Man Nien Schuchmann

HO2 ELIMINATION FROM α-HYDROXYALKYLPEROXYL RADICALS IN AQUEOUS SOLUTION

Abstract— In aqueous solutions α‐hydroxyalkylperoxyl radicals undergo a spontaneous and a base catalysed HO2 elimination. From kinetic deuterium isotope effects, temperature dependence, and the influence of solvent polarity it was concluded that the spontaneous reaction occurs via an HO2 elimination followed by the dissociation of the latter into H+ and O2‐. The rate constant of the spontaneous HO2 elimination increases with increasing methyl substitution in α‐position (k(CH2(OH)O2) < 10s‐1k(CH3CH(OH)O2) = 52s‐1k((CH3)2C(OH)O2) = 665 s‐1). The OH‐ catalysed reaction is somewhat below diffusion controlled. The mixture of peroxyl radicals derived from polyhydric alcohols eliminate HO2 at two different rates. Possible reasons for this behaviour are discussed. The mixture of the six peroxyl radicals derived from d‐glucose are observed to eliminate HO2 with at least three different rates. The fastest rate is attributed to the HO2 elimination from the peroxyl radical at C‐l (k > 7000s‐1). Because of the HO2 eliminations the peroxyl radicals derived from d‐glucose do not undergo a chain reaction in contrast to peroxyl radicals not containing an α‐OH group. In competition with the first order elimination reactions the α‐hydroxylalkylperoxyl radicals undergo a bimolecular decay. These reactions are briefly discussed.

The photolysis of potassium peroxodisulphate in aqueous solution in the presence of tert-butanol : a simple actinometer for 254 nm radiation

Aqueous potassium peroxodisulphate (0.01 mol dm−3) together with tert-butanol (0.1 mol dm−3) was photolysed with 254 nm light at 20 °C. The peroxodisulphate ion is cleaved, giving rise to sulphate radicals SO4−. These attack the tert-butanol under hydrogen abstraction and hydrogen sulphate formation. In deoxygenated solutions, Φ(H+) = 1.4, Φ(SO42−) = 1.4 and Φ((HOC(CH3)2CH2)2) is approximately 0.7; in the presence of oxygen, Φ(H+) = 1.8. The enhancement of Φ(H+) by oxygen comes about through the production of superoxide via one of the bimolecular decay channels of the tert-butanol-derived peroxyl radical. The superoxide radical reduces S2O82− under liberation of further SO4−. The oxygenated peroxodisulphate-tert-butanol system is shown to constitute a straightforward actinometer for 254 nm light. Some data and observations regarding the tert-butanol-free peroxodisulphate system are also presented.

The fate of peroxyl radicals in aqueous solution

The reactions of peroxyl radicals occupy a central role in oxidative degradation. Under the term Advanced Oxidation Processes in drinking-water and wastewater processing, procedures are summarized that are based on the formation and high reactivity of the OH radical. These react with organic matter (DOC). With O 2 , the resulting carbon-centered radicals O 2 give rise to the corresponding peroxyl radicals. This reaction is irreversible in most cases. An exception is hydroxycyclohexadienyl radicals which are formed from aromatic compounds, where reversibility is observed even at room temperature. Peroxyl radicals with strongly electron-donating substituents eliminate O 2 .− , those with an OH-group in a-position HO 2 . . Otherwise organic peroxyl radicals decay bimolecularly. The tetroxides formed in the first step are very short-lived intermediates and decay by various pathways, leading to molecular products (alcohols, ketones, esters and acids, depending on the precursor), or to oxyl radicals, which either fragment by scission of a neighbouring C-C bond or, when they carry an a-hydrogen, undergo a (water-assisted) 1,2-H-shift.

Oxidation of benzene by the OH radical. A product and pulse radiolysis study in oxygenated aqueous solution

Hydroxyl radicals [generated radiolytically in N2O/O2(4:1 v/v)-saturated aqueous solutions] have been reacted with benzene. The major product is phenol. At low dose rate (γ-radiolysis) it is formed in 53% yield with respect to the OH radical yield. This value increases to 93% in alkaline solution (pH 12.3). With deuteriated benzene it is reduced to 39%. In addition, more than fifteen different, ring-opened and fragment products are formed. A good material balance (based on primary OH radical yield and oxygen consumption) was obtained.At high dose rate (pulse radiolysis) the major products are phenol, hydroquinone and cyclohexa-2,5-diene-1,4-diol. An important intermediate is the HO2˙/O2˙– radical. Its rate of formation (kobsd= 800 s–1) has been followed by pulse radiolysis using tetranitromethane as a scavenger as well as conductimetrically (build-up of H+/O2˙–).The results have been interpreted as follows: in their reaction with benzene, hydroxyl radicals yield the hydroxycyclohexadienyl radical 1. In the presence of oxygen, radical 1 undergoes reversible oxygen addition yielding four different hydroxycyclohexadienylperoxyl radicals: the cis- and trans- isomers of 6-hydroxycyclohexa-2,4-dienylperoxyl radical 3 and the cis- and trans-isomers of 4-hydroxycyclohexa-2,5-dienylperoxyl radical 4. As reported previously, in the equilibrium mixture of the radicals 1, 3 and 4 the concentration of radical 3 represents only a few per cent of the total. It is suggested that 3 eliminates HO2. thereby yielding phenol. In basic solution deprotonation of 4 is followed by an O2˙–-elimination which opens up an additional route to phenol. The fact that phenol formation is not quantitative and its yield is reduced in the case of deuteriated benzene is due to another unimolecular decay route. The competing reaction is the intramolecular addition of the peroxyl radical function to a double bond (and subsequent fragmentation of the ring system). Since the HO2˙-elimination is not very fast, bimolecular decay of the radicals 1, 3 and 4(mainly of 4, 2k= 8.9 × 108 dm3 mol–1 s–1) plays an increasingly important role under the conditions of pulse radiolysis. As a consequence, the hydroquinone and cyclohexa-2,5-diene-1,4-diol yields increase with increasing dose rates under pulse radiolysis conditions (2–25 Gy pulse–1) as those of phenol and HO2˙ decrease.

γ-Radiolysis of DNA in Oxygenated Aqueous Solutions: Alterations at the Sugar Moiety

On gamma-irradiation of DNA in N2O/O2-saturated aqueous solutions alterations at the sugar moiety are observed. In the present study three new lesions were recognized: (i) 2-deoxytetrodialdose bound via a phosphoric acid ester linkage to a (broken) DNA strand, (ii) 2-deoxypentos-4-ulose bound to DNA via one (or two?) phosphoric acid ester linkage(s), and (iii) 2-deoxy-D-erythro-pentose bound to DNA via two phosphoric acid ester linkages. Lesion (i) is directly connected with a DNA strand break. Lesion (ii) might be related to a DNA strand break if bound via only one phosphoric acid ester linkage, or has to be considered as an alkali-labile site if bound via two phosphoric acid ester linkages. Lesion (iii) results from base damage, when the damaged base is hydrolysed from the sugar. This lesion is an alkali-labile site which turns into a strand break on alkali treatment. Attempts have been made to quantify these lesions. A lower limit of sugar damage (including lesions observed in preceding studies, but not lesion (iii) of G = 0.25 has been estimated.

Radiation chemistry of carbohydrates. Part 14. Hydroxyl radical induced oxidation of D-glucose in oxygenated aqueous solution

D-Glucose (5 × 10–3M) has been γ-irradiated in N2O–O2(80 : 20 v/v) saturated aqueous solutions (dose rate 1.1 × 1015 eV g–1 s–1), and the G values of 22 products determined. Major products (G values in parentheses) are : D-gluconic acid (1)(0.9). D-arabino-hexosulose (2)(0.9). D-ribo-hexos-3-ulose (3)(0.57), D-xylo-hexos-4-ulose (4)(0.50), D-xylo-hexos-5-ulose (5)(0.60). D-gluco-hexodialdose (6)(1.55), and L-threo-tetrodialdose (12)(0.20). Products (1)–(6) are thought to be formed by OH attack at C-1–C-6, followed by oxygen addition and and HO2 elimination. In competition, reactions between the glucose peroxyl radicals and HO2·(O2–·) give rise to fragmentation of C–C bonds. The major product from these fragmentation reactions is (12)(precursor. radical at C-5). From a dose rate study using electron pulses from a van de Graaff generator it has been shown that the rates of HO2· elimination are in the order: C-1 C-2, C-3, C-4 > C-6 C-5. Furthermore, it has been calculated that OH radicals abstract a hydrogen atom from C-1 and C-2 with a probability of ca. 20%. from C-3 and C-4 of ca. 10% from C-5 of ca. 15%, and from C-6 ca. 30%.

A common carbanion intermediate in the recombination and proton-catalysed disproportionation of the carboxyl radical anion, CO2*-, in aqueous solution.

The carboxyl radical anion, CO2*- was produced by the reactions of OH radicals with either CO or formic acid in aqueous solution. The pKa(*CO2H) was determined by pulse radiolysis with conductometric detection at pH approximately equals 2.3. The bimolecular decay rate constant of CO2*- (2k approximately equals 1.4 x 10(9) dm3mol(-1)s(-1)) was found to be independent of pH in the range 3-8 at constant ionic strength. The yields of the products of the bimolecular decay of the carboxyl radicals, CO2 and the oxalate anion were found to depend strongly on the pH of the solution with an inflection point at pH 3.8. This pH dependence is explained by assuming a head-to-tail recombination of the CO2*- radicals followed by either rearrangement to oxalate or a protonation of the adduct, which subsequently leads to the formation of CO2 and formate. The recombination of CO2*- to give oxalate directly is estimated to have a contribution of <25%.

Acetate Peroxyl Radicals, ·O2CH2 CO2-: A Study on the γ-Radiolysis and Pulse Radiolysis of Acetate in Oxygenated Aqueous Solutions

Abstract Hydroxyl radicals from the radiolysis of N2O/O2 (4:1 v/v)-saturated aqueous solutions have been reacted with acetate ions (10-2M). As measured by pulse radiolysis, the resulting ·CH2CO2- radicals react with oxygen yielding the corresponding peroxyl radicals, ·O2CH2CO2- (k = 1.7 x 109 M-1s-1). These peroxyl radicals decay bimolecularly (2k = 1.5 x 108 M-1s-1) giving rise to the products (G values in brackets) glyoxylic acid (2.7), glycolic acid (0.7), formaldehyde (1.4), carbon dioxide (1.4), organic hydroperoxide (0.7) and hydrogen peroxide (2.5). Oxygen is consumed with a G value of 5.3. Aided by data from pulse radiolysis it is concluded that the intermediate tetroxide formed upon the bimolecular decay breaks down by various routes to yield: (i) hydrogen peroxide and two molecules of glyoxylic acid (ca. 27%); (ii) oxygen, glycolic acid and glyoxylic acid (ca. 25%); (iii) hydrogen peroxide and two molecules of formaldehyde, carbon dioxide and OH- (25%). These reactions do not involve free radicals as intermediates; (iv) There is some O⨪2 (G ≈ 0.5) formed in the decay of the peroxyl radicals, which is attributed to the decay of intermediate oxyl radicals (tetroxide → O2 + 2 ·OCH2CO2-) by 1,2-H shift, oxygen addition and HO2· elimination, a reaction sequence which gives rise to glyoxylic acid (10%); (v) The reaction of O2⨪ with the organic peroxyl radical yields the hydroperoxide (13%). Reaction (iii) is a novel peroxyl radical reaction.

Hydroxyl Radical-Induced Oxidation of Ethanol in Oxygenated Aqueous Solutions. A Pulse Radiolysis and Product Study

Abstract γ-Radiolysis of N2O-saturated water or photolysis of aqueous H2O2 provided a source of OH radicals. These radicals react with ethanol by preferentially abstracting an H atom at C-1. In the presence of oxygen these radicals are converted into the corresponding peroxyl radicals. The a-hydroxyethylperoxyl radicals decay by first order kinetic(k=k1 + k2 [OH-]) acetaldehyde and HO2˙/H+ + O2 ⨪ being the products (k1 (20 °C) = 50 ± 10 s-1 , Ea = 66 ± 7 kJ·mol-1 , k2= (4± 1) X 109 M-1 s-1). In competition (favoured by low pH, low temperature and high dose rate) they also decay by second order kinetics (2k3 = (7 ± 2) x 108 M-1 s-1). The most important route in the bimolecular decay leads to acetaldehyde, acetic acid and oxygen (ca. 75%). This route might largely be concerted (Russell mechanism), but there might also be a contribution from the disproportionation of oxyl radicals within the solvent cage. There is also a concerted route that leads to two molecules of acetic acid and to hydrogen peroxide (ca. 10%). Another pathway (ca. 15%) yields two oxyl radicals and oxygen. The former may either decompose into formic acid and methyl radicals (ca. 5%) or rearrange into 1,1-dihydroxyethyl radicals (ca. 10%). These radicals add oxygen and the resulting peroxyl radicals rapidly decompose into acetic acid and HO2˙. The reaction of a-hydroxyethylperoxyl radicals with HO2˙/O2⨪ radicals appears to be slow (k≈107 M-1s-1).

H-atom abstraction from thiols by C-centered radicals. A theoretical and experimental study of reaction rates

The Arrhenius parameters and rates of reaction of three hydroxyradicals, methyl radical, and the hindered primary C-centred radical from t-butyl alcohol with dithiothreitol were measured by pulse radiolysis in water. The bimolecular rate constants were found to be in the order: ˙C(CH3)2OH > ˙CH(CH3)OH > ˙CH2OH > ˙CH3 > ˙CH2C(CH3)2OH. The reaction of three of these, ˙C(CH3)2OH, ˙CH2OH, and ˙CH3, with methanethiol were examined at the ab initio B3LYP/6311+G(d,p) level, coupled with transition state theory, both in the gas phase and in solution. The solvent effects are evaluated by two different continuum models (SCIPCM, CPCM), coupled with a novel approach to the calculation of the solution phase entropy. The reaction is discussed in terms of the charge and spin polarization in the transition state, as determined by AIM analysis, and in terms of orbital interaction theory. Rate constants, calculated by transition state theory are in good agreement with the experimental data.