Reaction Mechanism

The basic chemical equation for hydrogénation of an unsaturated carbon-carbon double bond is shown in equation 1(1).


As the equation indicates, and as alluded to earlier, hydrogénation takes place when the carbon to carbon double bonds in the liquid oil, the solid catalyst, and the gaseous hydrogen have been brought together—usually in a heated stirred reactor. The hydrogen must be dissolved in the oil since only dissolved hydrogen is available for reaction.

Although the hydrogénation reaction may appear straightforward, it is in actuality very complicated. The esters may contain one, two, three, or more unsaturated bonds, and each double bond may be hydrogenated at a different rate, depending on its position in the molecule. There may also be simultaneous isomerization of the unsaturated bonds. Isomerization is principally geometrical, but some positional isomerization can also occur. In addition, the position of the ester on the glycerol also has some effect in determining the physical properties of the molecule.

The complexity of the reaction is further illustrated by noting that the partial hydrogénation of soybean oil results in the production of a minimum of 30 different linolenic, linoleic, and oleic esters whose cis and trans forms could produce more than 4000 different triglycerides. The marvel is that producers have learned to not only control the reaction but actually utilize its many-faceted complexity to produce a great variety of oils, shortenings and margarines, each designed to have physical properties (functional characteristics) making them specifically desirable for a particular application. In recent years there has been an additional strong impetus to also make the final products nutritionally desirable.

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