Polyenes mechanism

There is a great deal of uncertainty as to the mechanism of PVC degradation but certain facts have emerged. Firstly dehydrochlorination occurs at an early stage in the degradation process. There is some infrared evidence that as hydrogen chloride is removed polyene structures are formed (Figure 12.18). 

There is significant bond alternation, however. The bond at the ring fusion is quite long 11.539 A). A molecular mechanics calculation on this molecule that included an SCF-MO treatment of the planar conjugated system found the molecule to be slightly destabilized (4kcal/mol) relative to a polyene reference. " ... 

In the stabilization of PVC, the principal mode of action of the various stabilizer systems has been explained in terms of the Frye and Horst mechanism, i.e., substitution of labile chlorines by more stable groups. Evidence for other actions, such as HCl neutralization, addition to polyene sequences, and bimetallic complex formation have also been given. Despite the wide acceptance of the Frye and Horst mechanism, researchers have frequently contended that this could not be the dominant mechanism in the stabilization of PVC. 

PORTER J w and Lincoln r e (1950) Lycopersicon selections containing a high content of carotenes and colourless polyenes II. The mechanism of carotene biosynthesis , yfrc/iiv Biochem Biophys, 27, 390-95. 

The ability of carotenoids to act as antioxidants is closely related to their long-chain conjugated polyene structures (see Section 2.2 in Chapter 2). Two main types of antioxidant actions can be distinguished singlet oxygen quenching and reactions with radicals. The first mechanism occurs in vivo in plants and has been extensively studied in vitro. Reactions with radicals of different types have also been extensively studied in vitro under different conditions but their occurrence in vivo is still a matter of discussion. 

There is a third method proposed for classifying colorants which is in terms of the mechanism of the electronic excitation process. According to this method, organic colorants may be classified as donor acceptor, polyene, cyanine or n-rc chromogens. While this method of classification is undoubtedly of importance theoretically, it is arguably of lesser practical importance, since the vast majority of commercial organic dyes and... 

These remarks represent only the barest outline of at least two aspects of PVC degradation which have been the focus of attention for several years and remain incompletely understood namely the mechanism involved and the related problem of the involvement of HC1. Several excellent reviews give more comprehensive summaries of the earlier work (10, 11, 12). More recent work has made it clear that under appropriate conditions the presence of HC1 can affect the initiation, propagation and termination steps as well as influencing the distribution of polyene sequence lengths. In addition it can undergo photochemical addition reactions with the polyenes, i.e. the reverse of the dehydrochlorination process, as well as forming colored polyene/HCl complexes. These various possibilities will be considered in turn. 

The Lewis acid, HX, promotes further decomposition eventually giving a conjugated polyene (>6 conjugated bonds are coloured). The mechanism is possibly,... 

Dienes and polyenes have been a subject of great interest due to their important role in biology, materials science and organic synthesis. The mechanism of vision involves cis-trans photoisomerization of 11 -civ-retinal, an aldehyde formed from a linear polyene. Moreover, this kind of molecule exhibits high linear and non-linear electrical and optical properties. Short polyenes are also involved in pericyclic reactions, one of the most important classes of organic reactions. 

The dienes and polyenes are compounds which intervene in a large number of organic reactions, as will be seen in different chapters of this book. Several excellent reviews have been devoted to theoretical studies about their reactivity, with special emphasis on the mechanism of pericyclic reactions3-5. As was mentioned in the introduction, this section will only treat, as an example, the Diels-Alder reaction, since it has been the most studied one by theoreticians. Our goal is not to cover all aspects, but instead to show the high potential and usefulness of theoretical methods in order to interpret and rationalize the experimental results. In the rest of the chapter we will concentrate on the last ab initio calculations. 

For most practical purposes—and certainly for the discussions in this chapter—much simpler models are entirely adequate for a qualitative or semiquantitative rationalization of the features of PE spectra of dienes and polyenes. Before using such models, we first situate them within the genealogy of quantum-mechanical treatments. 

X = 0, CH2, CHCOOH, C(COOH)2, NH, NCH3 N(CH2CH=CH2), N(CHs)2 Cl Bobrowski and Das published a series of papers on the transients in the pulse radiolysis of retinyl polyenes31-37, due to their importance in a variety of biomolecular processes. They studied32 the kinetics and mechanisms of protonation reaction. The protons were released by pulse radiolysis, on a nanosecond time scale, of 2-propanol air-saturated solutions containing, in addition to the retinyl polyenes, also 0.5 M acetone and 0.2 M CCI4. Within less than 300 ns, the electron beam pulse results in formation of HC1. The protonated products of retinyl polyenes were found to absorb optically with Xmax at the range of 475-585 nm and were measured by this absorption. They found that the protonation rate constants of polyene s Schiff bases depend on the polyene chain... 

Neumann and Khenkin28 review most of the various oxidation methods of dienes and polyenes and their mechanisms. They obviously emphasize the difference between non-conjugated and conjugated dienes and polyenes in selected oxidation reactions. 

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