Hydrogenation of linoleic acid

Recently, Ir/Al2C>3 catalyst was tested in the hydrogenation of linoleic acid at 140 °C and 300 torr hydrogen pressure. The A-12 double bond showed the highest reactivity in the reduction process49. 

Table 3.6 shows the results by Allen and Kiess on the proportion of the trans isomers in each positional isomer formed in the hydrogenation of linoleic acid over a nickel catalyst under nonselective and selective conditions.96 Nonselective conditions result in rather low amounts of trans isomers, especially in the 9- and 12-monoenoic acids, while more 9- and 12-trans monoenes are formed under selective hydrogena-... 

TABLE 3.6 Positional and Geometric Isomers in Monoenoic Acids from Partial Hydrogenation of Linoleic Acid ... 

Gerson, T., King, A.S.D., Kelly, K.E., and Kelly, WJ. 1988. Influence of particle size and surface area on in vitro rates of gas production, lipolysis of triacylglycerol and hydrogenation of linoleic acid by sheep rumen digesta or Ruminococcusflavefaciens. J. Agric. Sci. Camb. 110, 31-37. 

One important aim in oleochemistry hydrogenations is the selective hydrogenation of multiply unsaturated fatty compounds to singly unsaturated products. A typical example is the selective hydrogenation of linoleic acid (C18 2) to oleic acid (C181) without significant formation of stearic acid (D18 0) as shown in Eq. (1). This... 

A water-soluble rhodium hydrogenation catalyst has been found by Patin and co-workers [38]. Hydrogenations are carried out in very mild conditions, with Rh/ TPPTS in HzO/EtOH (1 1) and the catalyst is not air-sensitive and is easily prepared. However, the hydrogenation of linoleic acid is not selective, but yields only stearic acid. 

Scheme 1 Mechanistic proposal for the selective hydrogenation of linoleic acid with Pd SSCs (L,Pd—H = stabilized palladium hydrido species).

Acid hydrolysis of tristearoyl glycerol Reaction of decanoic acid with KOH Hydrogenation of linoleic acid... 

Vaccenic acid (frani-ll-18 l VA) is the predominant trans monounsaturated fatty acid in ruminant miUc and tissue fat (50,74). This acid is produced in the rumen as a stable intermediate by hydrogenation of linoleic acid-derived cis-9, trans-l 1-18 2 or from a- and y-linolenic acids by a series of biotransformadons (5). A linear relationship has been demonstrated in milk fat between VA and CLA over a wide range of concentrations. Regression equations show there is about twice the content of VA as CLA (5,75). Reporting the VA levels in food items is beyond the scope of this chapter, but information may be obtained from other publications (21,25,31,64,70). 

Figure 3.24 Course of selective hydrogenation of linoleic acid.

If first-order kinetics are assumed, k /is the linoleic selectivity ratio and k l is the selectivity ratio for reduction of linoleic acid to stearic acid. Figure 2 shows a typical course of hydrogenation for soybean oil the rate constants are = 0.367, = 0.159, and k = 0.013. With a selective nickel catalyst,... 

Inhibition and stimulation of LOX activity occurs as a rule by a free radical mechanism. Riendeau et al. [8] showed that hydroperoxide activation of 5-LOX is product-specific and can be stimulated by 5-HPETE and hydrogen peroxide. NADPH, FAD, Fe2+ ions, and Fe3+(EDTA) complex markedly increased the formation of oxidized products while NADH and 5-HETE were inhibitory. Jones et al. [9] also demonstrated that another hydroperoxide 13(5)-hydroperoxy-9,ll( , Z)-octadecadienoic acid (13-HPOD) (formed by the oxidation of linoleic acid by soybean LOX) activated the inactive ferrous form of the enzyme. These authors suggested that 13-HPOD attached to LOX and affected its activation through the formation of a protein radical. Werz et al. [10] showed that reactive oxygen species produced by xanthine oxidase, granulocytes, or mitochondria activated 5-LOX in the Epstein Barr virus-transformed B-lymphocytes. 

Many EOs also exhibit antioxidant activity and therefore several studies have been carried out in order to elucidate the activity of the components [139,153]. For instance, y-terpinene retarded the peroxidation of linoleic acid [139, 154-156], sabinene showed strong radical-scavenging capacity [139, 157], a-pinene [158] and limonene [146] showed low antioxidant activity in the 2,2-diphenyl-1-picrylhydrazyl (DPPH) test, while terpinene and terpinolene showed high hydrogen-donating capacity against the DPPH radical [146, 150, 155, 158],... 

These peroxyl radicals then attack another molecule of linoleic acid to abstract an allylic hydrogen and produce the conjugated diene hydroperoxide, LOOH, Equation 7. 

Ratnayake, W.M.N., Pelletier, G. 1992. Positional and geometric isomers of linoleic acid in partially hydrogenated oils. J. Am. Oil Chem. Soc., 69, 95-105. 

---