Radical rearrangement in gas-phase oxidation and related processes

Abstract

Introduction.-The reactions of organic molecules in the gas phase and in non-polar solvents often involve free-radical intermediates. Several modes of further reaction of such intermediates, including recombination, disproportionation, decomposition, displacement, and addition are often possible and the natures and amounts of the products formed during autoxidation, gaseous oxidation, and pyrolysis depend on the relative rates of these competing processes (e.g., ref. 1). Recent work has established that a further important reaction of the free radicals produced in such systems is intramolecular rearrangement or isomerisation. Rearrangement reactions of common radical species have important kinetic, mechanistic, and preparative applications and are of increasing interest, particularly in the oxidation field. It is the aim of this Review to classify anddiscuss recent work on these reactions.General Characteristics of Radical Reactions.-Radical reactions do not involve, in general, ionic or highly polarised species. They are insensitive therefore to the ionising power of media and take place quite similarly in the gas phase and in solution. Although their reactions in solution are usually studied in non-ionising solvents, this is not because radical reactions do not occur in ionising solvents but because these solvents encourage the occurrence of competing ionic processes. Similarly, radical reactions are insensitive to acid-and base-catalysis, but are catalysed by light and by radical sources such as peroxides, particularly when the radical reaction is a chain process. Free-radical chains are inhibited or retarded by stable radicals such as nitric oxide and diphenylpicrylhydrazyl and by substances such as amines, quinones, and aromatic polynitro-compounds, which react readily with free radicals to produce relatively stable radicals. Radical reactions are also sensitive to chain-termination processes such as destruction of active centres at the walls of the vessel.The concepts of activation energy, frequency factor, and transition complex and the effects of "resonance" stabilisation and strain energy apply to radical reactions just as they do to molecular and ionic processes. Measurement and calculation of bond energies in free radicals and in the corresponding saturated molecules shows that, in general, the former are lower. The presence of an odd electron in a molecule has a weakening effect on the surrounding bonds and this phenomenon accounts for the ease of radical decomposition, disproportionation, and rearrangement reactions. Radical Rearrangements.-Radical isomerisation reactions, or rearrange-* Present address: "Shell" Research Ltd., Thorton Research Centre, P.O. Box No.

The Oxidation of Propane at Low and Transition Temperatures

“…It can also abstract a hydrogen from another source to form a hydroperoxide. It has been determined by Fish (1964Fish ( , 1968) that another alternative path for the ROz is important and should be considered. This route involves the isomerization of the alkylperoxy radical by intramolecular hydrogen transfer from a carbon to the outer oxygen of the peroxy group, forming a hydroperoxyalkyl radical:…”

Intramolecular Hydrogen Transfer in Mercury Photosensitization of Pentene-1

“…These data illustrate that any empirical rule used to predict the aptitude of a positional shift must be properly, and perhaps extensively, modified by the nature of the processes involved; and, in this, the diradical systems apparently behave much like monoradical systems as t o relative ease of H-atom translocations (12). The modification is attributable t o conventional interpretation in terms of relative stability of various types of radicals.…”

The non-isothermal oxidation of 2-methylpentane. II. The chemistry of cool flames

“…The rearrangem ent by H -transfer of these alkylperoxy radicals m ay produce twenty-one hydroperoxyalkyl radicals of the general formula *C6H 12OOH (table 2). I t is established (Fish 1964;Cullis al. 1966a) th a t such hydroperoxy alkyl radicals decompose by several ro u tes:…”

“…I t has been suggested (Walsh 1963) th a t the detailed struc ture of the pressure/temperature locus of the limit of two-stage ignition is related to the number of possible modes of isomerization of alkylperoxy radicals derived from the fuel. Indeed, the formation during the gaseous oxidation of hydrocarbons of products such as O-heterocycles and carbonyl compounds with rearranged skeletons, which are diagnostic of the occurrence of alkylperoxy radical isomeriza tion (Fish 1964), has been amply demonstrated (Cullis, Hardy & Turner 1959;Jones & Fenske 1959;Jones, Allendorf, H utton & Fenske 1961;Zeelenberg & Bickel 1961;Zeelenberg 1962;Zeelenberg & de Bruijn 1965), and it has been shown th a t these reactions are most im portant in the cool-flame region (Cullis, Fish & Trimm 1963;Schroeder, Ohlmann & Leibnitz 1964;Cullis, Fish, Saeed & Trimm 1966 a). Moreover, O-heterocycles are formed in considerable quantities during preignition reactions even under engine conditions (Affleck & Fish 1966).…”