The photosensitised oxidation of amines (original) (raw)
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Oxidations of Amines. III. Duality of Mechanism in the Reaction of Amines with Chlorine Dioxide
Journal of the American Chemical Society, 1967
to the induction period gave straight-line plots when submitted to the pseudo-first-order treatment (chlorite was employed in excess). The results are summarized in Table V. Estimation of Activation Energy for Chlorine Dioxide Reaction with p-Nitrobenzyl-N,N-dimethylamine. The reaction of this amine was studied at 26.95 f 0.2", at 14.3 =k 0.2", and at 40.7 f 0.2". Although the Arrhenius plot showed some scatter, the Arrhenius activation energy, E,, was estimated at 12.9 Z!Z 2 kcal/mole, and AF* was estimated to be 13.3 =k 2 kcal/mole.
Quenching of singlet oxygen by hindered amine light stabilisers. A flash photolytic study
Polymer Degradation and Stability, 1984
Rate constants for the quenching of singlet oxygen (10 2 ' 1Ag) for a series of piperidines, piperidine-N-oxyl free radicals and some commercially used hindered amine light stabilisers (HALS) have been measured by a laser .[lash photolysis method. Quenching rate constants are in the order: piperidine-N-oxyl free radicals < secondary piperidines < tertiary piperidines. For some commercial HALS, 1 01 quenching rate constants and the light protective effect towards polypropylene photo-oxidation have been compared. No correlation has been found between the stabilizing action and the quenching efficiency towards 10 2. The data obtained point to little contribution of singlet oxygen to the key steps of polyolefin photo-oxidation.
Sensitized photo-oxidation of dihydroxybenzenes and chlorinated derivatives. A kinetic study
Journal of Photochemistry and Photobiology A: Chemistry, 1991
The dye-sensitized photo-oxidation of dihydroxybenzenes and chlorinated derivatives was studied as a function of solvent polarity. Rate constants for the disappearance of singlet molecular oxygen (O,('A,)), determined by time-resolved phosphorescence at A > 1050 nm, are in the range 5 X l@ to 3 X 10' M-' s-' depending on the solvent polarity and substitution pattern of the reactant. The ratio between the rate constant for reactive quenching k, and the total quenching constant k, is also strongly dependent on substitution pattern and solvent. The values of k, indicate that the O#A,)-mediated photo-oxidation of the dihydroxybenzenes is a viable possibility for their degradation in an aqueous environment. A charge-transfer mechanism is suggested for the photo-oxygenation. Dihydroxybenzenes are fairly good O,('AJ self-sensitizers of photo-oxidation on irradiation at 308 nm, with Oz('A,) quantum yields of approximately 0.2 in the most favourable case. For hydroquinones, this presumably occurs through the excitation of the corresponding quinone derivatives which are always present in traces in the aerated solutions.
Oxidation of aliphatic amines by aqueous chlorine
Tetrahedron, 1998
The oxidation of aliphatic amines by aqueous chlorine has been studied. The kinetic behaviour is similar for primary, secondary and tertiary aliphatic amines, the elementary step being the transfer of chlorine from the hypochlorous acid molecule to the nitrogen of the f~e amino group. Chlorination of aliphatic primary and secondary amines involves some water molecules in the transition state. Inductive effects are also discussed.
Chemistry-a European Journal, 1999
The generation of N-centered radicals (aminium and aminyl radicals) by photoionization and by pulse-radiolytic one-electron oxidation of N-phenylglycine (1), or by laser flash photolysis or reductive dechlorination of (Nchloro,N-phenyl)glycine (2) has been studied in order to model photodamage to amino acids and proteins, as well as degradation of nitrogen-containing drinking-water pollutants. The aminium radical . (H-N-phenyl)glycine (1 . ), a gdistonic radical zwitterion, was found as a product of photoionization (F PI (1) 0.34, 0.22, 0.30, and 0.01 at 193, 248, 266, and 308 nm) or chemical one-elec-tron oxidation of 1. The aminyl radical . (N-phenyl)glycine (1(N À H) . ), a g-distonic radical anion, is a product of reductive dehalogenation of 2, or of Nchloro photohomolysis, for which the upper limits of F NCl (2) at 193, 248, 266, or 308 nm excitation are 0.61, 0.43, 0.60, and 0.38, or of deprotonation of 1 . (pK a (1 . ) 7.35 AE 0.15). The reduction potential E8(1 . /1) at pH%6 is 0.89 AE 0.02 V versus a normal hydrogen electrode (NHE). The quantum yields of 193, 248, 266, or 308 nm photoionization of 2 are F PI (2) 0.29, 0.10, 0.08, and 0.01. For 1, an additional process leading to 1 . and H . takes place that is identified in terms of water-assisted ionization for which the quantum yields F WA (1) at 193, 248, 266, or 308 nm are 0.37, 0.26, 0.50, and 0.01, respectively. A full thermodynamic description of the system is given, including the different possible acid ± base forms, their redox potentials, and bond dissociation energies.
Journal of Photochemistry and Photobiology A: Chemistry, 2000
The kinetics of the dye-sensitized photooxidation of 2-amino-4-hydroxy-6-methyl-pyrimidine (AHMPD), a compound with the basic molecular structure of some systemic pyrimidine fungicides, has been studied in solution in water and in the mixture acetonitrile-water 4:1 v/v. Rate constants in the range 9×10 4 -2.7×10 7 M −1 s −1 for both the overall and the reactive singlet molecular oxygen [O 2 ( 1 g )] quenching processes were determined by time-resolved O 2 ( 1 g )-phosphorescence detection and polarographic methods. Photooxidation quantum yields were in the range 0.005-0.41, with values significantly higher in alkalinized media. On the contrary, pyrimidine, 2-aminopyrimidine, 4-hydroxypyrimidine and 6-methylpyrimidine do not react with O 2 ( 1 g ), in neutral or alkalinized D 2 O solutions. The presence of a 4-hydroxy group in pyrimidines plays a key role in the photooxidative process, because the formation of the tautomeric 4-oxo form, the predominant one in aqueous solution, diminishes or suppresses the interaction with O 2 ( 1 g ). However, in the presence of alkali the OH-ionization greatly enhances the photooxidation. The experimental evidence suggests a charge-transfer mediated mechanism involving an initial encounter excited complex. These results also show that the sensitized photooxidation is an alternative pathway for the environmental or programmed degradation of these colorless N-heteroaromatic compounds, in special in OH-ionized form.
Theoretical study of the oxidation mechanism of aromatic amines
Journal of the Chemical Society, Perkin Transactions 2, 1991
The overall mechanism of the chemical step involved in the electrochemical oxidation of tertiary aromatic amines has been investigated by means of the quantum-chemical A M 1 method. By using the parent compound aniline as a model it is found that, in good accord with previous electrochemical data, dimerization of t w o radical cations takes place before deprotonation, the dimerization step involving a higher energy barrier.
Mechanistic and Kinetic Aspects of Photosensitization in the Presence of Oxygen†§
Photochemistry and Photobiology, 2000
Determining whether the first step of photooxygenation is Type I or Type II is a necessary prerequisite in order to establish the mechanism of photodynamic action. But this distinction is not sufficient, because other processes, both consecutive and competitive, commonly participate in the overall mechanism. Thus, in both Type I and Type II reactions, the initial products are often peroxides that can break down and induce free radical reactions. These aspects of photosensitization are discussed and illustrated by sensitizer/substrate systems involving (1) only radical reactions (decatungstate/alkane) and (2) reactions of singlet oxygen occurring in competitive and consecutive processes and possibly followed by radical reactions (methylene blue/2-deoxyguanosine). Two other previously investigated systems involving, respectively, a Type II interaction followed by radical processes (methylene blue/alkene) and Type II reactions, some of which being competitive or consecutive (rose bengal/alkene), are briefly reconsidered.
Polymer Degradation and Stability, 1996
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