A formal [1,3] sigmatropic reaction involving free radical intermediates: a mechanistic investigation

1985 ◽  
Vol 63 (3) ◽  
pp. 745-754
Author(s):  
Gordon S. Bates ◽  
S. Ramaswamy
Keyword(s):  
Free Radical ◽  
Solvent Effect ◽  
Specific Mechanism ◽  
Radical Chain ◽  
First Order ◽  
Radical Intermediates ◽  
First Order Kinetics ◽  
Sigmatropic Shift ◽  
Mechanistic Investigation ◽  
Related Compounds

The quantitative isomerization of 2,2-bis(ethylthio)-3,3-dimethylpent-4-enal to 2,2-bis(ethylthio)-5-methylhex-4-enal was studied over the temperature range 130–170 °C. An investigation of the generality and specific mechanism of this formal [1,3] sigmatropic shift was conducted with six related compounds. The rearrangements were found to obey first-order kinetics, and on the basis of significant positive entropies of activation (52–106 J deg−1 mo−1), crossover and trapping experiments, and the lack of a solvent effect (decane vs. DMF), an intermolecular, free-radical chain pathway has been proposed for the isomerization. During the rearrangement of several of the compounds esr signals were observed that were consistent with the presence of the proposed free-radical intermediates. These esr signals have been computer simulated.

A thermal [1,3] sigmatropic rearrangement: thermolysis of 2,2-bis(ethylthio)-3,3-dimethyl-4-pentenal

1981 ◽  
Vol 59 (21) ◽  
pp. 3120-3122 ◽  
Author(s):  
Gordon S. Bates ◽  
S. Ramaswamy
Keyword(s):  
Solvent Effect ◽  
Wide Temperature Range ◽  
Sigmatropic Rearrangement ◽  
Positive Entropy ◽  
First Order ◽  
Temperature Range ◽  
First Order Kinetics ◽  
Sigmatropic Shift ◽  
Entropy Of Activation

2,2-Bis(ethylthio)-3,3-dimethyl-4-pentenal was found to quantitatively isomerize to 2,2-bis(ethylthio)-5-methyl-4-hexenal over a wide temperature range (130–170 °C). This rearrangement can formally be regarded as a [1,3] sigmatropic shift. The reaction, which could be conveniently monitored by 1Hmr spectroscopy, was found to obey first order kinetics. The substantial positive entropy of activation (+ 61.5 J deg−1 mol−1) for the reaction and the negligible solvent effect (decane vs. DMF) are both consistent with a proposed diradical pathway.

The Effects of Solvent and Oxygen Pressure on the Autoxidation of Benzoin Catalysed by Nickel Acetate

1979 ◽  
Vol 32 (2) ◽  
pp. 421 ◽  
Author(s):  
RP Chaplin ◽  
S Vorlow ◽  
MS Wainwright
Keyword(s):  
Free Radical ◽  
Oxygen Partial Pressure ◽  
Partial Pressure ◽  
Oxygen Pressure ◽  
Second Order ◽  
Nickel Acetate ◽  
Coordination Complex ◽  
First Order ◽  
First Order Kinetics

Kinetic results are reported for the autoxidation of benzoin, catalysed by nickel acetate in methanol and ethanol. The reaction in methanol is first order with respect to benzoin and the catalyst and is independent of the oxygen partial pressure. The reaction is non-free-radical and probably involves a coordination complex between the substrate and the catalyst. In ethanol the reaction is found to obey second-order reversible kinetics with respect to benzoin and first-order kinetics with respect to the catalyst. The oxidation is also at least 10 times faster in ethanol than in methanol at 303 K.

scholarly journals Stability of carotenoids toward UV-irradiation in hexane solution

2008 ◽  
Vol 73 (1) ◽  
pp. 15-27 ◽  
Author(s):  
Dragan Cvetkovic ◽  
Dejan Markovic
Keyword(s):  
Free Radical ◽  
Uv Irradiation ◽  
First Order ◽  
Chemical Structures ◽  
First Order Kinetics ◽  
Uv C ◽  
Hexane Solution

The stabilities of four selected carotenoids dissolved in hexane, two carotenes and two xanthophylls, toward UV-irradiation of three different ranges (UV-A, UV-B and UV-C) were studied in this work. The carotenoids underwent bleaching via a probable free radical mediated mechanism following first-order kinetics. The bleaching rates were highly dependent on the input of the involved photons and, although not consistently, on the chemical structures of the investigated compounds. For the two xanthophylls, a possible role of oxygen associated with their bleaching cannot be neglected.

The catalysis of the para -hydrogen conversion by the solid free radical αα -diphenyl- β -picryl hydrazyl

1953 ◽  
Vol 220 (1140) ◽  
pp. 77-90 ◽  
Keyword(s):  
Free Radical ◽  
Adsorbed Layer ◽  
Inhomogeneous Magnetic Field ◽  
Langmuir Adsorption ◽  
First Order ◽  
Heterogeneous Catalytic ◽  
Hydrogen Molecules ◽  
First Order Kinetics ◽  
Increasing Temperature ◽  
Kinetics Of

A detailed study of the kinetics of the heterogeneous catalytic ortho - para -hydrogen conversion on the solid free radical αα -diphenyl- β -picryl hydrazyl, shows that this conversion follows the usual first-order kinetics and that the rate is independent of pressure. Examination of the rate at 90, 190 and 290° K indicates that it decreases with increasing temperature. This type of behaviour has been observed for other paramagnetic heterogeneous conversions, and the ‘negative activation energy' has been assumed to indicate that the catalysis takes place in a physically adsorbed layer of hydrogen molecules. Some evidence that this is the true explanation of the phenomenon is shown by the results of studies of the adsorption of hydrogen on the free radical. Experiments carried out at 77 and 90° K indicate that hydrogen is physically adsorbed by the free radical to form a weakly sorbed layer which obeys the Langmuir adsorption isotherm for the case when the surface is only sparsely covered. A theoretical discussion is given for a heterogeneous conversion occurring during the collision of the hydrogen molecule with the catalyst surface both in the absence, and in the presence, of a physically adsorbed layer. It is shown that only the theory based on the latter model is in agreement with experimental results, and so it is concluded that in the present case the catalysis is due to the interaction between the hydrogen molecules in a physically adsorbed layer, and the inhomogeneous magnetic field at the surface of the catalyst, which enables the otherwise forbidden ortho - para transitions to take place.

scholarly journals THE THERMAL UNIMOLECULAR ISOMERIZATION OF 1,2-DICHLOROETHYLENE

1966 ◽  
Vol 44 (18) ◽  
pp. 2143-2148 ◽  
Author(s):  
L. D. Hawton ◽  
G. P. Semeluk
Keyword(s):  
Volume Ratio ◽  
No Decomposition ◽  
Radical Scavenger ◽  
Radical Chain ◽  
Limits Of Detection ◽  
First Order ◽  
First Order Kinetics ◽  
Trans Isomerization ◽  
Conversion Formula

The thermal cis–trans isomerization of 1,2-dichloroethylene in a toluene carrier was found to follow first order kinetics at temperatures below 846 °K. Within the limits of detection, no decomposition was observed, but there was evidence of a surface contribution to the isomerization. The results, extrapolated to a surface to volume ratio of zero, give for the cis to trans conversion[Formula: see text]and for the reverse action[Formula: see text]It is concluded that previous investigations which did not use a radical scavenger most probably were observations of a mixture of radical chain, heterogeneous, and unimolecular contributions to the isomerization.

Pyrolysis of Aryl Azides.X. Effects of Azide Concentration on Rate Constants and Product Yields

1990 ◽  
Vol 43 (6) ◽  
pp. 997 ◽  
Author(s):  
LK Dyall ◽  
PAS Smith
Keyword(s):  
Free Radical ◽  
Rate Constants ◽  
Aryl Azides ◽  
Radical Chain ◽  
Initial Concentration ◽  
Azo Compound ◽  
Product Yields ◽  
First Order ◽  
Induced Decomposition ◽  
Group Effects

First-order rate constants (k1) have been measured for pyrolysis of azidobenzenes in decalin solution, in the presence of a free-radical chain inhibitor to prevent any induced decomposition. The new values of k1 for the spontaneous unimolecular thermolysis are lower than previously reported ones, and require revision of published neighbouring group effects. Product yields ( azo compound and primary amine) vary with initial concentration of azide in ways which suggest the species responsible for induced decomposition is not triplet arylnitrene , but a solvent-derived free radical. There is no evidence for induced decomposition when nitrobenzene is the solvent. For aryl azides with no neighbouring group effects operating in their pyrolysis, the Arrhenius parameters Eact and ΔSactobey a precise linear relationship.

Free-radical intermediates in the reaction of the hydroxyl radical with amino acid derivatives and related compounds

1970 ◽  
Vol 74 (16) ◽  
pp. 3063-3065 ◽  
Author(s):  
Hitoshi Taniguchi ◽  
Hiroyuki Hatano ◽  
Hideo Hasegawa ◽  
Tetsuo Maruyama
Keyword(s):  
Free Radical ◽  
Amino Acid ◽  
Hydroxyl Radical ◽  
Amino Acid Derivatives ◽  
Radical Intermediates ◽  
Related Compounds

Kinetics and mechanisms of the pyrolysis of diethyl ether I. The uninhibited reaction

1964 ◽  
Vol 278 (1375) ◽  
pp. 505-516 ◽  
Keyword(s):  
Diethyl Ether ◽  
Chain Process ◽  
Chain Mechanism ◽  
Radical Chain ◽  
Elementary Reactions ◽  
First Order ◽  
First Order Kinetics ◽  
Steady State Rate ◽  
State Rate ◽  
Radical Chain Process

The uninhibited thermal decomposition of diethyl ether was studied from 560 to 620 °C and at pressures ranging from 15 to 320 mmHg . The order of the overall reaction was between 1 and 3/2, the order being greater the higher the pressure. Analytical and kinetic data provide strong evidence that there is a molecular split of diethyl ether into ethanol and ethylene. The reaction leading to acetaldehyde and ethane, on the other hand, is concluded to be almost entirely a free-radical chain process. A detailed chain mechanism is postulated, involving first-order initiation and the reaction between C 2 H 5 and CH 2 CH 2 OC 2 H 5 as the chain-ending step. This mechanism is shown to give a steady-state rate equation which leads to first-order kinetics at lower ether pressures and three-halves-order kinetics at higher ones. The kinetic results lead to activation energies which are in satisfactory agreement with values calculated on the basis of the elementary reactions.

scholarly journals Insight on TiO2-ZnO Photocomposite Competence Fabricated via Sonication Assisted with Gelatin for Rhodamine B Degradation

2020 ◽  
Author(s):  
Arafat Toghan ◽  
Kamal K. Taha ◽  
A. Modwi
Keyword(s):  
Rhodamine B ◽  
Rutile Phase ◽  
Maximum Activity ◽  
X Ray Diffraction ◽  
First Order ◽  
First Order Kinetics ◽  
Mechanistic Investigation ◽  
Phase Size ◽  
Kinetics Of ◽  
Tio2 Phase

Herewith we report a facile synthesis of zinc oxide doped with (5, 10, 15, and 20 wt%) titanium oxide nanocomposites in gelatin under ultra-sonication. The X-ray diffraction (XRD) data revealed ZnO the formation in addition to a rutile phase TiO2. The ZnO phase size decreased, and the rutile TiO2 phase increased with a TiO2 loading increment. The scanning electron microscopy (SEM) displayed a combination of spherical and hexagonal particles with a 60 – 80 nm size distribution. The prepared nanostructures photocatalytic activity was assisted using Rhodamine B dye, where they showed enhanced photodegradation competence under visible light irradiation. The kinetics of photodegradation followed the first-order kinetics with the 20 % wt sample having the maximum activity. The mechanistic investigation revealed the dominance of h+ and •O2- species during the dye photodegradation. The results indicate the potential application of such gelatin stabilized nanostructures for dye illumination from aqueous solutions under sunlight.

Sign in / Sign up

Export Citation Format

Share Document