Michael C. Flowers

Kinetics of the thermal gas-phase decomposition of 1,2-epoxypropane

The kinetics of the gas-phase decomposition of 1,2-epoxypropane have been studied over the temperature range 654–717 K at pressures between 5 and 326 Torr. Unimolecular isomerizations to propanal, propanone, methyl vinyl ether and allylol account for ∼90 % of the primary reactions. Secondary reactions, that make quantitative determination of the individual rate constants difficult, are reduced by studying the reaction in the presence of nitric oxide. The rate constants, expressed in Arrhenius form, for the reactions from 131 Torr initial reactant with 8.5 % nitric oxide added, are as follows:, kpropanal/s–1=1014.39 ± 0.18 exp (–244.7 ± 2.4 kJ mol–1/RT);, (kpropanone)131 Torr/s–1=1014.18 ± 0.18exp (–250.5 ± 2.4 kJ mol–1/RT);, kmethyl vinyl ether/s–1=1013.51 ± 0.23 exp (–245.9 ± 3.1 kJ mol–1/RT);,kallylol/s–1=1012.90 ± 0.15 exp (–239.1 ± 2.0kJ mol–1/RT)., Chemical activation effects result in the decomposition of some of the initially formed propanone. RRKM calculations satisfactorily reproduce the observed pressure dependence of propanone yields and allow an estimate to be made for the limiting high pressure rate constant for formation of propanone.(kpropanone)∞/s–1= 1014.23exp (–254 kJ mol–1/RT)Self-heating effects are considered and it is possible that the above activation energies and pre-exponential factors would be reduced by ∼900 J mol–1 and ∼100.07 respectively in the absence of self-heating.

Kinetics of the thermal gas-phase decomposition of 1,2-epoxybutane

The kinetics of the gas-phase decomposition of 1,2-epoxybutane have been studied over the temperature range 674–730 K at pressures between 5 and 70 Torr. Isomerization to butanal, butan-2-one, cis- and trans-methyl propenyl ether, and cis- and trans-but-2-en-1-ol accounts for ∼70 % of the primary reaction products and occurs by first-order, homogeneous, non-radical processes. kbutanal/s–1= 1013.95±0.41 exp(–238.4 ± 5.5 kJ mol–1RT), kbutan-2-one/s–1= 1014.12±0.78 exp(–248.7 ± 10.5 kJ mol–1RT), kcis+trans-methyl propenyl ether/s–1= 1012.21±0.75 exp(–227.1 ± 10.0 kJ mol–1RT), kcis+trans-but-2-en-1-ol/s–1= 1010.89±3.12 exp(–214.0 ± 41.8 kJ mol–1RT).

Kinetics of the thermal gas-phase decomposition of methoxycyclopropane

In the temperature range 635–694 K, 70–80% of the methoxycyclopropane decomposing initially results in the formation of the isomerization products (E)-and (Z)-1-methoxyprop-1-ene and 3-methoxyprop-1-ene. The residual decomposition results from approximately equal contributions from methyl–oxygen bond fission and the subsequent radical abstraction reactions. The isomerization products are formed by homogeneous, non-radical, unimolecular pathways with high-pressure rate constants given by the equations k[(E)-+(Z)-1-methoxyprop-1-ene]/S–1= 1013.29 ± 0.75 exp (–226.5 ± 9.5 kJ mol–1/RT) and k(3-methoxyprop-1-ene)/S–1= 1014.0 ± 1.1 exp (–254 ± 14 kJ mol–1/RT).The methoxy group lowers the activation energy for opening the cyclopropane ring adjacent to the point of substitution by ca. 45 kJ mol–1.

Kinetics of the gas-phase thermal decompositions of 1-methoxy-1-methylcyclopropane and cis- and trans-1-methoxy-2-methylcyclopropane

In the temperature range 665–737 K the thermal decomposition of 1-methoxy-1-methylcyclopropane follows first-order kinetics with a rate constant given by the equation, k/s–1= 1014.76±0.81 exp(–252±10 kJ mol–1/RT). The presence of the 1-methyl substituent destabilises the transition state for reaction. Secondary decomposition of the initially formed isomeric products precludes the determination of their individual rates of formation. Cis- and trans-1-methoxy-2-methylcyclopropane undergo first-order, reversible, geometric isomerisation in competition with structural isomerisation to give cis- and trans-1-methoxybut-1-ene and 1-methoxy-2-methylpropene in the temperature range 597–689 K: kcis→trans/s–1= 1015.25±0.23 exp (–235.4±2.8 kJ mol–1/RT), ktrans→cis/s–1= 1014.99±0.80 exp (–233.7±9.9 kJ mol–1/RT), kcis→cis-1-methoxybut-1-ene/s–1= 1013.79±0.29 exp (–233.1±3.6 kJ mol–1/RT), kcis→trans-1-methoxybut-1-ene/s–1= 1013.29±0.66 exp (–234.4±8.1 kJ mol–1/RT), kcis→1-methoxy-2-methylpropene/s–1= 1012.4±1.0 exp (–225±12 kJ mol–1/RT), ktrans→cis-1-methoxybut-1-ene/s–1= 1014.05±0.62 exp (–243.7±7.8 kJ mol–1/RT), ktrans→trans-1-methoxybut-1-ene/s–1= 1013.0±0.9 exp (–233±11 kJ mol–1/RT), ktrans→1-methoxy-2-methylpropane/s–1= 1013.5±1.0 exp (–235±13 kJ mol–1/RT).On the basis of a biradical mechanism the results provide evidence for the formation of distinguishable biradicals on opening the cis- and trans-1-methoxy-2-methylcyclopropane ring. Estimates are made of the relative rates of ring closing, internal rotation and hydrogen-atom transfer of the biradicals.

Kinetics of the thermal gas-phase decomposition of 2,3-epoxy-2-methylbutane

The kinetics of the gas-phase decomposition of 2,3-epoxy-2-methylbutane have been studied over the temperature range 665–715 K at pressures between 1.5 and 27 Torr. Isomerizations to 2,2-dimethylpropanal, 3-methylbutan-2-one, ethyl isopropenyl ether, 2-methylbut-3-en-2-ol and 3-methylbut-3-en-2-ol account for ∼95 % of the primary reactions and these isomerizations occur by first-order, homogeneous, non-radical processes. k2,2-dimethylpropanal/s–1= 1013.03 ± 0.26 exp(– 226.2 ± 3.5 kJ mol–1/RT)k3-methylbutan-2-one/s–1= 1013.05 ± 0.38 exp(– 224.9 ± 5.0 kJ mol–1/RT)kethyl isopropenyl ether/s–1= 1013.18 ± 0.29 exp(– 230.6 ± 3.8 kJ mol–1/RT)k2-methylbut-3-en-2-ol/s–1= 1012.01 ± 0.23 exp(– 228.9 ± 3.1 kJ mol–1/RT)k3-methylbut-3-en-2-ol/s–1= 1011.94 ± 0.62 exp(– 213.5 ± 8.2 kJ mol–1/RT)Comparisons of the results from this study and from studies of gas-phase thermal reactions of other epoxides are made and questions are raised regarding the validity of a biradical reaction mechanism.

Kinetics of the thermal gas-phase decomposition of 6-oxabicyclo[3,1,0]hexane

The kinetics of the gas-phase decomposition of 6-oxabicyclo[3,1,0]hexane have been studied over the temperature range 670–742 K at pressures between 1 and 28 Torr. Isomerization to cyclopentanone and cyclopent-2-en-1-ol accounts for ∼97 % of the primary reaction products and occurs by first-order, homogeneous, non-radical processes. kcyclopentanone/s–1= 1014.16∓0.11 exp(– 240 400∓1500/8.314T), kcyclopent-2-en-1-ol/s–1= 1013.56∓0.16 exp(– 242 200∓2200/8.314T)

Photochemical reactions of ketene and diazomethane with 2,3-dimethyl-2,3-epoxybutane

The photolyses of ketene (λ= 313 and 334 nm) and diazomethane (λ= 366 nm) in the presence of 2,3-dimethyl-2,3-epoxybutane (DMEB), oxygen and 2,2-dimethylpropane (as internal standard) have been investigated. Four main reaction products from DMEB were observed and identified: 2,3-dimethyl-2,3-epoxypentane (DMEP), 2,3-dimethylbut-2-ene, propanone and 2,3-dimethyl-2,3-epoxybutanal.CH2(1A1), formed from ketene or diazomethane, was found to insert into the C—H bonds of DMEB to give DMEP at 0.4 times the rate it inserted in to the C—H bonds of 2,2-dimethylpropane; however, no CH2(1A1) C—O insertion products were observed. 2,3-dimethylbut-2-ene and propanone were formed by pathways that did not involve methylene radicals and significant loss of 2,3-dimethylbut-2-ene, by secondary reactions, was observed in oxygen scavenged, ketene systems. 2,3-dimethyl-2,3-epoxybutanal was formed in both ketene and diazomethane systems but the amount produced varied with the photolysis wavelength and methylene precursor.Mechanisms for the formation of all reaction products are discussed.

Kinetics of the gas-phase thermal decomposition of 2,3-dimethyl-2,3-epoxypentane

In the temperature range 638–727 K 2,3-dimethyl-2,3-epoxypentane decomposes by homogeneous, unimolecular and non-radical mechanisms to give propene, propanone, but-1-ene, cis- and trans-but-2-ene, butanone, 2,2-dimethylpentan-3-one, 3,3-dimethylpentan-2-one, 2,3-dimethylpent-1-en-3-ol, 3-ethyl-2-methylbut-3-en-2-ol and 2,3-dimethylpent-3-en-2-ol as the major products. These products arise as the consequence of seven competing primary processes and the Arrhenius parameters for each of these processes are determined. The results and conclusions of this study are in accord with those of previous studies of the thermal decompositions of other alkyl-substituted oxiranes.

Kinetics of the thermal gas-phase decomposition of 2-(1-methylethoxy)propene and of 2,3-dimethyl-2,3-epoxybutane

In the temperature range 554–610 K 2-(1-methylethoxy)propene undergoes a homogeneous, unimolecular elimination reaction to give propene and propanone as the only products and with rate constants (kd) given by the equation kd/S–1= 1011.98 ± 0.46 exp (–168.1 ± 5.1 kJ mol–1/RT). A polar transition state for the reaction is supported.The decomposition of 2,3-dimethyl-2,3-epoxybutane has been reinvestigated over the temperature range 642–733 K. 3,3-dimethylbutan-2-one (kk), 2,3-dimethylbut-1-en-3-ol (ka), and propene + propanone (kp) are the major reaction products and are formed as the consequence of homogeneous, unimolecular processes with rate constants given by the equations kk/S–1= 1013.57 ± 0.46 exp (–233.4 ± 6.2 kJ mol–1/RT)ka/S–1= 1011.81 ± 0.94 exp (–209.8 ± 12.5 kJ mol–1/RT)kp/S–1= 1013.55 ± 0.34 exp (–235.0 ± 4.5 kJ mol–1/RT). A silanised glass surface is shown to be effective in eliminating surface reactions in the decomposition of oxirans. Data on all alkyl oxiran studies are summarised and conclusions are drawn regarding the transition states for the reactions. In contrast to previous conclusions support is given to the involvement of polar transition states.

Kinetics of the thermal gas-phase decomposition of 2,3-epoxy-1,1,1-trifluoropropane

In the temperature range 677–741 K, 2,3-epoxy-1,1,1-trifluoropropane decomposes at ≈ 100 Torr initial pressure by a unimolecular mechanism, to give initially 1,1,1-trifluoropropanone and 3,3,3-trifluoropropanal as the only major products. Rate constants for formation of these products were determined in the presence of 8% nitric oxide to reduce the importance of secondary reactions. At 100 Torr the reactions are close to their unimolecular high pressure limits and k1,1,1-trifluoropropanone/S–1= 1013.15 ± 0.57 exp (– 239 700 ± 7600/8.314T/K)k3,3,3-trifluoropropanal/S–1= 1014.49 ± 0.68 exp (– 259 000 ± 9400/8.314T/K).Comparison of this reaction with the decomposition of 2,3-epoxypropane shows that substitution of CF3 for CH3 on the oxiran ring apparently strengthens the C—O bond adjacent to the substituent but has little effect on the C—O bond opposite the substituent.