THERMAL DECOMPOSITION OF ACETONE

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. 

Application ot the steady-state treatment leads to the experimentally observed first-order expression provided that reaction (6) is the sole termination step. 

The question of molecular or chain decomposition had been the subject of several studies in the first part of this century. There is, however, decisive evidence available today supporting the chain mechanism. 

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

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

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. 

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. 

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

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. 

Derivation By spontaneous polymerization of ketene obtained by thermal decomposition of acetone or from bromoacetylbromide and zinc. 

Acetic Anhydride. Acetic oxide, acetyl oxide C4HeO,. mol wt 10209 C 4706-"- H 5.92 Sv O 47.02% (CHjCOljO Equivalent to 117 645, acetic acid. Made formerly from sod acetate and acetyl or sulluryl chloride now usually obtained from acetaldehyde or acetic acid Faith, Keyes Clark s Industrial Chemicals, F A Lowen -helm. M K Moran. Eds (Wiley -Inlerseicticc. New York. 4th ed, 1975) pp 16-20 Of industrial importance is also the kecene process, starting with the thermal decomposition of acetone- Schmidlin, Bergmann. Ber 43. 2821 (19)0). 

The thermal decomposition of acetone isfree-radical chain reaction. The initiating process may consist in the generation of carbon monoxide and methyl radicals. 

The ketene could be produced by thermal decomposition of acetone at 700800°C or dehydration at 750°C at reduced pressure over a dehydration agent, such as triethyl phosphate ... 

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