Hydrogenation conjugated polyenes

Raney-nickel was found to be selective in the hydrogenation of cyclopentadiene and cyclohexadiene and of their methyl and ethyl derivatives at 0-40 °C and 2-5 bar pressure137,138. The skeletal nickel proved to be selective in the semihydrogenation of conjugated polyenic compounds (equation 52)139. 

While initial aging did cause some loss in volume resistivity, it was no more severe in unstabilized than in stabilized compositions. The most startling effect was the marked increase in volume resistivity under severe aging conditions. This may be due to volatilization loss of DOP or cross-linking of polyvinyl chloride, both of which could reduce migration of ionic impurities, or it may involve adsorption of ionic impurities by the dark-colored, conjugated, polyene structure which forms by loss of hydrogen chloride. In any case it deserves further study. 

Strong evidence in confirmation of the above explanation of the chemical shifts of aromatic hydrogens is provided by a study of the cyclic conjugated polyene [18]annulene, which has hydrogens both inside and outside the ring ... 

The conjugated polyene, [18]annulene, IV, is a potential 4m+ 2 aromatic system. Because of steric repulsion between hydrogen atoms inside the ring, the molecule is distorted away from planarity. Nevertheless, its NMR shielding effects indicate an induced ring current in the mean molecular plane, once more in line with the decisive role of o-a-m. All evidence derived from the exclusion principle, conformational rigidity, aromaticity, electronic spectra and NMR shielding is therefore consistent with the alternative picture, and at variance with the conventional model. 

The dehydrochlorination reaction causes a color change. Marvel and co-workers (17, 19) attributed the discoloration to a conjugated polyene structure. They suggested that random dehydrochlorination to form an olefinic bond would be followed by allylically activated removal of successive hydrogen chloride units to form the polyene structure. 

Given that the myeloperoxidase enzyme see Peroxidases) within the neutrophils produces hypochlorous acid, an intriguing possibility is that it may combine with another hydrogen peroxide molecule. Such a reaction in the laboratory produces a stoichiometric quantity of singlet dioxygen ( O2), which is selectively reactive with conjugated polyenes. 

The thermal degradation of polyvinylalcohol has been studied by several workers [201—204]. The main products of degradation have been shown to be water from an elimination reaction similar to that observed with polyvinylacetate, and acetaldehyde from depolymerization. Small amounts of aldehydes and ketones of the general formula OCH-(CH=CH) -CH3 and CH3-CO-(CH=CH) -CH3 were also detected by Tsuchiya and Sumi [204]. Water is formed by a mechanism similar to that for hydrogen chloride production from polyvinylchloride, and acetic acid from polyvinylacetate, leaving a conjugated polyene... 

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