Rice-Herzfeld thermal decomposition

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) ... 

Thermal decompositions of gaseous organic compounds are found to involve free radicals (Rice Herzfeld, JACS 56 284, 1934). The process consists of an initiation step that forms free radicals, then propagation steps, and finally termination steps that destroy the free radicals. Three different kinds of termination steps will be examined. 

In any mechanistic study the proposed mechanism must fit the observed kinetics, and clues as to likely fits could help eliminate unnecessary trial and error analyses. In 1934 Rice and Herzfeld proposed a set of mechanisms from which systematic rules were inferred relating the observed order to the type of initiation and termination steps. These mechanisms are highly stylized, but even today they often form the basis for the interpretation of the thermal decomposition of organic molecules, though extra steps often have to be added. They demonstrate how only a small change in mechanism will alter the kinetic features in a systematic way. 

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. 

Other compounds. Rice-Herzfeld mechanisms appear to be the rule in thermal degradation of many other types of organic compounds, among them aldehydes [21,43,52-54] and ketones [21,55], Many of these reactions are approximately first order. Decomposition of acetaldehyde, quite extensively studied, is of order one-and-a-half, easily explained with a Rice-Herzfeld mechanism and eqn 9.18 or 9.19... 

The thermal decompositions (pyrolyses) of hydrocarbons other than the cyclic ones invariably occur by complex mechanisms involving the participation of free radicals the processes are usually chain reactions. In spite of this, many of the decompositions show simple kinetics with integral reaction orders, and this led to the conclusion by the earlier workers that the mechanisms are simple. Ethane, for example, under the usual conditions of a pyrolysis experiment, decomposes by a first-order reaction mainly into ethylene and hydrogen, and the mechanism was thought to involve the direct split of the ethane molecule. Rice et however, showed that free radicals are certainly involved in this and other reactions, and this conclusion has been supported by much later work. An important advance was made in 1934 when Rice and Herzfeld showed how complex mechanisms can lead to simple overall kinetics. They proposed specific mechanisms in a number of cases most of these have required modification on the basis of more recent work, but the principles suggested by Rice and Herzfeld are still very useful. 

The minor products of the thermal decomposition of acetaldehyde, around 500 °C, are Hj, CjHg, C2H4, CH3COCH3, C2H5CHO and CO2. A number ot these cannot be explained by the Rice-Herzfeld mechanism. Figs. 1 and 2 illustrate the amounts of these products as a function of time. 

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. 

F, O. Rice and K. F. Herzfeld. The thermal decomposition of organic com pounds from the standpoint of free radicals. VI. The mechanism of some chain reactions. J. Amer. Chem. Soc., 56 284-289, 1934. 

The Rice-Herzfeld mechanism for the thermal decomposition of acetaldehyde is ... 

The thermal decomposition of hydrocarbons in the gas phase occupies an important place both in industrial practice and in the development of the theory of chain reactions. A widely recognized approach to the kinetics of such reactions is that of Rice and Herzfeld [F.O. Rice and K.F. Herzfeld, J. Am. Chem. Soc., 56, 284 (1934)]. The chain steps proposed for these decompositions are... 

Since chain reactions are so common, it would be unreasonable to expect all mechanisms to fit neatly into the four examples of Table 1.3. Illustration 1.5 has already given some further information on thermal decomposition reactions following Rice-Herzfeld mechanisms. Before going on to additional reaction systems, we should add a few comments on the two-active-center reaction illustrated by the combination of hydrogen and bromine in the gas phase. This is probably the first reaction for which a suitable chain sequence was identified. The kinetics of the reaction were carefully studied and reported as early as 1907 [M. Bodenstein and S.C. Lind, Z. Physik. Chem., 57, 168 (1907)] and the chain reaction interpretation of the reported kinetics, as shown in the tabulation of Table 1.3, was given over a decade later [J.A. Christiansen, Kgl. Danske Videnskab. Selskab., 1, 14 (1919) K.F. Herzfeld, Ann. Physik. Chem., 59, 635 (1919) M. Polanyi, Z. Elektrochem., 26, 50 (1920)]. A through discussion of this is given in the text by Frost and Pearson. 

These reactions are known in industrial processes and occur simply by decomposition and in gas phase. However, it is a nonelementary chain reaction and occurs in different elementary steps. For reactions such as thermal cracking in gas phase, there are several kinetic models, but the most interesting one is the model of Rice-Herzfeld. The main idea is that in the first step of the reaction, the abstraction of the H from the reactant molecule occurs and subsequently there is decomposition with formation of a new radical. The process takes place throngh the steps of initiation, propagation, and termination. 

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] ... 

A Unear chain reaction is one in which chain carriers react with no net gain in the number of carriers. Although the rate laws obtained can be as complex as those seen earlier for the reaction between H2 and Btj, this is not necessarily the case. The thermal decomposition of hydrocarbons in the gas phase involves homolytic bond cleavage at the weakest chemical bond to produce two free radicals. In general, free radicals lead to chain reactions. However, the rate laws are often simple as was shown in 1934 by Rice and Herzfeld [15]. 

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