The Mechanism of the Photooxidation of Acetaldehyde at Room Temperature

Abstract

The initial rates of product formation in the photooxidation of acetaldehyde a t room temperature have been determined through the use of infrared spectrometry. The rates of forrnation of the products peroxyacetic acid, carbon monoxide, carbon dioxide, methanol, formic acid, and acetic acid were determined in experiments with various pressures of acetaldehyde, oxygen, and added gases. The amounts of methylhydroperoxide and acetylperoxide formed in all of the experiments arere below the detection limit of the analytical methods. The results require that some modification and corrections be made to the mechanism suggested by McDowell and Sharples. INTRODUCTIONRecently 14cDowell and Sharples (1) published a series of kinetic studies of the photooxidation of acetaldehyde and propionaldehyde. The reactions were followed by the titration of the peroxyacid formed. Reported here is a study of the photooxidation of acetaldehyde in which a more detailed product analysis has been made through infrared absorption spectroscopy. The results show that the simple mechanism which McDowell and Sharples have inferred from their rate studies is very incomplete, and in some respects, it is incorrect. Soine suggested modifications and corrections to their mechanism are presented. E X P E R I M E N T A L Apparatus and MaterialsOur experiments were basically similar to those of McDowell and Sharples (1). We used comparable amounts of acetaldehyde in oxygen, and we photolyzed a t rooin temperature with light in the range 3000-3300 A. In order to accelerate the rates and facilitate the identification of the products of the chain-initiating and chain-tern~iilatii~g reactions, we used intensities about 1000 times those enlployed by ;\iIcDowell and Sharples.The acetaldehyde (Eastman-Kodak, white-label, fractionated a t reduced pressure), oxygen (Linde tank gas), and other added gases were mixed in a 10-cm Pyrex-body infrared absorption cell which was mounted in the sample beam of a Perkin-Elmer Model 21 double beam infrared spectrometer. The side of the cell was exposed to the radiation from a Pyrex-jacketed, water-cooled, high pressure mercury arc (AH-6) which was fastened about 10 inches from the cell. The absorption characteristics of the water jacket and the cell body limited the effective radiation t o the range 3000-3300 A. The inaxirnuin intensity of the light absorbed by the aldehyde was near 3100 A.T h e other reactant gases and standards for analysis were obtained as follows: tetramethylethylene was the research grade Phillips product; peroxyacetic acid and acetylperoxide were obtained from Becco and purified by distillation a t reduced pressure; ozone was prepared as needed by a simple ozonizer apparatus; the oxides of carbon were reagent gases of the Air Reduction Co.; inethailol was the Mallinckrodt Ailalytical 'Ma?zuscript received M a y g6, 1969.

Photochemical Oxidations. IV. Acetaldehyde

“…Carbon dioxide and carbon monoxide are surely present. 7 The experimental results are listed in Tables I through III.…”

“…CHsCHO + hv ->• CH< + CO The concensus seems to be in favor of reaction 9 although there may be conditions favoring (7) or (8). As a part of the series of initial fast radical reactions, peroxy methyl radicals are believed to be converted to methoxy radicals 2CHaOO->-2CH,0 + 02 (10) Thus one expects important amounts of the following radicals promptly to follow the primary photochemical reaction: CH300, CH30, HO, H02, and perhaps HCO(OO) and HCOO.…”

Generation and Cyclization of Acyl Radicals from Thiol Esters Under Nonreducing, Tin-Free Conditions

“…It would seem reasonable that two of these species combine to give a diacyl tetraoxide which then looses oxygen to give two carboxyl radicals ( 51 ) and so the observed cyclization. Such processes have been discussed in the context of gas and liquid phase autoxidations of aldehydes. ,, Alternatively, and perhaps preferable in view of the dilution and the consequent improbability of bimolecular radical−radical reactions, reaction with iodine occurs to give a periodite which then furnishes the acyloxyl radical ( 51 ). A final point of note here is the preference of the acyloxyl radical ( 51 ) for cyclization in the 6-exo-trig mode over and above 1,5-hydrogen atom abstraction from a benzylic-allylic site …”

“…and isolation of the amines as their hydrochloride salts (18)(19)(20)(21)(22)(23)(24) in close to quantitative yield (Table 1). Finally, ethanolic solutions of these amine hydrochloride salts were exposed to tetrafluoroboric acid and isoamyl nitrite, permitting isolation of the desired diazonium ions as their tetrafluoroborate salts (25)(26)(27)(28)(29)(30)(31) in excellent yield (Table 1).…”