Decomposition of acetaldehyde

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). 

The following mechanism has been postulated for the gas phase decomposition of acetaldehyde ... 

For example, at 480°C the gaseous decomposition of acetaldehyde has an order with respect to concentration of 3/2. The main reaction is CH3CHO = CH4 +CO... 

Acetaldehyle, CH3CHO, occurs naturally in oak and tobacco leaves, and also is present in automobile and diesel exhaust. The initial rate of decomposition of acetaldehyde at 600°C... 

Once the order of the reaction is known, the rate constant is readily calculated. Consider, for example, the decomposition of acetaldehyde, where we have shown that the rate expression is... 

Having established the value of k and the reaction order, the rate is readily calculated at any concentration. Again, using the decomposition of acetaldehyde as an example, we have established that... 

Two examples are (1) the thermal, gas-phase decomposition of acetaldehyde at high temperatures and (2) the reaction of the hydrated 2-propylchromium ion with molecular oxygen in aqueous solution. The reactions and their rate laws are as follows ... 

The decomposition of acetaldehyde has Eq. (8-6) as the rate-controlling step, this being the one (aside from initiation and termination) whose rate constant appears in the rate law. In the sequence of reactions (8-20)—(8-23), the same reasoning leads us to conclude that the reaction between ROO and RM, Eq. (8-22), is rate-controlling. Interestingly, when Cu2+ is added as an inhibitor, rate control switches to the other propagating reaction, that between R and O2, in Eq. (8-21). The reason, of course, is that Cu2+ greatly lowers [R ] by virtue of the new termination step of reaction (8-30). 

Circle diagrams. Draw circle diagrams for (a) the decomposition of acetaldehyde and (b) the reaction of the organometal RM with 02. 

This agrees with experimental findings on the decomposition of acetaldehyde. The appearance of the three-halves power is a wondrous result of the quasisteady hypothesis. Half-integer kinetics are typical of free-radical systems. Example 2.6 describes a free-radical reaction with an apparent order of one-half, one, or three-halves depending on the termination mechanism. 

General acid Decomposition of acetaldehyde CH3CH(OH)2 = ch3cho + h2o... 

A possible free-radical chain mechanism for the thermal decomposition of acetaldehyde (to CH4 and CO) is the Rice-Herzfeld mechanism (Laidler and Liu, 1967) ... 

From the mechanism given in problem 7-8 for the decomposition of acetaldehyde, derive a rate law or set of independent rate laws, as appropriate, if H2 and C2Hs are major products (in addition to CH4 and CO). 

The decomposition of acetaldehyde was studied at 518°C with an initial pressure tt0 = 363 Torr (Hinshelwood Hutchinson, Proc Roy Soc 111A 380, 1926). The data are of time in sec and change in total pressure, An Torr. Verify that the reaction is second order. 

The rate of the second order decomposition of acetaldehyde was measured over a temperature range. Find the activation energy and the frequency factor. 

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... 

The decomposition of acetaldehyde is found to be overall first-order with respect to the acetaldehyde and to have an overall activation energy of 60 kcal/mol. Assume the following hypothetical sequence to be the chain decomposition mechanism of acetaldehyde ... 

Sorensen, R Jencks, W. Acid- and base-catalyzed decomposition of acetaldehyde hydrate and hemiacetals in aqneons solntion. J. Am. Chem. Soc. 1987, 109, 4675 690. 

In words, we describe the process as initiated by the decomposition of acetaldehyde to form the methyl radical CH3 and the formyl radical CHO. Then methyl attacks the parent molecule acetaldehyde and abstracts an H atom to form methane and leave the acetyl radical CH3CO, which dissociates to form another methyl radical and CO. Finally, two methyl radicals combine to form the stable molecule ethane. 

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. 

The decomposition of acetaldehyde and the union of hydrogen and iodine fit equally well into their places in this table. The decomposition of ozone, however, appears to have a greater value of E than would be expected from its velocity, and will be considered further in a subsequent section. 

These considerations do not, however, apply to examples such as the decomposition of acetaldehyde 2 CH3CHO = 2 CH4 + 2 CO, where the unimolecular change is under no disadvantage on purely thermochemical grounds. 

It may also be mentioned here that in specific molecular actions a particularly marked influence of like molecules upon one another is often to be observed. This is encountered in various ways in spectroscopy, in the extinction of the polarization of mercury resonance radiation with increasing vapour pressure, in the damping of fluorescence in concentrated solutions, and in various chemical reactions. As an example of the latter the decomposition of acetaldehyde (p. 70) may be quoted, where collisions between two molecules of the aldehyde are much more effective than collisions of aldehyde molecules with those of other gases. 

Thirdly, if in all cases there is one complete layer it is rather surprising that the molecules do not interact among themselves in that layer rather than await the arrival of molecules from the gas phase. This last argument is not, however, conclusive, since an example, the catalytic decomposition of acetaldehyde, is known where the reaction does apparently depend on collision between a molecule from the gas phase and an adsorbed molecule. (Proc. Boy. Soc., 1928, A, 121, 141.) But the whole behaviour of this reaction is markedly different from that of the reactions such as the decomposition of nitrous oxide. 

---