Reversed-phase high-performance liquid chromatographic separation of tert.-butyl hydroperoxide oxidation products of unsaturated triacylglycerols

Triacylglycerols containing monounsaturated fatty acids are known to be relatively resistant to autoxidation and require long periods of exposure to dilute oxidants. Use of concentrated solutions of synthetic hydroperoxides, however, yields in addition to the hydroperoxides also unidentified oxidation by-products. In the present study we have employed synthetic triacylglycerols containing one (18:0/18:1/18:0 and 18:1/16:0/16:0) and two (18:0/18:0/18:2 and 18:1/18:1/18:0) double bonds per molecule to reinvestigate the formation of oxotriacylglycerols using tert.-butyl hydroperoxide as an oxidant. Reversed-phase HPLC was used to separate and tentatively identify the oxidation products based on relative retention times of standards and the estimated elution factors for functional groups and their positional distribution. Hydroperoxides, diepoxides and hydroxides were the major components of the oxidation mixtures (50-95% of total). Previously unidentified peroxide-bridged tert.-butyl adducts were present in significant amounts (5-50% of total oxidation products) in all preparations. In several instances more than one functional group was present on a single fatty chain. The tentative reversed-phase chromatographic identification of the adducts was confirmed by determination of the molecular mass of each component by on-line LC with electrospray MS. The oxidation products were quantified by HPLC with light scattering detection.     

MEASUREMENT OF INDUCTION PERIODS IN INHIBITED FREE-RADICAL OXIDATION BY INFRARED CHEMILUMINESCENCE AT 1270 nm

Infrared chemiluminescence at 1.27 µ. appears after the consumption of phenolic inhibitors in solutions of 1-tetralyl hydroperoxide in tert -butylbenzene (BPh), initiated with di- tert -butyl hyponitrite (BHN) at 68_oC. The induction periods with phenols were in excellent agreement with ones calculated from rates of decomposition of the initiator. Di-(2,2,6,6-tetramethyl-4-piperidinyl) sebacate, benzhyd-rol, a derivative of benzhydrol, and di-rerr-butyl ketoxime under these conditions showed no inhibition or a weak effect. Initiated solutions 2.5 mM in p,p'-di- tert -octyldiphenylamine showed no detectable emission at 1270 nm because of efficient quenching of ₁O₂ by diaryl nitroxides formed from the amine, and because the oxidation products of the amine are peroxy scavengers (Thomas, J. R. 1960, J. Amer. Chem. Soc. 82, 5955). The relative yields of IR luminescence from several alkyl hydroperoxides and oxygen-hydrocarbon mixtures, initiated in each case by BHN, were found to vary substantially with the structure of the hydrocarbon and with the solvent.     

Interaction of the natural antioxidant echinochrome with hydroperoxides under anaerobic conditions

The action of cumene hydroperoxide and oxidized ethyl esters of the fatty acids of linseed oil on echinochrome (2,3,5,7,8-pentahydroxy-6-ethylnaphthalene-1,4-dione), an active scavenger of peroxide radicals, was investigated under anaerobic conditions (argon). The dependence of the rate of consumption of echinochrome on the initial concentrations of the components indicates the occurrence of two competitive reactions: a) with hydroperoxides and b) with the radicals formed in the thermolysis of hydroperoxides. The upper limit of the values of the rate constants of the reaction of echinochrome with cumene hydroperoxide (5·10^–7 M^–1·sec^–1) and with the total hydroperoxides of linseed oil (3·10^–6 M^–1·sec^–1) was determined.The numeration of the reaction used corresponds to the scheme of free-radical oxidation from [5].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 329–333, February, 1991.The authors would like to express sincere gratitude to V. A. Roginskii for participating in the discussion of the results.     

The analysis of combustion products : The effect of hydrogen peroxide on the reaction of potassium permanganate with organic peroxides in acid solution

Although methyl and ethyl hydroperoxides alone do not react with potassium permanganate in acid solution at room temperature, they can be oxidised by this reagent in the presence of hydrogen peroxide. The apparent equivalents (as compared with hydrogen peroxide) are much lower than usual but, in the presence of excess hydrogen peroxide, the titre due to the organic peroxide is proportional to its original concentration. This forms the basis of a simple titration method which, though empirical, is both rapid and accurate. Since t-butyl hydroperoxide does not react in this way, it may be possible to differentiate the higher peroxides from lower peroxides.     

Kinetic fluorimetric determination of organic peroxides and lipohydroperoxides at the nanomole level

A new kinetic fluorimetric method for determination of cumene hydroperoxide, tert-butyl hydroperoxide and lipohydroperoxides is reported. It is based on the manganese(II)-catalysed oxidation of 2-hydroxynaphthaldehyde thiosemicarbazone (HNTS) by a hydroperoxide. Measurements are made by the initial-rate method, which allows determination of as little as 0.1 nmole of peroxide and also permits establishment of the differences in reactivity between the hydroperoxides assayed. The method has been applied to the determination of lipohydroperoxides in six commercial oil samples (grape, corn, sunflower seed, cod-liver and linseed). The results obtained can be expressed as meq of peroxide per kg of oil, and are in close agreement with those obtained by the classical iodometric method.     

Colorimetric determination of lipid hydroperoxides in oils and fats with microperoxidase

The use of the peroxidase-like activity of microperoxidase for the calorimetric determination of lipid hydroperoxides in oils and fats has been investigated. The principle of the determination is that 4-aminoantipyrine and N,N-diethylaniline are coupled oxidatively by the hydroperoxides through the action of microperoxidase, yielding a violet colour with maximum absorbance at 554 nm. The response of the microperoxidase system is enhanced by the presence of acetonitrile. The method has been successfully applied to the determination of methyl linoleate hydroperoxide, tert-butyl hydroperoxide and the hydroperoxides in oil and fat samples (soybean oil, linseed oil, olive oil, salad oil, butter and lard). The results agreed closely with those obtained by the iodometric method. The proposed method permitted the determination of the hydroperoxides at 0.5-0.05 mumole levels, with the same sensitivity regardless of sample type tested, with satisfactory reproducibility compared with that obtained by the conventional assay methods.     

Determination of lipids and their oxidation products by IR spectrometry

We proposed a procedure for the IR spectrometric determination of lipid hydroperoxides in biological systems. The main bands in the IR absorption spectra of linoleic acid and its hydroperoxide were identified, and analytical bands suitable for the determination of both compounds in their mixtures were selected. It was demonstrated that tert-butyl hydroperoxide can be used as an external standard for determining fatty acid hydroperoxides. Using the external standard method (calibration curve) for tert-butyl hydroperoxides, we calculated the concentration of linoleic acid hydroperoxide in its mixture with linoleic acid; it agreed with the specified values. Using the developed procedure, we estimated the concentration of hydroperoxide groups in natural cardiolipin. The results were compared to those obtained by an independent method (activated chemiluminescence).     

Catalytic action of metallic salts in autoxidation and polymerization. VIII. Polymerization of methyl methacrylate by various metal acetylacetonate–cyclic ether hydroperoxides

Polymerization of methyl methacrylate by cyclic ether hydroperoxide–metal acetylacetonate systems for a number of different metals was carried out to compare with the tert-butyl hydroperoxide–metal acetylacetonate initiating systems. The rate of polymerization of methyl methacrylate with cyclic ether hydroperoxides as initiating systems was much higher than that with tert-butyl hydroperoxide. In cyclic ether hydroperoxide initiating systems, V(III), Co(II,III), Fe(III), Cu(II), and Mn(II) promoted the polymerization rate markedly, and Zn(II), Ni(II), Al(III), and Mg(II) had little or no effect; in the tert-butyl hydroperoxide initiating system only V(III), Co(II), and Mn(II) enhanced polymerization rate, and most of other metals showed little or no effect. Furthermore, noticeable differences in color of solution and appearance during polymerization, and in relation between conversion and the degree of polymerization were observed. The effect of metal acetylacetonates on hydroperoxide initiators in polymerization of methyl methacrylate was also compared with that on the decomposition of hydroperoxides.     

Evolution of H2O and CO2 during the copper-catalyzed oxidation of isotactic polypropylene

The kinetics and mechanism of H₂O and CO₂ evolution during uncatalyzed and copper(oxide)-catalyzed (Cu, CuO, CuO_0.67) oxidation of isotactic polypropylene have been investigated in detail for various catalysts over a range of temperatures (90–150°C). These volatiles were determined chromatographically; H₂O and CO₂ represent the main volatiles of the oxidation, comprising about 80 mol % of all volatiles. Uncatalyzed oxidation evolves ca. 1 mol of H₂O and 1 mol of CO₂ for each unit mole of polymer oxidized, while catalyzed oxidation produces 2 mol of H₂O and ca. 1.2 mol of CO₂ for each unit mole of polymer. These results indicate that secondary as well as tertiary H atoms on the polymer chains are involved in hydroperoxide formation and decay. The oxidation mechanism has been formulated and evaluated on this basis. It consists essentially of two parallel oxidation reactions involving tertiary and secondary groups (H atoms and hydroperoxides), respectively. The mechanism can be represented by first- and pseudo-first-order reactions in series: (1) oxygen absorption showing induction periods; (2) hydroperoxide formation and decay (plateaus are reached); (3) H₂O evolution from the decay of hydroperoxides; and (4) subsequent CO₂ production involving chain scission. Arrhenius parameters for all oxidation reactions (uncatalyzed and catalyzed) are also presented. It appears that CuO_0.67 is the most efficient catalyst of those investigated.     

Heterogeneous initiators for the synthesis of polymer nanocomposites

Nanocomposites are obtained by the radical polymerization of styrene and methyl methacrylate on the surface of a dispersed filler containing chemisorbed compounds of quaternary ammonium, which catalyze decomposition of cumene hydroperoxide. The heterogeneous catalysts of hydroperoxide decomposition are obtained via the adsorption of cetyltrimethyl ammonium bromide and acetylcholine chloride on sodium montmorillonite, cellulose, and chitosan. The highest rate of the polymerization of both monomers is provided by the cellulose–cetyltrimethyl ammonium bromide catalyst. For a more hydrophilic methyl methacrylate, the rate of radical initiation is significantly lower at the same concentrations of the catalyst and hydroperoxide compared with hydrophobic styrene; however, the rate of polymerization is higher than for styrene because of a higher activity of methyl methacrylate in chain-propagation reactions. Relatively high rates of radical generation upon contact of cellulose–cetyltrimethyl ammonium bromide and cellulose–acetylcholine with hydroperoxides open the possibility to create cellulose-based disinfecting and medical materials.     

Gas chromatography of trimethylsilyl derivatives of alpha-methoxyalkyl hydroperoxides formed in alkene-O3 reactions

The synthesis of trimethylsilyl (TMS) hydroperoxide derivatives for gas chromatography (GC) was studied using N-methyl-N-(trimethylsilyl) trifluoroacetamide (MSTFA) for derivatization of cumene hydroperoxide (CMOOH) (alpha,alpha'-dimethylbenzyl hydroperoxide) and alpha-methoxyalkyl hydroperoxides formed by liquid- and gas-phase ozonolysis of a series of terminal alkenes in the presence of methanol (CH3OH). Derivatization efficiencies >90% were achieved over a wide range of solution concentrations. The major compounds identified by GC-mass spectrometry of the derivatized products of alkene-O3 reactions were alpha-methoxyalkyl hydroperoxides, methyl esters, and aldehydes. Yields of alpha-methoxyalkyl hydroperoxides and methyl esters were quantified using effective carbon numbers (ECNs) and used to determine the yields of stabilized Criegee intermediates (SCIs) from gas-phase ozonolysis reactions. Such measurements are important for understanding the atmospheric chemistry of alkene emissions. SCI yields measured for the reactions of 1-octene [CH3(CH2)5CH=CH2], 1-nonene [CH3(CH2)6CH=CH2 ], and 2-methyl-1-octene [CH3(CH2)5C(CH3)=CH2] are consistent with previous measurements or predictions based on literature data. SCI yields measured for the reactions of 1-decene [CH3(CH2)7CH=CH2], 1-dodecene [CH3 (CH2)9CH=CH2], and 1-tetradecene [CH3(CH2)11CH=CH2] are much lower than expected, apparently due to side reactions with low volatility aldehydes that form peroxyhemiacetals, which are not amenable to GC analysis. In general, the results indicate that off-line MSTFA derivatization can be an efficient means for increasing the stability of thermally labile hydroperoxides for identification and quantitation by GC, and offers a new approach for the analysis of these environmentally important compounds.     

Decomposition of cumene hydroperoxide in the systems of normal and reverse micelles formed by cationic surfactants

Decomposition of cumene hydroperoxide into free radicals in aqueous and organic media in the presence of cationic surfactants at 37°C is studied by the method of inhibitors using quercetin as an acceptor of radicals. It is found that cationic surfactants catalyze the decomposition of cumene hydroperoxide into radicals, the catalytic effect in an organic medium being higher than that in an aqueous solution. Catalytic action of surfactants greatly depends on the counterion nature. Cetyltrimethylammonium chloride has the highest catalytic activity. Characteristics of surface activity of some cationic surfactants and hydroperoxides are obtained.     

Determination of Organic Peroxides by High Performance Liquid Chroma-Tography with Electrochemical Detection

Abstract

The application of electrochemical detection to the high performance liquid chromatographic determination of organic peroxides has been studied. The use of a buffered mobile phase was found to be critical to the successful analysis of samples containing hydroperoxides. Using amperometric detection, mixtures of peroxide containing compounds were readily determined. The sensitivity of the amperometric detector was in the one nanogram range for both benzoyl peroxide and cumene hydroperoxide. Polar-ographic detection was found to be a highly reproducible method for the analysis of samples containing peroxides as components of mixtures in the range of 5-2000 ng. The peroxide containing compounds determined in this manner were t-butyl hydroperoxide, cumene hydroperoxide, and 13-hydroperoxy-9(Z)-11(E)-octadecadien-oic acid. The polarographic detection system was used to obtain observed half-wave potentials for the peroxides under different chromatographic conditions. These observations correlated closely with literature results on the polarography of these compounds.     

Liquid‐phase oxidation of 1‐isopropyl‐ and 1‐ethyl‐4‐methoxybenzenes with oxygen to hydroperoxides

A study was carried out on the kinetics of free‐radical chain oxidation of 1‐isopropyl‐4‐methoxybenzene ( 1a ) and 1‐ethyl‐4‐methoxybenzene ( 1b ) with oxygen in the liquid phase to yield 1‐methyl‐1‐(4‐methoxyphenyl)ethyl hydroperoxide ( 2a ) and 1‐(4‐methoxyphenyl)ethyl hydroperoxide ( 2b ). The oxidizability of 1a and 1b was studied over the temperature range 50–100°C. Long‐term oxidations of 1a and 1b to the corresponding hydroperoxides were carried out and the properties and thermal stability of 2a were established. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 89–94, 2003     

Synthesis of poly(arylene ethylene) oligomeric hydroperoxides and their use as initiating agents of radical polymerization

The oxidation to hydroperoxide of poly(arylene ethylenes) (PAE) by oxygen carried out in solutions at 80–110°C. The effect of initiating additions and the nature of solvent relative to the content of hydroperoxide groups in oxidized PAE were investigated. The oxidation to hydroperoxides in PAE occurs at the methylene groups, and the synthesized hydroperoxides are secondary peroxides. The decomposition of PAE hydroperoxides in toluene and chlorobenzene at concentrations of 0.006–0.03 mole/l. for hydroperoxide in the presence and absence of N-phenyl-α-naphthylamine (PNA) was studied. The decomposition of one hydroperoxide has been studied in the presence of cobaltous and manganese resinates and of PNA in chlorobenzene at 30–50°C. The addition of PNA to a chlorobenzene solution of PAE hydroperoxide containing cobaltous or manganese resinate accelerates the hydroperoxide decomposition, reduces the activation energy, and changes the reaction order from the second-order to first-order. The synthesized hydroperoxides initiate the radical polymerization of styrene and methyl methacrylate. The initiating activity of one of the synthesized hydroperoxides of PAE for polymerization of styrene (60°C) in the presence and absence of activating addition of manganese resinate was also evaluated.     

Selective oxidation of unactivated C-H bonds by supramolecular control

Efficient methods for dioxirane-based selective C-H bond oxidation by supramolecular control in H(2)O have been developed. With β-cyclodextrin as the supramolecular host, site-selective oxidation of the terminal over the internal tertiary C-H bond of 3,7-dimethyloctyl esters 3a-c was achieved. In addition, β-cyclodextrin selectively enhanced the C-H bond oxidation of cumene in a mixture of cumene and ethyl benzene in H(2)O. Through (1)H NMR studies, the selectivity in C-H bond oxidation could be attributed to the inclusion complex formation between β-cyclodextrin and the substrates.     

Isolation of an oxomanganese(V) porphyrin intermediate in the reaction of a manganese(III) porphyrin complex and H2O2 in aqueous solution

The reaction of [Mn(TF(4)TMAP)](CF(3)SO(3))(5) (TF(4)TMAP=meso-tetrakis(2,3,5,6-tetrafluoro-N,N,N-trimethyl-4-aniliniumyl)porphinato dianion) with H(2)O(2) (2 equiv) at pH 10.5 and 0 degrees C yielded an oxomanganese(V) porphyrin complex 1 in aqueous solution, whereas an oxomanganese(IV) porphyrin complex 2 was generated in the reactions of tert-alkyl hydroperoxides such as tert-butyl hydroperoxide and 2-methyl-1-phenyl-2-propyl hydroperoxide. Complex 1 was capable of epoxidizing olefins and exchanging its oxygen with H(2) (18)O, whereas 2 did not epoxidize olefins. From the reactions of [Mn(TF(4)TMAP)](5+) with various oxidants in the pH range 3-11, the O-O bond cleavage of hydroperoxides was found to be sensitive to the hydroperoxide substituent and the pH of the reaction solution. Whereas the O-O bond of hydroperoxides containing an electron-donating tert-alkyl group is cleaved homolytically, an electron-withdrawing substituent such as an acyl group in m-chloroperoxybenzoic acid (m-CPBA) facilitates O-O bond heterolysis. The mechanism of the O-O bond cleavage of H(2)O(2) depends on the pH of the reaction solution: O-O bond homolysis prevails at low pH and O-O bond heterolysis becomes a predominant pathway at high pH. The effect of pH on (18)O incorporation from H(2) (18)O into oxygenated products was examined over a wide pH range, by carrying out the epoxidation of carbamazepine (CBZ) with [Mn(TF(4)TMAP)](5+) and KHSO(5) in buffered H(2) (18)O solutions. A high proportion of (18)O was incorporated into the CBZ-10,11-oxide product at all pH values but this proportion was not affected significantly by the pH of the reaction solution.     

Simultaneous determination of methyl tert.-butyl ether and its degradation products,other gasoline oxygenates and benzene,toluene, ethylbenzene and xylenes in Catalonian groundwater by purge-and-trap-gas chromatography-mass spectrometry

In Catalonia (northeast Spain), a monitoring program was carried out to determine methyl tert.-butyl ether (MTBE), its main degradation products, tert.-butyl alcohol (TBA), tert.-butyl formate (TBF), and other gasoline additives, the oxygenate dialkyl ethers ethyl tert.-butyl ether, tert.-amyl methyl ether and diisopropyl ether and the aromatic compounds benzene, toluene, ethylbenzene and xylene (BTEX) in 21 groundwater wells that were located near different gasoline point sources (a gasoline spill and underground storage tank leakage). Purge-and-trap coupled to gas chromatography-mass spectrometry was optimised for the simultaneous determination of the above mentioned compounds and enabled to detect concentrations at ng/l or sub-microg/l concentrations. Special attention was given to the determination of polar MTBE degradation products, TBA and TBF, since not much data on method performance and environmental levels are given on these compounds in groundwater. All samples analysed contained MTBE at levels between 0.3 and 70 microg/l. Seven contaminated hot spots were identified with levels up to US Environmental Protection Agency drinking water advisory (20-40 microg/l) and a maximum concentration of 670 microg/l (doubling the Danish suggested toxicity level of 350 microg/l). Samples with high levels of MTBE contained 0.1-60 microg/l of TBA, indicating (but not proving) in situ degradation of parent compound. In all cases, BTEX was at low concentrations or not detected showing less solubility and persistence than MTBE. This fact confirms the suitability of MTBE as a tracer or indicator of long-term gasoline contamination than the historically used BTEX.     

Reactions of alkoxy and peroxy radicals formed upon the decomposition of hydroxyperoxide groups in the artemisinin derivatives

The enthalpies of intramolecular reactions of alkoxy and peroxy radicals formed from polyatomic artemisinin hydroperoxides and of their bimolecular reactions with C—H, S—H, and O—H bonds of biological substrates were calculated. The activation energies and rate constants of these reactions were calculated using the intersecting parabolas method. The decomposition of artemisinin hydroperoxides can initiate the cascade of intramolecular oxidation reactions involving radicals R·, RO·, HO·, HO₂·, and RO₂·. The main sequences of transformation of these radicals were established. The oxidative destruction of the artemisinin peroxy derivatives generates radicals RO₂·, HO·, and HO₂· in an amount of 4.5 radicals per peroxide derivative molecule on the average. The kinetic scheme of oxidative transformations of the hydroperoxide with four OOH groups and radicals formed from it was constructed using this radical as an example.     

New insights into the titanium-mediated enantioselective oxidation of fluorinated aryl benzyl sulfides and aryl phenacyl sulfides

A fruitful switch from tert-butyl to cumene hydroperoxide was able to overcome a difficulty arose in the enantioselective oxidation of fluorinated aryl benzyl sulfide with hydroperoxides in the presence of a titanium/(S, S)-hydrobenzoin catalyst. New experiments show the complementarity of the old and the new protocols and indicate unequivocally the right choice leading to the corresponding highly enantioenriched sulfoxides. Moreover, in a totally unexpected way, the new protocol was able to overcome another difficulty arose in another field of research, that is the enantioselective oxidation of a fluorinated aryl phenacyl sulfide. Also in this case, the complementarity of behavior is acting. Finally, this investigation gives new support to the attribution of configuration of sulfoxides with ECD techniques, but only if the protocol outlined in our past research was followed thoroughly.