Autoxidation Of Ethyl Linoleate Research Articles

The autoxidation activity of new mixed-ligand manganese and iron complexes with tripodal ligands

The activity of new manganese and iron complexes of dianionic tripodal ligands in the autoxidation of ethyl linoleate (EL) is reported. EL consumption rates were monitored using time-resolved FTIR and the degree of oligomerisation was determined by SEC. Almost all complexes showed the same trend in the autoxidation of EL. After a short induction time, the reaction started at a relatively high constant rate; later, this rate changes to a lower rate, which was again constant and on average was the same for all complexes studied. The observation that the autoxidation rate eventually became the same for all complexes can be explained with the assumption that a catalytic species was formed that was structurally similar for each complex. The crystal structure of a representative new complex, [Mn(papy)(acac)] (H 2papy= N-(2-hydroxybenzyl)- N-(2-picolyl)-glycine; Hacac = acetylacetone), was elucidated, demonstrating a Jahn–Teller distorted octahedral geometry of the Mn(III) ion. To gain insight into the reaction mechanisms by which these manganese complexes react with peroxides, the reaction of [Mn(pppy)(dpm)] with alkyl hydroperoxides was studied using UV–vis and EPR spectroscopy (H 2pppy = 2-[bis(2-hydroxybenzyl)aminomethyl]-pyridine; Hdpm = dipivaloylmethane). It is proposed that in a reaction of [Mn(pppy)(dpm)] with t-butyl hydroperoxide ( t BuOOH), the species [Mn(III)(pppy)(OH)( t BuOO ⋅ )] is formed. In analogy with the formation of that species, which is postulated to occur through a reduction to Mn(II), it is proposed that in the direct initiation reaction of EL with a manganese(III) complex, a 1 electron–1 proton transfer reaction leads to the formation of Mn(II) and an EL radical. Considering parameters such as autoxidation activity, complex solubility and ease of complex preparation, the compound [Mn(III)(tbpppy)(dpm)] (H 2tbpppy = 2-[bis(2-hydroxy-3,5-di- tert-butylbenzyl) aminomethyl]pyridine) is expected to be the best potential alkyd paint drier.

Effect of [Mn(acac) 3] and its combination with 2,2′-bipyridine on the autoxidation and oligomerisation of ethyl linoleate

In this study we investigated the autoxidation and oligomerisation of ethyl linoleate (EL) catalysed by [Mn(III)(acac) 3] (acac=2,4-pentanedionate) and its combination with 2,2-bipyridine (bpy), in comparison with the EL catalysed by Co(II) 2-ethylhexanoate (Co-EH). EL is a model compound for the alkyd resin in alkyd paints, Co-EH is a common drying catalyst for alkyd paints, and [Mn(acac) 3] and the [Mn(acac) 3]/bpy combination are potential new drying catalysts. The autoxidation of EL was studied through time-resolved Raman spectroscopy, oxygen uptake measurements, and peroxide amount determination. To follow the oligomerisation of EL in time, size exclusion chromatography was used. Head-space GC-MS measurements were performed to determine the amounts of hexanal and pentanal that were formed as volatile byproducts during the autoxidation of EL. The autoxidation rates of EL in the presence of Co-EH and [Mn(acac) 3]/bpy were found to be similar, while the rate in the presence of [Mn(acac) 3] was slower. The extent of EL oligomerisation was much higher for [Mn(acac) 3] compared to the other catalysts. Different mechanisms are proposed for the mode of action for each of the catalysts: Co-EH is primarily a hydroperoxide decomposition catalyst, as is [Mn(acac) 3], only less active. The [Mn(acac) 3]/bpy combination probably forms the very reactive complexes [Mn(III)(acac) 2(bpy)] + and [Mn(II)(acac) 2(bpy)], which are responsible for a very high autoxidation rate, but also for significant degradation of the formed EL oligomers via β-scission reactions due to the promotion of alkoxy radical formation.

HPLC Analysis of Lipid Peroxides. I.

Reversed phase high performance liquid chromatography (HPLC) with an RI detector was used to separate and determine hydroperoxides of autoxidized ethyl oleate, linoleate and linolenate.Ethyl oleate hydroperoxide appeared as a single peak apart from unoxidized ethyl oleate on the chromatogram. The hydroperoxide isomers formed by autoxidation of ethyl linoleate were resolved into two components which were geometrical isomers having cis-trans and trans-trans conjugated diene structures. Autoxidized ethyl linolenate gave two peaks of conjugated diene monohydroperoxides and conjugated triene dihydroperoxides, respectively. Under the HPLC conditions used here, the hydroperoxides could be separated according to the number of their double bonds and hydroperoxy groups, and the configuration of the conjugated double bonds (cis-trans and trans-trans), but not on the basis of the difference in the location of hydroperoxy groups.When autoxidized esters were subjected to HPLC analysis directly, a good linear relationship was observed between the ratio of the peak area of hydroperoxides to that of all peaks on the chromatogram and POV of the sample ester in the range of 303, 000meq/kg. POV determined by this procedure agreed quite well with those measured by the official titration method.

The isolation and characterication of the polymers formed during the autoxidation of ethyl linoleate

SummaryEthyl linoleate autoxidized at 30°C. for 350 hours was separated into fractions of increasing polarity and molecular weight by extraction with solvent pairs of Skellysolve F and diethyl ether. The analytical results obtained on characterization of the more polar polymer fractions indicated that the oxidative polymers of ethyl linoleate were linked through carbon to oxygen bonds rather than carbon to carbon bonds. This postulation was substantiated by a successful depolymerization of the polymers and the catalytically hydrogenated polymers with hydrochloric, hydrobromic, or hydroiodic acid in aqueous ethyl alcohol. Characterization of the depolymerized product also indicated that the oxidative polymers were joined through carbon to oxygen bonds.

The volatile decomposition products and the organoleptic characteristics of the oxidative polymers of ethyl linoleate

SummaryThe polymers formed during autoxidation of ethyl linoleate at 30°C. were isolated by a solvent extraction method using pentane‐hexane and diethyl ether as solvents. These polymers were only partially decomposed by catalytic hydrogenation at temperatures ranging from 25 to 110°C. and pressures ranging from 60 to 1,800 p.s.i. with Raney Nickel as catalyst. Both the polymers and their hydrogenated products had a bitter rancid flavor and painty odor. However in a dilution up to 10 parts per million, the polymers as well as the hydrogenated polymers could not be detected organoleptically.Both the polymers and their hydrogenated products were reautoxidized at 30°C. and the volatile material obtained in each case characterized. The volatile material which was obtained from the polymers contained n‐propionaldehyde, n‐pentanal, and n‐hexanal; and the volatile material from the hydrogenated polymers contained n‐pentanal and 2‐heptenal. Since n‐hexanal was also identified in the volatile decomposition products from autoxidizing soybean oil and 2‐heptenal from the reversion compounds of hydrogenated soybean oil and since both these aldehydes were absent in the volatile cleavage products from autoxidizing linolenate, it is therefore possible that these reversion compounds could have arisen from the oxidative polymers of linoleate.