Thermal stability of primary amines

Tertiary-amyl amine has been decomposed in single-pulse shock-tube experiments. Rate expressions for several of the important primary steps are This leads to D(CH₃? H) – D(NH₂? H) = ?10.5 kJ and D[(CH₃)₃C? H] – D[(CH₃)₂NH₂C? H] = + 6 kJ. The present and earlier comparative rate single-pulse shock-tube data when combined with high-pressure hydrazine decomposition results-(after correcting for fall off effects through RRKM calculations) gives where k_r(…) is the recombination rate involving the appropriate radicals. This suggests that in this context amino radical behavior is analogous to that of alkyl radicals. If this agreement is exact, then Rate expressions for the primary step in the decomposition of a variety of primary amines have been computed. In the case of benzyl amine where data exist the agreement is satisfactory. The following differences in bond energies have been estimated:      

The reaction of O(1D) with CH4

The room-temperature photolysis of N₂O (10–100 torr) at 2139 Å to produce O(¹D) has been studied in the presence of CH₄ (10–891 torr). The reactions of O(¹D) with CH₄ were found to be The method of chemical difference was used to measure the rate constant ratio k₄/(k₂ + k₃), where reactions (2) and (3) are The CH₃ radicals produced in reaction (4) react with the O₂ and NO produced in reactions (2) and (3). Thus, near the endpoint of the internal titration, ?{C₂H₆} gives an accurate measure of k₄/(k₂ + k₃). For the translationally energetic O(¹D) atoms produced in the photolysis, k₄/(k₂ + k₃) = 2.28 ± 0.20. However, if He is added to remove the excess translational energy, then k₄/(k₂ + k₃) drops to 1.35 ± 0.3.     

The reaction of OH with CO

Hydroxyl radicals were prepared from the photolysis of N₂O at 213.9 nm in the presence of excess H₂. The O(¹D) produced in the primary photolytic act reacts with H₂ to produce OH radicals. If CO is also present, then OH can react either with H₂ or CO: The competition between reactions (1) and (2) was measured by measuring the CO₂ yield at various values of the ratio [CO]/[H₂] at 217–298°K. At 298°K the ratio of the rate coefficients k₁/k₂ increased with pressure from a low-pressure limiting value of 14 to a high-pressure limiting value of 50. The low-pressure limiting value agrees well with the low-pressure values found by others. At lower temperatures our high-pressure values of k₁/k₂ were larger than deduced from the accepted low-pressure Arrhenius expression and could be fitted to the expression The mechanism which seems to fit the results best is with k₁° = k_ak_b/k_-a and k₁^∞ = k_a.     

Pulse radiolysis and fourier transform infrared study of neopentyl peroxy radicals in the gas phase at 297 K

The ultraviolet absorption spectrum of the neopentyl peroxy radical (CH₃)₃CCH₂O₂, and the kinetics and products of its self reaction have been studied in the gas phase at 298 K. Absorption cross sections were quantified over the wavelength range 230–290 nm. The measured cross section at 250 nm was; Errors represent statistical (2σ) together with our estimate of potential systematic errors(15%). The kinetics of the decay of the UV absorption following the generation of the neopentyl peroxy radicals was complicated by the rapid decomposition of the (CH₃)₃CCH₂O radicals formed in channel (4a). By measuring the yield of t-butyl peroxy radicals, the branching ratio k_4a/(k_4a + k_4b) was determined to be 0.39 ± 0.03. The rate constant for the self reaction of neopentyl peroxy radicals was k₄ = (1.07 ± 0.22) × 10^?12 cm³ molecule^?1 s^?1. Quoted errors represent 2σ. These results are discussed with respect to the available literature data. © John Wiley & Sons, Inc.     

Reaction of CF3 radicals with H2O and D2O

The reaction of CF₃ radicals with H₂O (D₂O) has been studied over the range of 533–723 K using the photolysis and the pyrolysis of CF₃I as the free radical source. Arrhenius parameters for the reactions where X = H or D, relative to CF₃ radical recombination are given by where k/k is in cm^3/2/mol^1/2·s^1/2 and θ = 2.303RT/cal/mol. The activation energy and the primary kinetic isotope effect have been compared with those derived from the BEBO method.     

Decomposition of the t-butoxy radical — I. Studies over the temperature range 402–443 K

By allowing the t-butoxy radical to decompose in the presence of nitric oxide, it has been possible to determine rate constants for decomposition by the measurements of the relative rates (2) and (3) Process (3) is clearly pressure dependent. The value of k₃(∞) has been determined in the presence of several inert gases (CF₄, SF₆, N₂, and Ar) and a value of k₃ interpolated for atmospheric conditions. The results may be compared with those for other relevant alkoxy radicals at room temperature. Extrapolated values for k₃ in the presence of CF₄ lead to the result      

The kinetics of the gas phase decomposition reactions of the hexafluoro-t-butoxy radical

Hexafluoro-t-butoxy radicals have been generated by reacting fluorine with hexafluoro-2-methyl isopropanol: Over the temperature range of 406–600 K the hexafluoro-t-butoxy radical decomposes exclusively by loss of a CF₃ radical [reaction (-2)] rather than by loss of a CH₃ radical [reaction (-1)]: (1) The limits of detectability of the product CF₃COCF₃, by gas-chromatographic analysis, place a lower limit on the ratio k_?2/k_-1 of ～80. The implications of this finding in relation to the reverse radical addition reactions to the carbonyl group are briefly discussed. A thermochemical kinetic calculation reveals a discrepancy in the kinetics and thermodynamics of the decomposition and formation reactions of the related t-butoxy radical:      

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.     

Kinetics of some reactions of vinyl and isopropyl radicals in the gas phase

Vinyl and isopropyl radicals were generated by the pyrolysis of azoisopropane in the presence of acrolein at 473–563 K. Reaction products were analyzed by gas chromatography. Rate constant ratios k₂/k₁ = 0.02 ± 0.01 and k₄/k₃ = 0.01 ± 0.005 are suggested for the following reactions: The rate constant ratio of reactions (7) and (c) obeys the Arrhenius equation The Arrhenius equation was derived for (k₈ + k₉).     

Kinetics of recombination,dismutation, and disproportionation reactions involving neutral ketyl radicals and radical anions

The kinetics of fast elementary recombination of neutral ketyl radicals of benzophenone and its four derivatives (BPH?), the dismutation of benzophenone radical anions, the disproportionation between BPH? and stable nitroxyl radicals, ( ), and the electron transfer have been investigated in both individual solvents and binary mixtures of different viscosities. Reaction (1) for unsubstituted BPH in water, water glycerol, and n-hexane is controlled by diffusion with 2k₁ ? k_diff. In aliphatic alcohols and toluene, which form solvation complexes with BPH?, reaction (1) is diffusion-enhanced and activation-controlled, respectively, with 2k₁ < k_diff. In a viscous solvent such as 1-propanol–glycerol mixture (100 ? η ? 450 cP) reaction (1) is diffusion-controlled. Reaction (2) in alkaline 1-propanol and alkaline 1-propanol–glycerol mixture is activation controlled. The rates of reactions (3) and (4) for benzophenone radicals and nitroxyl radicals of the imidazoline series decrease as the viscosity of the water–glycerol and 1-propanol–glycerol mixtures is increased. The reactions are molecular mobility limited; nevertheless, the numerical values of k₃ (k₄) are 2–6 times as small as the corresponding k_diff values due to the low steric factor of the reactions (therefore called pseudodiffusion-controlled reactions). The theoretical estimates of k₃ (k₄) are in good agreement with the experimental results. The elimination of spin forbiddance in the process of radical recombination in viscous solvents is discussed.     

Intramolecular hydrogen isotope effect in the pyrolysis of diethyl carbonate-1,1,1,2,2-d5

The thermal unimolecular decomposition of diethyl carbonate-1,1,1,2,2-d₅ has been examined in the high-pressure-limiting region. The observed chemistry is consistent with a simple, competitive two-channel model: The intramolecular isotope effect k_H/k_D has been determined, and the relative Arrhenius parameters for the two channels are given by over the temperature range of 540–620 K. These Arrhenius parameters predict an isotope effect k_H/k_D = 5.4 at 300 K.     

The kinetics of the thermal bromination of CF3I. Determination of the bond dissociation energy D(CF3−I)

The kinetics of the thermal bromination reaction have been studied in the range of 173–321°C. For the step we obtain where θ=2.303RT cal/mole. From the activation energy for reaction (11), we calculate that This is compared with previously published values of D(CF₃?I). The relevance of the result to published work on k_c for a combination of CF₃ radicals is discussed.     

Reactions of alkoxy radicals with O2. I. C2H5O radicals

C₂H₅ONO was photolyzed with 366 nm radiation at ?48, ?22, ?2.5, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yield of CH₃CHO, Φ{CH₃CHO}, was measured as a function of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?_1a = 0.29 ± 0.03 independent of temperature. The C₂H₅O radicals can react with NO by two routes The C₂H₅O radical can also react with O₂ via Values of k₆/k₂ were determined at each temperature. They fit the Arrhenius expression: Log(k₆/k₂) = ?2.17 ± 0.14 ? (924 ± 94)/2.303 T. For k₂ ? 4.4 × 10^?11 cm³/s, k₆ becomes (3.0 ± 1.0) × 10^?13 exp{?(924 ± 94)/T} cm³/s. The reaction scheme also provides k_8a/k₈ = 0.43 ± 0.13, where      

Kinetic studies of free radical reactions by mass spectrometry. II. The reactions of SO + ClO,SO + OClO and SO + BrO

The rates of several novel elementary reactions involving ClO, BrO and SO free radicals in their ground states were studied in a discharge-flow system at 295 K, using mass spectrometry. The rate constant k₂ was determined from the decay of SO radicals in the presence of excess ClO radicals: The SO + OClO overall reaction has a complex mechanism, with the primary step having a rate constant k₅ equal to (1.9 ± 0.7) × 10^?12 cm³ sec^?1: A lower limit for the rate constant of the rapid reaction of SO radicals with BrO radicals was determined:      

Some reactions of the isopropyl radical

The decomposition of ethane sensitized by isopropyl radicals was studied in the temperature range of 496–548°K. The rate of formation of n-butane, isopentane, and 2,3-dimethylbutane were measured. The expression k₁/k₂^½ was found to be where k₁ and k₂ are rate constants of The decomposition of propylene sensitized by isopropyl radicals was studied between 494 and 580°K by determination of the initial rates of formation of the main products. The ratio of k₁₃/k₂^1/2 was evaluated to be where k₁₃ is the rate constant for The isomerization of the isopropyl radical was investigated by studying the decomposition of azoisopropane. The decomposition of the iso-C₃H₇ radical into C₂H₄ and CH₃ was followed by measuring the rate of formation of C₂H₄. On the basis of the experimental data, obtained in the range of 538–666° K, k₁₅/k₂^½ was found: where k₁₅ is the rate constant of      

Relative rate constants of the gas-phase decomposition reactions of the s-butoxy radical

s-Butoxy radicals have been generated by reacting fluorine with s-butanol: Over the temperature range 398.6 to 493.3 K the s-butoxy radical decomposes by two different pathways to yield acetaldehyde and propionaldehyde, acetaldehyde being the major product: The ratio k₁/k₂ was found to be temperature dependent. An Arrhenius plot of the data (398.6 to 493.3 K) yields the relative Arrhenius parameters, E₁ - E₂ = ?11.2 ± 0.8 kJ mol^?1 and (A₁/A₂) = 0.59 ± 0.14. The ratio of rate constants k₁/k₂ was shown to be independent of total pressure (80–600 torr) and of the pressure of s-butanol (2–13 torr). The kinetic results for these s-butoxy decomposition reactions are discussed in relation to the literature data and in terms of the thermochemistry of the reactions.     

Rate constants and kinetic isotope effects for hydrogen/deuterium abstraction by chlorine atoms from the chloromethyl group in CH2ClCH2Cl,CD2ClCD2Cl,CH2ClCHCl2, and CH2ClCDCl2

The abstraction of hydrogen and deuterium from 1,2-dichloroethane, 1,1,2-trichloroethane, and two of their deuterated analogs by photochemically generated ground state chlorine atoms has been investigatedin the temperature range 0–95°C using methane as a competitor. Rate constants and their temperature coefficients are reported for the following reactions Over the temperature range of this investigation an Arrhenius law temperature dependence was observed in all cases. Based on the adopted rate coefficient for the chlorination of methane [L.F. Keyser, J. Chem. Phys., 69 , 214 (1978)] which is commensurate with the present temperature range, the following rate constant values (cm³ s^?1) are obtained: The observed pure primary, and mixed primary plus α- and β3-secondary kinetic isotope effects at 298 K are k₃/k₆ = 2.73 ± 0.08, and k₁/k₂ = 4.26 ± 0.12, respectively. Both show a normal temperature dependence decreasing to k₃/k₆ = 2.39 ± 0.06 and k₁/k₂ = 3.56 ± 0.09 at 370 K. Contrary to some simple theoretical expectations, the kinetic isotope effect for H/D abstraction decreases with increasing number of chlorine substituents in the geminal group in a parallel manner to the trend established previously for C1-substitution in the adjacent group. The occurrence of a β-secondary isotope effect, k₄/k₅, is established; this effect suggests a slight inverse temperature dependence.     

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.     

Reactions of methyl radicals with acetaldehyde and acetaldehyde-d1. I. relative rates of atom abstraction reactions at 785°K

The role of ethenoxy radicals in the pyrolysis of CH₃CDO was studied by mass spectrometric analysis of the isotopic composition of the methane, ethane, and recovered aldehyde. Experimental evidence was obtained for the formation of ethenoxy radicals and for their reaction with acetaldehyde. Mixtures of CH₃CHO and CH₃CDO were pyrolyzed in order to minimize H-D scrambling in the methyl group of the aldehyde. A kinetic treatment of the methyl radical reactions and furnished the rate constant ratios (k_2a + k_2b)/k_1a = 2.7 and k_1b/k_1a = 0.62 at 785°K. It is concluded that at the usual temperatures of CH₃CHO pyrolysis the rate of alkyl hydrogen capture is comparable to that of formyl hydrogen abstraction. The results and conclusions are discussed and compared with previous work.     

The reaction of OH with NO2 and the deactivation of O(1D) by CO

NO₂ was photolyzed with 2288 Å radiation at 300° and 423°K in the presence of H₂O, CO, and in some cases excess He. The photolysis produces O(¹D) atoms which react with H₂O to give HO radicals or are deactivated by CO to O(³P) atoms The ratio k₅/k₃ is temperature dependent, being 0.33 at 300°K and 0.60 at 423°K. From these two points, the Arrhenius expression is estimated to be k₅/k₃ = 2.6 exp(?1200/RT) where R is in cal/mole – °K. The OH radical is either removed by NO₂ or reacts with CO The ratio k₂/k^α is 0.019 at 300°K and 0.027 at 423°K, and the ratio k₂/k⁰ is 1.65 × 10^?5M at 300°K and 2.84 × 10^?5M at 423°K, with H₂O as the chaperone gas, where k^α = k₁ in the high-pressure limit and k⁰[M] = k₁ in the low-pressure limit. When combined with the value of k₂ = 4.2 × 10⁸ exp(?1100/RT) M^?1sec^?1, k^α = 6.3 × 10⁹ exp (?340/RT)M^?1sec^?1 and k⁰ = 4.0 × 10¹²M^?2sec^?1, independent of temperature for H₂O as the chaperone gas. He is about 1/8 as efficient as H₂O.