Propane, decomposition photolysis

Thorium.—The production of nearly comparable quantities of propene and propane on photolysis of [Th(PrO( -C 5H 5)3] contrasts with the results of thermal decomposition studies and suggests that -hydride elimination pathways become available on photochemical excitation. The weakening of the metal-cyclopentadienyl ligand bonding e.g. promotion to 7 or ri excited states) and consequent reduction of metal co-ordinative saturation is a possible explanation for this change in behaviour. 

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

Free radicals formed from an initiator in the gas phase take part in other reactions and recombine with a very low probability (0.1-2%). The decomposition of the initiator in the liquid phase leads to the formation of radical pairs, and the probability of recombination of formed radicals in the liquid phase is high. For example, the photolysis of azomethane in the gas phase in the presence of propane (RH) gives the ratio [C2H6]/[N2] = 0.015 [76]. This ratio is low due to the fast reactions of the formed methyl radicals with propane ... 

The increased importance of the methylene elimination to give methane also accounts for the increase in propane, since the insertion of methylene into ethane gives propane directly through stabilization or indirectly through methyl radicals if decomposition occurs. n-Butane is formed by combination of ethyl radicals in the same manner as for the 1470 A photolysis. 

The photolysis of n-butane follows a pattern similar to that of propane, with many corresponding reactions. As found for previous hydrocarbons the photolysis includes both molecular and free-radical processes. The molecular elimination of Hj and Dj from C4H10-C4D10 mixtures was first shown by Sauer and Dorfman, who concluded that at 1470 A more than 90 % of the hydrogen came from molecular processes. On the basis of a study of the decomposition of excited -butane molecules generated by electron impact , they attributed hydrogen, methane, ethylene, and other hydrocarbon products to molecular processes, and concluded that free-radical reactions were minimal. 

Allyldiethylamine behaves similarly, but the yields are low since neither the starting amine nor the products are stable to the reaction conditions. For the efficiency of the cyclopropanation of the allylic systems under discussion, a comparison can be made between the triplet-sensitized photochemical reaction and the process carried out in the presence of copper or rhodium catalysts whereas with allyl halides and allyl ethers, the transition metal catalyzed reaction often produces higher yields (especially if tetraacetatodirhodium is used), the photochemical variant is the method of choice for allyl sulfides. The catalysts react with allyl sulfides (and with allyl selenides and allylamines, for that matter) exclusively via the ylide pathway (see Section 1.2.1.2.4.2.6.3.3. and Houben-Weyl, Vol. E19b, pll30). It should also be noted that the purely thermal decomposition of dimethyl diazomalonate in allyl sulfides produces no cyclopropane, but only the ylide-derived product in high yield.Very few cyclopropanes have been synthesized by photolysis of other diazocarbonyl compounds than a-diazo esters and a-diazo ketones, although this should not be impossible in several cases (e.g. a-diazo aldehydes, a-diazocarboxamides). Irradiation of a-diazo-a-(4-nitrophenyl)acetic acid in a mixture of 2-methylbut-2-ene and methanol gave mainly l-(4-nitrophenyl)-2,2,3-trimethylcyclo-propane-1-carboxylic acid (19, 71%) in addition to some O-H insertion product (10%). ... 

A mixture of 1-aryl-l-chlorodiazirine and an alkene in hexane sonicated with ultrasound (Fisher Scientific Solid State Ultrasound FS-9) at 40 °C for two hours afforded 1-aryl-l-chlorocyclo-propanes with yields similar to those obtained by photolysis. Electrophilic as well as nucleophilic alkenes were used. Photolytic, thermal or ultrasonic decomposition of 3-chloro-3-phenyl-3//-diazirine in 2-vinylpyridine did not give the corresponding 1-chloro-l-phenylcyclopropane derivative (see Section 1.2.1.3.1.2.2.1). 

The photochemical decomposition of methanal in a solid Xe matrix has been studied. Work has also been reported dealing with the photodissociation dynamics of methanal, and ab initio calculations have been carried out on the photochemical decomposition of acetaldehyde into methane and CO. The photocatalytic decomposition of acetaldehyde to yield carbon dioxide has also been reported. The threshold for CC bond fission in propanal and the release of the CHO fragment has been shown to be at a wavelength of 326.26 nm. Chowdhury has reported the dissociation of propynal using multiphoton irradiation. Gas-phase photolysis of butyraldehyde in the 280-330 nm range has shown that the CHO radical is produced. ... 

Bencsura et al. [12] in 1992 produced the most recent experimental data on the reaction kinetics of the propyl radical, based on previous work. Radicals were produced by pulsed laser photolysis and their unimolecular decay subsequently studied by photoionization mass spectrometry. Tsang [13] in 1988 produced a compilation of revised and evaluated data on the kinetics of reactions involving propane and the propyl radical, among other species. The general mechanism for the decomposition of propane was initially determined by Papic and Laidler [14, 15] who experimentally identified most of the products which are consistently predicted in the mechanism proposed in this paper. We followed their rationale for the development of our own model. 

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