Reactions acetaldehyde, decomposition

Students may have seen the acetaldehyde decomposition reaction system described as an example of the application of the pseudo steady state (PSS), which is usually covered in courses in chemical kinetics. We dealt with this assumption in Chapter 4 (along with the equilibrium step assumption) in the section on approximate methods for handling multiple reaction systems. In this approximation one tries to approximate a set of reactions by a simpler single reaction by invoking the pseudo steady state on suitable intermediate species. 

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. 

Acetaldehyde decomposition, reaction pathway control, 14-15 Acetylene, continuous catalytic conversion over metal-modified shape-selective zeolite catalyst, 355-370 Acid-catalyzed shape selectivity in zeolites primary shape selectivity, 209-211 secondary shape selectivity, 211-213 Acid molecular sieves, reactions of m-diisopropylbenzene, 222-230 Activation of C-H, C-C, and C-0 bonds of oxygenates on Rh(l 11) bond-activation sequences, 350-353 divergence of alcohol and aldehyde decarbonylation pathways, 347-351 experimental procedure, 347 Additives, selectivity, 7,8r Adsorption of benzene on NaX and NaY zeolites, homogeneous, See Homogeneous adsorption of benzene on NaX and NaY zeolites... 

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 reaction sequence is similar to that described for acetaldehyde decomposition to methane and carbon monoxide Reactions that produce stable products actually occur only... 

It was proved by a separate experiment that isotope mixing in a mixture of methane and methane-i proceeds very slowly even above 600 °C. Thus, it must be concluded that, in the pyrolysis, the formation of the partially deuterated methanes is a result of free radical reactions and not of the secondary exchange of the methanes. Consequently, these results support the free radical mechanism of the acetaldehyde decomposition. 

Any mechanism intended to describe the kinetics of acetaldehyde decomposition should be in accord with the experimentally well established f initial reaction order. It can be readily seen that the o = f requirement excludes certain possibil-ites as chain initiation and termination steps . 

At low pressures the products are those of the acetaldehyde decomposition. At high pressures yields of acetaldehyde (vibrationally stabilized) are increased. A biradical mechanistic interpretation and review of this reaction has been made by Benson . 

Termination reactions destroy free radicals. The major termination reaction postulated for the acetaldehyde decomposition is termination by combination ... 

The acetaldehyde decomposition discussed in Section 2.5.3 is observed to produce minor amounts of ethane as expected from the proposed termination mechanism. Minor amounts of hydrogen are also observed. A postulated mechanism adds two chain transfer reactions to account for the hydrogen ... 

Nonintegral Order. A nonintegral-order rate equation, such as that for acetaldehyde decomposition [the first reaction of Table 1.1], does not fit into the pattern expected for the rates of true elementary steps, but it is convenient to consider it here in succession with the other simple-order rate laws. Here we have, for example A B, where the reaction is -order with respect to A ... 

B. Excitation-Decomposition in Svbstitution Hoflf and Rowland (1957), in studying the reactions of tritium recoils with methanol, ethanol, and acetone, suggested the same reactions in order to account for most of their observed products in the liquid phase. In addition, they postulated an excitation-decomposition reaction as given in eqs. (10) and (11) in order to explain the formation of labeled acetaldehyde from ethanol. 

Whereas, at 800 K, small amounts of added O2 (up to 10 %) greatly accelerate acetaldehyde decomposition, at 1030 K little eflFect is observed. The changing influence of O2 may be explained by the relative importance of the two competing initiation reactions, (30) and (la). 

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. 

E8.1 Consider the decomposition of acetaldehyde, whose reaction mechanism is known ... 

The kinetics of the reactions is known. The acetaldehyde decomposition is of second order at variable volume and the ethane decomposition is of first order according to the unit of the rate constants. 

In the second series of experiments, the products from the photo-oxidation of diethyl ether, carried out in a Teflon bag reactor at ppm and ppb levels, have been determined by withdrawing vapour samples and monitoring by gas chromatography, HPLC and by chemiluminescence analysis. The major reaction products which have been measured are ethyl formate, ethyl acetate, acetaldehyde, formaldehyde, PAN, methyl nitrate and ethyl nitrate. The products observed arise from the decomposition reactions of the 1-ethoxyethoxy radical and from its reaction with oxygen. The data enable the establishment of a quantitative mechanism for the photo-oxidative reaction. In addition the rate of conversion of NO to NO2, determined by chemiluminescence analysis, shows that for each molecule of ether reacted only one molecule of NO is converted to NO2. In further end-product analyses experiments, the OH radical initiated photo-oxidation of n-hexane or the photolyses of 2- or 3-hexyl nitrites were studied to examine the... 

The major products were ethyl formate and formaldehyde and the minor products were ethyl acetate, acetaldehyde, peroxyacetyl nitrate, and methyl and ethyl nitrates. The products arise from the decomposition reactions of the 1-ethoxyethoxy radical and from its reaction with molecular oxygen ... 

It was proposed by Roh et al. [37] that the presence of partially oxidized Ce sites in Ce,cZri 02 suppresses CH4 formation by acetaldehyde decomposition, thus optimizing the hydrogen yield. In addition, Ce Zri c02 promotes noble metal and transition metals for the water-gas shift reaction (Eq. (24.3)) [38]. Moreover, in the reduced state, CexZii x02 niay reduce water to directly yield hydrogen [39]. Finally, Ce Zri, (02 improves the catalyst stability by (i) limiting the formation of ethylene and (ii) promoting carbon gasification [40]. 

This next example is not quite perfect because it gives a solution with a leftover radical unaccounted for. However, it is shown here as an example of what to expect in research. Suppose we want to understand the thermal decomposition of acetaldehyde. Rice and Herzfeld [10] studied the thermal cracking of hydrocarbons as part of a very important study related to petroleum processing. Here, we present the thermal cracking of acetaldehyde. Consider the following scheme for the thermal decomposition reaction [11] ... 

We observe that besides the ethanol desorption profile (a), there are other profiles related to the formation of water, acetaldehyde, hydrogen, CO, and methane at different temperature ranges (Fig. 6.18b). This suggests dehydrogenation and decomposition reactions of ethanol. Dehydrogenation and decomposition reactions occur on the metallic sites while dehydration on the support [30]. 

The decomposition of THF with n-butyllithium is a very efficient method for producing the lithium enolate of acetaldehyde. The reaction is easily rationalized by assuming deprotonation in the a-oxygen position, followed by a [2-1-3] cycloreversion of the anion 186. The enolate 187 resulting under concomitant liberation of ethylene has been trapped by different electrophiles among them the silylation proves the existence of the enolate [176]. 

In 1934, Rice and Herzfeld showed that the kinetics of acetaldehyde decomposition (CH3CHO — CH4 + CO), which proceeds via free radicals, can be kinetically treated by applying the steady state approximation principle. The following reactions should be considered ... 

Decomposition. Acetaldehyde decomposes at temperatures above 400°C, forming principally methane and carbon monoxide [630-08-0]. The activation energy of the pyrolysis reaction is 97.7 kj/mol (408.8 kcal/mol) (27). There have been many investigations of the photolytic and radical-induced decomposition of acetaldehyde and deuterated acetaldehyde (28—30). 

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