Distinguishing between abstraction and addition as the first step in the reaction of a nitroxyl radical with cyclohexene

An unambiguous method for distinguishing between abstraction-addition and addition-abstraction mechanisms (and mixtures thereof) in the reaction of 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl with a specifically deuterated cyclohexene, 1,2-dideuteriocyclohexene, is demonstrated.Peer reviewed: YesNRC publication: Ye

Mechanisms of reaction of aminoxyl (nitroxide), iminoxyl, and imidoxyl radicals with alkenes and evidence that in the presence of lead tetraacetate, N-hydroxyphthalimide reacts with alkenes by both radical and nonradical mechanisms

1,2-Dideuterio-cyclohexene, 1,2-dideuterio-cyclooctene, and trans-3,4-dideuterio-hex-3-ene were reacted with three >NO. radicals: 4-hydroxyTempo, di-tert-butyliminoxyl, both used as the actual radicals, and phthalimide-N-oxyl (PINO) generated from N-hydroxyphthalimide (NHPI) by its reaction with tert-alkoxyl radicals (t-RO.) and with lead tetraacetate. In all cases, except the NHPI/Pb(OAc)4 system, only mono >NO.-substituted alkenes were produced. The 2H NMR spectra imply that 88-92% of monoadducts were formed by the initial abstraction of an allylic H-atom, followed by capture of the allylic radical by a second >NO., while the remaining 12-8% appear to be formed by an initial addition of >NO. to the double bond followed by H-atom abstraction by a second >NO.. A substantial and sometimes the major product formed with the NHPI/Pb(OAc)4 system has two PINO moieties added across the double bond. Since such diadducts are not formed with the NHPI/t-RO. system, a heterolytic mechanism is proposed, analogous to that known for the Pb(OAc)4-induced acetoxylation of alkenes. A detailed analysis of the NHPI/Pb(OAc) 4/alkene products indicates that monosubstitution occurs by both homolytic and heterolytic processes. \ua9 2005 American Chemical Society.Peer reviewed: YesNRC publication: Ye

Influence of different oxidizing systems on cellulose oxidation level: introduced groups versus degradation model

Bleached cotton yarns were oxidized by potassium periodate and TEMPO-mediated oxidation systems, with different concentrations and treatment time, to obtain aldehyde and carboxyl oxy-cellulose with different oxidation levels. Oxidized celluloses were further treated with sodium chlorite to convert the created aldehyde to carboxyl groups. The oxidation level was evaluated by assessing the amount of introduced aldehyde and carboxyl groups, the changes in surface morphology and incurred degradation. Functional groups were determined by titration methods, while surface morphology by FTIR-ATR and SEM analyses. Degradation was analysed by determining the mechanical properties, degree of polymerization, alkali solubility and whiteness stability of the cotton samples. It has been established that a properly selected oxidation system, with appropriate working conditions, can provide satisfactory results for achieving low, medium and extensively oxidized celluloses, with a defined degradation profile. For a short treatment time, a higher oxidation level could be achieved by potassium periodate and TEMPO-mediated oxidation with sodium bromide, while by using the bromide-free TEMPO system a longer time was necessary to reach the same oxidation level. The type and oxidation level of the obtained oxy-celluloses influenced their mechanical properties, degree of polymerization, alkaline and whiteness stability. Low-level oxidized celluloses are suitable for producing stable, long-lasting materials with high added value, while extensively oxidized ones are more appropriate for developing disposable products