A kinetic study of the oxidation of α-hydroxy acids and α-hydroxy ketones by potassium bromate

The oxidation of α-hydroxy acids and α-hydroxy ketones by Br(V) follows the rate-law However, the former reaction exhibits a second-order dependence on hydrogen ion concentration while the latter reaction has a third-order dependence. A mechanism involving a slow formation of a bromate ester of the α-hydroxy acid followed by a fast decomposition is proposed. A rate-determining formation of a bromate ester from the conjugate acid of benzoin, followed by a rapid decomposition of the bromate ester, explains the kinetic data for the oxidation of benzoin.     

The thermal decomposition of fluorinated esters

The kinetics of thermal decomposition of ethyl, isopropyl, and t-butyl trifluoroacetates have been studied in the gas phase. In each case initial decomposition follows the normal ester route to give an olefin and trifluoroacetic acid, and elimination of hydrogen fluoride does not occur. However, trifluoroacetic acid is thermally unstable at ethyl and isopropyl ester decomposition temperatures, and further products result, including those from the difluorocarbene produced by decomposing trifluoroacetic acid. Placing a CF₃ group at an ester's γ carbon increases the polarity of its transition state and decreases its thermal stability. The activation energies of the ethyl and isopropyl esters are lowered by 3.8 and 4.7 kcal/mol compared to the corresponding acetates, and the primary decomposition kinetics, which are homogeneous and of the first order, are expressed by α-Methylation enhances the reactivity of the trifluoroacetates, and the t-butyl ester, the transition state for which is sufficiently polar for heterogeneous decomposition to occur, shows signs of thermal instability at room temperature. The equilibrium was also investigated and gave ΔH° = +13,580 cal/mol and ΔS° = +31.07 gibbs/mol in the forward direction. The results obtained extend and support the known structure–rate correlations in the gas-phase elimination of esters.     

Poly-α-ester degradation studies. II. Thermal degradation of poly(isopropylidene carboxylate)

The thermal degradation of poly(isopropylidene carboxylate) has been studied over the temperature range 200–800°C by using the kinetic and analytical techniques described in Part I of this series. Over a wide range of temperature, tetramethyl glycollide, acetone, carbon monoxide, and, to a lesser extent, methacrylic acid are observed when the products are rapidly swept away from the reaction zone. As decomposition temperature is increased, tetramethyl glycollide takes on the role of an intermediate, the more stable products carbon monoxide and acetone being formed from it. At the highest temperature examined, carbon monoxide begins to predominate, and its formation is accompanied by formation of small amounts of a carbonaceous residue. When the reaction products are allowed to remain in the reaction zone, which is possible at the lower end of the temperature range, methacrylic acid becomes the major product. This is interpreted as a dual decomposition route, involving the tetramethyl glycollide intermediate. Kinetic studies have shown that the decomposition is a first-order process and that the first-order rate constant k is related to temperature (T) by the expression: The results are interpreted in terms of an intramolecular ester interchange process involving tetramethyl glycollide as the primary decomposition product.     

Effect of olefins on the thermal decomposition of propane part I. The model reaction

Study of the thermal decomposition of propane at very low conversions in the temperature range 760–830 K led to refinement of the mechanism of the reaction. The quotient V/V characterizing the two decomposition routes connected with the 1- and 2-propyl radicals proved to depend linearly on the initial propane concentration. This suggested the occurrence of intermolecular radical isomerization: in competition with decomposition of the 2-propyl radical: The linearity led to the conclusion that the selectivity of H-abstraction from the methyl and methylene groups by the methyl radical is practically the same as that by the H atom. The temperature-dependence of this selectivity ( μ = kCH₃/kCH₂) was given by Further evaluation of the dependence gave the Arrhenius representation for the ratio of the rate coefficients of the above isomerization and decomposition reactions. Steady-state treatment resulted in the rate equation of the process, comparison of which with measurements gave further Arrhenius dependences.     

Addition of methoxy radicals to olefins

The addition of methoxy radicals to several olefins has been studied by a competitive method at 127°C in gas phase. The thermal decomposition of dimethyl peroxide was used as methoxy radical source. The rate of addition to the double bond was measured relative to the oxidation of carbon monoxide. For the addition to ethylene it was obtained that This rate constant is similar to the one shown by methyl radicals under similar conditions. From the relative rate of addition to several chlorinated and fluorinated olefins it can be concluded that methoxy radicals show very little “electrophilic” character.     

Steric factors in the gas-phase elimination of esters: The pyrolysis of 2,3-dimethyl-3-pentyl acetate

The kinetics of the thermal decomposition of 2,3-dimethyl-3-pentyl acetate have been studied at a temperature range of 212–260°C and a pressure range of 30–300 mm Hg. The olefins produced are 2-ethyl-3-methyl-1-butene, 3,4-dimethyl-trans-2-pentene, 3,4-dimethyl-cis-2-pentene, and 2,3-dimethyl-2-pentene. The reaction is homogeneous, obeys first-order law, and the value of the rate constant is given by the Arrhenius equation The directions of elimination and their corresponding partial rates are best explained in terms of purely steric factors.     

Gas phase thermolysis of sulfur compounds. II. Ditertiary butyl sulfide

The pyrolysis of di-tert-butyl sulfide has been investigated in static and stirred-flow systems at subambient pressures. The rate of consumption of the sulfide was measured in some experiments, and the rate of pressure increase was followed in others. The results suggest that the reaction is essentially homogeneous in a seasoned reactor and proceeds through a free radical mechanism. In the initial stages, the decomposition rate follows first-order kinetics, and the rate coefficient in the absence of an inhibitor is given by between 360 and 413°C. The stoichiometry of the uninhibited reaction at 380°C and 50% decomposition is approximately between 360 and 413°C. The stoichiometry of the uninhibited reaction at 380°C and 50% decomposition is approximately.     

The thermal decomposition of 4-methyl-3,6-dihydro-2H-pyran

The thermal decomposition of the title compound has been studied in the gas phase in the temperature range of 584–634 K. The decomposition was found to be a first-order homogeneous process yielding 2-methylbuta-1,3-diene and formaldehyde. The rate constants obtained at 11 temperatures within the quoted range fitted the Arrhenius equation The decomposition is probably unimolecular and concerted.     

On the gas-phase free radical displacement reaction CH3 + CD3COCD3 → CD3 + CH3COCD3

The gas-phase free radical displacement reaction has been studied in the temperature range of 240–290°C and at 140°C with the thermal decomposition of azomethane (AM) and di-tert-butylperoxide (DTBP), respectively, as methyl radical sources. The reaction products of the CD₃ radicals were analyzed by mass spectrometry. Assuming negligible isotope effects, Arrhenius parameters for the elementary radical addition reaction were derived: The data are discussed with respect to the back reaction and general features of elementary addition reactions.     

The kinetics and the mechanism of the thermal decomposition of bis(trifluoromethyl) trioxide. The influence of carbonmonoxide on its decomposition

The kinetics of the thermal decomposition of CF₃O₃CF₃ has been investigated in the pressure range of 15–599 torr at temperatures between 59.8 and 90.3°C and also in the presence of CO between 42 and 7°C. The reaction is homogeneous. In the absence of CO the only reaction products are CF₃O₂CF₃ and O₂. The rate of reaction is strictly proportional to the trioxide pressure, and is not affected by the total pressure, the presence of inert gases, and oxygen. The following mechanism explains the experimental results: In the presence of CO there appear CO₂, (CF₃OCO)₂, and CF₃O₂C(O)OCF₃ as products. With increasing temperature the amount of peroxicarbonate decreases, while the amounts of oxalate and CO₂ increase. The rate of decomposition of the trioxide above a limiting pressure of about 10 torr CO is strictly first order and independent of CO pressure, total pressure, and the pressure of the products. The addition of larger amounts of O₂ to the CO containing system chaqnges the course of the reaction.     

Effects of Olefins on the Thermal Decomposition of Propane Part IV. Influence of Propylene

On the basis of the thermal decomposition of mixtures of propylene and propane with molar ratios of 0.0–0.33 in the temperature range 779–812K, the influencing functions describing the inhibition by propylene of the decomposition of propane were determined. The rate-reducing effect is explained mainly by the reactions (in which ^.R = ^.H, ^.CH₃ and 2-?₃H₇) and also by the addition reactions It was established that the bulk of the allyl radicals formed participate in the chain step, but, due to their lower reactivity, they restore the decomposition chain more slowly than the original radicals do. From the characteristic change in the ratio υ/υ, the rate ratios of hydrogenabstraction reaction by radicals from propylene and propane could be determined. In these reactions there was no significant difference between the selectivities of the radicals. For an interpretation of the changes, the decomposition mechanism must be completed with the reaction Evaluation of the influencing curves revealed that the initiation reactions must be taken into account. By parameter estimation we have determined the rate ratios characterizing the above initiation reactions, the unimolecular decomposition of propane, hydrogen abstraction by radicals from propane and propylene, intermolecular isomerization of the 2-propyl radical via propane and propylene, and abstraction of propane hydrogens by the ethyl and methyl radicals; these are given in Tables II.     

The photochemistry of methyl cyclobutyl ketone. Part 2. Temperature dependence and the acetyl radical decomposition

Following earlier room-temperature studies, gaseous mixtures of methyl cyclobutyl ketone (MCK) diluted in argon have been photolyzed at temperatures up to 205°C. Experiments have been carried out at a variety of pressures (up to ca. 2 atm) at wavelengths of 313 nm (steady state conditions) and 308 nm (pulsed photolysis). The results are consistent with a mechanism dominated by radical-radical reactions involving acetyl, methyl, and cyclobutyl radicals. Acetyl radical processes predominate at lower temperatures while methyl radical reactions are more important at high temperatures. The results are interpreted via kinetic modelling of a mechanism in which a key role is played by the acetyl radical decomposition reaction Values for k₃ have been obtained and its temperature and pressure dependence are fitted by RRKM theory and a weak-collisional activation model to yield This high-pressure limiting Arrhenius equation is consistent with other studies in the same temperature range, but is difficult to reconcile with higher temperature investigations.     

The thermal decompositions of carbamates. IV. The ethyl N?methyl?carbamate system

Ethyl N? methylcarbamate decomposes thermally over the temperature range of 600–650 K by competing first-order reactions, one forming methylamine, carbon dioxide, and ethylene, the other forming methyl isocyanate and ethanol. The first-order rate constants are described in S^{? 1} units by the equations where R = 1.986 cal/deg mol. The appareance of sym? dimethylurea among the products raised the possibility of gas-phase transesterifications. These were ruled out by the study of the reactions of sym-dimethylurea at 604 K which showed its behavior to be well explained by the rapid decomposition in the gas phase which is reversed in the condensation stage in the analysis.     

Effects of olefins on the thermal decomposition of propane part III. Some remarks on the kinetics of decomposition

Conditions of applicability of quasi-steady-state kinetic treatment have been investigated with respect to the explanation of the decomposition of propane and the influence of ethylene on this. From the measured rate of accumulation of ethane and from the relations between the kinetic equations describing product formation, the rate parameters of the initiation reactions were determined, for which the temperature-dependences and were found. In the decomposition of propane under the examined conditions, the chain length exceeds 500. In response to ethylene the chain length significantly decreases, but even in this case the decomposition chains are long enough for it to be assumed that the ratios of radical concentrations are governed by the propagation steps. Calculations demonstrated that the actual radical concentration during a sufficiently short induction period approximates to the stationary concentration, so that it does not seriously affect the accuracy of the kinetic treatment.     

The gas-phase decomposition of methylsilane. Part I. Mechanism of decomposition under shock-tube conditions

The gas-phase decompositions of methylsilane and methylsilane-d₃ have been investigated in a single-pulse shock tube at 4700 torr total pressure in the temperature range of 1125–1250 K. For CH₃SiD₃ at 1200 K three primary steps occur in the homogeneous decomposition with efficiencies in parentheses: , , and . For CH₃SiH₃ at 1200 K the primary CH₄ elimination efficiency is 0.09 while the total primary H₂ elimination efficiency is 0.91. Minor product formations of C₂H₄, acetylene, dimethylsilane, and SiH₄ are discussed.     

The kinetics of the gas-phase thermal isomerization and decomposition of Trans- and Cis-1,2-Bis(trifluoromethyl)-1,2,3,3-tetrafluorocyclopropane

The kinetics of the gas-phase thermal isomerization between trans- and cis-1,2-bis(trifluoromethyl)-1,2,3,3-tetrafluorocyclopropane as well as their decomposition to trans- and cis-perfluoro-2-butene, respectively, and CF₂, was studied in the temperature range of 473–533°K, with an initial pressure of reactant of 1.5 to 7.0 Torr. Some runs were also made with the addition of SF₆ as an inert gas up to a total pressure of 100 Torr. The reactions are first order and homogeneous. The rate constants for the geometrical isomerization fit the following Arrhenius relations: and the corresponding equations for the decomposition of the trans and cis-cyclopropane are .     

Effects of alkenes on the thermal decomposition of propane part II. Influence of ethylene

Experiments with propane-ethylene mixtures in the temperature range 760–830 K resulted in refinement of the role of ethylene inhibition in the decomposition of propane. The source of the rate-reducing effect of ethylene is the reaction This replaces the decomposition chains more slowly by means of the reactions than H-atoms do by direct H-abstraction from propane. Analysis of the ratios of the product formation rates showed that the selectivity of the ethyl radical for the abstraction of hydrogen of different bond strengths from propane was practically the same as that of the H-atom. The ratio of the rate constants of hydrogen addition to ethylene and methyl-hydrogen abstraction from propane by the H-atom (3) was determined as was that of the decomposition and the similar H-abstraction of the ethyl radical Interpretation of the influence of ethylene required the completion of the mechanism with further initiation of the reaction besides termination via ethyl radicals.     

Effects of olefins on the thermal decomposition of propane part V. Effect of butene-2-Cis

The thermal decomposition of butene-2-cis at low conversion and its effect on the pyrolysis of propane have been studied in the temperature range 779-812 K. It was established that 2-butene decomposes in a long-chain process, with the chain cycle (Besides the radical path, the molecular reaction can also play a role in the formation of the products.) The thermal decomposition of propane is considerably inhibited by 2-butene, which can be explained by the fact that the less reactive radicals formed in the reactions between the olefin and the chain-carrying radicals regenerate the chain cycle more slowly than the original radicals in the above chain cycle or in the reactions The reactions of the 2-propyl radical are further initiation steps. The ratios of the rate coefficients of the elementary steps of the decomposition (Table III) have been determined via the ratios of the products. Estimation of the radical concentrations indicated that only the methyl, 2-propyl and methylallyl radicals are of importance in the chain termination. On the basis of the inhibition-influenced curves, the role of the bimolecular initiation steps. could be clarified in the presence of 2-butene.     

Absolute rate constants for reactions of free radicals in the high-temperature photolysis of formamide vapor. II. Amino radicals

The rotating-sector method has been applied to the photoinitiated radical-chain decomposition of formamide at 300°C to measure the rate constant for the bimolecular disappearance of NH₂ radicals. The decomposition is propagated by the reactions (1) (2) Conditions were chosen so that reaction (1) was rate controlling and NH₂ the terminating radical. A flow system was employed with C₂F₆ as a carrier gas at a pressure of 300 Tort, and the chain reaction was initiated by the photolysis of either formamide or NH₃. A value of 4.7(±2.0) × 10¹⁰ (M ·sec)^?1 was estimated for the termination reaction (3) and a value of 8.4 × 10⁶ (M ·sec)^?1 for reaction (1) in the same system, both at 300°C.     

Thermal stability of cyclohexane and 1-hexene

The mechanism and initial rates of decomposition of cyclohexane and 1-hexene have been determined from single-pulse shock-tube experiments. The main initial processes involve isomerization of cyclohexane to 1-hexene, followed by decomposition of 1-hexene. From comparative rate experiments the following rate expressions have been derived: The 1-hexene bond-braking reaction leads to an allylic resonance energy of 42.7 kJ and a heat of formation of allyl radicals of 176.6 kJ (300°K). There appear to be general relations relating the rate expressions for the decomposition of alkynes, alkanes, and alkenes. Studies on the induced decomposition of cyclohexane have also been carried out.