Determination of Free Radicals in Acetone Photolysis

Abstract

Acetone has been photolyzed in a flow system at several temperatures in the pressure range 0.5 to 11 mm with exciting radiation of wave-length 2650–2900A and 2537A. The free methyl radicals formed in the decomposition have been determined as lead tetramethyl. Approximately one methyl radical is formed for every four molecules of carbon monoxide, the actual proportion depending on temperature and pressure. Relatively more free radicals are formed at 100°C than at lower temperatures and more are formed at lower pressure than at high. A process involving the existence of a long lived metastable state for photoactivated acetone has been suggested to explain the results. The proposed scheme is found consistent with most of the published work on acetone photolysis.

The Gas-phase Photochemical Decomposition of the Simple Aliphatic Ketones.

“…CH3COCH2 -+ CH2CO + CHS (24) This reaction is probably unimportant except at temperatures above 400°C.…”

“…Thus, from acetone at these wave lengths is about 0.7. With the formation of the more unstable propionyl radical, would be expected to be greater than 0.7 on the basis of the free-radical theory (see table 24). In general, the photolysis of ethyl methyl ketone appears to follow a completely free-radical mechanism, the formation of ethylene possibly being explained by 2C2H6 C2H4 + C2H6 (48) and at higher temperatures by reaction 27 or 46, followed by CH3COC2H4 -> CH3CO + C2H4 (49) At room temperature and low pressures biacetyl is probably formed according to the reaction: CH3CO KCH.CO), (11) The presence of biacetyl in the photolysis of ethyl methyl ketone has been demonstrated by Matheson and Zabor (46), who found the green fluorescence characteristic of biacetyl when this ketone is irradiated with light from a mercury arc.…”

“…Further, the acetone remaining after illumination should be a mixture of CH3COCH3, CH3COCD3, and CD3COCD3 if the free-radical mechanism is obeyed; i.e., if the low quantum yield of acetone decomposition is due to radical recombination. Before discussing the effect of temperature upon the products of the photolysis of acetone, it might be well to consider some recent work of Feldman, Burton, Ricci, and Davis (24). The excited-molecule mechanism was presented by Spence and Wild to account for the results of the photolysis of acetone only in the "banded" region of absorption above about 3050 A.…”

“…CHgCOCHs + C1I3 -* CH,COCHjCH, (23) CH3COCH2 -> CH2CO + CH3 (24) Reaction 23 has been assigned an energy of activation of about 8 kg.-cal. ( 14), while the 48 kg.-cal.…”

Acetone photoactivated process: application to the degradation of refractory organic pollutants in very saline waters

“…In this approach, acetone photoactivation may be integrated as an emerging method for seawater treatment. Acetone photolysis (190–300 nm) has been studied by many workers in the pure liquid as well in the gas phase for over 50 years (Damon and Daniels, 1933; Feldman et al ., 1945; Trotman‐Dickenson and Steacie, 1950; Ridge and Steacie, 1954; Pieck and Steacie, 1955; Matsuura et al ., 1969). The analysed gaseous products in all cases are methane, ethane and CO, which are formed mainly through methyl radical‐induced chain reactions (Eqs. )…”