Hydrogenation of linolenate. VII. Separation and identification of isomeric dienes and monoenes

Isomeric dienes and monoenes produced by partial hydrogenation of linolenic acid have been separated by the combined use of low-temp crystallization and countercurrent distribution.Cis, trans dienes have been separated fromcis, cis dienes.Cis, cis conjugatable dienes have been partially separated fromcis,cis nonconjugatable dienes. Dienes with onetrans double bond were separated by gas chromatography into two groups:cis, trans andtrans, cis. Individual positional isomers could not be separated. Cis-9 monoene was separated fromcis-12,cis-15, andtrans monoenes by low-temp crystallization. Countercurrent distribution at 3,000 transfers only partially separated this mixture ofcis-12,cis-15, andtrans monoenes. The double bond in bothcis andtrans monoenes was found in all carbon positions, 7 through 16, showing for the first time that the 15, 16 bond of linolenic acid had moved away from the carboxyl. The majorcis bonds remained at carbons 9, 12, and 15. Combination of countercurrent distribution fractions has produced samples containing 95%cis, cis dienes; 90%cis, trans ortrans, cis dienes; 95%cis monoenes; and 90%trans monoenes. Presented at the AOCS meeting at Chicago, Ill., 1961. A laboratory of the No. Utiliz. Res. & Dev. Div., ARS, U.S.D.A.     

Selective homogeneous hydrogenation of triunsaturated fats catalyzed by tricarbonyl chromium complexes

The need for a selective catalyst to hydrogenate linolenate in soybean oil has prompted our continuing study of various model triunsaturated fats. Hydrogenation of methylβ-eleostearate (methyltrans,trans,trans-9,11,13-octadecatrienoate) with Cr(CO)₃ complexes yielded diene products expected from 1,4-addition (trans-9,cis-12- andcis-10,trans-13-octadecadienoates). Withα-eleostearate (cis,trans,trans-9,11,13-octadecatrienoate), stereoselective 1,4-reduction of thetrans,trans-diene portion yielded linoleate (cis,cis-9,12-octadecadienoate). However,cis,trans-1,4-dienes were also formed from the apparent isomerization ofα- toβ-eleostearate. Hydrogenation of methyl linolenate (methylcis,cis,cis-9,12,15-octadecatrienoate) produced a mixture of isomeric dienes and monoenes attributed to conjugation occurring as an intermediate step. The hydrogenation ofα-eleostearin in tung oil was more stereoselective in forming thecis,cis-diene than the corresponding methyl ester. Hydrogenation of linseed oil yielded a mixture of dienes and monoenes containing 7%trans unsaturation. We have suggested how the mechanism of stereoselective hydrogenation with Cr(CO)₃ catalysts can be applied to the problem of selective hydrogenation of linolenate in soybean oil. No. Market. Nutr. Res. Div., ARS, USDA.     

Deuteration of methyl linoleate with nickel,palladium, platinum and copper-chromite catalysts

Samples taken during deuteration of methyl linoleate with the title catalysts were separated into saturate, monoene and diene fractions. Monoenes were further separated intocis andtrans fractions. A comparison of the double bond distribution in monoenes with those from hydrogenation of alkaliconjugated linoleate indicated that up to 59% of the linoleate was reduced through a conjugated intermediate with nickel catalyst. The respective percentages for palladium and platinum catalysts were 51 and 23. Copper catalysts have previously been shown to reduce linoleate solely through conjugated intermediates. Copper-chromite catalyst showed infinite selectivity for the reduction of linoleate, because stearate did not form. The decreasing order of various catalysts for the selective reduction was copper-chromite>>>Ni at 195 C>Pd>Ni at 100 C>Pt. Computer simulation of platinum reduction indicated that ca. 20% of the linoleate was directly reduced to stearate through a shunt. Geometrical isomers of linoleate were formed during reduction with all catalysts except copper-chromite. Nickel catalyst formed bothtrans,trans- andcis,trans-isomers, as well as nonconjugatable dienes. These isomers were favored at the higher temperature and deuterium was incorporated into them. Palladium and platinum did not isomerize linoleate to nonconjugatable dienes. Because conjugated dienes are more reactive than linoleate, they were not found in appreciable amounts during reduction. Conjugated dienes were the only isomers formed with copper-chromite catalyst. Deuterium was found in these conjugated dienes, which were also extensively isomerized. As a result of isomerization and exchange during reduction of linoleate-as well as further exchange between deuterium and monoenes-a wide distribution of isotopic isomers in monoenes was found with nickel, palladium and platinum catalysts. Since isomerization of monoenes with copper-chromite is negligible, the isotopic distribution of monoenes must be due to exchange of intermediate conjugated dienes followed by addition. Presented at the AOCS Meeting, Ottawa, September 1972. ARS, USDA.     

Homogeneous catalytic hydrogenation of unsaturated fats: Iron Pentacarbonyl

Iron pentacarbonyl is an effective homogeneous catalyst for the reduction of polyunsaturated fats. Hydrogenation of soybean oil and its methyl esters has been achieved at 180C, hydrogen pressures of 100-1,000 psi, and 0.05–0.5 molar concentrations of catalyst. Analyses of partially reduced products show considerable isomerization of double bonds, reduction of linolenate and linoleate with little or no increase in stearate, and accumulation ofcis,trans- andtrans, trans-conjugated dienes, and isolatedtrans monoenes. The unreduced trienes include diene conjugated fatty esters. The nonconjugated dienes contain large amounts oftrans and nonalkali conjugatable unsaturation. Considerable scattering of double bonds is evident in different fractions between the C₄ and C₁₆ positions. Complex formation between iron carbonyl and unsaturated fats is also indicated. The course of the homogeneous hydrogenation catalyzed by iron pentacarbonyl appears similar to the heterogeneous catalytic reaction. Metal carbonyls are well known for their isomerizing effects and their ability to form stable complexes with olefins. These homogeneous complexes provide suitable model systems to study the mechanism of catalytic hydrogenation of fats.     

Isomerization during hydrogenation of soap: II. Potassium elaidate and potassium linoleate

Potassium elaidate in slightly alkaline solution was hydrogenated for up to 7 hr with 1.5% of Rufert nickel catalyst at 150 C and 20 kg/sq cm pressure. Potassium linoleate was similarly hydrogenated with 1.0% catalyst for 7 hr, and the hydrogenation continued for another 7 hr after addition of 0.5% fresh catalyst. Periodic samples from each were analyzed for component acids. The positional isomers in thecis andtrans monoenes, isolated by preparative argentation thin layer (TLC) or column chromatography, were estimated after oxidation to dicarboxylic acids. Some diene fractions were isolated for further examination. In potassium elaidate hydrogenation,cis monoenes were initially produced in considerable amounts, but to a lesser extent thereafter. Positional isomers were similarly distributed in bothcis andtrans monoenes after prolonged hydrogenation. In the hydrogenation of potassium linoleate, a drop in iodine value (IV) of 60 units occurred in the first hour, and 38% oftrans monoenes (in which the 10- and 11-monoenes constitute 32% each) were formed. The IV then fell only slowly, and up to 38% ofcis monoene (mostly 9- and 12-isomers) was formed. Addition of fresh catalyst caused a major shift ofcis monoenes totrans forms. The diene fraction was mostly nonconjugated material with the first double bond at the 9, 8 and 10-positions. Minor amounts of conjugated dienes were present as well as a dimeric product.     

Homogeneous Selective Catalytic Hydrogenation of Soybean Oil

Chelates derived from the Schiff bases of the 2,2-dialkyl propylene-1,3-diamine with salicylaldehyde have been used as homogeneous catalysts in the hydrogenation of soybean oil. Copper, iron, cobalt, nickel, palladium chelates were tested and different catalytic trends observed. The most active catalysts were the copper chelates. A marked reduction of trienes to give mostly monoenes accompanied by no marked increase of conjugated dienes or trans double bonds was observed in some of the hydrogenation experiments. Selectivity values have been calculated; the iron catalysts showed the highest selectivity values but their catalytic activities were usually lower than that of the copper chelates.     

Homogeneous catalytic hydrogenation of unsaturated fats: Metal acetylacetonates

Hydrogenation of linseed and soybean methyl esters was achieved at 100–180C, 100–1000 psi H₂ and 0.05–0.25 moles catalyst per mole of ester. The relative activity of metal acetylacetonates in decreasing order was: nickel (III), cobalt (III), copper (II) and iron (III). Reduction occurred readily in methanol solution but only slowly in dimethylformamide and acetic acid. No reduction occurred in the absence of solvents. Soybean oil was also hydrogenated rapidly with nickel (III) acetylacetonate in methanol, but in this system the triglycerides were converted to methyl esters. Nickel (III) acetylacetonate was the most selective catalyst toward linolenate hydrogenation. Methyl linoleate and linolenate hydrogenated with nickel(III) acetylacetonate were fractionated into monoenes, dienes and trienes. Thecis monoenes separated in 62 to 68% yield had double bonds in the original position. The remainingtrans monoenes had extensively scattered unsaturation. The dienes and trienes showed no conjugation, but some of the double bonds in the dienes were not conjugatable with alkali. Little stearate was formed. Presented at AOCS meeting in Chicago, 1964 No. Util. Res. and Dev. Div. ARS, USDA     

Chromatographic separation ofcis andtrans fatty esters by argentation with a macroreticular exchange resin

Methyl oleate and methyl elaidate, as well as other monoenecis andtrans isomers of fatty esters, can be separated quickly and conveniently by a preparative chromatographic procedure in which a silver-saturated ion-exchange resin is used. Separations are based on differences in stabilites of the silver-olefin complexes. Recoveries of better than 95% were made, and puretrans andcis monoene fractions were collected. This method can be used to separate saturates fromcis andtrans monoenes. Thecis,trans,cis,cis, andtrans,trans-9-12-octadecadienoates were separated. Whilecis,trans andtrans,trans dienes were cluted separately, thecis,cis diene isomer remained on the column. Presented at AOCS Meeting, in Minneapolis, 1963. A laboratory of the No. Utiliz. Res. and Dev. Div., ARS, USDA.     

Isomerization during hydrogenation of soap. I. Potassium oleate

Potassium oleate in slightly alkaline solution was hydrogenated for up to 7 hr with Rufert nickel catalyst at 150C and 20 kg/sq cm pressure. With 1% catalyst, the iodine value dropped by 12 units in the first hour, and only slightly thereafter. With 2% catalyst there was a drop of 24 units in iodine value in the first hour, a steady state for the next 3 hr, and a second sharp drop of 30 units prior to the seventh hour. Samples of fat hydrogenated over 1% catalyst for 3 hr and 7 hr respectively were analyzed by gas-liquid chromatography, thecis andtrans monoenes were separated by argentation thin-layer chromatography, and the positional isomers in each were determined by oxidation of the total fraction to dicarboxylic acids, which were then estimated by GLC. Apart from double-bond saturation during the first 3 hr of hydrogenation, extensive double-bond migration yielded 23.5% oftrans 8- to 13-monoene, accompanied by small amounts only of positionalcis monoenes other than the starting material. After 7 hr of hydrogenation, extensivecis tocis isomerization occurs, accompanied by lesscis totrans shift; thecis:trans ratio for each monoene consequently tended toward 1:1. The results are explained on the sorption mechanism of hydrogenation and suggest that soap hydrogenation, involving catalyst poisoning, may represent a magnified version of normal fat hydrogenation.     

Influence of Various Reaction Parameters on the Reactivities of Highly Unsaturated Fatty Acids During a Partial Hydrogenation on Nickel Catalyst and on the Formation of Isomeric Mono- and Di-Ethylenic Acids

A refined menhaden oil of iodine value (IV) of 189 was hydrogenated on nickel catalyst under different temperatures, rates of agitation, hydrogen pressures and nickel content. Samples were collected during each hydrogenation in order to calculate the selectivities of the different C₂₀ fatty acid classes (dienes, pentaenes). In each case, a large difference in reactivities was observed between the 20:1 and 20:2 acids, but the 20:3, 20:4 and 20:5 acids had similar behaviours toward the nickel catalyst. The reaction parameters had a great influence on the 20:2 selectivity. However they did not apparently affect the 20:3, 20:4 or 20:5 selectivity values. The 20:1 and 20:2 isomer distributions also depended on the reaction parameters. For the 20:1 acids, an accumulation of isomers with the ethylenic bonds close to the carboxyl group, especially cis Δ 5, trans Δ 5 and trans Δ 6, was observed at high temperatures. The results can be interpreted as the combination of a substantial migration of ethylenic bonds and simultaneously a preferential hydrogenation of those ethylenic bonds farthest removed from the carboxyl group.     

Linoleic acid isomers in heat treated sunflower oils

Heat treatment of sunflower oil resulted in the formation of linoleic geometrical and positional isomers. These isomers were isolated using a combination of column chromatography, urea fractionation, high performance liquid chromatography (HPLC) on a C18 reverse phase column and silver nitrate thin layer chromatography (TLC). Each component was submitted to hydrazine reduction and the resulting monoenes to AgNO₃-TLC. The resultingcis andtrans fractions were submitted to ozonolysis in BF₃-MeOH in order to determine the position of the ethylenic bonds. The major isomers were thecis, trans andtrans, cis 18∶2 Δ9, 12, thetrans, trans 18∶2 Δ9, 12 and somecis, trans, trans, cis andtrans, trans 18∶2 conjugated dienes. Thecis, trans andtrans, cis conjugated dienes were the Δ9, 11, Δ10, 12, Δ11, 13 and Δ12, 14 while thetrans, trans isomers were the Δ9, 11, Δ10, 12 and Δ11, 13. These C18∶2 isomers also were detected in oils collected from restaurants and market vendors. Presented in part at the AOCS/JOCS annual meeting in Honolulu, Hawaii, in May 1986.     

Homogeneous hydrogenation of methylcis-9,cis-15-octadecadienoate catalyzed by platinum-tin,palladium and nickel complexes

Methylcis-9,cis-15-octadecadienoate was used as a model for the hydrogenation of methyl linolenate. Homogeneous catalysis by platinum, palladium and nickel complexes produced a mixture of isomeric monoenes similar to that from the hydrogenation of methyl linolenate. These catalysts are, therefore, capable of promoting isomerization of isolated double bonds and of producing conjugated dienes which are necessary for the formation of monoenes. N. Market, and Nutr. Res. Div., ARS, USDA.     

Deuteration of methylcis-9,cis-15-octadecadienoate with nickel catalyst

Methylcis-9,cis-15-octadecadienoate was partially deuterated with nickel catalyst, and the product was separated into saturate, monoene and diene fractions. Monoenes were separated intotrans andcis fractions, and dienes intotrans,trans, cis,trans andcis,cis fractions. Monoene isomers with double bonds at the 9 and 15 positions predominated in bothcis- andtrans-monoene fractions. Considerable amounts of isomers with double bonds situated on either side of the original 9 and 15 positions were found in thetrans-monoene fraction. Diene was extensively isomerized to positional and geometrical isomers, and deuterium was incorporated into these isomers. Double bond migration was greatest intrans,trans-dienes and smallest incis,cis-dienes. The amount of deuterium in the dienes was proportional to the extent of isomerization experienced by the dienes. ARS, USDA.     

Nonmetallic palladium-on-resin: A very active and selective catalyst for the hydrogenation of diunsaturated fatty acid esters I

Methylcis-9,cis-12-octadecadienoate (methyl linoleate;c9,c12) and the correspondingcis,trans andtrans.trans geometric isomers (c9,t12 andt9,t12) were hydrogenated at 40 C and atmospheric hydrogen pressure in acetone as solvent, with nonmetallic palladium-on-resin catalyst. These catalysts were prepared by impregnation of cationic exchange resins with an aqueous solution of palladium dichloride. The methyleneinterrupted dienes were hydrogenated to the monoene stage with almost infinite selectivity, especially withc9,c12, whereast9,t12 was hydrogenated somewhat less selectively. The latter isomer was reduced considerably more slowly than the first, whereasc9,t12 occupied an intermediate position. The hydrogenation proceeded for an important part via a straightforward reduction of one of the double bonds, though conjugation prior to hydrogenation also occurred. The methylene-interrupted dienes isomerized to a high degree geometrically during hydrogenation, but they scarcely isomerized positionally, resulting in small amounts of inactive ethylene-interrupted dienes.     

Modification of vegetable oils. XI. The formation of trans isomers during the hydrogenation of methyl oleate and triolein

Summary 1. During the hydrogenation of methyl oleate, trans isomers are formed at a very rapid rate. As much as 38% of trans isomers formed while the first 10% of methyl stearte was formed. 2. The rate of formation of trans isomers in methyl oleate undergoing hydrogenation is increased by increasing the temperature, increasing the catalyst concentration, and decreasing the degree of dispersion of the hydrogen. 3. The hydrogenation of methyl oleate always resulted in the establishment of an equilibrium between cis and trans isomers, and irrespective of the conditions employed the concentration of trans isomers was always 67%, calculated on the basis of total unsaturated constituents. 4. It is concluded that all of the iso-oleic acids formed during the hydrogenation of methyl oleate adsorb hydrogen at the same rate as oleic acid and are adsorbed and desorbed from the nickel catalyst with equal ease. 5. Trans isomers are formed at a slightly lower rate during the hydrogenation of triolein than in the case of methyl oleate. 6. Partial hydrogenation of triolein also results in the establishment of an equilibrium between cis and trans isomers of oleic acid but at values of less than 67% of trans constituents (based on the total unsaturated constituents) observed with methyl oleate. The equilibrium concentration was found to vary with the conditions of hydrogenation and was found to be 62% at 200°C. and 57% at 175°C. Report of study made under the Research and Marketing Act of 1946. Presented at the 24th Fall Meeting of the American Oil Chemists’ Society, San Francisco, Calif., Sept. 26–28, 1950. One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Department of Agriculture.     

Selective hydrogenation of soybean oil: IV. Fatty acid isomers formed with copper catalysts

Two samples of soybean oil hydrogenated with copper-containing catalysts at 170 and 200 C were analyzed for their natural and isomeric fatty acids. Methyl esters of the hydrogenated oils were separated into saturates, monoenes, dienes and trienes by countercurrent distribution between acetonitrile and pentane-hexane. Monoenes were further separated intocis- andtrans-isomers on a silver-saturated resin column. Double bond location in these fractions was determined by a microozonolysis-pyrolysis technique. The diene fraction was separated with an argentation countercurrent distribution method, and linoleate was identified by infrared, ozonolysis and alkaliisomerization data. The double bonds in thecis-monoenes were located in the 9-position almost exclusively. However, the double bonds in thetrans-monoene were quite scattered with 10- and 11-isomers predominating. About 86% to 92% of the dienes consisted of linoleate as measured by alkali isomerization. Other isomers identified as minor components includecis,trans andtrans, trans conjugated dienes and dienes whose double bonds are separated by more than one methylene group. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Distribution of Isomeric Octadecenoic Fatty Acids in Commercially Hydrogenated Olive Oil

Samples of cooking fats and margarines prepared from hardened olive oil were analyzed for positional isomers of the cis and trans monoene fraction. The fats were transesterified and the methyl esters were separated by silver nitrate thin layer chromatography. The monoene fraction was recovered from the plates and the positional isomers were determined by epoxidation, cleavage of methyl epoxy-octadecanoates and gas chromatographic analysis of the aldehyde esters. Hydrogenated olive oil products contain 70–75% monoenes. 25–27% of the fatty acids are in the form of trans 18:1 isomers. The position of the double bond appears to be distributed from Δ7 to Δ13. This distribution over the carbon chain is 2–4 times higher in the trans fraction of the monoenes.     

Conjugation of polyunsaturated fats: Methyl linoleate with tris(triphenylphosphine) chlororhodium

Tris(triphenylphosphine) chlororhodium in methanol produces nearly 95% conjugated dienes from methyl linoleate. The dienes are principally cis,trans- and cis,cis-conjugated. Since deuterium is introduced into the products when deuterated methanol is the solvent, a dihydride complex is the probable intermediate. The reaction is sluggish in chloroform, possible because the dihydride cannot easily form. The catalyst is activated in chloroform with hydrogen. Since no reduction occurs in the absence of hydrogen, hydrogenation likely occurs by hydrogenolysis of the alkyl complex of the dihydride olefin rather than a transfer of both hydrogens from the hydride complex. Presented at the AOCS Meeting, New Orleans, April 1970. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Analytical13C NMR: A rapid,nondestructive method for determining the cis,trans composition of catalytically treated unsaturated lipid mixtures

High resolution natural abundance 13C Fourier transform nuclear magnetic resonance (NMR) has been found to be an effective tool for the rapid and direct determination of the cis/trans composition in partially hydrogenated and isomerized unsaturated lipids. With the eis and trans allylic carbon resonances as representative probes for double bond stereo-chemistry. Evaluation of the cis/trans composition of complex, positionally isomerized mixtures can be made without the necessity of carrying out detailed analyses of multishift olefinic carbon resonances. Migration of double bonds in monoenes and polyenes and formation of conjugated unsaturation in cata-lytically treated fats are discussed and assessed as possible sources of error in the evaluation of cis/trans isomer ratios. Carbon spin lattice relaxation times T₁ were measured for both cis and trans allylic reson-ances in isomeric mixtures of varying composition to assure quantitative intensity relationships. 13C NMR compositional analysis of complex mixtures is demonstrated.     

Analytical Problems in the Investigation of Unsaturated Acids Conversion I Determination of the Content of trans Isomers

The paper discusses the methods suitable for serial investigations of vegetable oils modification by catalytic isomerization, which comprises geometrical and positional isomerization, as well as partial polymerization and cyclization of unsaturated acids. The first part discusses the methods relating to the determination of stereoisomers. Infrared spectrophotometric method is used for the determination of isolated and conjugated trans isomers total content. Gas chromatography method in packed columns filled with cyanosilicones is suitable for the determination of the trans bonds in monoenes, dienes and trienes. In further parts of the paper discussion on the selection of methods of determination of the total content of conjugated dienes, cyclic monomers and polymers is envisaged.