Photodecomposition of acetone

Photodecomposition. Owing to its simple structure and great importance as a model compound, there have been many studies of acetone photochemistry. The bulk of this work has been reviewed earlier (148,175). The photodecomposition of acetone has shown the major products to be carbon monoxide, ethane, and methane. Under proper conditions, other minor products are produced. 

Gardner, E. P., R. D. Wijayaratne, and J. G. Calvert (1984). Primary quantum yields of the photodecomposition of acetone in air under tropospheric conditions. J. Phys. Chem. 88, 5069-5676. 

Acetone is primarily removed from the atmosphere by photolysis and by reaction with OH. Blitz et al. (2004) discussed the competition between these two reactions, showing that reaction with OH is faster at the surface, with photolysis becoming more important at higher altitudes. Here we estimate the lifetime for photodecomposition of acetone in the lower troposphere with an overhead Sun to be about 12 to 16 days (see table lX-M-1). For [OH] = 10 moleculecm , while the lifetime at the Earth s surface with respect to reaction with OH is about 64 days. 

Gierczak et al. (1998) also studied the photodecomposition of acetone in dilute mixtures in air in experiments at discrete wavelengths between 248 and 337 nm and at several pressures of air. In these studies, the quantum yields of CH3C(0)CH3 loss and CO2 formation were monitored. In experiments in which both 4>-ch3C(0)CH3 and cc>2 could be measured, the authors observed a near equality between these quantities as observed previously by Gardner et al. (1984). Both CO2 formation and acetone loss were used as a measure of process (I). However, in view of other evidence of the occurrence of process (II), particularly at the shorter wavelengths, it may be more appropriate to use the CO2 product alone as a monitor of process (I). The acetone loss... 

NMasdi et al. (2007) studied the photodecomposition of acetone at 248 nm and 298 K (0.23 mbar of acetone in air). They found that the total quantum yield of acetone loss was very near unity and independent of the added air pressure 1.040 0.046 at 133 mbar and 0.999 0.066 at 1000 mbar. In studies at 308 nm, Nidasdi et al. (2007) also reported that the quantum yields of acetone loss for photolyses at 308 nm was both temperature (233-323 K) and pressure dependent, in confirmation of the conclusions ofBlitzetal. (2004, 2006). 

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

Quenching of Singlets of Carbonyl-Containing Compounds, a. Acetone. Recent reports have shown that singlet complications are not restricted to hydrocarbons for example, the photodecomposition of 1,4-dichlorobutane (52) to free radicals is sensitized by the (n, n) singlet state of acetone.216 Besides the observations that 52 quenches acetone fluorescence and that... 

The (CH3CO-CH3) bond dissociation energy has been measured by the kinetic, and more recently by the electron impact, methods, and concordant values40 41 of 71 kcal. mol.-1 were obtained so that when acetone suffers a photodecomposition of type A, the fragments still carry excess energy. 

The energy required to dissociate the acetone molecule into two methyl radicals and a molecule of carbon monoxide may be calculated from thermocheinical data and amounts to 89 kcal. mol.-1. It would be energetically possible, therefore, for a photodecomposition of type B to occur,... 

With the exception of the photolysis of acetone itself, no other photodecomposition of a ketone has been studied so extensively as that of hexafiuoroacetone. 

Surely, many such common reactions could be utilized for the intentional destruction of unwanted herbicides and their residues (41). Metham might be caused to react simply with aqueous ammonia to form harmless methylthiourea many herbicides including prometryne and metribuzin (Sencor) might be degraded by dilute hypochlorite ("chlorinated lime") of the type used to purify swimming pools, and the photodecomposition of others (such as 2,4,5-T) might be accelerated by cheap nontoxic photosensitizers such as acetone (Table III) (42). The variations of environmental chemistry applications to control and direct herbicide persistence and effectiveness now appear endless. 

It is obvious that the whole process is catalytic in both Eu2+ and H+ ions and the net reaction is the photodecomposition of 2-propanol into acetone and hydrogen. 

The conversion of the benzohydroxamic acid (191) into the amide (192) is the result of nitrogen-oxygen bond homolysis.Photodecomposition of aminophenazone (193) in aqueous solution gave N-acetyl-N-methyl-N -phenylhydrazine (194), whereas on irradiation in acetone products arising by... 

Photodecomposition of carbohydrate-derived geminal diazides appears to proceed via intermediate carbenes, ° whereas irradiation of the diazidomethylpyrimidine (70) in acetone affords the uracil carboxylate (71), but only in the presence of oxygen. 

SCHEME 13.17 Photodecomposition and self-initiation mechanism of acetone. 

The excited state of ketones can thus initiate free-radical reactions, and this is probably the mechanism for many examples of enhanced photodecomposition of environmental pollutants sensitized by acetone or other simple carbonyl compounds. A good example of such reactions is the acetone-promoted photooxidation of atrazine (24) and related triazine herbicides described by Burkhard and Guth (1976). In water, atrazine absorbs almost no solar UV and was accordingly quite stable to photolysis, but in the presence of large amounts of acetone (about 0.13 M), its half-life was decreased to about 5 hr. The produets were N-dealkylation products and ring-hydroxylated triazines. Similar products were also identified in riboflavin-sensitized photooxidation of triazines (Rejto et al., 1983). Presumably, a principal mechanism of photodecomposition would be H-abstraction from the N-alkyl substituents of atrazine, perhaps in conjunction with electron transfer from the unshmed pairs of the nitrogen atoms. 

Acetone and a stable dimer of nitroso methane are major products of the photodecomposition of butyl nitrite at 25 °C [12]. It was suggested that dimer formation from nitroso methane monomer follows the primary process ... 

Figure IX-D-3. Experimental determinations of the quantum yields of acetone photodecomposition (01 4- 0ii) as a function of wavelength for photolyses in air at 760Torr and 298 K. The dashed gray curves define an envelope that describes the range of possible values. Values from the two dashed curves were used in estimating the possible range of photolysis constants shown in figure IX-D-6. Authors of the current study recommend use of the values given by the short-dashed line. The maximum values are also given in table IX-D-2.

Hi) Minor processes in acetone photodecomposition Primary process (III), H-atom formation, has been observed by Takahashi et al. (2004) and Lightfoot et al. (1988) in photolysis of acetone at 193 nm. Lightfoot et al. (1988) reported that 3% of acetone... 

In photolysis at 313 nm, quenching of 2-hexanone by formation of triplet 2,3-butanedione is only l/20th of that seen for 1-pentanone. The quantum yields of propene and acetone are also little affected by 2,3-butanedione addition at 35°C, d csHe = 0.41 0.01 and 4>ch3C(0)CH3 = 0.44 0.04. At 253.7 nm, there is no observable effect of added 2,3-butanedione on the photodecomposition of 2-hexanone, and no evidence for a sensitized emission of 2,3-butanedione was observed (Michael and Noyes, 1963). Obviously, a very short-lived triplet or singlet state is the precursor to all photodecomposition in 2-hexanone. 

In the following sections the limited data on the photodecomposition of the substituted acetone molecules are described, largely as reviewed by Calvert et al. (2008). 

The photodecomposition of another OH-substituted acetone molecule, diacetone alcohol [(CH3)2C(0H)CH2C(0)CH3l has been studied by Magneron et al. (2003) in the EUPHORE facility in Valencia, Spain. In this molecule, the OH group is located two carbon atoms down-chain from the carbonyl group. Its estimated photolysis frequency in sunlight was too low to measure (j < 5x 10 s ). 

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