Photolysis Of Acetone Research Articles

Wall reactions in acetone photolysis

The photolysis of acetone has been re-examined at temperatures between 107 and 225°C, and pressures between 0.13 and 133 mbar. In a Pyrex reaction vessel the wall reaction, CH3·+(CH3COCH3)ads→ CH4+ CH3COCH2·, contributes significantly to methane formation at acetone pressures below 10 mbar. The rate of this reaction is proportional to the methyl radical concentration in the gas phase, and varies markedly with the nature of the surface. On PTFE and PbO surfaces the rate of reaction appears independent of acetone pressure in the range 0.13–2.7 mbar, but on Pyrex it increases slowly with increasing acetone pressure. On Pyrex surfaces the reaction has an activation energy of 6.6 ± 1.0 kcal mol–1 and is always faster on a per collision basis than the analogous reaction in the gas phase.

The liquid-phase photochemistry of acetone

Acetaldehyde, diacetone alcohol, acetonylacetone and 2,2,3-trimethyl-3-hydroxyoxetan were produced by photolysis of neat acetone, together with pinacol as an artifact.

Photolysis of Acetone at 185 nm

AbstractThe photolysis of acetone at 185 nm yields carbon monoxide and ethane with quantum efficiencies of about unity. Carbon monoxide arises in the primary process, ethane is formed almost exclusively by methyl radical recombination. Besides, methane and hydrogen are formed at 185 nm in small amounts that depend on various parameters. At room temperature methane is mainly formed via hot methyl radicals as precursors. Hydrogen arises from atom abstraction by hydrogen atoms. At 185 nm acetyl radical cannot be an intermediate in acetone photolysis. It is probable that at least two distinct excited electronic states of the acetone molecule are involved in the mechanism.

Methyl radical reactions with isopropanol and methanol, their ethers and their deuterated derivatives

The reactions of methyl radicals with three isotopically different isopropanols three isotopically different methanols and the undeuterated ethers (diisopropyl and dimethyl ethers) have been investigated. The photolysis of acetone or acetone-d6 has been used to generate methyl radicals or trideuteromethyl radicals. An isotopic exchange reaction between acetone and alcohols makes it necessary to work with isotopically compatible pairs. Values for Arrhenius parameters, based on Shepp's rate-constant for the combination of methyl radicals, are as follows : [graphic omitted] Primary isotope effects in the alkyl groups of these alcohols are larger than expected solely on the basis of zero-point-energy difference : at 150°, (kCH/kCD) is ca. 8 for methanol and ca. 6.5 for isopropanol. These values, which resemble those for ethanol and for isobutane, taken together with the corresponding activation-energy differences, suggest that tunnelling may again be important. A secondary reaction, disproportionation between the radicals · C(CH3)2OH and CH3, is important in attack on isopropanol. It gives rise to an apparent enhancement of reactivity of the hydroxyl groups, to a spurious secondary isotope effect and, unless allowance is made, to a low value for the A-factor. The corresponding reaction does not appear to be significant in methanol or ethanol systems. In dimethyl ether, the reactivity of each methyl group is the same as in methanol but in diisopropyl ether there is steric hindrance and the alpha hydrogen atoms are less accessible than in the alcohol.

Reaction of methyl radicals with oxygen

Methyl radicals produced by the photolysis of acetone have been allowed to react with oxygen and the kinetics of the reaction studied over a range of conditions from 200 to 400°C. The results of earlier workers have been confirmed, and between 200 and 300 °C, the reaction is of the third order with a rate constant of approximately 10–31 cm6 molecule–2 sec–1 which decreased slightly with increasing temperature. Among the products of the reaction below 300°C, methyl hydroperoxide has been definitely identified and estimated quantitatively. Also, the major decomposition product of methyl hydroperoxide at room temperature is methanol. The formation and further reactions of the methylperoxy radical are discussed in relation to its thermochemistry, with emphasis on the possible role of hydroperoxides in combustion reactions.

The photo-induced oxidation of acetylene

The oxidation of acetylene catalysed by the photolysis of acetone between 190° and 250°C has been studied using a static system by following the small pressure changes occurring and by analysis. The uncatalysed oxidation has also been investigated, but less extensively. With excess acetylene the pressure decreased to a minimum and then rose to a maximum, which was very sharp in the induced oxidation. The products were carbon monoxide, carbon dioxide and water with less formaldehyde, formic acid, glyoxal and acetaldehyde. An unidentified polar compound containing probably more than two atoms of carbon was also formed and, in the induced oxidation, methanol was produced. The rates of consumption of reactants and of formation of the main products increased slightly at first and then fell sharply to zero at a time, in the induced oxidation, coinciding with the sharp pressure maximum. The concentration of glyoxal rose to a sharp maximum early in the reaction, the amounts presents in the latter stages being very small. The rate of formation of acetaldehyde was very low at first but increased markedly just before the end of the oxidation. The activation energy of the uncatalysed oxidation was about the same (30 to 34 kcal/mole), whether it referred to the consumption of reactants or to the formation of carbon monoxide, carbon dioxide or water. The kinetics of the photo-induced oxidation were investigated more fully. The average rates of consumption of acetylene and oxygen and of formation of carbon monoxide were all proportional to I0.5[CH3COCH3]0.25[C2II2], where I is the intensity of the incident u.v. radiation, and independent of oxygen concentration. The kinetics of the formation of carbon dioxide and methanol were somewhat different, however. The activation energies for the consumption of reactants and the formation of products were less than the value for the uncatalysed oxidation, although most of the Arrhenius plots were curved. The mechanisms of both catalysed and uncatalysed oxidations are probably similar. It is like y that degenerate chain branching due to glyoxal is of minor importance only, and that products such as carbon monoxide, glycxal, formaldehyde and acetaldehyde are formed in elementary steps in the main chain propagation process subsequent to the reaction of OH and HO2 radicals with acetylene, whereas methanol is the result of the addition of CH3O2 and CH3O to acetylene.

Reactions of acetyl radicals. Part 1.—Photolysis of acetone in presence of nitric oxide and of nitrogen dioxide

The photolysis of acetone in presence of nitric oxide and of nitrogen dioxide was studied in order to obtain information about the kinetics of acetyl radical reactions. The ratio of nitric oxide to nitrogen dioxide, and the temperature, determine whether nitrogen dioxide is produced or consumed during photolysis. The initial yields of nitrogen dioxide are explained in terms of a competition between nitric oxide and nitrogen dioxide for acetyl radicals. In the temperature range 20–60°C, the reaction CH3CO+NO2→CH3+CO2+NO is 5 times as rapid as the reaction CH3CO+NO→CH3CONO. Evidence for the dissociation CH3CONO→CH3CO+NO has been found. The ratio of the dissociation rate constant to that for the reaction CH3CONO+2NO→CH3CO+N2+NO3 has been evaluated.

The Gas Phase Photolysis of Acetone at 3130 A in the Presence of Hydrogen Bromide. A Study of the Primary Photochemical Decomposition Processes of Acetone

The Gas Phase Photolysis of Acetone at 3130 A in the Presence of Hydrogen Bromide. A Study of the Primary Photochemical Decomposition Processes of Acetone

The mechanism of the gas phase photolysis of acetone

A mechanism for the photolysis of acetone has been derived from experimental data including studies on the photosensitized isomerization of olefines. Dissociation into methyl and acetyl radicals occurs from both the first excited singlet and triplet states; the rate constants for excitation by light of 3130 Å wavelength are 2.5 x 10 16 exp ( —16900/ RT ) and 2.5 x 10 10 exp ( — 6400/ RT ) 1. mole -1 s -1 respectively. At low intensities of absorption, decomposition of acetyl radicals predominates over radical-radical interactions. The triplet state is extensively populated at moderate temperatures, the yield at 48 °C being 0.98. The low phosphorescence efficiency of acetone can be ascribed to intersystem crossing to the ground state for which the first order rate constant is 1.25 x 10 5 s -1 . The isomerization data requires the recognition of at least two vibrational levels of the acetone triplet, the rate of electronic energy transfer depending upon the excess energy possessed by the donor

Repeated Gas Chromatographic Sampling Technique in Gas Phase Chemical Kinetics. Photolysis of Acetone.

Repeated Gas Chromatographic Sampling Technique in Gas Phase Chemical Kinetics. Photolysis of Acetone.

Hydrogen-abstraction reactions by methyl radicals from nitrogen-containing compounds

From the photolysis of acetone in the presence of the nitrogen compounds the following Arrhenius parameters have been determined for hydrogen-abstraction reactions of methyl radicals: [graphic omitted] The results are compared with hydrogen-abstraction reactions involving other nitrogen compounds.

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.

EFFECT OF NON-ZERO EXTINCTION COEFFICIENT ON ABSTRACTION–RECOMBINATION RATE RATIOS IN PHOTOCHEMICAL SYSTEMS

The data of Marjury and Steacie on the photolysis of acetone and acetone-d6 in the presence of H2 and D2 respectively have been re-analyzed in terms of the integrated rate equations. It is concluded that only part of the discrepancy between the results for the methyl and hydrogen reaction can be accounted for in this way.In an appendix, it is shown that for high extinction coefficients, the integrated rate expression reaches a limit which may sometimes be attainable experimentally without undue complication from diffusion effects.

THE REACTION OF METHYL RADICALS WITH METHYL ALCOHOL

The photolysis of acetone in the presence of methyl alcohol and of methyl-d3 alcohol has been studied between 406 and 472° K. The rates and Arrhenius parameters for the following abstraction reactions have been determined:[Formula: see text] [Formula: see text] [Formula: see text]At 182 °C the rate of reaction [b] is found to be approximately four times greater than the rate of reaction [c].

Photochemical Studies in Flash Photolysis. III. Photolysis of Acetone in Different Wavelength Regions

The flash photolysis of acetone was studied at wavelength regions centered around 260, 280, and 300 mμ, using absorbed intensities of the order of 1019 quanta/cc/sec for each wavelength region. The light source was an exploding wire, and the maximum temperature increase per flash was calculated to be not more than 5°C. The products, analyzed by gas chromatography using a sensitive electric discharge detector, consisted of C2H6, CO, biacetyl, and, in smaller amounts, CH4. A search was made for other products as well, and detection limits are given. The C2H6/CO ratio decreased with increasing acetone pressure at all wavelength regions and was independent of light intensity in the range investigated. The RCO/pacet ratio, measured over a wide pressure range, was pressure-independent at 260 and 300 mμ but increased appreciably with pressure at 280 mμ. At low pressures, where the C2H6/CO ratio approached a limiting value, the ratio decreased with wavelength according to the order 280>260>300 mμ. The lifetime of the excited acetone molecules at the respective wavelengths was estimated, from a steady-state treatment, to be 0.6×10—9, 1×10—9, and 4×10—9 sec at 260, 280, and 300 mμ, assuming a collision deactivation efficiency of unity. At 300 mμ there was a marked difference in value of C2H6/CO at flash and low intensities: certain second-order reactions involving excited states appear to occur completely at the relatively high concentrations prevailing under flash conditions. The transition region of intensity effects is described in subsequent papers of this series.

Photolysis by pulsed illumination

A technique is described in which photolysis is effected by a sequence of light pulses. It is shown that if a single train of pulses is employed with accurately known (variable) pulsing frequency, and if the light absorbed per pulse is measured, it is possible to obtain absolute rate constants for competing radical-abstraction reactions and dimerization reactions. First results from this method, applied to the photolysis of acetone, lead to mean rate constants at 134 °C ofk2= 2.3 x 1013ml. mole-1s-1for 2CH3-> C2H6andk3= 2.0 x 106ml. mole-1s-1for CH3+ CH3COCH3-> CH4+ CH2COCH3. A second mode of operation, employing a sequence of pairs of light pulses with variable time delay between members of the pair, is also discussed, in relation to the measurement of rate constants and to the detection of reaction among 'hot’ radicals.

Carbon Monoxide Yields in the Photolysis of Acetone

<b>Carbon Monoxide Yields in the Photolysis of Acetone</b>

REACTIONS OF METHYL RADICALS WITH NITRIC OXIDE

Methyl radicals have been produced by the photolysis of acetone at 200 °C and their reaction with nitric oxide has been studied. The reaction CH3 + NO + M → CH3NO + M is shown to be third order with a rate constant of 8.8 × 10−31 cm6 molecule−2sec−1, with acetone as M. It is further shown that the nitrosomethane formed reacts with either methyl radicals or nitric oxide. The probability of reactions such as CH3CO + NO → CH3CO•NO taking place is shown.

Photolysis of Acetone in the Presence of HI and the Decomposition of the Acetyl Radical

Acetone over a range of pressures and temperatures (1.2–15 cm Hg; 126–295°C) was photolyzed with light of 3130 A in the presence of small amounts of hydrogen iodide. The primary process was found to be the C–C split to form acetyl and methyl radicals. Hydrogen iodide acted as an excellent radical ``trap'' for these radicals. No spontaneous decomposition of acetyl radicals was observed. Relative rates of methyl radical reactions with iodine and hydrogen iodide were measured. Complications due to surface effects in these reactions are considered. Kinetic constants for the reactions CH3+HI→CH4+I(1); CH3+I2→CH3I+I(2) are E1—E2=1.3±0.5 kcal, A2/A1=2.4, and log(k1/k2) = —0.38–1300/4.575T. Above 200°C the acetyl radical decomposition is measurable and the rate constant is observed to be pressure dependent. The pressure dependence is treated by an approximate method related to the Hinshelwood-Lindemann equation and the high- and low-pressure limiting rate constants are given by logk∞=10.3–15 000/4.575T sec—1, logk0=11.5–12 000/4.575T (l/mole-sec). The number of vibrational modes S contributing to the decomposition is 4.5 and the lifetime of the excited acetyl radical is t∼10—8 sec. The abnormally low frequency factor of the high-pressure rate constant implies a nonadiabatic decomposition of the acetyl radical. From the kinetic constants, the heat of formation of the acetyl radical is found to be ΔHf0 (CH3CO)—6.2 kcal and the values of the appropriate bond dissociation energies in a series of acetyl derivatives are tabulated.

Photochemical Studies in Flash Photolysis IV. Intensity and Wavelength Effects

AbstractWavelength and intensity effects which we have observed in the flash photolysis of acetone are reviewed. Marked nonmonotonic wavelength effects occurred in the different regions investigated (260, 280 and 300 mμ) and serve to emphasize the problem of interpreting any data obtained with unfiltered light. The data at 300 mμ were compared with those obtained with continuous light sources 106 fold less intense. A considerable difference occurred in the C2H6/CO ratio. To investigate this intensity effect, flash photolyses were made over a wide range of reduced intensities, as low as 104 fold less than the maximum one, and the transition intensity region for change in C2H6/CO ratio and CO quantum yield was located. This phenomenon illustrates the more general fact that the apparent primary processes observed at typical flash intensities need not be the same as the primary processes found at low intensities.