Hydroxyl radical, attack

Only 20—40% of the HNO is converted ia the reactor to nitroparaffins. The remaining HNO produces mainly nitrogen oxides (and mainly NO) and acts primarily as an oxidising agent. Conversions of HNO to nitroparaffins are up to about 20% when methane is nitrated. Conversions are, however, often ia the 36—40% range for nitrations of propane and / -butane. These differences ia HNO conversions are explained by the types of C—H bonds ia the paraffins. Only primary C—H bonds exist ia methane and ethane. In propane and / -butane, both primary and secondary C—H bonds exist. Secondary C—H bonds are considerably weaker than primary C—H bonds. The kinetics of reaction 6 (a desired reaction for production of nitroparaffins) are hence considerably higher for both propane and / -butane as compared to methane and ethane. Experimental results also iadicate for propane nitration that more 2-nitropropane [79-46-9] is produced than 1-nitropropane [108-03-2]. Obviously the hydroxyl radical attacks the secondary bonds preferentially even though there are more primary bonds than secondary bonds. 

In an unusual example of displacement of fluonne by hydroxyl, hydroxyl radicals attack fluorinated benzenes Hexafluorobenzene is the least reactive The hydroxyl radical generates the pentafluorocyclohexadienonyl radical from it [13] (equation 13) These unstable species are detected spectroscopically Their disap-... 

Slivka, A. and Cohen, G. (1985). Hydroxyl radical attack on dopamine. J. Biol. Chem. 260, 15466-15472. 

Aqueous alkaline luminol solutions can be excited to chemiluminescence by pulse radiolysis, the only additional requirement being oxygen 119h The suggested mechanism is that hydroxyl radicals attacking luminol monoanions, followed by reaction of the luminol radical anion thus formed with oxygen ... 

Y. Z. He, W. G. Mallard, and W. Tsang, Kinetics of hydrogen and hydroxyl radical attack on phenol at high temperatures,/. Phys. Chem. 92,2196-2201 (1988). 

In one of the earlier reports hydroxyl radical footprinting was employed to analyze the interaction of distamycin and actinomycin with the 5s ribosomal RNA genes of Xenopus (Churchill et al, 1990). The two drugs showed different hydroxyl radical footprints. Distamycin gives a conventional (albeit high-resolution) footprint, while actinomycin does not protect DNA from hydroxyl radical attack, but instead induces... 

Hydroxyl radicals attack the C4 position of the sugar resulting in sugar decomposition and phosphodiester cleavage. However, if nucleotides are protected... 

Bunce, N.J., Nakai, J.S., and Yawching, M. A model for estimating the rate of chemical transformation of a VOC in the troposphere by two pathways photolysis by sunlight and hydroxyl radical attack, Chemosphere, 22(3/4) 305-315, 1991. 

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). 

Rate Constants for Hydroxyl Radical Attack on Aromatic Compounds... 

The QSAR models for phenols using resonance constants are apparently more accurate than QSAR models using Hammett constants. The R2 values for QSAR models using resonance constants, oresonancer and Hammett constants, a, were 0.9492 and 0.5473, respectively. The R2 value demonstrates that the QSAR model using resonance constants has a better fit. Figure 5.25 shows the reaction pathway for hydroxyl radical attack on phenols. 

The reaction mechanism for the reaction of substituted benzenes with hydroxyl radical appears to be hydroxylation. Hydroxyl radical attack on... 

To evaluate the effect of the number of chlorines on the degradation rate constants of different chlorophenols, Table 6.2 shows the rate constants of elementary, oxidation, and dechlorination for the ratios of k2 CP/k2/l DcP and 2,4,6-tcp/ 2,4-dcp The relative rate constants are plotted against the number of sites unoccupied by chlorine atoms on the chlorinated phenols in Figure 6.3.A linear correlation between the rate constants and the number of sites available is found with a standard deviation of 0.132. Clearly, the more chlorine atoms the aromatic rings contain, the fewer sites are available for hydroxyl radical attack however, the correlation should not be used for... 

Using the developed model, the k values for 2-CP, 3-CP, and 4-CP are 1.12 x 107, 1.004 x 109, and 1.005 x 108 (1/s), respectively therefore, the dechlorination constants for monochlorophenols follow a decreasing order 3-CP > 4-CP > 2-CP. Because chloride ion can be released only after the rupture of the aromatic ring, the faster the hydroxylation of the parent compounds, the faster the dechlorination process should be. Therefore, the above order can be understood in terms of the effect of the substituents on the reactivity of their parent compounds. It is known that both OH and Cl are ortho and para directors. Under the influence of these directors, the following preference of hydroxyl radical attack is expected ... 

The first step in this sequence is the hydroxyl radical attack on chlorobenzene (reaction 2), which likely results in the formation of chlorohydroxycy-clohexadienyl (C1HCD) radical I (Dorfman et al., 1962). This may initiate one of several possible further reactions. In the absence of strong oxidants, two predominant reactions are dimerization, to produce dichlorobiphenyls (reaction 3), and bimolecular disproportionation, to produce chlorophenol and chlorobenzene (reaction 4). Both reactions showed the stoichiometry of 2 mol... 

Three different degradation mechanisms were proposed. In the first mechanism, the hydroxyl radical attacks atrazine by hydrogen abstraction from the secondary carbon of the ethylamino side chain, producing a free radical as shown in Equation (6.135). 

Figure 9.15 plots the kinetics of Ch formation by hydroxyl radical attack on chlorinated phenols. It shows that the kinetic rate constants of Ch formation are linear with respect to o.,.. This is consistent with the findings of D Oliviera et al. (1993) on the study of photodegradation of dichlorophenols... 

The plot shown in Figure 9.18 for the kinetics of COz formation from hydroxyl radical attack on chlorinated phenols appears to be random. This can be justified by the various intermediates formed along the reaction pathway. Each of these intermediates will have different degradation rates, so COz formation will be different according the reaction. 

Figure 9.25 shows the correlation coefficient of 0.9636 between the hydroxyl radical rate constants and UV/Ti02 rate constants for chlorinated alkanes. Therefore, the reaction mechanism for the degradation of chlorinated alkanes by UV/Ti02 is similar to the reaction with hydroxyl radical. This correlation suggests that the reaction proceeds via hydroxyl radical attack on the chlorinated alkane. 

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