Fundamental reactions of free radicals relevant to pyrolysis reactions

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Abstract

Free-radical mechanisms are ubiquitous for pyrolysis of organic substrates. Background information is supplied to assist in the interpretation of such reactions by dissection into the elementary radical-forming, radical-consuming, and radical-interconverting reactions that form the building blocks of overall mechanisms. Particular attention is given to homolysis, hydrogen abstraction, β-scission–addition, and rearrangement steps, and to bond-breaking and bond-forming processes. Since the thermochemistry of such elementary reactions is dependent on the stability of the radicals involved, the major influences of structural changes on radical stability are reviewed. Kinetics of prototypical elementary reactions and their responses to structural changes are rationalized in terms of their Arrhenius parameters. The fundamental relationships between thermochemical parameters and kinetic parameters are summarized, and the use of these to estimate rate constants for cases where data is not available is demonstrated. Some kinetic consequences of the combination of elementary reactions to produce chain reactions are reviewed. Critical structural and kinetic features that determine the selectivity among product types and the response of reactions to temperature and substrate concentration are indicated.

Introduction

Free-radical pathways dominate mechanisms for pyrolysis reactions of organic materials. The general reaction conditions — high temperature; gas-phase or relatively nonpolar liquid-phase media; typical absence of strongly acidic or basic catalysts — are favorable for free-radical behavior. Alternatives such as concerted molecular processes and ionic reactive intermediates do occur, but it is unwise to accept them without rigorous testing and simultaneous evaluation of alternate radical pathways. We will summarize some of the common elementary radical reactions relevant to pyrolysis and how they combine, often in chain fashion, to produce overall mechanisms. We will sketch their responses to variations in radical and substrate structure, to temperature, and to concentration (or pressure), as these follow from the underlying thermochemistry and kinetics [1].

Radical mechanisms often appear more complicated than alternatives because of the number of elementary steps involved. Whereas concerted decomposition processes will normally involve only a single elementary step and acid-catalyzed decompositions only a few steps, it is not uncommon for a radical mechanism for pyrolysis of a moderately complex molecule, let alone a mixture of substrates, to contain tens or even hundreds of elementary steps. While this might seem to offer considerable latitude in formulating mechanistic hypotheses, it is actually quite constraining. The available data base on prototypical elementary radical steps — their thermochemistry, based on experimental and calculated thermochemical parameters of the species involved, and their corresponding rate constants, again measured or estimated from analogs by thermochemical kinetic approaches — has become extensive enough, and the ability to simulate the kinetic consequences of complex postulated pathways by numerical integration has become accessible enough, that any postulated pathways can and should be rigorously tested in a quantitative sense. Thus in older literature, it is relatively common to find complex proposed reaction networks for which one of the critical steps may now be shown not to be kinetically competent to rationalize the observed facility of the chemical transformation involved. Similarly, simulation of previously proposed mechanisms may often predict co-products, whose experimental absence must now cast serious doubt on the validity of the mechanism. In summary, while it is unwise not to first consider radical pathways for a new pyrolytic process, the quantitative bar for evaluating such hypotheses is continually being raised.

Pyrolysis reactions occur not only in the gas phase, for which the largest thermochemical and kinetic data base exists, but also in the liquid or even solid phases. We will make the common approximation that gas-phase equilibrium constants and rate constants for reactions involving carbon-centered radicals are transferable to reactions in relatively nonpolar liquid media. While there is ample basis for this simplification [2], it is likely a poorer approximation for radicals centered on heteroatoms, especially in hydrogen-bonding media where specific complexation may occur. For reactions in condensed phases, one must also remain aware of possible additional diffusional restraints on kinetics that are not present in the gas phase.

Section snippets

Thermodynamic and kinetic relationships and constraints

Before considering mechanisms, we must remind ourselves that overall thermodynamic constraints must be met. A very common reaction result under pyrolytic conditions is cracking of the substrate to form smaller products: S→P1+P2. Such cracking reactions are typically endothermic and therefore must be driven thermodynamically by the favorable increase in translational entropy associated with the formation of more molecules of product than reactant. Consider the simple prototype:CH3CH2CH2CH3→CH3CH3

Radical stability

We will need a working definition of radical ‘stability’ as a basis for applying thermochemical kinetic considerations to elementary reactions. Although unambiguous information on stability of radicals is contained in their numerical ΔfH° values, a more useful, albeit less rigorous, methodology involves normalizing such values of ΔfH° relative to the ΔfH° values of the stable molecules from which the radicals can be formally derived by breaking a CH bond (RH→Rradical dot+Hradical dot). Thus we will use a ‘bond

Major elementary reactions

We will categorize the elementary radical reactions relevant to pyrolysis as radical-forming, radical-interconverting, and radical-consuming. These become the building blocks to produce actual reaction networks, often of the chain variety.

Chain kinetics

Sets of elementary radical reactions typically combine in chain sequences to accomplish an overall pyrolysis reaction. The minimum requirements for a chain are at least one radical-forming reaction to produce reactive radicals, a repetitive alternation of at least 2 radical-interconverting steps that forms a closed cycle with respect to the radicals involved, and a radical-consuming step to destroy reactive radicals. In chain-reaction parlance, these constitute the ‘initiation,’ ‘propagation,’

Acknowledgements

Preparation of this overview was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, US Department of Energy under contract number DE-AC05-96OR22464 with the Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corporation.

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