Acetaldehyde, chain reaction

Reactions with molecular species above the arrow e.g. RIO) involve subsequent reactions with these species to produce the indicated products. In most cases the reactants shown to the left of the arrow participate in the slowest or rate-determining step]. The CH3O radical formed in Rll then follows reaction R7. The H02 radical formed in RIO is the other member of the family and is linked with HO in a variety of chain reactions. These radicals are produced following HO attack on hydrocarbons or by photodissociation of oxygenated hydrocarbons such as formaldehyde (RIO) and acetaldehyde ... 

For the thermal decomposition of acetaldehyde, a chain reaction propagated by free radicals is postulated ... 

The energy of activation in such chain reactions can be evaluated from those of the individual steps in the process. For example, in decomposition of acetaldehyde, we have... 

This reaction system is an example of a chain reaction, which we will consider in more detail in Chapter 10. However, here we will just use the pseudo-steadly-state approximation to find the approximate rate expression above. The only reactant is acetaldehyde, and there are six products listed CH4, CO, CH3, CH3O, CHO, and C2H6. The first two, CH4 and CO, are the major products C2H6 is a stable but minor product. The other species, CH3, CH3CO, and CHO, are free radicals that are very reactive and never build up to high concentrations. 

We are repeating a discussion from Chapter 4, but here we emphasize that this is a chain reaction. This reaction is found experimentally to be irreversible and proportional to the power of the acetaldehyde concentration,... 

Figure 10-1 Sketch of the chain reaction for acetaldehyde decomposition. The chain cycles between CHs and CH3CO" radicals in the propagation steps and is fed by acetaldehyde and terminated by methyl recombination.

This generic chain reaction can be sketched similarly to the acetaldehyde decomposition reaction as shown in Figure 10-2. The circular chain propagates itself indefinitely with a rate rp once initiated by rate ri, but it is terminated by rate ri, and in steady state Ti and rt control how fast the cycle runs. The overall reaction rate is controlled by the concentration or the chain-propagating radical Cr because this controls how many molecules are participating in the chain. This is why r and rt are so important in deterniining the overall rate. 

Our first example of a chain reaction, the decomposition of acetaldehyde to methane and CO, is endothermic so the reactor tends to cool as reaction proceeds. However, the oxidation of H2 is exothermic by 57 kcal/molc of H2, and the oxidation of CH4 to CO2 and H2O is exothermic by 192 kcal/mole of CH4. Thus, as these reactions proceed, heat is released and the temperature tends to increase (strongly ). Thus thermal ignition is very important in most combustion processes. 

For a simple system such as 2-propanol, which decomposes to acetone, with no appreciable chain reactions, the QY reaches a plateau at very low light intensity.74) The precise value depends sensitively upon the characteristics of the particular film, probably depending upon the presence of trace impurities and defects, which could act as recombination sites. For a system in which there are radical chain reactions, such as acetaldehyde, QY can continue to increase, even at very low lnorm values, and can in principle exceed unity.75)... 

The G-values of all products depend on the dose rate, not only that of acetaldehyde, the product of the chain reaction (reaction 53). At high dose-rates, the elimination of water (reaction 49) cannot compete effectively with the disproportionation and combination reactions of... 

Schuchmann MN, von Sonntag C (1988) The rapid hydration of the acetyl radical. A pulse radiolysis study of acetaldehyde in aqueous solution. J Am Chem Soc 110 5698-5701 Schuchmann MN, von Sonntag C, Tsay YH, Kruger C (1981) Crystal structure and the radiation-induced free radical chain- reaction of 2-deoxy-p-D-erythro-pentopyranose in the solid state. Z Naturforsch 36b 726-731... 

In the study of conjugated processes, of special attention are works by Emanuel [5], who has considered conjugated chain reactions to be of great importance and promising directions of radical reactions. Emanuel et al. have implemented detailed studies in this area. In particular, they suggested an effective method of propylene and acetaldehyde conjugated oxidation [6] for a one-stage synthesis of propylene oxide and acetic acid [7],... 

Other types of chain reactions which propagate by hydrogen abstraction have also been observed. The increased yield of acetaldehyde in irradiated aqueous solutions of ethylene glycol with increase in solute concentration has been explained by a chain involving reactions (64) and (65) (von Sonntag and Thoms, 1970 Burchill and Perron, 1971 Schulte-Frohlinde and von Sonntag,... 

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. 

The low quantum yield tends to exclude chain reactions but it is possible that complex reactions of the products might act in such a way as to give a low apparent quantum yield in spite of a chain reaction. Leermakers28 found that the quantum yield of acetaldehyde increases from less than unity at room temperature... 

McDowell and Sharpie 2 studied the photooxidation at 23 °C. using 3130 A. radiation. They found that propionaldehyde was very similar in its behavior to acetaldehyde. The main product was perpro-pionic acid produced in a chain reaction with a quantum yield proportional to the square root of the absorbed intensity and of the order of 50 for medium intensity (10-9 einstein l.-1s ec.-1). This was in-... 

Photodecomposition. The photochemistry of acetaldehyde has been studied by several investigators in the past. Both the primary processes and the free-radical chain processes have been studied, but it has not been until recently that a reasonable understanding of the primary processes has been achieved. If free-radical chain reactions are assumed to be secondary processes, three decomposition pathways can be envisaged ... 

In a similar manner it has been shown that the photolysis of acetaldehyde at high temperatures is also a chain reaction. The number of such reactions studied is by now too great to be adequately summarized here. 

The decomposition of acetaldehyde (CH3CHO) to methane and carbon monoxide is an example of a free radical chain reaction. The overall reaction is believed to occur through the following sequence of steps ... 

The continuous photochemical decomposition of trifluoroacetaldehyde closely resembles that of acetaldehyde, the main products at A 3130 A being tri-fluoromethane and carbon monoxide. A chain reaction involving trifluoromethyl radicals is probably involved... 

Morris analysed the methanes formed in the pyrolysis of acetaldehyde and acetaldehyde-d by infrared spectrophotometry. If the decomposition were an intramolecular reaction then only CH4 and CD4 could be expected, while CH3D, CH2D2 and CHD3 could not. However, in a chain reaction partially deuterated methanes should be formed. With equimolar mixtures of acetaldehyde and acet-aldehyde-d4 at temperatures 480 and 535 °C, respectively, only CH4 and CD4 were formed. On the basis of these findings it was concluded that in the thermal decomposition of pure acetaldehyde mainly a unimolecular mechanism is operative the contribution of the chain decomposition was regarded to be only 10-20 % at the most. 

In order to explain the autoxidation processes, one generally has recourse to the chain-reaction mechanism, depending on the existence of highly unstable intermediates often of a hypothetical nature only. Examples of such unstable compounds are found in the papers by Bodenstein on the autoxidation of acetaldehyde (2) and by Haber and Franck on the oxidation of sodium bisulfite (26). This mode of explanation was not pursued in the present work, as it would have necessitated the addition of new hypotheses to explain the incorporation of the ozone molecule into the proposed chain reactions. But one may consider, in a simplified manner, that ozone starts the chain by means of its active oxygen. It has been stated (27) that peroxides, especially benzoyl peroxide, act as autoxidation catalysts. 

Thoms (156), Burchill and Perron (157), as well as Venter, van der Linde, and Basson (158) confirmed the chain processes 20 and 21. Hence, the temperature effect on [Pg.80]

Reductive alkylation. In a synthesis of mew-chimonanthine and me o-ealycanthine, two acetaldehyde chains in the masked form are introduced as a ( 2I-2-buten-l,4-diyl unit by reductive alkylation of A. lV -dibenzylisoindigo with fZ)-l,4-dichloro-2-butene. The reaction is mediated by Sml -LiCl. 

The quantum yield can be in excess to 100% (Cassano etal, 1995). To explain this behavior Ohko et al, (1998) invoke a chain reaction mechanism involving the following steps for the photocatalytic conversion of acetaldehyde ... 

It not infrequently happens that a chain reaction and a molecular reaction take place concurrently and make contributions of comparable magnitude to the total observed chemical change. In the thermal decomposition of acetaldehyde vapour, for example, there are probably two major mechanisms, a direct molecular rearrangement CH3CHO = C0-[-CH4, and a chain process similar to (3) above. The activation energy, Ui, for the formation of radicals is very much higher than that for the rearrangement, Ii, and in consequence the number of molecules which initiate chains is smaller in about the ratio than the number which suffer simple... 

We were able to observe clear evidence for the chain-type mechanism in experiments, involving acetaldehyde decomposition in the gas-phase [98], similar to those already discussed for 2-propanol. With acetaldehyde, the values exceeded the maximum value obtained for a similar film for 2-propanol oxidation (0.28) (Fig. 6). As already discussed, the latter value may be considered to be an intrinsic maximum value for this particular film. Therefore, if < > exceeds the intrinsic maximum value, it indicates that radical chain reactions are important, that is, a single photon can cause more than one photodecomposition reaction. 

O2 consumption decreased significantly in the case of ethanol but negligibly in the case of acetaldehyde (Fig. 18). Thus it appears that the tetroxide pathway may be important for ethanol. However, aldehydes can decompose via radical-initiated chain reactions, consuming O2 but not 02 [95-97]. 

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