1. Selectivity

During the hydrogenation process not only do some of the double bonds get saturated, but some may even get relocated and/or transformed from the common cis to a trans configuration. The resulting isomers are known as iso acids. Thus, partial hydrogenation may end up in the formation of a quite complex mixture of reaction products, depending on which of the double bonds are hydrogenated, what kind and degree of isomerisation occurs and the relative rates of reaction. The following diagram illustrates some reaction paths linolenate (9c,12c,15c) may go through during hydrogenation.

For natural fats the task is yet more difficult due to the complexity of the mixture of starting materials present in each. It is noteworthy that no double bond migration was considered. One important result is that when hydrogenation is complete only one saturated product is obtained, irrespective of the pathway.

Selectivity defines the relative rate of hydrogenation of the more unsaturated fatty acids when compared with that of the less saturated acids. In terms of a ratio (selectivity ratio) it is possible to obtain a quantitative measure of selectivity in more absolute terms. This ratio is expressed by the quotient of the rate of hydrogenation of linoleic acid to produce oleic acid ÷ the rate of hydrogenation of oleic acid to produce stearic acid. Constants for each reaction rate may be determined from the starting and ending fatty acid compositions and hydrogenation time. For the above reaction and under certain conditions a selectivity ratio of 12 states that linoleic acid is undergoing hydrogenation 12 times faster than oleic acid, may be typical.

The use of different catalyts as well as operating parameters and pressures will induce varied selectivities. As indicated on the following table, greater selectivity ratio (SR) values occur at high temperatures, low pressures, high catalyst concentration and a low rate of agitation. The effect of changes of processing environments upon rate of hydrogenation and on the formation of trans acids are also shown. Quite a few mechanistic speculations have been undertaken in order to explain the significance of process conditions on selectivity and rate of hydrogenation. These concepts are important whenever attempting to partially hydrogenate an oil (a process termed brush hydrogenation).The nutritional inadequacy of trans fatty acids, currently a subject of some dispute, and the fact that when led to its completion hydrogenation no longer results in any trans fatty acids has led processors to devise fat hardening systems which are based upon total hydrogenation of part of the feedstock, followed by interesterification.

In this way the fact that industrially interesting higher rates obtained at higher temperatures invariably results in high temperatures.