Photolysis Of Acetone Research Articles

The Rate-Constant for the Recombination of Methyl Radicals

Rate constants for the recombination of CH3 radicals have been measured by means of the rotating sector technique in the temperature range 373- 463 K, and at a pressure of 30 Torr . CH3 radicals were produced by the photolysis of acetone, and the experimental data were fitted to sector curves generated from Shepp's theory. The results give kb = (2.81�0.22)×1013 cm3 mol-1 s-1, which, under the chosen experimental conditions, is close to its high-pressure limiting value. A comparison is made with the other values of the rate constant reported in the literature, and a best value is suggested.

The Photochemistry of Acetone: The Primary Decomposition of Acetone and Biacetyl Yields

AbstractIn the photolysis of acetone at 265nm at 27°C the quantum yield of biacetyl has been determined to be 0.34 and the primary decomposition yield is evaluated to be about 0.68. The fraction of triplet acetone decomposed into methyl and acetyl radicals is estimated to be 0.14 at 313nm. The acetyl radicals undergo reaction to form biacetyl 50 times faster than decomposition to methyl radicals and carbon monoxide.

Type I cleavage of triplet acetone in solution studied by time-resolved CIDNP

The time evolution of 1H nuclear polarization of reaction products generated by laser flash photolysis of acetone in D 2O is studied by FT NMR. A theoretical model for the analysis of net polarization is extended to the case of slow geminate pair formation. Application yields the triplet lifetime of acetone, the nuclear relaxation time of protons in the methyl radical and absolute polarization intensities. The latter agree well with predictions derived from the Pedersen-Freed high-field radical-pair theory.

A time-resolved technique for measuring rates of alkyl radical-atom reactions: The pressure dependence of the CH 3 + H reaction at 504 K

The rate constant for the CH 3 + H reaction has been measured as a function of pressure at 504 K. A new technique was employed, in which the temporal behaviour of both the atom and radical was measured, by resonance fluorescence and UV absorption spectroscopy respectively, following excimer laser flash photolysis of acetone at 193 nm. A minor complication, resulting from the production of small amounts of CH 2 from a two-photon process, was eliminated by studying the atom and radical decays as a function of laser pulse energy.

Photolysis of acetone at 100 – 170 K: an electron spin resonance study of radical formation and decay in the solid state

Solid acetone was photolysed in an electron spin resonance (ESR) spectrometer at 100 – 170 K at wavelengths above 250 nm. The nature of the trapped radicals depended on the photolysis temperature. From 100 to 110 K, methyl and acetyl radicals were formed, with acetyl radicals predominating. Above 110 K, the spectrum consisted of a seven-line signal, attributed to the isopropanol radical CH 3Ċ(OH)CH 3, resulting from hydrogen atom addition to the carbonyl group. The singlet spectrum of the acetyl radical was also observed, but no methyl radicals. This difference between photolysis below and above 110 K may reflect a solid state transformation. In the range 100 – 110 K, the concentration of acetyl radicals increased linearly with the photolysis time, but the concentration of methyl radicals showed distinct curvature, which could be analysed quantitatively in terms of zero-order formation and first-order disappearance kinetics. The variation in concentration of the CH 3Ċ(OH)CH 3 radical with the photolysis time at 100 – 170 K was analysed similarly. Acetonyl radicals ĊH 2COCH 3 were photolabile and hence were observed only as a minor component of the ESR spectra at 100 – 170 K.

The stationary-state approximation in the photolysis of acetone

A mechanism has been written for the photolysis of acetone at 313 nm and 253.7 nm and the differential equations involved solved rigorously. Testing this mechanism has revealed inconsistencies between different sets of experiments that were not previously obvious. The stationary-state approximation is valid up to ca. 0.5% decomposition but its practical usefulness is minimal. Graphs are given showing the variation of all significant quantum yields with temperature, pressure, and light intensity.

Reactions of methyl radicals. IV. Hydrogen abstraction from tetramethylsilane by methyl radicals produced by the photolysis of both acetone and azomethane

Studies of hydrogen abstraction by CH3 radicals from (CH3),Si with two different radical sources, acetone and azomethane, have yielded data on the reactions CH3 + Ch3COCH3 → CH4 + CH3COCH2 (1) CH3 + CH3NNCH3 → CH4 + CH3 NNCH2 (2) Ch3 + (Ch3)4Si → Ch4 + (CH3)3SiCH2 Their rate constants, based on the value of 1013.34 cm3 mol-1 s-1 for the recombination of CH3 radicals, together with the corresponding experimental temperature ranges, are given by (k in cm3 mol-1 s-1, E in J mol-1): log k2 = (11.54±0.02) - (40 000±220)/19.145T 398-522 K (2) log k8 = (10.98±0.02) - (32 550±160)/19.145T 334-463 K (8) log k8' = (11.03±0.02) - (32 920±180)/19.145T (8') With acetone as the radical source: log k5 = (11.85±0.06) - (44 810±160)/19.145T 395-523 K (5) With azomethane as the radical source: log ks = (11.88±0.03) - (44 610±260)/19.145T 363-463 K (5) log k5' = (12.02±0.04) - (45 630±310)/19.145T (5') where k5' and k5' are the values for ks and k5, respectively, obtained after allowance is made for the formation of C2H6 by way of the intramolecular decomposition of azomethane. From a consideration of these and previously reported data on CH3 and CD3 attack on (CH3)4Si, we conclude that the source of CH3 radicals, and the intramolecular formation of C2H6 in the photolysis of azomethane, both have a negligible effect on the rate constants for the reaction, and that CH3 and CD3 radicals are of equal reactivity. We also recommend best values for the Arrhenius parameters for the reaction.

The reactions of methyl radicals with chloromethanes

AbstractThe reaction of methyl radicals with CCl4 and CCl3Br have been reinvestigated in the gas phase over a wide range of temperatures and pressures using both the photolysis of acetone and the pyrolysis of di‐tertiary butyl peroxide (dtBP) as the methyl radical sources. The results are in essential agreement with previous work; however, these new studies provide evidence that at higher pressures the major source of HCl in the reactions is due to methyl radical attack on CH3CCl3, formed via the combination of methyl and trichloromethyl radicals.From these investigations Arrhenius parameters for the reactions equation image have been determined: Pyrolysis of dtBP in the presence of relatively high‐pressure mixtures of CCl4 and CCl3Br resulted in no enhanced methane formation, since, under these conditions, the only termination product is C2Cl6, and the HCl precursor CH3CCl3 is not formed. A competitive technique has been used in which dtBP was pyrolysed in the gas phase in the presence of high‐pressure mixtures of CCl3Br and a chloromethane. Arrhenius parameters were obtained for the reactions equation image and the results were used to provide information on the importance of polar effects for hydrogen abstraction from halogenated methanes.

Reactivity of Excited Triplet Alkyl Ketones in Solution. I. Quenching and Hydrogen Abstraction of Triplet Acetone

Abstract The photolysis of acetone in various solutions has been investigated by analysis of reaction products as well as absorption spectroscopy. The rate of photolysis of acetone in aqueous solution decreases on addition of methanol or with increasing concentration of acetone, being much higher than that in methanol and heptane solutions. The results suggest that intermolecular hydrogen abstraction plays a significant role in quenching of triplet acetone. This is supported by spectroscopic studies which reveal formation of the enol form of acetone as an intermediate in the decay process of triplet acetone.

Réactions des radicaux CH 3 avec les molécules CCl 4, CFCl 3 et CF 2Cl 2

The Cl abstraction reactions from CCl 4, CFCl 3 and CF 2Cl 2 by CH 3 radicals have been investigated within the temperature range 20 – 150 °C. Two radical sources were used: the photolysis of acetone at λ ⩾ 3130 Å and the thermal decomposition of di-t-butylperoxide. An important increase of the reactivity is observed for the fluorinated molecules at 128 °C; the rates obtained for these three reactions are in the ratio 1:24:126. The polar effects are discussed.

Studies on the process of sensitization in the benzene-ketones system by fluorescence quenching

Studies were made on the quenching of fluorescence of benzene state S 1, excited with λ = 254 nm radiation, by acetone, 2-butanone and 3-pentanone in cyclohexane, n-heptane, acetonitrile and ethyl acetate. The calculated values of quenching rate constants k q suggest that this process is in most cases controlled by diffusion. To furnish an example, the results are presented of investigations on the sensitizing effect of benzene upon the photolysis of acetone in cyclohexane and attempts were made at an analysis of the kinetics of the process.

A pulse photolysis system for use in kinetic electron spin resonance spectroscopy

The requirements of a combined pulse photolysis/e.s.r. spectrometer system suitable for use in the measurement of lifetimes and concentrations of free radicals in solution are discussed. The design and operation of such a system are described and some typical results are presented in order to demonstrate the performance of the system. These results were obtained during studies of the photolysis of acetone, chloranil and maleimide in alcohol solvents.

Product Analysis of Flash Photolysis of Acetone in Gas Phase.

Product Analysis of Flash Photolysis of Acetone in Gas Phase.

Photochemistry in the Adsorbed Layer. III. Photolysis of Acetone Adsorbed on Porous Vycor Glass

Abstract Photolyses of acetone adsorbed showed that the ratio of ethane to methane in the products as well as the kinetic isotope effect for the methane formation is much smaller than that in the gas phase photolysis. The methane formed from acetone-d6 contains a considerable amount of CD3H. Ethane formation is quenched more efficiently by nitric oxide than methane formation. It is proposed that there are two types of methyl radicals, i.e., long- and short-lived ones, the ethane resulting from the long-lived radicals. The results suggest that methane arises from hydrogen abstraction from surface OH groups as well as disproportionation reaction CH3+CH3CO→CH4+CH2CO, in addition to hydrogen abstraction from acetone molecules.

Gas phase photolysis of acetone in the far ultra-violet

The photolysis of acetone (pressure: 2 to 25 Torr, T: 300 K) was investigated at 147 nm (8.4 eV), 123.6 nm (10 eV) and 106.7-104.8 nm (11.6–11.8 eV). The quantum yields of the products H 2, CO, CH 4, C 2H 6, C 2H 4, C 2H 2, C 3H 8, C 3H 6, C 3H 4, and CH 3COC 2H 5 were measured both in the presence and absence of free radical scavengers (C 2H 4 or NO). These data, in conjunction with the results of deuterium-labelling experiments, demonstrated the occurrence of the following primary processes: ▪ It is also suggested that the primary process: ▪ occurs with a quantum yield of ≈ 0.02. The quantum yield of “molecular” methane (process d) is 0.02 (147 nm), 0.028 (123.6 nm) and 0.043 (104.8–106.7 nm). The quantum of “molecular” hydrogen is 0.03 (147 nm) 0.26 (123.6 nm) and 0.28 (104.8–106.7 nm). The quantum yield of process (a) is 0.7 ± 0.1 at 147 nm. At shorter wavelengths the quantum yield of the latter process could not be estimated because of the uncertain fate of the cations [Φ(CH 3COCH 3 + ) = 0.25 at 123.6 nm and 0.21 at 104.8–106.7 nm] and the extensive dissociation of the internally excited species formed in the primary processes.

Intermediates produced in the flash photolysis of acetone and amides in aqueous solution

Intermediates produced in the flash photolysis of acetone and amides in aqueous solution

The photolysis of acetone in the presence of triethylborane

The photolysis of acetone in the presence of triethylborane has been investigated at 20 and 126°. From an analysis of the hydrocarbons produced and the diminution of the rate of carbon monoxide formation, it is concluded that a reaction between the acetone triplets and triethylborane takes place and that this reaction produces ethyl radicals. The values of the rate constant of this reaction were found to be ≈ 1 x 10 11 and 3.4 x 10 11 ml·mole −1·s −1 at 20 and 126° respectively. The small temperature dependence of these values points to a very low activation energy. This result is similar to that previously found for the biacetyl/triethylborane system. The rates of attack of methyl and ethyl radicals on triethylborane have also been measured.

Hydrogen Abstraction from Methyl Formate by Methyl Radicals

The reactions of CH3 radicals with methyl formate and CD3 radicals with methyl formate and methyl formate-d have been studied. The CH3 and CD3 radicals were produced by the photolysis of acetone and acetone-d. The Arrhenius parameters (log A, A in 1 mol−1 s−1; E, in kcal mol−1) for hydrogen abstraction are as follows[Formula: see text]

Acetone photolysis: Kinetic studies in a flow reactor

AbstractThe photodecomposition of acetone vapor was studied in a tubular flow, differential reactor at atmospheric pressure and from 82 to 112°C. The products included significant amounts of biacetyl along with carbon monoxide and ethane. A rate equation was developed from a sequence of elementary reactions believed to explain the photolysis of acetone. The effects of light intensity and acetone concentration, indicated in the derived rate equation, were confirmed by the data.The rate is less than first‐order with respect to acetone, and this is believed to be due to reformation of stable acetone molecules by deactivation of CH3COCH3*.

Hydrogen abstraction from amides. Isotope effects, reactivity of different sites, and secondary effects in the reaction of methyl radicals with formamide and acetamide

The gas-phase attack by methyl radicals on acetamide, acetamide-d3, formamide, and formamide-N-d2 has been studied, and hydrogen and deuterium abstraction rates have been measured. Methyl and methyl-d3 radicals were produced by photolysis of acetone and acetone-d6. Measurements on acetamide were made in the temperature range 122–259°C; the maximum temperature regularly employed with the formamides was 202°C. Rate constants for the reactions below have been derived, giving the following Arrhenius parameters: [graphic omitted] Throughout the temperature range studied, the rates of abstraction of hydrogen from the different sites in acetamide are approximately equal; e.g., at 150°C abstraction from the nitrogen atom accounts for 52% of the attack. The individual reactivities are compared with those of methyl and amino groups in other molecular environments, and the reactivities of the methyl and methyl-d3 groups in acetamide and acetamide-d3 are discussed in terms of theories of kinetic isotope effects. Comparison of the present work with published data for methyl radical attack on N-dimethyl acetamide enables the reactivity of the N-methyl groups in that compound to be assessed.