Hydrogen scission

The chain scission also can start truly randomly and not only at the weaker bond. For polymers containing linear backbones, in addition to p-scissions, it is possible to have a-scissions, methyl scissions or even hydrogen scissions. The scission of a C-H bond is thermodynamically unfavorable at low temperatures and is not too common at temperatures where the other scission can take place. The a-scission is more frequent. It refers to the breaking of a o bond to an sp2 carbon. For polystyrene for example, the a-scission leads to the formation of a phenyl radical and a polymeric radical (and it is not a chain scission). 

Some of the radicals formea in the propagation sequence are incapable of allylic carbon-carbon bond cleavage to yield diolefinic product. For these systems, decomposition is via the much slower carbon-hydrogen scission or disproportionation reactions. 

Density functional and chemical model study of the competition between methyl and hydrogen scission of propane and P-scission of the propyl radical... 

Alkoxy radicals can abstract hydrogen (eq. 12), but they may undergo a -scission reaction (eq. 16). 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

Polymer units formed by either 3,4- or 1,4-addition do not evolve hydrogen chloride under these conditions in the absence of oxygen. Also, chain scission resulting in the formation of lower molecular weight polymers does not occur (104). 

When )3-scission can occur in the radical, further reactions compete with acid amide formation. Thus oxaziridine (112) with iron(II) ion and acid yields stabilization products of the isopropyl radical. If a-hydrogen is present in the Af-alkyl group, radical attack on this position in (113) occurs additionally according to the pattern of liquid phase decomposition. 

Liebbrandt have prepared arecaidine by bromination of methyl jV-methylpiperidine-3-carboxyIate, scission of hydrogen bromide from the resulting bromo-compound (VI) and hydrolysis of the resulting arecoline, but Preobrachenski and Fischer were unable to confirm this observation. 

The formation of cotar none from cotar nine methine methiodide by the action of potash (IX—X) led Roser to represent cotarnine and its salts by the following formulae, the loss of a molecule of water in the formation of cotarnine salts being explained by the production of a partially reduced pyridine ring, which is fully hydrogenated in the reduction of cotarnine to hydrocotarnine. In the reverse process, oxidation of liydrocotarnine to cotarnine, Roser assumed the scission of the ring at the point indicated, with the formation of a hydration product, and oxidation of the latter to cotarnine thus —... 

Heliotrine, CigH2,05N, m.p. 125-6°, [ajn — 75° (CHCI3), yields a methiodide, m.p. 108-111°, and contains one methoxyl group, two hydroxyl groups and a tertiary nitrogen atom. On alkaline hydrolysis it forms heliotridine (p. 607) and heliotric acid (p. 613). In presence of platinic oxide it absorbs two molecules of hydrogen and affords, as scission products, heliotric acid and hydroxyheliotridane (p. 607). 

Extreme differences between 5% palladium-on-carbon and platinum oxide were found on reduction of the 5-aryl substituted oxazole 14. Over palladium, 15 was formed in quantitative yield by hydrogenolysis of the benzyl hydroxyl, whereas over Pt, scission of the oxazole occurred to give 13 quantitatively (48). Hydrogenation of 15 over platinum oxide gave the phenethylamide 16. 

Alpha-scission is not favored thermodynamically but does occur. Alpha-scission produces a methyl radical, which can extract a hydrogen atom from a neutral hydrocarbon molecule. The hydrogen extraction produces methane and a secondary or tertiary free radical (Equation 4-3). 

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