Conjugated dienes structure

On the other hand, BF3 (see Basic Protocol 1), as well as other acidic catalysts, will change the double-bond configuration of fatty acids that contain conjugated dienes. As research on conjugated linoleic acid (CLA) and other conjugated fatty acids becomes more popular, it is essential not to provide misinformation about compositional analysis due to improper application of a methylation protocol (Li and Watkins, 1998). The basic catalysts perform better on lipids rich in fatty acids with unique conjugated diene structures. Isomerization and artifacts are not produced when sodium methoxide or TMG are used as transesterification agents... 

Reversed-phase HPLC has been used to analyze the oxidation products of triacylglycerols in edible oils. The detection is often based on monitoring the conjugated dienes with an ultraviolet detector (234-235 nm). However, the UV detector provides no information about oxidation products without a conjugated diene structure, e.g., products of oleic acid. Information about these compounds is important when oils with a high oleic acid content are studied. The most common universal detector types—refractive index and flame ionization detectors—are not sensitive enough to detect small amounts of oxidation products. 

Let us bear in mind that for compounds of the 3-ionone series we can consider two possibilities of reaction through six-center transition states leading respectively to heterocyclic rings (lactone or oxide) and to isomers with exocyclic conjugated diene structures. The two mechanistic t3rpes are shown in Figure 1. 

Hydroperoxides have geometric as well as positional isomers on lipid chains. When the hydrogen is abstracted at an allylic carbon, the double bond shifts one carbon to a position (3 to the abstraction site, and it reforms in the trans rather than cis configuration. The fran5,c/i-conJugated diene structure is retained whether oxygen adds or not, and provides the first detectable intermediate in lipids during autoxidation (238). 

The detector of choice for CLA is UV spectrophotometry, due to the strong chro-mophore of the conjugated diene system. A diode array detector has also been used that increased the capacity to identify the presence of the conjugated diene structure because it allows the UV spectrum of each eluting peak to be revealed (7). 

RGaCtions. Because of the rich chemistry associated with conjugated diene structures, the reactivity of 1,3-butadiene has been studied extensively. However, the hundreds of polymers and copolymers described in the literature represent by far the most important commercial uses for butadiene. 

Note that the details of a conjugated diene structure will determine relative stabilities of 1,2- and 1,4-addition products, so the preceding statements concerning kinetic and thermodynamic product ratios of butadiene should be considered guidelines only and each new molecule needs to be carefully analyzed. 

Another difficult structural problem where NMR spectroscopy was used to confirm the correct structure of a complex compound was in the analysis of photosantonic acid [ ]. The structure proposed for this compound contained a conjugated diene group however, the ultraviolet spectrum of the compound did not have an absorption band in the region where conjugated dienes absorb. Using NMR spectroscopy, it was shown that the conjugated diene structure did exist. 

The mechanism by which BHT, a simple phenolic, functions is shown in Figure 14.3. A hindered phenol, when used alone, will react with two radicals. It then becomes spent and it wiU no longer protect the polymer. These reactions are only simplihed overviews. More complex and undesirable side reactions can occur. For example, quinone formation by the reaction of an alkoxy radical at the para position of the phenoxy radical leads to color development because of the conjugated diene structure [18]. 

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