Hydrogenation of linolenate. II. Hydrazine reduction

Observations by Aylward and Rao that hydrazine is a reducing agent for a number of unsaturated fatty acids were extended. The hydrazine reaction on linolenic acid was followed by periodic sampling until methyl esters prepared from the reduced acids had an iodine value of 162. These esters were shown by countercurrent distribution to consist of 26% triene, 43% diene, 26% monoene, and 5% stearate and by infrared analysis to contain notrans bonds. Oxidation of the separated monoene and diene fractions by permanganate-periodate mixtures and gas chromatography of the dibasic acids showed that the double bonds were in the original 9, 12, and 15 positions and that the double bonds farthest from the carboxyl were reduced at a slightly faster rate. Gas chromatography of the monoene fraction indicated three components that were identified in the order of elution from the column as 9, 12, and 15 monoenes; in the diene fraction three components were identified in the order of elution as 9,12; 9,15; and 12,15 dienes. After alkali isomerization of this diene fraction, the conjugated material was reacted with maleic anhydride; the unreacted 9,15 diene isomer was separated by distillation. Presented at fall meeting, American Oil Chemists’ Society, New York, October 17–19, 1960. This is a laboratory of the Northern Utilization Research and Development Division, Agricultural Research Service, U.S. Department of Agriculture.     

Hydrogenation of linolenate. I. Fractionation and characterization studies

Methyl linolenate hydrogenated at 140°C, with 0.5% Ni catalyst and 1.1 mole of hydrogen at atmospheric pressure was separated into octadecenoate, octadecadienoate, and octadecatrienoate fractions by countercurrent distribution. Gas chromatography on a 200-ft. capillary Apiezon L column revealed one component in the triene fraction, four in the diene fraction, and nine in the monoene fraction. These components were partially fractionated by low-temperature crystallization, and their solubilities were correlated with alkali conjugation results, with infrared data forcis andtrans configuration of bonds and with dibasic acids isolated from the fractions after oxidative cleavage. Approximately 45% oftrans acids were present in both the monoene and diene fractions. Considerable migration of double bonds from the original 9, 12, and 15 positions occurred.Cis,cis dienes which could not be conjugated by alkali were formed. Little alteration of the residual methyl linolenate was observed. The results demonstrate the applicability and utility of new techniques of fractionation and analysis to the study of the hydrogenation mechanism. Presented at 51st annual meeting, American Oil Chemists' Society, Dallas, Tex., April 4–6, 1960. This is a laboratory of the Northern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture.     

Hydrogenation of linolenate. VI. Survey of commercial catalysts

A survey of commercially available hydrogenation catalysts has been made in a search for high-selectivity and low-isomerizing characteristics. Selectivity—the ratio of hydrogenation rates for linolenate to linoleate—was determined on a linoleate-linolenate equimixture under standardized conditions. Ratios of reaction rate constants for nickel catalysts at 140C ranged between 1.48 and 2.71; for palladium catalysts at 25C, 1.68 to 1.99; and for platinum catalysts at 25C, 1.33 to 1.61.Trans contents of ester mixtures reduced with these three metal catalysts ranged from 18.0 to 22.8, 16.7 to 20.5, and 6.3 to 8.4%, respectively. Although platinum catalysts produced the lowest isomerization, their selectivities were also low. Presented at Spring meeting, American Oil Chemists' Society, 1961. A laboratory of the Northern Utilization Research and Development Division. Agricultural Research Service, U.S.D.A.     

Hydrogenation of linolenate. XI. Nuclear magnetic resonance investigation

Nuclear magnetic resonance (NMR) spectra have been obtained during the hydrogenation of methyl linolenate with platinum, nickel and sulfur-poisoned-nickel catalysts and during the reduction of linolenic acid with hydrazine. Structural changes have been studied by “proton counting” techniques and include those for the total unsaturation (olefinic protons), 15,16-double bond (β-olefinic methyl protons), 1,4-pentadienes (di-α-olefinic methylene protons) and allylic methylene (α-olefinic methylene protons). The number of olefinic protons is inversely related to the degree of saturation as determined by iodine value, GLC and hydrogen absorption. Amounts of double bonds in the 15,16-position and in the 1,4-pentadiene structures decrease linearly with increasing saturation, but the slopes of lines differ for specific catalysts. Sulfur-poisoned pickel has the most negative slope, followed by electrolytic nickel, and then platinum and hydrazine. Amounts of α-olefinic structures with increasing saturation are roughly constant for hydrazine and platinum during reduction with the first mole of hydrogen; they fall to zero at complete saturation. For the two nickel catalysts, the α-olefinic structures increase during absorption of the first mole and a half of hydrogen before dropping to zero. The possibility of substituting NMR measurements for iodine value, alkali-isomerization spectrophotometric determination and other structural analyses is discussed. Presented at AOCS Meeting in Minneapolis, 1963. A laboratory of the No. Utiliz. Res. & Dev. Div., ARS, USDA.     

Hydrogenation of linolenate. XII. Effect of solvents on selectivity

Selectivity of heterogeneous catalysts for hydrogenation of linolenate over linoleate is increased by the presence of certain polar solvents. A ratio of specific reaction rate constants for linolenate to linoleate of 4 was obtained with a 5% palladium-on-alumina catalyst when dimethyl formamide (DMF) was used as the solvent. This high selectivity of DMF was independent of temperature and catalyst concentration. Other solvents that improved selectivity include furfural, acetonitrile, tetramethyl urea and trimethyl phosphate. Presented at AOCS meeting in Chicago, 1964. A laboratory of the No. Utiliz. Res. & Dev. Div., ARS, USDA.     

Hydrogenation of methyl linolenate. I. The formation of methyl isolinoleate on the hydrogenation of methyl linolenate

Summary A highly purified sample of methyl linoleate was prepared from linolenic acid obtained by debromination of hexabromostearic acid. The methyl linolenate was hydrogenated to varying degrees of saturation, using palladium on barium sulfate as the catalyst. Ethyl acetate was used as the solvent and all hydrogenations were conducted at room temperature and atmospheric pressure. The hydrogenated samples were analyzed for their fatty acid composition (as methyl esters). The relative reactivities of the various polyunsaturated acids towards hydrogenation were calculated and may be represented by the following whole numbers: oleic (including isooleic) acid, 1; isolinoleic acid, 5; linoleic, 27; linolenic, 27. A procedure was outlined for effecting a concentration of an isolinoleic acid (methyl ester) by low temperature crystallization. The fraction isolated contained 95.8% methyl isolinoleate. Contribution No. 798 from the Department of Chemistry, University of Pittsburgh.     

Hydrogenation of linolenate. IV. Kinetics of catalytic and homogeneous chemical reduction

Kinetics for consecutive reactions of octadecatrienoate to octadecadienoate to octadecenoate have been studied with the aid of radioisotopic tracers and gas chromatography. Evidence for a triene to monoene shunt has been obtained. Similarly, the chemical reduction with hydrazine has been studied, but no evidence for this anomalous behavior was obtained. Methods to determine reaction rates from these kinetic measurements are discussed. Presented at spring meeting, American Oil Chemists' Society, St. Louis, Missouri, May 1–3, 1961. This is a laboratory of the Northern Utilization Research and Development. Division, Agricultural Research Service, U. S. Department of Agriculture.     

Hydrogenation of linolenate. V. Procedure of evaluating hydrogenation catalysts for selectivity

Equations for determining the ratio of hydrogenation rates for linolenate and linoleate acyl groups are derived from kinetic theory. They are based upon the analysis for linolenate after absorption of 0.5 mole of hydrogen by an equal mixture of linoleate and linolenate. This method finds routine application in the evaluation of hydrogenation catalysts for selectivity. Presented at spring meeting, American Oil Chemists' Society, May 1–3, 1961, St. Louis, Mo. This is a laboratory of the Northern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture.     

Quantitative GLC determination of methyl linolenate in the presence of conjugated dienes

Methyl linolenate and cis,trans conjugated isomers of methyl linoleate are not resolved by gas liquid chromatography because they have the same retention time on polar columns used for separation of methyl ester mixtures. A new column of intermediate polarity made from a mixture of OV-17 and OV-225 separated these critical pairs and enabled quantitative determination of methyl linolenate in partially hydrogenated or conjugated esters. Presented at the AOCS meeting, San Francisco, April, 1979.     

Hydrogenation of dienes with cationic rhodium complexes

Methylcis-9,cis-12-octadecadienoate (methyl linoleate;c9,c12), itst10,t12 andt10,c12 isomers and methylcis-9-octadecenoate (methyl oleate;c9) were hydrogenated with rhodium complexes, the active species of which consisted of [RhL₂]⁺ and [RhL₂H₂]⁺ with ligands L=P(C₂H₅)₂C₆H₅ (catalyst A) P(i-C₄H₉)₃ (catalyst B) and P(CH₃)₃ (catalyst C). Using these catalysts the influence of steric effects on the reaction mechanism of hydrogenation of dienes was studied. The reactions were carried out in 2-propanol at atmospheric hydrogen pressure and ambient temperature. During hydrogenation ofc9 on catalysts A and B, geometrical isomerization mainly occurred, whereas on catalyst C some positional isomerization also took place.C9,c12 was almost exclusively hydrogenated via conjugated intermediates on catalyst A. On catalyst C, one of the double bonds was hydrogenated directly, in most cases. In the absence of hydrogen, catalysts A and B conjugatedc9,c12 very fast. The conjugation activity of catalyst C was much lower. Catalyst C showed a high 1,5-shift activity for the conjugatedcis, trans andtrans, cis intermediates during hydrogenation, in contrast to catalysts A and B, which showed a poor activity in this respect.T10,t12 was hydrogenated almost exclusively via 1,4-addition of hydrogen to thecisoid conformation, whereas only a slight preference was found in this mechanism over 1,2-addition for the hydrogenation oft10,c12. On the sterically unhindered catalysts A and C thetrans double bond int10,c12 was preferentially hydrogenated whereas on catalyst B, with its bulky ligands, thecis double bond was reduced faster than thetrans double bond.     

Hydrogenation of linolenate. X. Comparison of products formed with platinum and nickel catalysts

One mole of hydrogen/mole of ester was added to methyl linolenate over a platinum catalyst at 20C and atmospheric pressure. The product was separated into trienoic, dienoic and monenoic esters by countercurrent distribution (CCD) with acetonitrile and hexane. Bach of these ester fractions was further separated by CCD with methanolic silver nitrate and hexane. Comparison with hydrogénations, in which a commercial nickel catalyst at 140C and atmos-pheric pressure was used, shows that with plati-num more stéarate is formed ; i.e., the platinum hydrogénation was less selective. Also, a smaller amt oftrans esters was formed with platinum, and there was less shift of double bonds from the original 9, 12 and 15 positions. Presented at AOCS Meeting, Atlanta, 1963. A laboratory of the No. Utiliz. Res. & Her. Div., ARS, USDA.     

Mass spectrometry of the 4,4‐dimethyloxazoline derivatives of isomeric octadecenoates (monoenes)

Although 4,4‐dimethyloxazoline (DMOX) derivatives of fatty acids have been widely used for structural analysis of fatty acids by mass spectrometry, spectra of relatively few authentic standards have been published. Confusion can result when double bonds are located near either end of the molecule, and errors have been promulgated in the literature. Mass spectra of DMOX derivatives of the complete series of isomeric octadecenoates are described. Even when spectra are not easily interpreted mechanistically in terms of the double bond location, they usually give distinctive fingerprints.     

Hydrogenation of linolenate. VIII. Effects of catalyst concentration and of temperature on rate,selectivity, andtrans formation

The effects of catalyst concentration and of temperature on linolenate selectivity,trans formation, and rate of hydrogenation have been studied for a commercial electrolytic nickel catalyst. Results obtained with an equimixture of linoleate and linolenate, following the procedure previously described, are presented as isometric drawings, which cover the experimentally practicable temperature ranges from 70–230C and nickel concentration from 0.05–10%. Whereas the rate of hydrogenation depends upon both temperature and catalyst concentration,trans formation is essentially a function of temperature while selectivity is little influenced by either parameter. Presented at the AOCS meeting in New Orleans, La., 1962. A laboratory of the No. Utiliz. Res. & Dev. Div., ARS, U. S. D. A.     

Separation and identification of isomeric conjugated fatty acids by high-performance liquid chromatography with photodiode array detection

A simple, high-performance liquid chromatographic method is described for the separation of tetraenoic, trienoic and dienoic conjugated fatty acids on a Zorbax ODS reversed-phase column using acetonitrile/tetrahydrofuran (95∶5, vol/vol) at a flow rate of 1.2 mL/min as mobile phase. Also described is the separation of the isomeric conjugated fatty acids with acetonitrile/water/tetrahydrofuran (90∶90∶1, by vol) as mobile phase. The simultaneous detection and identification of the separated geometrical isomers in the eluant was accomplished using photodiode array detection.     

Separation of isomeric lysophospholipids by reverse phase HPLC

A reverse-phase high performance liquid chromatography (HPLC) method was developed which resolved isomers of lysophosphatidylcholine (LPC) differing in the location of the aliphatic chain (sn-1 orsn-2 position) and the position (Δ⁶ or Δ⁹) or geometric configuration (cis ortrans) of the olefin group in monounsaturated species. LPC isomers containing an acyl substituent at thesn-2 position eluted before their 1-acyl-sn-glycero-3-phosphocholine (1-acyl LPC) counterparts. The retention times of both thesn-1 andsn-2 isomers of monounsaturated species increased in the order Δ⁹-cis < Δ⁹-trans < Δ⁶-cis. The integrated ultraviolet absorbance (203 nm) in binary mixtures of the Δ⁹-cis and Δ⁶-cis 2-acyl lysophospholipid isomers correlated with the lipid phosphorus content of corresponding column eluates (r-0.994). Thus, the present method will facilitate synthesis of isomerically pure diradylphospholipids by providing homogeneous lysophospholipid precursors and help simplify the quantitative analysis of unsaturated lysophospholipid species.     

Selective hydrogenation with copper catalysts: I. Isolation and identification of isomers formed during hydrogenation of linolenate

Methyl linolenate was hydrogenated with 10% copper chromite catalyst at 150 C and atmospheric hydrogen pressure. The product was separated into monoene, diene and triene fractions by countercurrent distribution. These fractions were further separated into various geometrical isomers. The double bond location in the various fractions was determined by reductive ozonolysis. Double bonds in bothcis andtrans monoene fractions, as well as incis,trans andtrans,trans conjugated dienes, were extensively isomerized. A monoene containing vinylic unsaturation was one of the major products. The nonconjugated dienes were mostly dienes whose double bonds were widely separated. Results are explained on the basis of conjugation of the double bonds in linolenate followed by hydrogen addition. Presented in part at the symposium “Hydrogenation Process,” Division of Industrial Engineering Chemistry, 157th American Chemical Society Meeting, Minneapolis, April 1968. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Hydrogenation and deuteration of conjugated isomers of linoleate and linolenate with palladium,platinum, nickel and lindlar catalysts

Conjugated isomers of methyl linoleate and linolenate were reduced with palladium, platinum, nickel and Lindlar catalysts at atmospheric hydrogen or deuterium pressure. After the products were separated, positions of their double bonds were determined by ozonolysis. Palladium and platinum catalysts reduced β-eleostearate directly to monoene. Nickel reduced β-eleostearate to dienes chiefly by 1,2-addition and to a lesser extent by 1,4- and 1,6-addition, whereas Lindlar catalyst reduced by 1,2-and 1,6-addition only. All catalysts reduced conjugated linoleate isomers by both 1,2- and 1,4-addition, with nickel being somewhat preferential for 1,2-addition. Selectivity for the catalytic reduction of dienes to monoenes decreased in the order: nickel, palladium and platinum. Lindlar catalyst did not isomerize or reduce monoenes that formed during reduction. Palladium and platinum did not isomerize conjugated dienes and trienes during their reduction, whereas nickel and Lindlar catalysts isomerized them slightly. Some deuterium was found in unreacted conjugated diene and triene with nickel and Lindlar catalysts, but none with palladium or platinum. Deuterated products contained a wide range of isotopic isomers with some products having up to 31 deuterium atoms. This wide deuterium distribution resulted from (a) exchange followed by addition, (b) addition followed by exchange and (c) exchange-addition-exchange reactions. Presented at the AOCS Meeting, Atlantic City, October 1971. ARS, USDA.     

Photosensitized oxidation of methyl linolenate. Secondary products

Previous studies of secondary oxidation products by high-pressure liquid chromatography (HPLC) of autoxidized methyl oleate, linoleate and linolenate and photosensitized-oxidized linoleate are extended to photosensitized-oxidized linolenate. Photosensitized-oxidized linolenate was fractionated by silicic acid chromatography with diethyl ether/hexane mixtures. Selected silicic acid chromatographic fractions were separated by polar phase HPLC and characterized by thin layer and gas liquid chromatography and by ultraviolet, infrared, nuclear magnetic resonance and mass spectrometry. Secondary products from the photosensitized oxidation mixtures (containing 8.2 to 29.0% monohydroperoxides) included keto- and epoxy-dienes (0.4–1.6%), hydroperoxy epidioxides (0.8–4.9%), hydroperoxy bicyclic monoenes (0.1–0.3%), dihydroperoxides (1.0–5.6%), and hydroperoxy bisepidioxides (0.7–1.6%). Some of these secondary products are new and unique to photosensitized oxidation. Cyclization of the 10-, 12-, 13- and 15-hydroperoxides of linolenate would account for their lower relative concentration than that found for the 9- and 16-hydroperoxides. Dihydroperoxides may be derived from monohydroperoxides by singlet oxygenation or free radical oxidation. The hydroperoxy bis-epidioxides may be formed by further serial cyclization of the hydroperoxy epidioxides from 10- and 15-monohydroperoxides. Dihydroperoxides, hydroperoxy epidioxides and hydroperoxy bis-epidioxides are suggested as important flavor precursors in oxidized fats. The mention of firm names or trade products does not imply that they are endorsed by the US Department of Agriculture over other firms or similar products not mentioned.