Molecular Hydrogen Elimination Research Articles

Strahlenchemie von Alkoholen XVII

In the 185 nm photolysis of liquid O2-free isopropanol the following products (quantum yields) are formed: hydrogen (0.75), acetone (0.72), pinacol (0.036), methane (0.046), acetaldehyde (0.04), propane (0.02β), ane (0.0023) and carbon monoxide (0.0015). A detailed reaction scheme is proposed. The major primary processes are the formation of H-atoms by homolytic scission of the O —H-bond (61 — 69%), elimination of molecular hydrogen (21%) and molecular methane (5%). Φ(Η2) is strongly decreased by adding water which does not absorb an appreciable portion of the 185 nm light in the mixtures down to 1 mole/l isopropanol (Φ(Η2) =0.21). In contrast to the strong effect of water there is no effect on Φ(Η2) by diluting isopropanol with n-hexane. From experiments with isopropanol-OD and 2-deutero-isopropanol it is tentatively concluded that H-atoms stemming from the O—H-group of the alcohol are to about 65% the precursors of the hydrogen in these mixtures.

Photolysis of Formaldehyde at 1470 and 1236 Å

The photolysis of formaldehyde vapor has been examined at 1470 and 1236 Å. Mixed isotope (H2CO+D2CO) and radical scavenger (C2D4) experiments have been utilized in addition to inert gas pressurization and lamp intensity attenuation. The quantum yields for H2 and CO formation are both equal to 2; and both are independent of pressure and lamp intensity at 1470 and 1236 Å. The primary quantum yield for molecular hydrogen elimination (ΦII) was determined from the scavenger results and also from the mixed isotope work. The two values obtained are in reasonably good agreement, viz., ΦII = 0.5. The mechanism most consistent with the experimental observations is as follows: H2CO+hυ→H2+CO, H2CO+hυ→2H+CO, H+H2CO→H2+HCO, 2HCO→H2+2CO, where the quantum yields of primary process (II) and (III) are both equal to 0.5.

Thermal decomposition of 1-methyl-3-phospholene

The homogeneous gas-phase thermal decomposition of the 5-ring 1-methyl-3-phospholene (C4H6PCH3) has been followed from 356.0 to 444.6°C in excess toluene and at pressures of phospholene ranging from 8.3 to 27.4 mmHg (1106–3652 N m–2). Butadiene, apparently a primary reaction product, partially decomposes to give isomeric butenes. Rate constants calculated on the basis of the observed C4-hydrocarbon formation satisfy a first-order rate expression and yield the following Arrhenius relationship (with standard errors): log k(s–1)= 12.10±0.22)–(50.0±0.7)/θ where θ= 4.58 × 10–3T K in kcal mol–1; log k(s–1)=(12.10±0.22)–(209±3)/θ where θ= 19.14 × 10–3T K in kJ mol–1. The observed activation parameters are in agreement with a mechanism involving ring opening to a biradical followed by fragmentation into butadiene, phosphorus and methyl radicals. Methane, hydrogen, phosphorous and a phosphorus-containing polymeric material are the other reaction products. The molecular elimination of hydrogen from the starting material is a possible side reaction. The results suggest a dissociation energy for the secondary phosphorus carbon bond of 68.1±1.3 kcal mol–1(285±5 kJ mol–1).

Nonequilibrium Unimolecular Decomposition of Ethylene

Some of the experimental data in the literature on the nonequilibrium decomposition of ethylene have been examined in the light of calculations using a simple Rice–Ramsperger–Kassel–Marcus treatment. An activation energy of approximately 80 kcal mole−1 is indicated for the molecular elimination of hydrogen. Of the energy available to the products of the initial processes by which excited ethylenes are formed, the fraction retained by ethylene does not appear to be constant, as was previously concluded using the classical RRK theory of unimolecular reactions. In the case of ethylene produced in the photolysis of 3-methyldiazirine, an alternative explanation for the limiting yield of acetylene at low pressure is offered, based upon an arbitrary energy distribution selected to match the experimental data.

Isotope effects on metastable transitions. III. CH 3CD 3 and C 2H 6

Metastable-ion intensities were determined at 70 cV for CH 3CD 3 and C 2H 6. There is no indication for a reshuffling of hydrogen atoms in the CH 3CD 3 ∸ ion, prior to molecular hydrogen elimination. An isotope effect is observed on both the intensity and kinetic energy release of the molecular hydrogen elimination from the parent. The results are discussed in view of recent electron impact experiments on isotopic methanes 8 and methanols 7 and the photoionization of ethane 5.

Far-Ultraviolet Photolysis of Acetone

A survey of the trends in acetone decomposition in the range from the near-ultraviolet photolysis to gamma radiolysis has been made using a xenon discharge (1470 and 1295 Å) and a krypton discharge (1236 and 1165 Å) for room temperature photolysis. The variation of the ratios to CO of products as a function of acetone pressure and wavelength has been noted. Analysis of the mixed products from mixtures of acetone and acetone-d6 presents evidence for methyl radicals in the production of ethane, of hot methyl radicals in the production of methane and of hydrogen atoms in the production of hydrogen. Deviation from the H2—D2 equilibrium indicates molecular hydrogen elimination. The conclusions are confirmed by scavenger studies using propylene, iodine, and oxygen, as well as by the effects of added xenon and neon. The results are consistent with the existence of two excited states of acetone, the higher state eliminating either H or H2; the lower state, methyl. The far-ultraviolet photolysis appears to be closer in over-all behavior to the gamma radiolysis than to the near-ultraviolet photolysis and the trend of the decomposition may be explained by the increase in energy of the radiation.

Dissociative Charge-Exchange Reactions of Rare-Gas Ions with Propane

Dissociative charge-exchange reactions of rare-gas ions with propane and with partially deuterated propanes have been investigated using a modified commercial mass spectrometer. The experimental results for propane are in good agreement with data previously obtained using much more sophisticated instrumentation. Charge-exchange spectra obtained with 2,2-dideuteropropane and with 1,1,1,3,3,3-hexadeuteropropane permit the determination of details of the fragmentation mechanism. The data suggest that both s-propyl and n-propyl ions are formed by charge exchange, but that the latter is a relatively unimportant process. Ethylene is formed chiefly by 1,3 elimination of methane from the molecule ion, but 1,2 elimination becomes relatively more important with increasing recombination energy of the rare-gas ion. In contrast to observations on the vacuum uv photolysis of deuterated propanes, the present data indicate that elimination of molecular hydrogen as HD is of substantial importance in the over-all ionic decomposition of both compounds.

Vapor-Phase Radiolysis of Propane-d8 in the Presence of Other Hydrocarbons

The radiolysis of propane-d8 has been investigated in the presence of saturated and unsaturated hydrocarbons. Radical scavengers have been added in all experiments in order to prevent the production of hydrogen, methane, ethane, and ethylene by neutral thermal radical reactions. The formation of ethane could be ascribed mainly to the occurrence of the hydride-transfer reactions C2D5++C3D8→ lim k1C2D6+C3D7+and C2D5++RH→ lim k2C2D5H+R+.When RH=saturated hydrocarbon, the values of k2/k1 increase with increase in the molecular weight of RH. The observation that olefins and aromatic compounds lower the yield of ethane, but do not affect the rates of formation of molecular methane and ethylene, led to the conclusion that ethyl ions may add to the unsaturated hydrocarbons. Evidence has been obtained for a molecular elimination of hydrogen, methane, ethane, and ethylene from several of the hydrocarbons.

Vacuum Ultraviolet Photolysis of Methane

The photolysis of CH4—CD4 mixtures, CH2D2, and CHD3 with 1236 Å light shows that molecular elimination of hydrogen from methane is an important but not an exclusive primary photochemical process. The photolysis products are principally those expected from the insertion reaction of the methylene radical.

Elimination of Molecular Hydrogen from Alkyl Free Radicals

Elimination of Molecular Hydrogen from Alkyl Free Radicals