Fatty acid carbon-hydrogen bond positions

FIGURE 12.3 Carbon-hydrogen bond positions in fatty acids. (Reaney, Hertz, RB. and McCalley, WW. Vegetable oils as biodiesel. In Bailey s Industrial Oil Fat Products. Vol. 6. Shahidi F. ed. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission.)... 

Since hydrogenation is not carried to completion there is considerable scope for isomerization to occur. The mechanism by which this takes place has been described by Allen and Kiess (1955). They indicate that selective conditions tend to favor isomer formation. Both positional and geometric isomers are formed. In positional isomers, double bonds have wandered from their original position along the fatty acid carbon chain in geometric isomers the position of groups attached to carbon atoms have changed relative to each other in space from the natural cis to trans. Recently the nutritional properties of isomeric fatty acids have been questioned. Applewhite (1981) in a literature review has concluded that this concern is not substantiated by the available data. 

Odor and color stability problems were also related to the alkyl chains used for SAI. These could be traced to the oxidation of unsaturated carbons, such as oleic acid (Ci8 fatty acid with a single double bond between carbon 9 and 10, i.e. bond position 9 counted from the carboxyl carbon), linoleic acid (Cis fatty acid with two double bonds at position 9 and 12), and linolenic acid (Cis fatty acid with three double bonds at position 9, 12, and 15). Natural coconut fatty acid contains about 6% oleic acid, about 3% linoleic acid, and less than 1% linolenic acid. Tallow fatty acid contains nearly 44% oleic and about 6% of other unsaturates [20]. Partial hydrogenation of the coconut fatty acid used in the manufacture of SCI served to eliminate linoleic and linolenic acids for improved odor stability, while not eliminating oleic acid, which is important for good lather. 

Fatty acids (the carboxylic acids produced by saponification of fats) usually have an even number of carbons and may also have one or more cis double bonds. The fatty acid that has 18 carbons with a cis double bond at the 9 position, (Z)-9-octadecenoic acid, is called oleic acid. Catalytic hydrogenation of oleic acid produces stearic acid, or octadecanoic acid, another fatty acid. One of these fatty acids melts at 13.4°C and the other melts at 69.6°C. Explain which fatty acid melts at the higher temperature. 

The physical properties of TAGs are determined by the types of fatty acids that compose them for example, the number of saturated and unsaturated chains, cis-and frawi-double bonds, short and long chains, chains with even and odd numbers of carbon atoms, and esterified positions of fatty acids with glycerol carbon atoms. Fats are modified by hydrogenation, interesterification, and fractionation to produce desirable physical properties for fat-based products. 

Translating this into observed behavior, isolated double bonds behave as if there were two separate resonant systems of equal probability, so oleic acid yields (C9 + C11) and (C8+C10) hydroperoxides from the two resonance systems, respectively, in approximately equivalent amounts (18 1, Figure 5). In 1,4-diene systems, H abstraction occurs preferentially at the doubly allylic hydrogen between the two double bonds, and the resonance system with the unpaired electron extends across both double bonds with electron density focused at the central carbon (11) and electron deficient positions at external carbons 9 and 13 (18 2, Figure 5). In higher polyunsaturated fatty acids with multiple 1,4-diene structures (18 3 and 20 4, Figure 5), the resonant systems from multiple doubly allylic radicals... 

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