Selective hydrogenation with copper catalysts: IV. Reaction of stearolate,oleate and conjugated esters with deuterium

β-Eleostearate andtrans,trans-conjugated diene were reduced with deuterium in the presence of copper chromite. Considerable exchange of hydrogen atoms of the starting material for deuterium atoms was observed. Conjugated double bond systems isomerized (positional and geometric) extensively during hydrogenation. Isomerization and exchange reactions occurred at a faster rate than hydrogen addition reaction. During early stages of the reaction, a large amount of the product formed contained no deuterium. Of the three possible mechanisms of hydrogen addition toβ-eleostearate (1,2; 1,4 and 1,6), no one mechanism could account for all the products formed. Reduction oftrans-9,trans-11-octadecadienoate at a higher temperature and pressure (200 C, 500 psi) caused a minimum of exchange and isomerization apparently due to sintering of the catalyst. Monoenes were formed fromtrans,trans-conjugated diene by both 1,2 and 1,4 addition. Methyl oleate was not reduced, but extensive isomerization occurred. Deuterium was incorporated into isomeric monoenes. Mechanistic schemes are proposed to account for exchange, isomerization and hydrogen addition. Presented in part at the AOCS Meeting, New Orleans, April 1970.     

Selective hydrogenation with copper catalysts: V. Kinetics and mechanism at high pressure

The mechanism of hydrogenation at 900~950 psi with copper-chromite catalyst was investigated with pure methyl esters as well as their mixtures. A comparison of double bond distribution intrans-monoenes formed during hydrogenation of linoleate and alkali-conjugated linoleate revealed that 85~95% of the double bonds in linoleate conjugated prior to hydrogenation. The mode of hydrogen addition to conjugated triene and diene at high pressure is similar to that at low pressure but positional and geometric isomerizations of unreduced conjugated esters were less at high pressure. Geometric isomerization of methyl linoleate and linolenate was considerable at high pressure whereas it was negligible at low pressure. The absence of conjugated products during hydrogenation of polyunsaturated fatty acid esters resulted from their high reactivity. Conjugated dienes are 12 times more reactive than the triene, methyl linolenate, and 31 times more reactive than the diene, methyl linoleate. The products of methyl linolenate hydrogenation were the same as those predicted by the conjugation mechanism. Presented at the 70th Annual Meeting of the American Oil Chemists' Society, San Francisco, April 29~May 3, 1979.     

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.     

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.     

Kinetics of hydrogenation of conjugated triene and diene with nickel,palladium, platinum and copper-chromite catalysts

A mixture of methyl linoleate and alkali-conjugated methyl linoleate was reduced with nickel, palladium, platinum and copper-chromite catalysts. The course of hydrogenation was followed by gas liquid chromatography of samples withdrawn at intervals. Relative rate constants of reactants and inermediates were calculated by a computer. Conjugated linoleate was 10–18 times more reactive than methyl linoleate with all catalysts except platinum, which showed no selectivity at 60 C. At 150 C conjugated diene reacted four times faster than methyl linoleate with platinum catalyst. A conjugated diene-to-stearate shunt was observed with palladium and platinum catalysts. When β-eleostearate was hydrogenated with the same catalysts, 50–97% of the triene was reduced directly to monoene with all catalysts except copper chromite, which selectively reduced conjugated triene to conjugated diene. On the basis of present kinetic data and previous knowledge about the mode of hydrogen addition to conjugated systems, a scheme has been proposed to account for the products formed during hydrogenation of methyl linolenate. ARS, USDA.     

Homogeneous catalytic hydrogenation of unsaturated fats: Cobalt carbonyl

Homogeneous hydrogenation of unsaturated fats by cobalt carbonyl has been compared with the previously reported catalysis by iron carbonyl. Soybean methyl esters, methyl linoleate and linolenate have been hydrogenated at 75–180C, 250–3,000 psi H₂ and 0.02 molar concn of catalyst. The cobalt carbonyl catalyst is more active at lower temp than iron carbonyl. The partially reduced products are similar to those observed with iron carbonyl, but the reaction differs in showing much less accumulation of conjugated dienes, no selectivity toward linolenate, almost complete absence of monoene hydrogenation to saturates, less double bond migration and moretrans isomerization. No evidence was found for a stable complex between cobalt carbonyl and unsaturated fats as previously observed with iron carbonyl. The rates of hydrogenation/double bond were the same for linoleate and linolenate on one hand, and for alkali-conjugated linoleate and nonconjugated linoleate on the other. Presented at AOCS Meeting in Minneapolis, 1963. A laboratory of the No. Utiliz. Res. & Dev. Div., ARS, USDA.     

Hydrogenation of unsaturated fatty esters with copper-chromite catalyst: Kinetics,mechanism and isomerization

Hydrogenation of linolenate with copper chromite produced a large amount of conjugated diene and minor amounts of nonconjugatable dienes. The double bonds in conjugated dienes and monoenes were scrambled all along the chain. This product distribution can be explained if it is assumed that conjugation of the double bonds is followed by hydrogenation. In competitive hydrogenation, fatty esters with conjugated double bonds were reduced preferentially over fatty esters with methylene-interrupted double bonds. Isomerization of conjugated double bonds (geometric and positional) occurred more rapidly than reduction. Reduction of conjugated double bonds in the presence of deuterium resulted in a majority of the products containing no deuterium. Most of the added deuterium was incorporated into the unreacted material. Mechanisms are proposed to account for the products formed during the hydrogenation of linolenate, linoleate and their isomers. One of 10 papers to be published from the Symposium “Hydrogenation,” presented at the AOCS Meeting, New Orleans, April 1970. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Homogeneous hydrogenation of dienes with rhodium complexes

Different Rh complex catalysts were compared for the hydrogenation of methyl sorbate and linoleate in the absence of solvents. At 100 C and 1 atm H₂ the following complexes, RhCl(Ph₃ P)₃ (Ph= phenyl), [RhClNBD]₂ (NBD=norbornadiene) and RhH(CO)(Ph₃P)₃, produced mainly methyltrans-2-hexenoate (34 to 56%). Their diene selectivity was not particularly high as they produced 14 to 41% methyl hexanoate. With RhCl(Ph₃ P)₃ constant ratios between rates of methyl sorbate disappearance and formation of methyltrans-2- andtrans-3-hexenoate indicate approximately the same activation energy for 1,2-addition of H₂ on the Δ4 double bond of methyl sorbate and for 1,4-addition to this substrate. In the hydrogenation of methyl linoleate with RhCl(Ph₃ P)₃, the kinetic curves were simulated by a scheme in which 1,2-reduction was more than twice as important as 1,4-addition of H₂ via conjugated diene intermediates. Although the complexes RhCl(CO)(Ph₃ P)₃ and [Rh(NBD)(diphos)]⁺PF₆ (diphos=diphosphine) were inactive in the hydrogenation of methyl sorbate, they catalyzed the hydrogenation of methyl linoleate at 100 C and 1 atm. Catalyst inhibition apparently was caused by stronger complex formation with methyl sorbate than with the conjugated dienes formed from methyl linoleate.     

Selective hydrogenation with tris(triphenylphosphine) chlororhodium (I) catalyst: Preparation of octadecenoate isomers

Fractionation of products obtained from partial catalytic hydrogenation of methylcis-9,cis-12-octadecadienoate (9c,12c-18:2) with tris(triphenylphosphine) chlororhodium [RhCl(Ph₃P)₃] provided a facile method for preparation of a nearly equal molar mixture of methylcis-9- andcis-12-octadecenoate (9c-18∶1 and 12c-18∶1). Isolation of products was achieved by silver resin and C18 reverse phase liquid chromatography. Catalytic deuteration of 9c,12c-18∶2 yields a mixture of 9c-18∶1-12,13-d₂ and 12c-18∶1-9,10-d₂ with an isotopic purity of 85%. Final isolated yield of the mixture of 9c- and 12c-18∶1 products was 30%. Isolation of products from partial hydrogenation of conjugated octadecadienoates (9c,11t-18∶2 or 10t,12c-18∶2) provided a convenient method for synthesis of an almost equal molar mixture of methyltrans-10 andtrans-11-octadecenoate (10t-18∶1 and 11t-18∶1). Characterization of the reaction products from hydrogenation of 9c,12c-28∶2 indicates that the 9c- and 12c-18∶1 products are formed by the expected 1,2-hydride addition. The presence of small amounts of 10t- and 11t-18∶1 and conjugated octadecadienoates was evidence for a secondary isomerization-1,4-hydride addition pathway. Isolation and characterization of products from RhCl(Ph₃P)₃-catalyzed hydrogenation of 9c,11t-18∶2 and 10t,12c-18∶2 indicate that both 1,2- and 1,4-hydride addition to the conjugated diene isomers occurs at about equal rates, but only thecis bond is reduced by the 1,2-hydride addition pathway and the 1,4-hydride addition pathway yields only atrans-18∶1. Because of this unusual selectivity for acis bond conjugated with atrans bond, hydrogenation of both 9c,11t-18∶2 and 10t,12c-18∶2 yields the same mixture of t-18∶1 isomers.     

Isomerization during hydrogenation. III. Linoleic acid

Summary The isomerization that takes place during the catalytic hydrogenation of linoleic acid and methyl linoleate producescis andtrans 9, 10, 11, and 12 monoenes. The double bond at the 12 position appears to hydrogenate slightly faster than that in the 9 position. More octadecenoic acids with double bonds at the 10 or 11 positions are produced during a selective (high temperature, low pressure) hydrogenation than during a non-selective process. Although the degree of selectivity of the hydrogenation is determined by the amount of isomerization of the original pentadiene system to a conjugated diene, only part of the methylene-interrupted diene goes through this type of isomerization even during a highly selective hydrogenation. The half hydrogenation-dehydrogenation reaction mechanism is applied to explain the simultaneous positional and geometrical isomerizations.     

Conjugation of polyunsaturated acids

The isomerization reaction of methyl linoleate and methyl linolenate with potassiumt-butoxide has been investigated. The compositions of the reaction products formed at three temperatures have been determined and the relationships between these analyses and observed differences in absorptivities by UV spectrometry are discussed. Conclusions concerning the reaction mechanisms are based on compositional analysis and results of experiments using radioactive or stable isotope labeled reagent. Double bonds in molecules which are not conjugated during the reaction retain the originalcis configuration. The double bond in the Δ12 position is the most susceptible to positional isomerization to form the conjugated system. With the diene, this selectivity is small, while with the triene, the shifting of the Δ12 bond is the preponderant initial reaction. Isotopic experiments yielded direct evidence for the postulated carbanion mechanism of reaction. An activated methylene group is generally required for the formation of the carbanion. While the UV spectra of the reaction products formed from methyl linolenate at 140 C showed no peak in the diene region, 34% conjugated diene-triene was present. The intact conjugated systems can migrate when the reaction is sufficiently energentic to produce conjugated trienes with double bonds other than the 10, 12, 14 system. The conjugation of triene is a stepwise reaction through the conjugated dienetriene. This paper reports in part research submitted to satisfy thesis requirements for a Master’s Degree at Bradley University. Bond Award paper. Award presented at the 43rd AOCS Fall Meeting, Minneapolis, October 1969. No. Utiliz. Res. Dev. Div., ARS, USDA.     

Selective hydrogenation of soybean oil: X. Ultra high pressure and low pressure1

Soybean oil was partially hydrogenated with copper-chromite catalyst at 170 C and up to 30,000 psig hydrogen pressure. Catalyst activity increased with increase in pressure up to 15,000 psig. The linolenate selectivity (S_Ln) of the reaction remained essentially unchanged over 50–1000 psig pressure range. A S_Ln of 5.5 to 5.6 was achieved at 15,000 to 30,000 psig pressure range. This value is somewhat lower than the selectivity at 50–1000 psig, but much higher than that obtained with nickel catalysts. Geometric isomerization increased as pressure increased up to 200 psig; above this pressure, the percenttrans remained the same up to 500 psig.trans Isomer content decreased when the pressure was increased to 30,000 psig. cis,trans Isomerization of linoleate was greater at 1000 psig and 15,000 psig than at 50 psig. At 15,000 psig, part of the linoleate in soybean oil was hydrogenated directly without prior conjugation, whereas at low pressures, all of the double bonds first conjugate prior to hydrogenation. This difference in mechanism might explain the lower selectivities obtained at high pressures. Conjugated diene isomers were found in the products up to 200 psig. Above this pressure conjugated diene was not measurable. No significant differences were found in the double bond distribution oftrans monoenes even though the amount oftrans monoene formed decreased as pressure was increased to 30,000 psig. 1 Presented at the AOCS meeting, San Francisco, May 1979.     

Mechanism of hydro-isomerization of methylene-interrupted dienes on nonmetallic palladium-on-resin catalysts III

A mixture of conjugated dienes, obtained by alkali-isomerization of methylcis-9,cis-12-octadecadienoate (c9,c12) has been hydrogenated on a nonmetallic palladium-on-resin catalyst at 40C under atmospheric hydrogen pressure in acetone as solvent. The results have been used to quantify the contribution of the conjugation mechanims to the hydrogenation process ofc9,c12. In a 50∶50 mixture conjugated dienes are hydrogenated 4.6 times faster thanc9,c12 at 40 C. This diene is hydrogenated very selectively both by straightforward reduction of one of the double bonds and by reduction of conjugated intermediates. The contribution of the former reaction route is more than 50% under the reaction conditions used. It is assumed that the crucial intermediate in the associative mechanism is formed by hydrogen transfer from the metal ion toc9,c12 resulting in a chelating ring consisting of an ethylene bridge between the π-coordinated double bond and the σ-coordinated bond. This σ, π-complex is responsible for the straightforward, selective hydrogenation ofc9,c12 as well as for the geometric isomerization of this diene.     

Hydrogenation of alkali-conjugated linoleate: Isomeric monoene profiles obtained with nickel,palladium and platinum catalysts

Alkali-conjugated linoleate (cis-9,trans-11- andtrans-10,cis-12-octadecadienoate) was hydrogenated with nickel, palladium and platinum catalysts. Thetrans andcis monoenes formed during reduction were isolated, and their double bond distribution was determined by reductive ozonolysis and gas liquid chromatography. About 44–69% of the monoenes were composed of δ¹⁰ and δ¹¹ trans monoene isomers, whereas the δ⁹ and δ¹² cis monoenes amounted to 20–26%. With nickel catalyst, composition of monoene isomers remained the same, even when the hydrogenation temperature was increased. The monoene isomer profiles between nickel and palladium catalysts were indistinguishable. Isomerization of monoenes with platinum catalyst was suppressed at 80 psi. The position of the double bonds in unreacted conjugated diene was always retained, except with nickel at both temperatures and with platinum at 150 C when a slight migration occurred. Geometrical isomerization totrans,trans-conjugated diene was observed in the unreacted diene with nickel (ca. 15% of diene) at both 100 C and 195 C, and with platinum (ca. 7% of diene) at 150 C. ARS, USDA.     

The heats of hydrogenation of unsaturated hydrocarbons

Recent precise experimental work on the heats of hydrogenation of hydrocarbon compounds containing one or more double bonds shows a variation from compound to compound which can be correlated with the structure of the molecule. For compounds containing one double bond or several non-conjugated bonds, the heat of hydrogenation per double bond is nearly constant. However, in compounds containing conjugated double bonds the average heat of hydrogenation per bond is less than in non-conjugated systems due to the interaction energy between the double bonds tending to stabilize the system. This effect is most pronounced in aromatice compounds. Although the hydrogenation of benzene to form cyclohexane (addition of three molecules of hydrogen per molecule of benzene) is an exothermic reaction, the hydrogenation of benzene to form cyclohexadiene −1,3 (addition of one hydrogen molecule per molecule of benzen) is an endothermic reaction. This signifies that the energy associated with the aromatic character of benzene is greater than the energy of hydrogenation of a double bond, so that the resultant of these two effects is an endothermic reaction.     

Isomerization phenomena during hydrogenation of methyl oleate and methyl elaidate over nickel-silica catalysts

Methyl oleate was hydrogenated at temperatures varying from 50–175 C over three nickel-silica catalysts of different pore-size distribution. Methyl elaidate was reduced over one of these catalysts at temperatures between 75–150 C. From the detailed double bond distributions information was obtained on transport phenomena in the pores of the catalyst. It was established that the migration of the double bond in methyl oleate proceeds with a stepwise mechanism, and evidence was obtained that the double bond in methyl elaidate migrates significantly faster than that in methyl oleate, while the rate of hydrogenation of these esters was equal. Thetrans-cis ratio of the geometrical isomers which are formed by double bond migration varies strongly during hydrogenation.     

Reaction mechanism of poly(vinyl chloride) degradation. Molecular orbital calculations

Semiempirical Molecular Orbital Calculations (MNDO AM1) support kinetic results concerning the molecular mechanism of thermal degradation of PVC and show that under special conditions radical and ionic mechanisms are also possible. The degradation of poly(vinyl chloride) is a complex chain dehydrochlorination that consists of an initiation process to generate an active intermediate followed by chain reactions that generate additional active intermediates with progressively increased numbers of double bonds. Each intermediate partitions between an intermediate with one more double bond and a stable conjugated polyene with the same number of double bonds. At low and moderate temperatures thermal degradation of PVC in an inert atmosphere is a succession of molecular concerted reactions. The initiation process is a 1,2-elimination through a four center transition state requiring a synperiplanar conformation. There are two main chain reactions: the first is a 1,4-elimination from allylic chlorine atoms and methylenes cis to a double bond through a transition state of six centers; the second is a 1,3-rearrangement of hydrogen atoms catalyzed by hydrogen chloride. The chain reaction is interrupted when a relatively stable trans double bond is formed and no hydrogen chloride is present to catalyze trans-cis isomerization or 1,3-rearrangement. Macro carbocations formed by heterolysis of carbon-halogen bonds in the presence of strong Lewis acids react much faster than does the original PVC in concerted elimination by 1,2-syn or 1,4-cis mechanisms, promoting a so-called catastrophic, very fast degradation. Macro radicals formed by thermal homolysis, irradiation or reaction with promoters can also promote very fast hydrogen chloride elimination because of a special mechanism consisting of a 1,2-rearrangement of a chlorine atom followed by a concerted 1,3-elimination through a five center transition state.     

Reactions of conjugated fatty acids. VIII. Dibasic acids by hydrogenation and oxidative cleavage

Summary Conjugated linoleic acid can be hydrogenated as sodium soap in an aqueous or ethylene glycol solution with commercial nickel catalysts. Under suitable conditions the acid is reduced predominantly to monounsaturated acids with only a slight increase in saturated acids. An alkali-conjugation reaction mixture may be hydrogenated without isolating the conjugated acids. One set of conditions found suitable for hydrogenation is as follows: 10 g. of conjugated linoleic acid, 7 g. of sodium hydroxide, 250 ml. of water, and 0.05 g. of nickel placed under 40 p.s.i. hydrogen pressure and heated at 140°C. for 1 hour. Acids prepared from this reaction mixture have an iodine value of about 90. Oxidation and chromatographic analyses of the resultant dibasic acids indicate that with alkali-conjugated linoleic acid, 1,2, 1,4, and 3,4 addition of hydrogen take place with equal ease. The reduced acids contain 66%trans acids. Withtrans,trans conjugated linoleic acid, 1,4 addition takes place to a greater extent that 1,2 and 3,4 addition, and the reduced acids are allrans. Presented at the fall meeting, American Oil Chemists' Society, Cincinnati, O., September 30–October 2, 1957. Part VII is in press, Journal of Organic Chemistry.     

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.     

On the mechanism of the copper‐catalysed hydro‐genation; a reinterpretation of published data

The copper‐catalysed hydrogenation of triglyceride oils differs from the nickel‐catalysed reaction in that copper catalysts only hydrogenate double bonds in methylene interrupted polyunsaturated fatty acid moieties. Accordingly, the copper‐catalysed reaction stops when the triglycerides present in the reaction mixture are only monounsaturated fatty acids and polyunsaturated fatty acids in which the double bonds are separated by more than one methylene group. Copper catalysts thus exhibit a very high oleic acid selectivity. This observation has been explained in the literature by assuming that copper catalysts can only catalyse the hydrogenation of conjugated polyenes and that they are also capable of catalysing the conjugation of methylene interrupted polyenes. Accordingly, the hydrogenation of linolenic acid and linoleic acid moieties starts with their conjugation which is then followed by hydrogen addition to these conjugated acids. For both reactions (conjugation and hydrogenation) the literature assumes the Horiuti‐Polanyi mechanism stipulating the addition of a hydrogen atom to a double bond as the first step. Reinterpretation of the literature data now leads to the hypothesis that the first step in the conjugation mechanism could well be the abstraction of a hydrogen atom from an allylic methylene group rather than the addition of a hydrogen atom to a doubly bonded carbon atom. A conjugated double bond system then results from the addition of a hydrogen atom to the allylic radical formed by the foregoing hydrogen abstraction.