Aromatic compounds, radiolysis

In the photolysis of vitreous matrices containing some additives of aromatic compounds, as well as in the radiolysis of vitreous matrices both with and without these additives, it is frequently possible to observe a luminescence after the irradiation is over [49U57, 123 125]. As a rule, the luminescence lasts for a long time and its intensity varies with time according to the Debye-Edwards law [49]... 

Because hydroxyl radicals have indiscriminate reactivity, they can react with almost all types of organic and inorganic compounds. Most aromatic compounds undergo radical attack on the aromatic ring in a manner similar to that of benzene systems. The products and the rate constants for hydroxyl radical attack on aromatic compounds are listed in Table 5.11. The data were obtained from the pulse radiolysis studies (Buxton et al., 1988). 

Aromatic compounds are especially stable and are, therefore, important persistent pollutants. They include the polyaromatic hydrocarbons (PAHs), and may be halogenated, such as the polychlorinated biphenyls (PCBs) and many pesticides. Also included are the substituted benzenes, such as phenol. A large body of literature has examined aromatic radiation chemistry [22— 38]. The discussion that follows examines benzene and substituted benzenes as a model for the radiolysis of more complicated aromatic compounds. 

Neta P, Dorfman LM. Pulse radiolysis studies. XIII. Rate constants for the reaction of hydroxyl radicals with aromatic compounds in aqueous solutions. Adv Chem Ser 81. Washington, DC American Chemical Society, 1968 222-230. 

As is known, the estimation of G values of water decomposition products can be done by pulse radiolysis techniques or steady state radiolysis with product analysis methods. For pulse radiolysis, although a direct measurement of these transient species is desirable, it is difficult to be effectuated by nanosecond pulse radiolysis because of the acceleration of spur reactions and/or the limitation of detection techniques (e.g. the absorption of OH radical is in deep UV with a rather small absorption coefficient). One is forced to adopt the scavenging method, that is, to use a chemical additive to react with the transient species and form another easy-to-detect and relatively stable product. In this section, we mainly introduce the estimation of G values by pulse radiolysis, with the support by y-radiolysis of some aromatic compounds. 

Figure 6 shows the experimental results of 7 radiolysis of benzophenone from room temperature to 400°C, in deaerated solutions or N2O saturated solutions. Apparently, the tendency of the temperature-dependent yields of benzophenone decomposition and various products formation [Fig. 6(a)], as well as the water density effects on the yields at 400°C [Fig. 6(b)], are also similar to those of G(e q) and G. The studies on other aromatic compounds such as phenol and benzene gave also similar results. All these strongly imply that the decomposition of the aromatic compounds reflects the yields of water decomposition under irradiation. However, it should... 

The products of the radiolysis of butyne-2, propyne, pentyne-2, hexyne-3, and butyne-1, have been determined by Rondeau et Dimers, trimers and tetra-mers, in decreasing importance, are the most significant but many minor products including aromatic compounds, are observed. A free-radical mechanism is suggested. 

The radiolysis of other aromatic compounds is not understood with any more certainty than that of benzene and is sometimes complicated by additional sites for reaction. The reader is referred to the following publications toluene, Cooper and Thomas S , Weiss and Collins ethylbenzene, Hentz and Burton isopropylbenzene, Hentz , Sworski et t-butylbenzene, Sworski et stilbenes, Caldwell et... 

Aromatic compounds frequently protect other more radiosensitive compounds from radiolysis. For example, liquid cyclohexane is protected from extensive radiolysis by the addition of a small amount of benzene. This is probably due to energy transfer from cyclohexane to benzene, followed by dissipation of the excitation energy by the aromatic Ti-system. 

The irradiation of aromatic compounds results in considerably lower yields of radiolysis products than does irradiation of aliphatic compounds of similar molecular weight and functional group composition. This has been attributed to effectiveness of the delocalized 7t-orbitals in accommodating excitation energy without permitting the molecule to dissociate. Nevertheless, some radiolysis does occur. Benzene is known to yield biphenyl, phenylcyclohexadiene, and a polymeric material of average composition (C6H7) which behaves as if it were an unsaturated hydrocarbon. Dimerization and polymer formation are also characteristic of the radiation... 

However, hydroxyl radicals are very reactive and known to react with aromatic compounds not only by electron abstraction but also by adding to the ring. Well-established, one-electron oxidizing radicals such as Br2 and N3, formed by pulsing a nitrous oxide saturated solution of potassium bromide or sodium azide, are used to produce cation radicals of the drug molecules. For example, the reactions that take place when a nitrous oxide aqueous solution of lO2 M potassium bromide in the presence of 10 4 M chlorpromazine (C1P) is subjected to pulse radiolysis (Asmus et al., 1979 Davies et al., 1979) are given below ... 

Absolute rate constants for electron transfer reactions of aromatic molecules in solution have been determined by the pulse radiolysis method for three additional pairs of aromatic compounds. In two of these cases in which an electron transfer equilibrium is established, the rate constant for the back reaction has also been determined. The equilibrium constant has been estimated from the kinetic data. A correlation of the experimental rate constants with the theory for homogeneous electron transfer rates is considered. 

A bsolute rate constants for electron transfer reactions of some aromatic molecules in solution have been reported in our earlier work (2) using the pulse radiolysis method. The transfer of an electron from various radical anions to a second aromatic compound in solution was observed directly. Of the rate constants for nine donor-acceptor pairs investigated, two were found to be lower than the diffusion controlled values, and a correlation with such parameters as the reduction potential difference of the pair was considered. These measurements have been extended to additional transfer pairs for which the reduction potential difference is small. The objective of this work, in addition to furnishing new data for electron transfer rates, is to provide an adequate test of theories of the rate of homogeneous electron transfer in polar liquids (10, 11,12,13, 14, 15,16,17). 

Pulse Radiolysis Studies. XIII. Rate Constants for the Reaction of Hydroxyl Radicals with Aromatic Compounds in Aqueous Solutions... 

Numerous synthetic, technical and natural processes involve formation of aromatic radical cations as reaction intermediates. [1,2] In general, aromatic radical cations are formed upon ionization, e.g., by direct photolysis or radiolysis, or one-electron oxidation of aromatic compounds (reaction 1). 

Labeled compounds experience self-radiolysis induced by the radioactive decay. The extent of such radiation effects depends on the half-life, the decay energy, the specific activity of the sample, and the G-value for decomposition. The presence of other substances can considerably affect the amoimt of damage. Aromatic compounds such as benzene (as a solvent) can serve as a protective medium to minimize radiation self-decomposition, whereas water or oxygen enhance it. 

In addition to main chain scission or cross-linking, gas formation is also observed as a result of irradiation. The gas in the hydrocarbon-based polymers mostly consists of hydrogen. The amount of gas produced depends on the nature of the polymer and also on dose, temperature, type of radiation, etc. In the case of polyethylene the G-value of gas production is high, G 0.32 pmol comparable to the gas yields observed in the radiolysis of hquid n-alkanes G 0.5-0.6 pmol In the radiolysis of polystyrene and polymethylstyrene the yield of gaseous products is only G 0.01 pmol 1 , that value is typical of aromatic compounds. The benzene rings attached to the main chain exert a protective effect against both the C-H and the C-C decompositions in the chain. 

Anion radicals of aromatic hydrocarbons are known to undergo protonation in protic media (1-5). The spectrophotometric pulse radiolysis technique has been used to demonstrate this process in alcoholic solutions of several aromatic compounds ( ). In such experiments, the anion radicals are produced by the reaction of the hydrocarbon with the solvated electron k. 

Radiolysis and photodecomposition of chlorite and chlorate have been studied in aqueous solution. Reaction schemes, rate constants, and quantum yields are reported.The first stage in the reaction of chlorine with phenol is the second-order formation of [CI2 PhOH], with a rate constant 2 = 2.3 x 10 lmol- s at 5 °C. This [Clg-PhOH] intermediate, whose properties are similar to those of known charge-transfer compounds of halogens with aromatic compounds, decomposes by three parallel pathways. Chlorine reacts more slowly with chlorophenols than with phenol itself. The reaction of chlorine dioxide with phenol follows a comparable rate law ... 

Physical properties of carbon black-filled EPR and EPDM elastomers have been found to be comparable with the suUur-cured analogues [372]. Aromatic oils increase the optimum dose requirement for these compounds due to the reaction of the transient intermediates formed during radiolysis of the polymer with the oil as well as energy transfer which is particularly effective when the oil contains aromatic groups. The performance and oxidative stability of unfilled EPDM as well as its blend with PE [373], and the thermal stabdity and radiation-initiated oxidation of EPR compounds are reported by a number of workers [374,375]. 

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