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Title: Oxidation, in the gas-phase and in solution, and with reactions of cyclopropane
Author: Tipper, Charles Frank Howlett
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1959
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Peroxides and hydroperoxides are important since they are the primary products of the reaction of hydrocarbons with molecular oxygen in solution. They then react to give the final products, alcohols, ketones, etc. The decomposition of a typical peroxide [benzoyl peroxide] and two typical hydroperoxides [tetralin and decalin hydroperoxide] have been studied under a wide variety of conditions mainly by kinetic measurements. It has been found that the decomposition of benzoyl peroxide is more complicated than has been suspected hitherto. Besides the main free radical chain reaction complex side effects occur, especially in the presence of water, which may lead to some hydrolysis. Oxygen may retard the reaction due to the formation of a yellow secondary product, which eliminates the induced chain decomposition. However, even under favourable conditions, the amount of heterolytic fission of the peroxide giving ions is small. This is in contrast to unsymmetrical peroxides and hydroperoxides. The mechanism of the decomposition of the two hydroperoxides is greatly affected by conditions, e.p. the solvent used. In neutral solvents of low dielectric constant it takes place by a chain reaction involving free -radicals produced from the solvent, whereas in the presence of acids [and bases with the secondary hydroperoxide from tetralin] the intermediates are ionic, e.g. carbonium ions. It has been shown that, because of this change in mechanism, the autoxidation of diphenylmethane also depends on the nature of the system. In the absence of acids autocatalysis by a first -order decomposition of the diphenylmethylt hydroperoxide formed, initiating a free- radical chain oxidation, occurs. In the presence of acids, however, ionic decomposition of the hydroperoxide takes place, leading to partial or complete retardation of the oxidation. The reaction of hydrocarbons and their simple derivitives with oxygen in the gas -phase has been studied for many years, but, owing to the great complexity of such systems and the different phenomena met with [e.g. slow combustion, cool flames, ignition] there is still uncertainty as to the mechanism. In particular there is still controversy over the respective roles of aldehydes and peroxidic compounds as intermediates in the slow combstion, especially in view of the great importance of peroxides and hydroperoxides in hydrocarbon oxidation in solution [Part 1]. The slow combustion of three compounds, cyclopropane, cyclopentane and methanol, has been studied in considerable detail by kinetic and analytical methods. The effect of the nature and extent of the surface of the reaction vessel has been particularly investigated. This is an aspect of the subject which has been neglected until recently. One advantage of using the cyclic hydrocarbons is that all the C-H bonds are equivalent. The main products of the oxidations as well as carbon monoxide and water are: methanol-formaldehyde and hydrogen peroxide; cyclopropane-formaldehyde; cyclopentane- carbon dioxide, cyclopentene and lower olefins, and aldehydes mainly acetaldehyde. It has been shown that aldehydes are the important intermediates, responsible for degenerate branching, in the oxidation at "high" temperatures [> 350°C]. Probable mechanisms of the reactions have been deduced. These involve free-radical chains, and the rates of the slow combustions, which vary in a complex way with pressure of reactants, depend markedly on the termination reactions, the rates and nature of which are affected by the temperature, wall conditions, and the fuel. RO₂ radicals [fuel = RH] are formed and those derived from methanol and cydopropane decompose readily to give formaldehyde plus other radicals, rather than react with the fuel or oxygen. Hydroperoxy [HO₂] radicals are present in the systems and either propagate the chain by hydrogen abstraction from fuel molecules or are destroyed at the wall. In contrast, the cyclopentylperoxy radical [C₅H₉O₂], as well as decomposing to give carbon dioxide, olefins or aldehydes, appears to have a long enough life due to its complexity to be able to abstract hydrogen from aldehyde or cyclopentane or to terminate the chains in the gas phase by reaction with oxygen. Termination by HO₂ radicals is relatively unimportant and the hydroperoxide seems to play only a minor role at the higher temperatures. The slow combustion of cyclopropane resembles that of methane rather than that of propylene or higher olefins [c.f. Part III, and see concluding remarks, page 124]. In contrast to the gas -phase oxidation [Part II], in many reactions cyclopropane behaves in a similar manner to the lower olefins. Some physical characteristics of the cyclopropane ring, e.g. apparent conjugation with double bonds, also indicate that it may have some unsaturated character. This is in agreement with wave -mechanical calculations, which suggest that there is considerable delocalisation of the six electrons associated with the ring. The reactions of cyclopropane with strong acids, e.g. HClO₄, platinous salts, aromatic compounds in the presence of aluminium chloride and lithium aluminium hydride have been studied, and it was found to behave in a similar way to propylene. The dependence of the rate of reaction on acid concentration is of a high apparent order. Probably the first step is the addition of a proton to the ring to give a n-c₃H₇⁺ ion. A co- catalyst, e.g. water, is required for the Friedel -Craft reaction. The kinetic results suggest that several complexes involving the various reagents and solvent are present in the solution, and that the A final reaction with the aromatic compound proceeds by a carbonium ion mechanism. The strongest evidence for the unsaturated nature of the cyclopropane ring is the fact that several complexes of cyclopropane with platinous chloride [e.g. PtCl₂. C₃H₆], in which the ring is intact, have been prepared. Also the reaction with lithium aluminium hydride gives aluminium tripropyl, the initial step in the formation of which is probably coordination of the cyclopropane with AlH₃.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (D.Sc.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available