scholarly journals The Reactions of Hydroxyl Radicals with 1,4-and 1,3-Cyclohexadiene in Aqueous Solution

1988 ◽  
Vol 43 (9) ◽  
pp. 1201-1205 ◽  
Author(s):  
Xian-Ming Pan ◽  
Eugenie Bastian ◽  
Clemens von Sonntag
Keyword(s):  
Hydroxyl Radicals ◽  
Pulse Radiolysis ◽  
Hydrogen Abstraction ◽  
Material Balance ◽  
Oh Radicals ◽  
Oh Radical ◽  
Product Analysis ◽  
Γ Irradiation ◽  
Abstraction Reaction ◽  
G Value

Abstract The reactions of radiolytically generated hydroxyl radicals and H atoms with 1,4- and 1,3-cyclohexadiene were studied by pulse radiolysis and product analysis. Hydrogen abstraction from these substrates by the OH radical yields the cyclohexadienyl radical (ε (310 nm) = 4400 dm3 mol-1 cm-1 from the reaction of the H atom with benzene) with an efficiency of 50% (0.29 ,μmol J-1) in the case of 1,4-cyclohexadiene and 25% (0.15 ,μmol J-1) in the case of 1,3-cyclohexadiene as determined by pulse radiolysis. The remaining OH radicals add to the olefin. In 1.4-cyclohexa- diene the yield of the resulting adduct radicals has been determined in a steady-state 60Co-γ-irradiation experiment by reducing it with added 1.4-dithiothreitol (DTT) to 4-hydroxycyc- lohexene. There are two sites of OH radical attack in the case of 1.3-cyclohexadiene, and only the alkyl radical is reduced quantitatively by DTT (G(3-hydroxycyclohexene) = 0.15 ,μmol J-1). From material balance considerations it is concluded that the allylic radical must be formed with a G value of 0.28 ,μmol J-1 but largelv escapes reduction by DTT (G(4-hvdroxycyclohexene) = 0.03 ,μmol J-1). H atoms add preferentially to the double bonds of 1,4- and 1,3-cyclohexadiene (78% and 93%, respectively), while the O.- radical (the basic form of the OH radical) undergoes mainly H- abstraction (92% and 83%, respectively). The radicals formed in these systems decay bimolecularly (2k = 2.8 x 109 dm3 mol-1 s-1). In their combination reactions the cyclohexadienyl radicals form the four possible dimers in propor­tions such that the dienyl radical moiety shows a 2:1 preference to react from its central (1a) rather than from a terminal carbon atom (1b). Cyclohexadienyl radicals and the OH- and H-adduct radicals also cross-tcrminate by disproportionation and dimerization. Material balance has been obtained for the 1,4-cyclohexadiene system in N2O-Saturated solution (10-2 mol dm-3) at a dose rate of 0.14 Gy s-1, the products (G values in ,μmol J-1) being: benzene (0.085), 4-hydroxycyclohexene (0.25), cyclohexadienyl-dimers (0.144). cvclohexadienyl-OH-adduct- dimers (0.02), OH-adduct-dimers (0.02). Some of the 4-hydroxycyclohcxene is formed in an H-abstraction reaction by the OH-adduct radical from 1,4-cyclohexadiene.

β-HydroxyethyIperoxyl Radicals: A Study of the γ-Radiolysis and Pulse Radiolysis of Ethylene in Oxygenated Aqueous Solutions

1984 ◽  
Vol 39 (9) ◽  
pp. 1262-1267 ◽  
Author(s):  
Alicja Piesiak ◽  
Man Nien Schuchmann ◽  
Henryk Zegota ◽  
Clemens von Sonntag
Keyword(s):  
Ethylene Glycol ◽  
Hydroxyl Radicals ◽  
Pulse Radiolysis ◽  
Material Balance ◽  
Peroxyl Radicals ◽  
Oh Radicals ◽  
Rate Determining Step ◽  
Primary Water ◽  
Hydroxyethyl Radicals ◽  
Ethyl Radicals

Hydroxyl radicals (and H atoms) generated in the radiolysis of N2O-containing water add to ethylene forming β-hydroxyethyl radicals (and ethyl radicals). In the presence of oxygen these are converted into the corresponding peroxyl radicals which decay bimolecularly (2k = 2 x 108 M-1s-1) as measured by pulse radiolysis. The major products (G values in brackets) are glycolaldehyde (3.3). formaldehyde (1.6), ethylene glycol (0.8), acetaldehyde (0.5). organic peroxidic material (0.5). and hydrogen peroxide (2.4) (dose rate 0.26 Gy s-1, pH 5.6). A material balance has been obtained with respect to the primary water radicals (6.0) and oxygen uptake (4.8). The products and their yields can be accounted for if in the rate determining step atetroxide is formed which decomposes via three major pathways leading to: (i) H2O2 and two molecules of glycolaldehyde (ca. 45%), (ii) O2, glycolaldehyde and ethylene glycol (ca. 30%). and (iii) O2. two molecules of formaldehyde and two CH2OH radicals (ca. 15%). A 1,2-H shift of intermediate oxyl radicals is unimportant in this system.

Reaktionen von OH-Radikalen mit Acetaldehyd in wäßriger Lösung in Gegenwart von Sauerstoff / Reactions of OH-Radicals with Acetaldehyde in Oxygenated Aqueous Solution

1974 ◽  
Vol 29 (1-2) ◽  
pp. 91-95 ◽  
Author(s):  
Hartmut Schultze ◽  
Dietrich Schulte-Frohlinde
Keyword(s):  
Acetic Acid ◽  
Peracetic Acid ◽  
Oh Radicals ◽  
Oh Radical ◽  
Order Process ◽  
Chain Reaction ◽  
Γ Irradiation ◽  
First Order ◽  
G Value ◽  
A Chain

γ-Irradiation of aqueous acetaldehyde in the presence of N2O and O2 at 0°C leads to the formation of 1.5 molecules of acetic acid per OH radical consumed. As intermediates peroxidic compounds are produced with a half life of about 50 min. H2O2 is not formed.The primary step of the reaction is the interaction of the OH radical with acetyldehyde yielding water and acetyl radicals which add oxygen. The so formed acetylperoxy radicals form peroxidic intermediates which decompose to give oxygen, acetic acid, and peracetic acid. The peracetic acid interacts with additional acetaldehyde forming a peroxide that is converted into two molecules of acetic acid in a first order process.At temperatures above 10°C, a chain reaction takes place leading to acetic acid with a G-value of 40 at 40°C. The propagating species are the acetylperoxy and the acetyl radicals. The reaction steps are addition of oxygen to the acetyl radicals and transfer of hydrogen from acetaldehyde to the acetylperoxy radical leading to peracetic acid. In this reaction no hydrogen peroxide is formed.

Phenylalanine: Its ˙OH and SO4˙⁻-Induced Oxidation and Decarboxylation. A Pulse Radiolysis and Product Analysis Study

1993 ◽  
Vol 48 (6) ◽  
pp. 761-770 ◽  
Author(s):  
Degui Wang ◽  
Heinz-Peter Schuchmann ◽  
Clemens von Sonntag
Keyword(s):  
Radical Cation ◽  
Pulse Radiolysis ◽  
Side Reactions ◽  
Intramolecular Electron Transfer ◽  
Oh Radical ◽  
Product Analysis ◽  
Water Elimination ◽  
Deprotonation Reaction ◽  
Type Radical ◽  
Radical Yield

Phenylamine has been oxidized by radiolytically generated hydroxyl and sulfate radicals, the ensuing intermediates and their reactions have been studied by pulse radiolysis and product analysis in the absence and presence of oxidants such as Fe(CN)63- and O2. Upon OH radical attack, hydroxycyclohexadienyl-type radicals are mainly formed while Η-abstraction reactions can be neglected. In the presence of Fe(CN)63- these radicals are for the most part oxidized to the corresponding tyrosines (80%), except for the ipso-OH-adduct radicals (≈ 20%). It is concluded that ˙OH-addition is almost random, but with a slight avoidance of the metaposition relative to the ortho-, para- and ipso-positions. Oxygen adds reversibly to the OH-adduct radicals (kf = 1.8 × 108 dm3 mol-1 s-1, kr = 5.4 × 104 s-1). In this case, tyrosine formation occurs by HO2˙-elimination. However, due to side reactions, tyrosine formation only reaches 52% of the OH radical yield. The tyrosine yield drops to 10% in the absence of an oxidant.Upon SO4˙⁻-attack, decarboxylation becomes a major process (33% of SO4˙⁻) alongside the production of tyrosines (43%). Here, with Fe(CN)63- as the oxidant the formation of p-Tyr (18.5%) and m-Tyr (16.5%) is preferred over o-Tyr formation (8.5%). It is believed that in analogy to other systems a radical cation is formed immediately upon SO4˙⁻-attack which either reacts with water under the formation of hydroxycyclohexadienyl-type (“OH-adduct”) radicals, or decarboxylates after intramolecular electron transfer. The radical cation can also arise indirectly through H+-catalysed water elimination from the ˙OH-adduct radicals. At pH 2 and a dose rate of 0.0046 Gy s-1 CO2 formation matches the OH radical yield when ˙OH is the attacking radical. Below pH 2, G(CO2) decreases with falling pH. This indicates the occurrence of another, unimolecular, pathway under these conditions competing effectively with decarboxylation. This appears to be a relatively slow deprotonation reaction of the carboxylprotonated phenylalanine radical cation which gives rise to the benzyl-type radical.

Reactions of OH Radicals with Acetylacetone in Aqueous Solution. A Pulse Radiolysis and Electron Spin Resonance Study

1982 ◽  
Vol 37 (3) ◽  
pp. 368-375 ◽  
Author(s):  
R. K. Broszkiewicz ◽  
T. Söylemez ◽  
D. Schulte-Frohlinde
Keyword(s):  
Double Bond ◽  
Pulse Radiolysis ◽  
Visible Region ◽  
Alkaline Condition ◽  
Main Reaction ◽  
Electron Spin Resonance Study ◽  
Oh Radicals ◽  
Oh Radical ◽  
Keto Form ◽  
Optical Absorbance

Abstract Pulse radiolysis experiments monitoring optical absorbance as well as conductivity and in-situ ESR radiolysis studies show that the OH radical reacts with the enol (k=8.6 x 109 M-1 s-1) and the enolate (k = 7.4 X 109 M-1 s-1) forms of acetylacetone by addition to the C = C double bond in aqueous N2O saturated solution. The OH reaction with enol leads to equal amounts of two radicals, CH3COCHOHCOHCH3 (2) and CH3COCHC(OH)2CH3 (4), as determined by scavenger reactions. At pH less than 1 the radical CH3COCHCOCH3 (1) is observed by ESR spectroscopy showing that radical 2 and/or 4 eliminate water by proton catalyzed reactions. Under alkaline condition the OH adducts to the enolate eliminate OH -with rate constants larger than 105 s-1 leading to radical 1. G(OH-) is determined to be 5.6 showing that addition is the main reaction of OH with enolate. To a much smaller degree the OH radical is proposed to abstract an H atom from that CH3 group which is attached to the C -C double bond in enol and enolate, producing substituted allyl radicals which absorb in the visible region. The reaction of OH with the keto form has not been observed indicating that the rate constant of this reaction is significantly smaller than those with enol and enolate.

scholarly journals Reaction of Br3·2- with 2-Deoxy-D-ribose. A Preferred Attack at C-1

1978 ◽  
Vol 33 (6) ◽  
pp. 666-668 ◽  
Author(s):  
Barry J. Parsons ◽  
Dietrich Schulte-Frohlinde ◽  
Clemens von Sonntag
Keyword(s):  
Aqueous Solutions ◽  
Hydroxyl Radicals ◽  
Pulse Radiolysis ◽  
Hydrogen Abstraction ◽  
Major Product ◽  
Alternative Route ◽  
Sugar Moiety ◽  
Bromide Ions

Abstract In the photolysis of 5-bromouracil containing DNA Br atoms are expected inter mediates. In order to evaluate the possible site of attack of the Br atom at the sugar moiety of DNA the reaction of 2-deoxy-D-ribose with the Br atom (complexed with two bromide ions) was investigated. Hydroxyl radicals generated by the radiolysis of N2O saturated aqueous solutions were converted into Br3·2- radicals by 1 M bromide ions. Br3·2- reacts with 2-deoxy-D-ribose (k = 3.7 · 104M-1s-1, pulse radiolysis). The major product is 2-deoxy-D-erythro-pentonic acid (G = 2.4, γ-radiolysis). It is formed by hydrogen abstraction from C-l and oxidation of this radical by other radicals. An alternative route via the radical at C-2 is neglible. It follows that Br3·2- reacts preferentially at C-1 of 2-deoxy-D-ribose

Site of H Atom Attack on Uracil and Its Derivatives in Aqueous Solution

1985 ◽  
Vol 40 (3-4) ◽  
pp. 292-294 ◽  
Author(s):  
Suresh Das ◽  
David J. Deeble ◽  
Clemens von Sonntag
Keyword(s):  
Aqueous Solution ◽  
Double Bond ◽  
Pulse Radiolysis ◽  
Hydrogen Abstraction ◽  
Methyl Groups ◽  
Oh Radicals ◽  
Hydrogen Atoms ◽  
Sugar Moiety ◽  
Reducing Properties

Hydrogen atoms from the radiolysis of water at pH 1.6 add to the 5,6-double bond of pyrimidines. The preferen­tial site of attack is the C(5) position (values in brackets) in the case of 6-methyluracil (87%), 1,3-dimethyluracil (71%), uracil (69%) and poly(U) (60%). This reaction yields a radical of reducing properties which can be monitored by its reaction with tetranitromethane in a pulse radiolysis experiment. In thymine (37%), thymidine (32%) and 1,3-dimethylthymine (25%) H-addition no longer pre­ferentially occurs at C(5), but addition is now mainly at C(6). Hydrogen abstraction from the methyl groups or the sugar moiety is negligible (≦ 5.5%). A comparison is made with literature values for the equivalent reactions of OH radicals.

Peptide Free-Radicals: The Reactions of OH Radicals with Glycine Anhydride and its Methyl Derivatives Sarcosine and Alanine Anhydride. A Pulse Radiolysis and Product Study

1989 ◽  
Vol 44 (8) ◽  
pp. 959-974 ◽  
Author(s):  
Oliver J. Mieden ◽  
Clemens von Sonntag
Keyword(s):  
Hydrogen Atom ◽  
Dose Rate ◽  
Rate Constant ◽  
Pulse Radiolysis ◽  
Solute Concentration ◽  
Oh Radicals ◽  
Initial Attack ◽  
Product Analysis ◽  
H Nmr ◽  
Methyl Derivatives

The reactions of radiolytically generated OH radicals and H atoms with the cyclic dipeptides of glycine, alanine and sarcosine in deoxygenated aqueous solutions and the subsequent reactions of the transient peptide radicals were studied in the absence and presence of K3Fe(CN)6 as oxidant by pulse radiolysis and product analysis.Hydroxyl radicals and H atoms react with glycine anhydride and alanine anhydride by abstracting an H atom bound at C-3; there is no evidence for any other site of attack at these two peptides. The resulting radicals have pKa values of 9.8 and 10.6, respectively.In the absence of an oxidant the radicals decay by second order (2k = 7.0×108 dm3 mol-1 s-1 and 2k = 4.4×108 dm3 mol-1 s-1, resp.), the main fraction (94% of the glycine anhydride-derived radicals, 90% of the alanine anhydride-derived radicals) yielding dehydrodimers (G = 0.58 μmol J-1 and 0.56 µmol J-1 (in monomer units), resp.). A small portion however disproportionates via abstraction of a C-6-bound Η atom followed by isomerization to 2,5-dihydroxypyrazines (pKa values of the parent 2,5-dihydroxypyrazine at about 7.9 and 10.1) and subsequent addition of water to 2,5-diketo-3-hydroxypiperazines, thus indicating that the transfer of a carbon-bound hydrogen atom is prefered to the transfer of a nitrogen-bound hydrogen atom.No disproportionation products but three different dehydrodimers (G = 0.36, 0.18 and 0.04 µmol J-1 (in monomer units)) were found after irradiation of sarcosine anhydride. In this case a dose rate and solute concentration dependence of dehydrodimer formation indicates a radical-solute reaction converting part of the N-methyl radicals (21% of ‘initial’ attack) into the C-3-yl radicals. A rate constant of k = 600 ± 50 dm3 mol-1 s-1 was obtained for this reaction by measuring and computing the dehydrodimer yields as a function of dose rate and solute concentration. Thus the observed transient spectrum accounts only for about 79% of the radicals from the ‘initial’ attack at C-3.The rate of oxidation of the glycine anhydride-derived radicals by Fe(CN)63- reflects the pKa of the transient radical. The rate constant for oxidation of the (protonated) radical derived from glycine anhydride is: k = 1.0x 108 dm3 mol-1, the corresponding radical anion is oxidized with k = 3.1 × 108 dm3 mol-1 s-1. No change with pH was observed in the case of the alanine anhydridederived radicals (k = 7.9x 108 dm3 mol-1 s-1). In contrast to the disproportionation, oxidation by Fe(CN)63- leads to the removal of a proton from the heteroatom, a carbocation being the intermediate. The resulting dehydropiperazines rapidly add water to yield the corresponding 2,5-diketo-3-hydroxypiperazines (G = 0.61 μmol J-1 after oxidation of the glycine anhydride-derived radicals, G = 0.58 µmol J-1 after oxidation of the alanine anhydride-derived radicals). The radicals derived from sarcosine anhydride are readily oxidized with k = 4.0×108 dm3 mol-1 s-1, independent of pH.1H and 13C{1H} NMR-spectroscopic and mass-spectroscopic data of the products are given.

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