Mechanism Rice-Herzfeld

The basic premises on which Rice-Herzfeld mechanisms are based are as follows. 

A reaction rate expression that is proportional to the square root of the reactant concentration results when the dominant termination step is reaction (4c), that is, the termination reaction occurs between two of the radicals that are involved in the unimolecular propagation step. The generalized Rice-Herzfeld mechanism contained in equations 4.2.41 to 4.2.46 may be employed to derive an overall rate expression for this case. 

For Rice-Herzfeld mechanisms the mathematical form of the overall rate expression is strongly influenced by the manner in which the chains are broken. It can also be shown that... 

Although the above Rice-Herzfeld mechanisms lead to simple overall rate expressions, do not get the impression that this is always the case. More detailed discussions of these types of reactions may be found in textbooks (31-34) and in the original literature. 

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) ... 

The authors suggested a Rice-Herzfeld mechanism of the form... 

Rice—Herzfeld mechanism for alkane pyrolysis is an example which, in its... 

Although OH radicals are produced by sonolysis of water, the corresponding spin adducts could not be detected using PBN due to the very short half-life of the PBN/OH adduct in aqueous solutions at neutral and slightly acidic pHs. According to the Rice-Herzfeld mechanism, the primary pyrolysis step is cleavage of the weakest bonds in the molecule, such as C-N (-85 kcal/ mol) or C-C bonds (-80 kcal/mol). 

STYLIZED MECHANISMS ATYPICAL RICE-HERZFELD MECHANISM... 

Stylized Mechanisms A Typical Rice-Herzfeld Mechanism... 

This behaviour is reflected in the presence of the third body, M, in the Rice-Herzfeld mechanism. The third body produces suitably activated molecules by an energy transfer collision. These can then pass through the activated complex and react. M can be a reactant molecule or an added inert substance capable of efficient energy transfer. It can also remove energy by an energy transfer step. 

Further problems on steady states and the Rice-Herzfeld mechanism... 

A general mechanism based on the Rice-Herzfeld mechanism used previously... 

CH3CHO decomposition - determination of the mechanism, 211-213 the steady state analysis, 233-238 setting up of the steady state expression for the overall activation energy in terms of the activation energies for the individual steps, 238-239 H2/Br2 reaction - a steady state analysis on the reaction with inhibition, 213-216 without inhibition 216-217 determination of the individual rate constants, 217-218 Stylised Rice-Herzfeld mechanisms, 221-224, with surface termination, 240-243 RH/Br2 reaction - a steady state analysis, 225-227... 

Thus in the pyrolysis of n-butane at about 550°C, the initial distribution of products is roughly II2 = 3 per cent, CH4 = 34 per cent, C2H4 = 15 per cent, C2H6 =14 per cent, and CsHe = 34 per cent. These products can be reasonably well accounted for by the Rice-Herzfeld mechanism ... 

Again the Rice-Herzfeld mechanism seems to give qualitative coherence to the data. A reasonable sequence could be the following ... 

There has been much controversy regarding the order of the reaction (see Steacie, loc, cit.) and the importance of a chain mechanism vs. a molecular decomposition of acetaldehyde. The bulk of the present evidence, however, indicates that the chain decomposition is the important path for the reaction, - and in view of the simplicity of the products a fairly simple Rice-Herzfeld mechanism may be presented. 

Examples of indirect initiation will be encountered later in this chapter in the Rice-Herzfeld mechanisms and hydrocarbon autoxidation (see next section). Also, initiation of free-radical polymerization usually is a two-step process (see Section 10.3). 

The principal features of what has come to be called Rice-Herzfeld mechanisms are [21,29] ... 

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... 

Very common in thermal degradation of organic substances, especially hydrocarbons, are so-called Rice-Herzfeld mechanisms. One of the two chain carriers abstracts a hydrogen atom from the reactant to form a product plus the other chain carrier, which then reacts to yield another product plus the first chain carrier. 

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. 

It appears to be established that, at low temperatures, the reaction order is close to i and that the experimental results can be interpreted by a simple Rice-Herzfeld mechanism. At higher temperatures, the decomposition of the C2H5 radical becomes significant and the mechanism discussed above describes the kinetic data (at small conversions). There are, however, definite indications that at higher conversions and temperatures several secondary reactions occur resulting in the formation of a number of minor products. The kinetics of the reaction is rather complex under such circumstances. If these reactions can be neglected (small conversions), the mechanism is resonably described by steps (3)-(9). The steady-state treatment leads to... 

On the basis of a Rice-Herzfeld mechanism, with long chains and termination by the recombination of methyl and acetonyl radicals, the overall activation energy can be calculated as... 

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