Formation mechanisms preservation products

I.A. Taub, Reaction mechanisms, irradiation parameters, and product formation, in Preservation of Food by Ionizing Radiation, Vol. II, E.S Josephson and M. Peterson (eds ), CRC Press, Boca Raton, 1983, pp. 125-166. 

Ice formation is both beneficial and detrimental. Benefits, which include the strengthening of food stmctures and the removal of free moisture, are often outweighed by deleterious effects that ice crystal formation may have on plant cell walls in fmits and vegetable products preserved by freezing. Ice crystal formation can result in partial dehydration of the tissue surrounding the ice crystal and the freeze concentration of potential reactants. Ice crystals mechanically dismpt cell stmctures and increase the concentration of cell electrolytes which can result in the chemical denaturation of proteins. Other quaHty losses can also occur (12). 

The active nickel catalyst contains one bidentate phos-phinite ligand and the overall mechanism of the reaction is believed to be similar to butadiene hydrocyanation except that the final reductive elimination step is irreversible under the conditions of the reaction. jr-Allyl intermediates (7) are believed to play an important role in the exclusive formation of the branched nitrile product observed. Formation of the C-CN bond in the final reductive ehmination from the r-allyl intermediate occurs at C(2) and not C(4), because the aromaticity of the naphthalene ring is preserved only when the bond forms with C(2). A a-alkyl complex see a-Bond) with the Ni bound to C(l), which could give the linear (anti-Markovnikov) nitrile product, does not contribute because of the much greater stability of intermediate (7), accounting for the high regioselectivity observed. 

Photolyses of 3-ch 1 oro-3-pheny 1 -3//-dia/irine (63) were carried out within CyDs and FAU zeolites to foster the ring expansion of the carbene intermediate, chloro(phenyl)carbene (64). However, benzaldehyde (PhCHO) was formed instead of products derived from l-chloro-l,2,4,6-cycloheptatetraene (65). Since CyD innermolecular products were also formed, control experiments in relevant solvents were performed. The true structure of the innermolecular product that was formed in alkaline buffer, and then observed via FAB MS and RP HPLC, has not yet been determined. The ability of the basic medium to preserve the innermolecular product and reduce the amount of PhCHO formed seems to indicate that the latter stems from the former, e.g., O-H insertion product 82. However, the exact mechanism of PhCHO formation within the CyD ICs could not be pinpointed. 

According to different mechanisms of coke formation, we have observed different products of polycondensation using chromatographic, luminiscent, and UV-spectroscopic methods. For example, in the case of decomposition of benzene on different catalysts only products of the dehydrocondensation of benzene with preservation of nuclei were observed (biphenyl, biphenylbenzenes, triphenylene, products of condensation of more than four benzene nuclei, etc.) and such products as naphthalene, anthracene, and phenanthrene were not observed. In tar and coke formation from ethylene on silica gel and aluminosilicates the formation of naphthalene, chrysene, 1,2-dibenzanthracene, fluorene, its derivatives, and others, takes place and if the process is carried out on alumina at a temperature lower than 500°, mainly anthracene, phenanthrene, pyrene, and coronene are formed, but aliphatic hydrocarbons, etc., do not appear. 

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