The photolysis of acetone

This reaction has been studied quantitatively in three different laboratories and all the results22 are in good agreement on the quantum yield and the most important factors, although some minor points such as the chemical analysis of the products are still unsettled. It is a simple reaction with a low quantum yield—considerably less than unity. It offers a good example of a reaction in which free radicals are produced. The chief reaction is CH3 

Experimental Procedure. The photochemical technique used in this investigation has been described. Practically monochromatic 

The acetone was purified by crystallization with sodium iodide, thoroughly dried, and distilled into a small quartz bulb fused to the cell, after thorough heating and drying of the cell. After partial evaporation the acetone was distilled into the cell, and sealed off while cooled with solid carbon dioxide to prevent decomposition of the vapor. 

In a typical experiment radiation at 3,130 Awas absorbed by the acetone vapor at the rate of 85,200 ergs per second (corrected for reflections) for a period of 23,000 seconds. The pressure of acetone was 760.0 before the radiation and 790.4 after it. Two molecules are produced for each molecule of acetone decomposed (neglecting minor products) and hence the total number decomposing can be easily calculated from the pressure increase Ap (30.4 mm. at 56.0°), the volume v (60.3 cc.) and the Avogadro number. Thus 

The number of photons absorbed is then calculated from the radiation at 3,130 A. 

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Although a very great deal of work has been carried out on the photolysis of acetone, it is not possible to decide unequivocally whether both processes occur simultaneously or whether a primary step of type A is succeeded by a further partial breakdown of the acetyl radical. 

In fact, no appreciable quantities of hydrogen have been detected in the photolysis of acetone under these conditions. 

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

Actually, type I cleavage occurs from both singlet and triplet excited states, as evidenced by the incomplete quenching of the photolysis of acetone by high pressures of biacetyl308 or 2-butene.243 The extent to which dissociation can be quenched is subject to large temperature and wavelength effects and depends markedly on the stability... 

Hoare203 examined the photolysis of acetone at 200°C in the presence of NO. He monitored both CH4 production and NO consumption and computed k7/k15 from the competition between reaction (7) and... 

There are isolated reports of Sh2 reactions involving attack by C-centered radicals. These are typically gas-phase reactions using the photolysis of acetone (sometimes the per-deutero or per-fluoro analog) as a source of Me, which gives CD3 with Hg(CD3)2 examples involving B and Sn have also been reported. 

The theory of the photolysis of acetone has been discussed in a comprehensive manner by Howe and Noyes.38 They have obtained new, exact experimental data in the short ultraviolet near 1,900 A which they have correlated with the earlier work in the longer ultraviolet. 

In the photolysis of acetone it is accepted96 that the primary process is the formation of a methyl and an acetyl radical ... 

Similar methods have been employed in the photolysis of acetone in the presence of H2/ the photolysis of light and heavy acetone mixtures, the pyrolysis of ethane,- and the decomposition of C2H6Br in the presence of radioactive BrS, HBr, and DBr. ... 

Table XIII.13. Thermodynamic Data for Some Reactions of Interest in the Photolysis of Acetone (All Gas Phase)...

In the photolysis of acetone-l2 mixtures, reported by Benson and Forbes, the ratio (CIl4)/(CO)(X = 2537 A) was always about 0.15 to 0.20, indicating either a very low efficiency for reaction 3 relative to CHs + acetone or a hot radical effect in this photolysis as well. 

Products of the photolysis of acetone include CO, C2H6, CH4, (CH3-C0)2, CH2CO, and C2H6COCH3. From these products one assumes the elementary steps... 

Thus under carefully controlled conditions, the photolysis of acetone is a source of methyl radicals. 

McNesby et studied mixtures of acetone and D2. Methyl radicals were generated by the photolysis of acetone at 140 and 424 °C. The experimentally... 

Studies made at 1236 A show that the photolysis of acetone becomes more and more similar to its radiolysis as the energy of the irradiating light increases . 

O Neal and Benson S studied the photolysis of acetone at 3130 A in the presence of hydrogen iodide. The main products of the reaction were methane and acetaldehyde CO could be detected only at high temperatures. The dependence of the CH3CHO/CO ratio on pressure was considered to be a result of the pressure-dependent decomposition of CH3CO. The experimental results concerning the radical decomposition obeyed the relation predicted by the Hinshelwood-Linde-mann theory. The values reported for the limiting high-, and low-pressure rate coefficients were... 

Finally, the reactions of the acetonyl radical have to be dealt with. The thermal stability of this radical is manifested by the facts that (/) the ketene formation is negligible below 200°C , and (ii) chains do not occur in the photolysis of acetone up to about 350°C . The acetonyl radicals disappear from the system by recombination reactions, viz. 

Methyl ethyl ketone was detected in experiments above lOO C. Acetonyl acetone has not been identified in the products so far. Actually, not more than a small amount of this compound is expected to be formed, since (i) the acetyl radical is unstable at higher temperatures, and (ii) the rate of formation of acetonyl radical is slow at lower temperatures. The formation of biacetonyl was observed in the investigation of the reaction between CH3 and acetone . Brin-ton ° has succeeded in detecting biacetonyl also among the products of acetone photolysis in the temperature range 200-475°C. Most of the evidence on the formation of biacetonyl in the photolysis of acetone is, however, based on material bal-ance and hence is only of secondary importance. 

Consequently, one would expect chains to occur in the photolysis of acetone at high temperatures. However, the values of < co hardly exceed unity even above 400 °C ° . It is likely that ketene traps the methyl radicals very efficiently and thus prevents development of the chains. 

In the photolysis of acetone, as was seen earlier, several reaction steps have to be taken into consideration. However, one may select experimental conditions where the mechanism of the reaction is simple. For example, the quantum yield of acetone consumption is unity and the stoichiometry is simple in the wavelength region 3130-2537 A, within the temperature range 120-200 °C and at pressures higher than 100 torr. Under these circumstances all the experimental results can be interpreted by a reaction scheme consisting of steps (22), (26), (32) and (38). 

QUANTUM YIELDS OF CASEOUS PRODUCTS OF THE PHOTOLYSIS OF ACETONE IN THE LIQUID PHASE AND IN SOLUTION AT 2537 A AND 25 °C... 

Fig. 6. Effect of the amount adsorbed upon the photolysis of acetone-dg on Vycor glass at 298 K and the content of CDgH in the produced methane.

The photolysis of acetone adsorbed on Ni /Vycor glass was found to... 

All of the reaction products can be rationalized in terms of primary cleavage to acetyl radicals and analogy to the photolysis of acetone. However, the fact that appreciable carbon monoxide was observed 113> in iodine trapping experiments, in contrast to the results with acetone, suggests that some other primary dissociation also occurs. 

Acetylacetone is one of the major products of the photolysis of acetone (20). In the presence of chlorobenzene, the self-quenching reaction competes with the energy treuisfer and the radicals induce the formation of by-products. This self-quenching reaction is much reduced with hexadeuterated acetone. Up to 0.1 mol.l, the major photoproduct from CD--CO-CD. 

Another class of atmospherically important compounds is ketones, which are almost of biogenic origin. The most simple is acetone, which is ubiquitous in air. With the exception of acetone, higher ketones react preferably with OH at any carbon atom by H abstraction. With subsequent O2 addition the fate of RO2 is known, either forming aldehyde (HCO) at terminal carbon or ketone (C=0) in the middle of the chain. The photolysis of acetone (Emrich and Warneck 2000) provides the known radicals, which finally convert into HCHO and form the acetyl-peroxo radical (see 5.46 and 5.47). 

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