Decomposition of acetone

The following mechanism has been proposed for the thermal decomposition of acetone. 

An example of a reacting system with a network involving reactions in series is the decomposition of acetone (series with respect to ketene) (C)... 

The following mmechanism has been proposed for the decomposition of acetone by Rice Herzfeld ... 

In problem P2.03.24 it is shown that the decomposition of acetone is the result of a chain of five reactions with a net rate equation... 

Problem 3.7 (Chain Reaction) The thermal decomposition of acetone is found to follow the rate law as... 

Acetone cyanohydrin nitrate will not nitrate amines with branching on the carbon a to the nitrate group. For these substrates the use of ethyl nitrate and lithium bases is favoured. a-Aminonitriles are frequently observed as impurities under the reaction conditions because of the slow decomposition of acetone cyanohydrin nitrate to hydrogen cyanide and acetone. The need for an excess of amine during these reactions is wasteful and only practical if this component is cheap and widely available. Other cyanohydrin nitrates are less efficient N-nitrating agents. ... 

The wavelength thresholds are 338 nm for (31a) and 299 nm for (31b). Thus, (31a) is expected to predominate at the earth s surface. As might be expected, once excited, acetone can be collisionally quenched in competition with decomposition, and hence the quantum yields decrease with increasing total pressure. Figure 4.30, for example, shows the measured quantum yields for the decomposition of acetone at 760 Torr total pressure and the values extrapolated to zero pressure (Gierczak et al., 1998). In the tropospherically important wavelength region, the yields are small beyond about 330 nm at 1 atm pressure. 

As well as almond meal, Sorghum bicolor shoots have also found application in the synthesis of aromatic cyanohydrins [55]. The enantiomeric purity obtained in transhydrocyanation experiments with acetone cyanohydrin as the cyanide source suffers from the high water content (>14% v/v) necessary for the decomposition of acetone cyanohydrin. In contrast, the application of HCN allows the use of low amounts of water (2% v/v), leading to yields and optical purities comparable with those obtained by the isolated enzymes. 

The quantum yield for decomposition of acetone from the triplet state is about 0.40 at 40°, and the quantum yield of phosphorescence is about 0.02.308 At least 407o of the initially excited molecules are not accounted for by either emission or chemical reaction, and thus must undergo some kind of radiationless decay to the ground state, presumably mostly from the triplet state. Recent studies of hexafluoro-acetone317 indicate that approximately half the triplet molecules formed by intersystem crossing undergo radiationless decay. 

SCHMIDLIN KETENE SYNTHESIS. Formation of kelene by thermal decomposition of acetone over electrically heated wire at 500-750 degrees by a reaction involving radical formation with generation of methane and carbon monoxide. 

Acetic anhydride is produced by three major processes (I) dehydration of acetic acid, t2) decomposition of acetone, and t.3) oxidation of acetaldehyde The formates und ucct3tes are prepared hy reacting the appropriate alcohol and organic acid in the presence of p-toluene-sulfonic acid... 

The possible extent to which free radical chains may account for the thermal decomposition of organic molecules in the gas phase was first emphasized by Rice and Herzfeld.26 They gave three examples showing how all the known facts in the decomposition of acetone, acetaldehyde and ethane could be explained by chain reactions involving free radicals. Their calculations showed that the first order character of the reaction could be maintained under proper conditions and they estimated reaction rates and temperature coefficients in agreement with the facts. 

Quantum calculations can make some selections in this case even though thermochemistry can not. The reverse of reaction (2) involves free radicals and the activation energy of free radicals is very low, 5,000 to 10,000 calories or less. The energy of activation for decomposition of acetone by reaction (2) then can not be much greater than 75,000 or at the most 80,000 calories and it can not be much less than 70,000. 

The presence of methane in the decomposition products together with the fifty per cent of carbon monoxide indicates a reaction more complicated than (1) or (4) and favors reaction (2) or (5). Rice and Herzfeld32 have proposed a series of chain reactions for the thermal decomposition of acetone which would give both methane and ethane. The recombination of CH3 and CH3CO according to (2) might account for the low quantum yield. 

The primary quantum yield was then shown to decrease from 0.59 to a steady value of 0.20 for 50 mm. acetone as up to 0.5 mm. biaeet.yl was added. The quantum yield of carbon monoxide decreased correspondingly from 0.16 to 0.05. It is clear that under such conditions deactivation of the triplet state by biacetyl causes a major decrease in the decomposition of acetone. The ratio of the total acetone emission intensity to the absorbed intensity is small (0.02 at 40°C.5li). 

The rates for the thermolysis of cyclic peroxides are also only slightly solvent-dependent. For example, the first-order decomposition of acetone cyclic diperoxide (3,3,6,6-tetramethyl-l,2,4,5-tetroxane) increases only 21-fold on going from n-octane to acetic acid as solvent, corresponding to homolytic 0-0 bond rupture [824],... 

The body of literature on the photochemical decomposition of acetone is by now quite imposing, and excellent critiques of the %work are available. " This particular photolysis owes its importance to the fact that it has been one of the chief sources of quantitative data on the behavior of methyl radicals. It is therefore of some consequence to examine the principal features of the reaction. 

Through a stirred suspension of HgO (21.7 g, 100 mmol) and Hg(OAc)2 (31.8 g, 100 mmol) in dry MeOH (217 mL) at rt was gently bubbled ketene (generated by the thermal decomposition of acetone). The reddish suspension turned into a white suspension. After stirring for 12 h, a gray clear soln was obtained... 

The first quantitative investigation on the thermal decomposition of acetone vapour was carried out by Hinshelwood and Hutchison by pressure measurement in the temperature range 506-632 °C. The authors concluded that the thermal decomposition of acetone is a unimolecular reaction. In contradiction to this conclusion Rice and Herzfeld suggested a chain mechanism, viz. 

The wall-effect was investigated by Winkler and Hinshelwood . They concluded that the thermal decomposition of acetone is almost completely homogeneous in a seasoned vessel. 

However, according to Allen , surface increase (/) completely altered the shape of the rate versus pressure curves, (i7) had practically no effect at high temperatures and high pressures, though considerably reduced the rate for instance at 465 °C, and Hi) eliminated the induction period which occurred in an empty vessel. The observations of Allen concerning the induction period and the effect of surface/volume ratio show, without doubt, that the decomposition of acetone is a complex process. 

The participation of free radicals in the thermal decomposition of acetone has been proved by Patat and Sachsse by the para-ortho technique. Talrose et al identified methyl and acetyl radicals by mass spectrometry. 

In contradiction to Bairstow and Hinshelwood , Gantz and Walters found that iodine catalyzes the thermal decomposition of acetone between 470 and 517 °C. The rate is roughly proportional to the square root of the acetone pressure. Inhibitors, such as NO, C2H4 or C3H6, retard the reaction. 

Relatively more attention had been paid to the study of the sensitized thermal decomposition of acetone. In the temperature range 350-400 °C, Rice et al investigated the decomposition of acetone sensitized by dimethyl mercury. The a-mount of acetonyl acetone formed was equal to that of dimethyl mercury decomposed, indicating the absence of chains. At higher temperatures, however, sensitized chain decomposition has been observed. According to Kodama and Takezaki ,... 

Staveley and Hinshelwood studied the thermal decomposition of acetone in the presence of NO and observed no inhibition. Reinvestigation revealed a slight inhibition at low NO concentrations at higher concentrations catalysis was observed. 

Though radicals react with acetone, chains are not propagated below about 450 °C. On the other hand, at higher temperatures where the thermal decomposition of acetone has been generally studied, the acetonyl radical is unstable and decomposes into ketene and methyl radical. Thus, under such conditions, the reaction is a chain process. 

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