown below for benzophenone sensitized photoinitiation ... 

The reactions of free radicals in ethylene are well known (5, 24), and the applicability of this information to radiation chemistry has been demonstrated (32). Kinetic analysis of radical reactions revealed that the formation of methyl radicals and methane cannot be ascribed to excited precursors alone (32, 37). 

Chemical interactions also occur in the condensed phases. Some of these are expected to be quite complex, e.g., the reactions of free radicals on the surfaces of or within aerosol particles. Simpler sorts of interactions also exist. Perhaps the best understood is the acid-base relationship of NH3 with strong acids in aerosol particles and in liquid water (see Chapter 16). Often, the main strong acid in the atmosphere is H2SO4, and one may consider the nature of the system consisting of H2O (liquid), NH3, H2SO4, and CO2 under realistic atmospheric conditions. Carbon dioxide is not usually important to the acidity of atmospheric liquid water (Charlson and Rodhe, 1982) the dominant effects are due to NH3 and H2SO4. The sensitivity the pH of cloud (or rainwater produced from it) to NH3 and... 

Reactions of free radicals either give stable products (termination reactions) or lead to other radicals, which themselves must usually react further (propagation reactions). The most common termination reactions are simple combinations of similar or different radicals ... 

The time-dependent nature of migration and chemical reaction of free radicals [30] in irradiated polymers can play an important role in altering the polymer structure and properties, e.g., cross-link formation via reactive sites or chain scission, or postirradiation oxidative influences (irradiation in presence of air or oxygen). 

A characteristic reaction of free radicals is the bimolecular self-reaction which, in many cases, proceeds at the diffusion-controlled limit or close to it, although the reversible coupling of free radicals in solution to yield diamagnetic dimers has been found to be a common feature of several classes of relatively stable organic radicals. Unfortunatly, only the rate constants for self-termination of (CH3)jCSO (6 x 10 M s at 173 K) and (CH3CH2)2NS0 (1.1 X 10 M s at 163K) have been measured up to date by kinetic ESR spectroscopy and consequently not many mechanistic conclusions can be reached. 

Although NMR spectrometers of operating frequencies > 400 MHz are cosdy and require specialist technical support staff, the technique provides a broad picture of the chemical modifications arising from the reactions of free radicals or related oxidants in complex, multicomponent systems such as intact biofluids, tissue sample... 

Class II second-order rate expressions are one of the most common forms one encounters in the laboratory. They include the gas phase reaction of molecular hydrogen and iodine (H2 + I2 -> 2HI), the reactions of free radicals with molecules (e.g., H -f Br2 -> HBr -f Br), and the hydrolysis of organic esters in nonaqueous media. 

A particularly interesting question which remains unanswered is whether dinuclear photoproducts are produced directly from the photoexcited parent molecule or whether they are formed by reaction of free radicals within the solvent cage. In principle this question can be answered by making time-resolved IR measurements on the molecules in the gas phase, where no solvent cage can interfere. Thus, it may transpire that a full understanding of the photolysis of these dinuclear compounds will require complementary experiments in solution and in the gas phase. 

Free-Valence Persistence in Reactions of Free Radicals with Molecules... 

Alkyl radicals react in solution very rapidly. The rate of their disappearance is limited by the frequency of their encounters. This situation is known as microscopic diffusion control or encounter control, when the measured rate is almost exactly equal to the rate of diffusion [230]. The rate of diffusion-controlled reaction of free radical disappearance is the following (the stoichiometric coefficient of reaction is two [233]) ... 

Hydrogen bromide was found to initiate the autoxidation of decane [197]. The reactions of free radicals generation are the following ... 

Bolland and Gee [5] proposed the following reaction of free radical initiation in oxidized hydrocarbon as the most probable ... 

REACTIONS OF FREE RADICAL GENERATION BY HYDROPEROXIDES 4.3.1 Unimolecular Decomposition of Hydroperoxides... 

Kinetic Parameters of the Bimolecular Reactions of Free Radical Generation Calculated by the IPM Method [116]... 

We can see that the rate of initiation increases during autoxidation from 10 7 to 10 4 mol L 1 s-1 (about 1000 times). Due to increasing concentrations of 2-methylbutyl alcohol and acetic acid, the latter becomes a very important reactant in the reactions of free radical generation. 

The chain mechanism is complicated when two hydrocarbons are oxidized simultaneously. Russell and Williamson [1,2] performed the first experiments on the co-oxidation of hydrocarbons with ethers. The theory of these reactions is close to that for the reaction of free radical copolymerization [3] and was developed by several researchers [4-9], When one hydrocarbon R H is oxidized in the liquid phase at a sufficiently high dioxygen pressure chain propagation is limited only by one reaction, namely, R OO + R H. For the co-oxidation of two hydrocarbons R1 and R2H, four propagation reactions are important, viz,... 

Experimental data on the substitution reactions of free radicals with peroxides were analyzed by the IPM method [64]. The calculated parameters are collected in Table 6.27. The activation energies and the rate constants of radical substitution reactions calculated by the IPM method are presented in Table 6.28. 

The reaction of the peroxyl radical with the sulfonyl radical was studied by pulse photolysis technique [38]. Both radicals were generated photochemically by a light pulse (A = 270 -380 nm) in the system DBP-cyc/o-CgHnSCkCI ryc/o-CgH lRH) air (T 293 K). The reactions of free radical formation were the following ... 

Diffusion of particles in the polymer matrix occurs much more slowly than in liquids. Since the rate constant of a diffusionally controlled bimolecular reaction depends on the viscosity, the rate constants of such reactions depend on the molecular mobility of a polymer matrix (see monographs [1-4]). These rapid reactions occur in the polymer matrix much more slowly than in the liquid. For example, recombination and disproportionation reactions of free radicals occur rapidly, and their rate is limited by the rate of the reactant encounter. The reaction with sufficient activation energy is not limited by diffusion. Hence, one can expect that the rate constant of such a reaction will be the same in the liquid and solid polymer matrix. Indeed, the process of a bimolecular reaction in the liquid or solid phase occurs in accordance with the following general scheme [4,5] ... 

Therefore the problem of searching retardants for these chain reactions of free radicals is critical. For instance, it is known that sulfur-containing amino acid (cysteine) attracts unpaired electrons of protein [2,3], Similar properties are reported about selenium, the element of the same subgroup Vl-a of the System as sulfur [4],... 

Atoms of S and Se can sufficiently structurally influence fragments of CH3 that are frequently located on the ends of hydrocarbon chains or in the form of free radicals. The data given confirm high reactivity of sulfur and selenium atoms as retardants of chain reactions of free radicals as elements drawing back impaired valence electrons of free radicals, but at the same time preserving the basic structure of hydrocarbon chain. 

Describe the principal unimolecular and bimolecular reactions of free radicals and explain the usefulness of electron spin resonance in detecting radical species. 

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. 

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