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The oxidation of 2-butenes Tse, Ronald Siu-Man

Abstract

In the first part of this work, the reaction between cis- and trans-2-butenes and oxygen atoms was studied at room temperature in a flow reactor in the essential absence of molecular oxygen. Products were mainly analysed by gas chromatography. These reaction products were found to consist of hydrocarbons and oxygenated products in the ratio of approximately 2:1. The hydrocarbon products were mainly ethane, some ethylene, a little propane and isobutane, and trace amounts of methane, n-butane and isobutene. The oxygenated products were mainly acetaldehyde and propanal, some cis- and trans-2, 3-butene oxide, acetone, isobutanal, butanone-2 and traces of other products. Peroxides were also detected. Cis-trans isomerisation, previously unobserved by other workers, was found to occur during the course of the oxidation reaction. The disappearance of the oxygen atoms could not be accounted for by the amount of oxygenated products nor by the disappearance of the 2-butene, as observed by several previous authors. The reaction order for the rate of production of the various products and for the rate of disappearance of the 2-butene was found not to be simply unity with respect to each of the reactants 2-butene and oxygen atoms. The formation of products is explained in terms of an initial oxygen atom attack at the 2-butene to form a biradical [formula omitted] which is thought to decompose in one of three ways: [formula omitted]. Other products are formed through the interaction of the radicals thus produced, and from the reactions between these radicals and the parent 2-butene. Ring closure occurs to produce epoxides. Cis-trans isomerisation is effected by rotation about the centre C-C bond in the biradical and subsequent scission of the C-0 bond. The disappearance of oxygen atoms is explained through an olefin-catalysed recombination of oxygen atoms via a complex (π or charge transfer) formation between 2-butene and the oxygen atom [formula omitted] or by way of a 4-centre reaction scheme involving the biradical [formula omitted]. The formation of peroxides is thought to be due to the reaction between the radicals present and molecular oxygen thus produced. In the second part of this work, the thermal oxidation of. cis- and trans-2-butenes by molecular oxygen was investigated in a conventional static system. Pressure-time studies were made at 289°C and at a total initial pressure of 51 mm Hg. Cool flames were observed at the end of an induction period when the ratio (po₂/p₂-butene) was greater than unity but not very large. When this ratio became very large, no cool flame was observed but the pressure-time curve took on a simple S-shape. When this ratio became less than unity, no pressure increase was detected at the end of the induction period. The relationship between the overall pressure increase and the initial pressure of 2-butene at a total initial pressure of 51 mm Hg was found to be ΔP/p₂-butene = 1.66 - 6.0xl0⁻² p₂-butene where ΔP is the overall pressure increase and p₂-butene the initial 2-butene pressure. The induction period ⊤ at 289°C and at a total pressure of 51 mm Hg was found to relate to the initial 2-butene pressure by the following expression [formula omitted] where ⊤ is in minutes and p₂-butene is the initial pressure of 2-butene in mm of Hg. Products formed during the induction period were mainly acetaldehyde, some propanal, isobutanal and butanone-2. After the end of the induction period, large quantities of hydrocarbons and other products such as formaldehyde, methanol and carbon dioxide appeared. Methane and carbon monoxide appeared in large amounts immediately after the end of the induction period but it appeared they were subsequently consumed. The maximum in the methane concentration occurred later than that observed for the carbon monoxide concentration. Crotonaldehyde was not detected. Kinetic studies were made at total pressure of 10-75 mm Hg. It was found that [formula omitted]. The value of [formula omitted] was determined over six temperatures each for cis- and trans-2-butenes in the range of 289-357°C. There was no appreciable difference in [formula omitted] between using cis- and trans-2-butenes as reactant. It was found that [formula omitted] Thermal isomerisation of 2-butenes to isobutene and the catalysed cis-trans isomerisation of 2-butenes have also been observed. A mechanism has been proposed to explain the observations involving the initial abstraction of hydrogen from the 2-butene by oxygen to form a butenyl radical and a HO₂ radical: [formula omitted] The butenyl radical is thought to react rapidly with oxygen to form a peroxy radical [formula omitted] Acetaldehyde is probably produced through the isomerisation and subsequent decomposition of the A-butenyl peroxy radical [formula omitted] The B-butenyl radical is likely to undergo isomerisation to the A-form. [formula omitted] Other products are probably formed through the interaction of radicals produced and through the reaction between these radicals and the parent 2-butene and oxygen. This mechanism must be incomplete since the rate law thus derived, employing the stationery state treatment, is in disagreement with the observed expression. A satisfactory mechanism which would give the observed rate law has not been found.

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