Hydrogenation of linoleic esters

CATALYTIC HYDROGENATION OF LINOLEIC ESTERS REACTION COURSES ACCORDING TO THE HYPERBOLIC FORMULA... 

Fig. 92. Hydrogenation of linoleic esters at a relatively high temperature (i8o°C). 2% Ni-guhr, i8o°C, 1 atm.

The catalytic hydrogenation of linoleic esters has also been treated kinetically by Boelhouwer et al,88. If the velocity constants of the consecutive reactions are represented by kx and k2 and first order kinetics are assumed, the following equations hold ... 

Hydrogenation of linoleic esters (Cig) a shell catalyst with Pd on activated carbon is recommended... 

The hydrogen atoms of the -CH2- group located between the two double bonds of linoleic ester (Lin-H) are especially susceptible to abstraction by radicals. 

Hydrogen abstraction from another molecular of the linoleic ester... 

Figure 10.7 Autoxidation of a linoleic acid ester. In step 1 the reaction is initiated by the attack of a radical on one of the hydrogen atoms of the -CH2-group between the two double bonds this hydrogen abstraction produces a radical that is a resonance hybrid. In step 2 this radical reacts with oxygen in the first of two chain-propagating steps to produce an oxygen-containing radical, which in step 3 can abstract a hydrogen from another molecule of the linoleic ester (Lin-H). The result of this second chain-propagating step is the formation of a hydroperoxide and a radical (Lin ) that can bring about a repetition of step 2.

A mixture of palladium chloride and triphenylphosphine effectively catalyzes carboxylation of linoleic and linolenic acids and their methyl esters with water at 110°-140°C and carbon monoxide at 4000 psig. The main products are 1,3-and 1,4-dicarboxy acids from dienes and tricarboxy acids from trienes. Other products include unsaturated monocar-boxy and dicarboxy acids, carbomethoxy esters, and substituted a,J3-unsaturated cyclic ketones. The mechanism postulated for dicarboxylation involves cyclic unsaturated acylr-PdCl-PhsP complexes. These intermediates control double bond isomerization and the position of the second carboxyl group. This mechanism is consistent with our finding of double bond isomerization in polyenes and not in monoenes. A 1,3-hydrogen shift process for double bond isomerization in polyenes is also consistent with the data. 

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. 

An accurate investigation of these problems was, however, only possible after the development of reliable methods of analysis. Particularly Bertram s method55 for the determination of saturated fatty acids should be mentioned. Van Vlodrop70 proved its usefulness for the study of the hydrogenation of several fatty acid esters (oleic, linoleic, elaeostearic). 

Ethyl linoleate is prepared by debromination of the tetra-bromide by action of zinc, or nascent hydrogen from zinc and glacial acetic acid 3 by zinc and alcoholic-hydrochloric acid 2, 4 and by zinc and alcoholic-sulfuric acid.5 The pure acid can be obtained by saponification of the ester, and directly by action of zinc and pyridine (quinoline, aniline, piperidine) on tetrabromo-stearic acid.6... 

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. 

We found previously (10) that in a natural mixture of mono-, di-, and triunsaturated fatty esters the hydrogenation of monoenes with Fe(CO)s was minor. Therefore, competitive hydrogenation studies were carried out with an equal mixture of methyl oleate and linoleate. Diene hydrogenation in such a mixture was indeed dominant (Figure 2). At O.IM initial concentration of Fe(CO)s the formation of stearate was a minor reaction the diene-Fe(CO)3 complex reached a maximum of 4% and remained constant. On the other hand, at 0.5M Fe(CO)5, stearate formation became a more important reaction diene-Fe(CO)3 reached a maximum of 7% and decreased during the course of hydrogenation. Free conjugated dienes were minor products. 

The Fe(CO)4 intermediates of types III and VIII in Scheme IV explain the direct reduction paths evidenced in the hydrogenation of mono- and diunsaturated fatty esters. Competition between monoene and diene hydrogenation can be related to the stability of the Fe(CO)3-and Fe(CO)4-complexes. At a low concentration of Fe(CO)5, the formation of Fe(CO)a complexes is favored because they are more stable. At a high concentration of Fe(CO)s, formation of mono- and di-Fe(CO)4 complexes becomes important, and selectivity for diene hydrogenation is decreased. Although the occurrence of olefin-Fe(CO)4 complexes has precedence in the literature (i9), no such species has yet been identified with either methyl oleate or linoleate. 

Liquid vegetable oil is a mixture of linolenic, lin-oleic, oleic, and stearic esters. The hydrogenation process of these esters displays a chart of a reaction chain shown in Scheme 10. The processes of preferential hydrogenation of more unsaturated acids with minimum formation of completely saturated fatty acids are preferred by the food industry. The selectivity is expressed as the ratio imo/ oie or k o/ko, where the variables k are the reaction rate constants, and the term fiino represents the conversion of linolenic and linoleic compounds, while ole is for oleic. Thus, good processes have high k o/ko values. 

Fig. 4.3. (A) Analytical catalytic reactor with bypass. 1 = Catalyst 2 = glass-wool 3 = stainless-steel capillary. (B) Chromatograms of methyl esters of Cj, acids after hydrogenation using the analytical catalytic reactor with bypass. Peaks 1 = methyl stearate 2 = methyl oleate 3 = methyl linoleate 4 = methyl linolenate. Reprinted with permission from ref. 79.

The fatty acid methyl esters were analyzed using GC (Model 5890, Hewlett-Packard, Wilmington, DE) equipped with a capillary column (SPB-1,15 m x 0.32 mm, 0.25-pm film thickness, Supelco). Helium was used as the carrier gas at 1 mL/min. The initial column temperature was 160°C, heated at a rate of 5°C/min to 210°C, and then kept at 210°C for 30 min. Detection was done by a flame ionization detector with hydrogen, helium (make-up gas), and air at 27, 29, and 445 mL/min, respectively. The amounts of linoleic acid, THOA, and DEOA were calculated from the peak area relative to that of palmitic acid (internal standard) using stan-... 

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