Methyl linoleate, hydrogenation

In 1977, Kellogg and Fridovich [28] showed that superoxide produced by the XO-acetaldehyde system initiated the oxidation of liposomes and hemolysis of erythrocytes. Lipid peroxidation was inhibited by SOD and catalase but not the hydroxyl radical scavenger mannitol. Gutteridge et al. [29] showed that the superoxide-generating system (aldehyde-XO) oxidized lipid micelles and decomposed deoxyribose. Superoxide and iron ions are apparently involved in the NADPH-dependent lipid peroxidation in human placental mitochondria [30], Ohyashiki and Nunomura [31] have found that the ferric ion-dependent lipid peroxidation of phospholipid liposomes was enhanced under acidic conditions (from pH 7.4 to 5.5). This reaction was inhibited by SOD, catalase, and hydroxyl radical scavengers. Ohyashiki and Nunomura suggested that superoxide, hydrogen peroxide, and hydroxyl radicals participate in the initiation of liposome oxidation. It has also been shown [32] that SOD inhibited the chain oxidation of methyl linoleate (but not methyl oleate) in phosphate buffer. 

A significant portion of the neutral ethyl ether extract from the salts of carboxylated methyl linoleate consists of ,/3-unsaturated cyclic ketones. This material is produced in small but significant amounts (4-10%) (Table I) from the carboxylation of polyunsaturates but not from the corresponding monounsaturated fatty acids and esters (19). These ,/ -unsaturated cyclic ketones were identified spectroscopically (IR, UV, and NMR) as 4. This structural assignment was firmly established by mass spectral analyses before and after hydrogenation of the carbon-carbon double bond. 

Table I. The Homogeneous Hydrogenation of Methyl Linoleate (4 g) in the Presence of 0.65 mmol [ML2CL2] and 5.7 mmol SnCl2 in a Mixture of Benzene (30 mL) and Methanol (20 mL) at 90°C...

Quantitative information on the antioxidant actions of allicin (by radical scavenging through one-step allylic hydrogen transfer) against the oxidation of cumene and methyl linoleate (ML) initiated with 2,2/-azobis(isobutyronitrile) in chlorobenzene has been obtained.283... 

TABLE 3.9 Effects of Additives on the Hydrogenation of Methyl Linoleate over Palladium Catalysts 1 ... 

Direct initiation through higher valence metals involves direct electron transfer from the metal to a bond in the lipids and is the simplest mechanism for metal catalysis. Electron transfer to methyl linoleate is exothermic (AH= 62.8kJ, ISkCal), so is probably the dominant initiation mechanism with lipids (23, 27). Ab initio hpid radicals are formed directly by removing an electron from a double bond (Reaction 2) (28, 29) or, more generally, from the C H bond of any labile H in lipid molecules (e.g., allylic hydrogens) (Reaction 3), or via subsequent secondary hydrogen abstraction reactions, as designated in the bracketed reactions. 

The bis-triphenylphosphine nickel halides are not activated by tin (II) halides. However, the iodide, (03P)2Nil2, is an effective catalyst for the hydrogenation of methyl linoleate to the monoene stage. The bromide is less effective, and the chloride has very little catalytic power. None of these nickel compounds has much ability to bring about isomerization. The nickel compounds are unstable in alcoholic solutions, and the experiments reported in Table III were carried out in either benzene, tetrahydro-furan, or toluene. 

Figure 12-11 Slurry reactor for the hydrogenation of methyl linoleate.

To illustrate the principles of slurry operation, we shall consider the hydrogenation of methyl linoleate, L, to form methyl oleate, O ... 

Hydrogen is absorbed in liquid methyl linoleate, diffuses to the external surfaee of the eatalyst pellet, and then diffuses into the catalyst pellet, where it reacts with methyl linoleate, 1 to form methyl oleate, O. Methyl oleate then diffuses out of the pellet into the bulk liquid. 

The Rate Law The rate law is first-order in hydrogen and first-order in methyl linoleate. However, because the liquid phase is essentially all linoleate, it is in exeess and its eoncentration. Cl, remains virtually constant at its initial concentration, L0> for small to moderate reaction times. 

The catalytic hydrogenation of methyl linoleate was carried out in a laboratory-scale slurry reactor in which hydrogen gas was bubbled up through the liquid and catalyst. Unfortunately, the pilot-plant reactor did not live up to the laboratory reactor expectations. The catalyst particle size normally used was between 10 and 100 pm. In an effort to deduce the problem, the experiments listed in Table E12-5.1 were carried out on the pilot plant slurry reactor at 121°C. 

P12-19 The following table was obtained from the data taken in a slurry reactor for the hydrogenation of methyl linoleate to form methyl oleate. 

Dependence on the nature of the ligands of the ability of the complex to function as a catalyst is shown by the extent of conversion of methyl linoleate by the complexes PtCl2L2 and tin(II) chloride in Table V. Hydrogenation with PtBr2(PPh3)2 and tin(II) bromide is more effective than with the corresponding chloro analogs, but neither Pt(CN)2(PPha)2... 

To elucidate the mechanism of homogeneous hydrogenation catalyzed by Fe(CO)s, kinetic studies were carried out with mixtures of unsaturated fatty esters containing a radioactive label. A C-labeled methyl octadecadienoate-Fe(CO)3 complex was prepared to serve as a catalytic intermediate. Hydrogenation of methyl oleate (m-9-octa-decenoate) and palmitoleate (cis-9-hexadecenoate) and of their mixtures with methyl linoleate was also studied to determine the selectivity of this system, the function of the diene-Fe(CO)3 complex, and the mechanism of homogeneous isomerization. Mixtures of reaction intermediates with a label helped achieve unique simulation of the kinetic data with an analog computer. 

Figure 4. Rate curves for hydrogenation of a mixture of methyl linoleate-l- C and trans,trans-con ugated diene. Run 5, 0.2M Fe(CO)s (A) chemical data, (B) radioactivity data...

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