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The thermal decomposition of 1-butene and 1-butene-4-d₃

University of British Columbia

The thermal decomposition of 1-butene and 1-butene-4-d₃ was studied in the temperature range 490° to 560°C in a static system. The majority of the reaction products were determined qualitatively and quantitatively by gas chromatography and mass spectrometry. The reaction products consisted of a gaseous and a liquid fraction at S.T.P. The main products in the gaseous fraction were methane, propylene, ethylene and ethane. The principal liquid products were cyclohexadiene, benzene, cyclopentene, cyclopentadiene, and toluene. The liquid fraction also contained a large number of other compounds in trace amounts. The concentrations of the methane, propylene, ethylene and ethane were found to increase almost linearly with the time of reaction over the temperature range. The rates of formation of these products were found to follow a first order dependence on the initial concentration of the butene. The overall activation energy for the butene decomposition was found to be approximately 66 kcal/mole. The overall activation energies for the formation of the individual gaseous hydrocarbons also were determined. The thermal decomposition of the 1-butene-4-d₃ was found to be nearly identical with that of the 1-butene. The distribution of deuterium in the pyrolysis products was determined. The major components of the light hydrocarbons were: methanes, CHD₃; propylenes, C₃H₆, C₃H₅D, C₃H₃D₃; ethylenes, C₂H₄, C₂H₂D₂, C₂H₃D; ethanes, C₂H₃D₃.The addition of 5% by volume of mercury dimethyl to the initial 1-butene or 1-butene-4-d₃ was found to produce a large acceleration of the decomposition at 490°C. The products of these sensitized reactions were determined quantitatively and the deuterium distribution in the products from the 1-butene-4-d₃ experiment was obtained. The results of this investigation provide strong evidence for the existence of complex free radical reactions. A mechanism is proposed which accounts at least qualitatively for the main features of the kinetics and predicts the observed distribution of deuterium isomers in the pyrolysis of the deuterated butene. A feature of the mechanism is the extensive use of disproportionation reactions in which disproportionation is assumed to occur by addition of a free radical to the double bond of the butene, followed by rapid subsequent decomposition of the addition product. The mass spectrum of 1-butene-4-d₃ was measured in a 90° Nier-type mass spectrometer using 50-volt electrons. High resolution nuclear magnetic resonance measurements showed that the methyl group was fully deuterated and that there were no D atoms located elsewhere in the molecule. A comparison of the mass spectrum with that of 1-butene shows that the total intensities of each group of C₄, C₃, C₂ and C₁ fragments are the same for both compounds. This indicates equal probabilities of C – C bond rupture in the dissociation of the corresponding parent ions. The distribution of fragments within the groups in the deuterated compound shows, however, that extensive migration of the D atoms has occurred during ionization. Migration is also evident at much lower energies (approximately 15 e.v.)

Text

2012-01-27

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http://hdl.handle.net/2429/40342

Chemistry

University of British Columbia

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