Rice-Herzfeld

Employing mechanistic equations based on the Rice-Herzfeld postulation yields ... 

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. 

In order for the overall rate expression to be 3/2 order in reactant for a first-order initiation process, the chain terminating step must involve a second-order reaction between two of the radicals responsible for the second-order propagation reactions. In terms of our generalized Rice-Herzfeld mechanistic equations, this means that reaction (4a) is the dominant chain breaking process. One may proceed as above to show that the mechanism leads to a 3/2 order rate expression. 

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) ... 

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. 

The following mmechanism has been proposed for the decomposition of acetone by Rice Herzfeld ... 

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... 

This section ends with two problems, which between them cover a lot of the aspects of chain reactions and use of one of the Rice-Herzfeld rules. The second problem explains how it is that many chain reactions have observed overall activation energies which are considerably less than the bond dissociation energy involved in the first step of the sequence. 

Under certain conditions the reaction appears to have 3/2 order kinetics. Use the Rice-Herzfeld rules to identify the dominant termination step. 

Using the Rice-Herzfeld rules, predict the overall kinetics, if each of the other termination steps were dominant in turn. 

The Rice-Herzfeld rules indicate that 3/2 order occurs for termination via like radicals produced in initiation, which then undergo reaction of the type R + reactant —> . Hence step 4 is the dominant termination. 

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... 

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