Very low-pressure pyrolysis (VLPP) of but-1-yne the heat of formation and stabilization energy of the propargyl radical

The unimolecular decomposition of but-1-yne has been investigated over the temperature range of 1052° – 1152°K using the technique of very low-pressure pyrolysis (VLPP). The primary process is C? C bond fission yielding methyl and propargyl radicals. Application of RRKM theory shows that the experimental rate constants are consistent with the highpressure Arrhenius parameters given by where θ = 2.303 RT kcal/mol. The parameters are in good agreement with estimates based on shock-tube studies. The activation energy, combined with thermochemical data, leads to DH°[HCCCH₂? CH₃] = 76.0, ΔH(HCC?CH_2,g) = 81.4, and DH° [HCCCH₂? H] = 89.2, all in kcal/mol at 300°K. The stabilization energy of the propargyl radical SE° (HCC?CH₂) has been found to be 8.8 kcal/mol. Recent result for the shock-tube pyrolysis of some alkynes have been analyzed and shown to yield values for the heat of formation and stabilization energy of the propargyl radical in excellent agreement with the present work. From a consideration of all results it is recommended that ΔH(HCC?CH_2,g) = 81.5±1.0, DH[HCCCH₂? H] = 89.3 ± 1.0, and SE° (HCC?CH₂) = 8.7±1.0 kcal/mol.     

Very low-pressure pyrolysis (VLPP) of pentynes. III. Pent-2-yne. Heat of formation and resonance stabilization energy of the 3-methylpropargyl radical

The thermal unimolecular decomposition of pent-2-yne has been studied over the temperature range of 988–1234 K using the technique of very low-pressure pyrolysis (VLPP). The main reaction pathway is C₄? C₅ bond fission producing the resonance-stabilized 3-methylpropargyl radical. There is a concurrent process producing molecular hydrogen and penta-1,2,4-triene presumably via the intermediate formation of cis-penta-1,3-diene. The 1,4-hydrogen elimination from cis-penta-1,3-diene is the rate-determining step in the molecular pathway. This is supported by an independent VLPP study of cis- and trans-penta-1,3-diene. RRKM calculations show that the experimental rate constants for C? C bond fission are consistent with the following high-pressure rate expression at 1100 K: where θ = 2.303RT kcal/mol and the A factor was assigned from the results of shock-tube studies of related alkynes. The activation energy leads to ΔH[CH₃C?C?H₂] = 70.3 and DH[CH₃CCCH₂? H] = 87.4 kcal/mol. The resonance stabilization energy of the 3-methylpropargyl radical is 10.6 ± 2.5 kcal/mol, which is consistent with previous results for this and other propargylic radicals.     

Very low-pressure pyrolysis (VLPP) of cyclopentene

The unimolecularity of the thermal dehydrogenation of cyclopentene has been confirmed using the technique of very low-pressure pyrolysis (VLPP). Application of RRKM theory shows that the experimental unimolecular rate constants obtained over the temperature range of 942°–1152°K are consistent with the high-pressure Arrhenius parameters given by where θ = 2.303 RT kcal/mol. These parameters are in good agreement with static and shock tube studies. No firm evidence could be found for any side reactions or reversibility under the experimental conditions used.     

Very low-pressure pyrolysis (VLPP) of alkyl cyanides. II. n-propyl cyanide and isobutyl cyanide. The heat of formation and stabilization energy of the cyanomethyl radical

The very low-pressure pyrolysis (VLPP) technique has been used to study the pyrolysis of n-propyl cyanide over the temperature range of 1090–1250°K. Decomposition proceeds via two pathways, C₂? C₃ bond fission and C₃? C₄ bond fission, with the former accounting for >90% of the overall decomposition. Application of unimolecular reaction rate theory shows that the experimental unimolecular rate constants for C₂? C₃ fission are consistent with the high-pressure Arrhenius parameters given by where θ=2.303RT kcal/mole. The activation energy leads to DH₂₉₈⁰[C₂H₅? CH₂CN]=76.9±1.7 kcal/mole and ΔH(?H₂CN, g)=58.5±2.2 kcal/mole. The stabilization energy of the cyanomethyl radical has been found to be 5.1±2.6 kcal/mole, which is the same as the value for the α-cyanoethyl radical. This result suggests that DH[CH₂(CN)? H] ～ 93 kcal/mole, which is considerably higher than previously reported. The value obtained for ΔH_?⁰(?H₂CN) should be usable for prediction of the activation energy for C₂? C₃ fission in primary alkyl cyanides, and this has been confirmed by a study of the VLPP of isobutyl cyanide over the temperature range of 1011–1123°K. The decomposition reactions parallel those for n-propyl cyanide, and the experimental data for C₂? C₃ fission are compatible with the Arrhenius expression A significant finding of this work is that HCN elimination from either compound is practically nonexistent under the experimental conditions. Decomposition of the radical, CH₃CHCH₂CN, generated by C₃? C₄ fission in isobutyl cyanide, yields vinyl cyanide and not the expected product, crotonitrile. This may be explained by a radical isomerization involving either a 1,2-CN shift or a 1,2-H shift.     

Very low-pressure pyrolysis (VLPP) of t-butylmethyl ether

The thermal decomposition of t-butylmethyl ether has been studied using the VLPP technique. The recommended Arrhenius parameters for the molecular elimination, reaction (1), are A(800°K) = 10^{1 3, 9} sec^?1 and E_a (800°K) = 59.0 ± 1.0 kcal/mole. No radical reactions occur under the conditions used. These parameters are in good agreement with earlier experimental work and with theoretical estimates of both A and E.     

Very low-pressure pyrolysis (VLPP) of pentynes. I. Kinetics of the retro-ene decomposition of pent-1-yne

The thermal unimolecular decomposition of pent-1-yne has been investigated over the temperature range of 923–1154 K using the technique of very low-pressure pyrolysis (VLPP). Under the experimental conditions the reaction proceeds predominantly via a molecular retro-ene pathway to yield allene and ethylene. There was some evidence for the occurrence of the higher energy C₃? C₄ bond fission pathway at the high end of the temperature range. Interpretation of the data with the aid of RRKM theory and taking into account a decrease in gas-wall collision efficiency with temperature yields the following high-pressure rate constant expression for the retro-ene pathway: at 1100 K where θ = 2.303 RT kcal/mol and the A factor was assigned from the results of shock-tube studies of similar molecules. These rate parameters are independent of the inclusion of the bond fission pathway in the RRKM calculations. The results are compared with previous data on the retro-ene decomposition of alkynes.     

Very low-pressure pyrolysis (VLPP) of pentynes. II. 4-methylpent-2-yne and 4,4-dimethylpent-2-yne. Heats of formation and resonance stabilization energies of methyl-substituted propargyl radicals

Studies of the unimolecular decomposition of 4-methylpent-2-yne (M2P) and 4,4-dimethylpent-2-yne (DM2P) have been carried out over the temperature range of 903–1246 K using the technique of very-low pressure pyrolysis (VLPP). The primary reaction for both compounds is fission of the C? C bond adjacent to the acetylenic group producing the resonance-stabilized methyl-substituted propargyl radicals, CH₃C??H(CH₃) from M2P and CH₃C?C?(CH₃)₂ from DM2P. RRKM calculations were performed in conjunction with both vibrational and hindered rotational models for the transition state. Employing the usual assumption of unit efficiency for gas-wall collisions, the results show that only the rotational model with a temperature-dependent hindrance parameter gives a proper fit to the VLPP data over the entire experimental temperature range. The high-pressure Arrhenius parameters at 1100 K are given by the rate expressions log k₂ (sec^?1) = (16.2 ± 0.3) ? (74.4 ± 1.5)/θ for M2P and log k₃ (sec^?1) = (16.4 ± 0.3) ? (71.4 ± 1.5)/θ for DM2P where θ = 2.303RT kcal/mol. The A factors were assigned from the results of recent shock-tube studies of related alkynes. Inclusion of a decrease in gas-wall collision efficiency with temperature would lower both activation energies by ～1 kcal/mol. The critical energies together with the assumption of zero activation energy for recombination of the product radicals at 0 K lead to DH⁰[CH₃CCCH(CH₃)? CH₃] = 76.7 ± 1.5, ΔH_f⁰[CH₃CCCH(CH₃)] = 65.2 ± 2.3, DH⁰[CH₃CCCH(CH₃)? H] = 87.3 ± 2.7, DH⁰[CH₃CCC(CH₃)₂? CH₃] = 72.5 ± 1.5, ΔH[CH₃CC?(CH₃)₂] = 53.0 ± 2.3, and DH⁰[CH₃CCC(CH₃)₂? H] = 82.3 ± 2.7, where all quantities are in kcal/mol at 300 K. The resonance stabilization energies of the 1,3-dimethylpropargyl and 1,1,3-trimethylpropargyl radicals are 7.7 ± 2.9 and 9.7 ± 2.9 kcal/mol at 300 K. Comparison with results obtained previously for other propargylic radicals indicates that methyl substituents on both the radical center and the terminal carbon atom have little effect on the propargyl resonance energy.     

Very low-pressure pyrolysis (VLPP) of methyl- and ethynyl-cyclopentanes and cyclohexanes

Studies of the kinetics of thermal unimolecular decomposition of methylcyclopentane, methylcyclohexane, ethynylcyclopentane, and ethynylcyclohexane have been carried out at temperatures in the range 861–1218 K using the technique of very low-pressure pyrolysis (VLPP). Multiple reaction pathways and secondary decomposition of primary products results in a complex array of reaction products. VLPP rate data (fall-off regime) were obtained for the overall decompositions and interpreted via the application of RRKM theory. The data for methylcyclopentane and methylcyclohexane were interpreted in terms of ring-opening bond fission pathways and bond fission to methyl and cycloalkyl radicals. By selecting Arrhenius parameters consistent with the analogous pathways in open-chain alkanes, a good fit to the overall decomposition is obtained. The data for ethynylcyclopentane and ethynylcyclohexane were interpreted in terms of ring-opening bond fission and alkyne to allene isomerization. The A factors for ring opening were based on known values for C-C fission in open-chain alkynes and the Arrhenius parameters for isomerization were chosen to be consistent with previously reported alkyne to allene isomerizations. The VLPP data are consistent with the following high-pressure rate expressions (at < T > = 1100 K) for the dominant primary reaction channel of ring opening adjacent to the substitutent group: where θ = 2.303RT kJ mol^?. Comparison of the activation energies for the ethynyl-cycloalkanes with those for the methyl-cycloalkanes shows that the effect of the ethynyl substituent is consistent with the propargyl resonance energy. This evidence supports the assumption of a biradical mechanism for ring opening in these cycloalkanes.     

Very low-pressure pyrolysis of cyclobutyl cyanide. The cyano stabilization energy

A very low-pressure pyrolysis (VLPP) apparatus has been constructed and shown to yield kinetic data consistent with other VLPP systems. The technique has been applied to the pyrolysis of cyclobutyl cyanide over the temperature range of 833–1203°K. The reaction was found to proceed via a single pathway to yield ethylene and vinyl cyanide. If A_∞ is based on previous high-pressure data for this reaction and for cyclobutane pyrolysis, then RRKM theory calculations show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by where θ=2.303 RT in kcal/mole. If A_∞ is adjusted relative to the more recent parameters for cyclobutane pyrolysis suggested by VLPP studies, then the Arrhenius expression becomes The cyano group reduces the activation energy for cyclobutane pyrolysis by 6±1 kcal/mole, and on the basis of a biradical mechanism this value may be attributed to the cyano stabilization energy.     

Very low-pressure pyrolysis (VLPP) of hex-1-ene. Kinetics of the retro-ene decomposition of a mono-olefin

The thermal unimolecular decomposition of hex-1-ene has been investigated over the temperature range of 915–1153 K using the technique of very low-pressure pyrolysis (VLPP). The reaction proceeds via the competitive pathways of C₃?C₄ fission and retro-ene elimination, with the latter dominant at low temperatures and the former at high temperatures. This behavior results in an isokinetic temperature of 1035 K under VLPP conditions (both reactions in the unimolecular falloff regime). RRKM calculations, generalized to take into account two competing pathways, show that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by log k₁ (sec^?1) = (12.6 ± 0.2) -(57.7 ± 1.5)/θ for retro-ene reaction, and log k₂ (sec^?1) = (15.9 ± 0.2) - (70.8 ± 1.0)/θ for C-C fission, where θ = 2.303 RT kcal/mol. The A factors were assigned from the results of a recent shock-tube study of the decomposition in the high-pressure regime, and the activation energies were found by matching the RRKM calculations to the VLPP data. The parameters for C-C fission are consistent with the known thermochemistry of n-propyl and allyl radicals. A clear measure of the importance of the molecular pathway in the decomposition of a mono-olefin has been obtained.     

The Very low-pressure pyrolysis of 2-phenylethylamine. The enthalpy of formation of the aminomethyl radical

The thermal unimolecular decomposition of 2-phenylethylamine (PhCH₂CH₂NH₂) into benzyl and aminomethyl radicals has been studied under very-low-pressure conditions, and the enthalpy of formation of the aminomethyl radicals, ΔH°_{f, 298K} (H₂NCH₂·) = 37.0 ± 2.0 kcal/mol, has been derived from the kinetic data. This result leads to a value for the C—H bond dissociation energy in methylamine, BDE(H₂NCH₂—H) = 94.6 ± 2.0 kcal/mol, which is about 3.4 kcal/mol lower than in C₂H₆ (98 kcal/mol), indicating a sizable stabilization in α-aminoalkyl radicals.     

Very low-pressure pyrolysis (VLPP) of alkanes: n-butane, 2,3-dimethylbutane, 2,2′,3,3′ -tetramethylbutane,and isobutane

The four species in the title were decomposed under VLPP conditions at temperatures in the vicinity of 1100°K. Three model transition states were constructed that fit the low-pressure data thus obtained and that also yield (1) E₂₉₈ = ΔE₂₉₈; (2) E₁₁₀₀ = ΔE₁₁₀₀; (3) log A₁₁₀₀ = 16.4 per C–C bond broken. The predictions of these models as to values of the high-pressure rate constants for bond scission and the reverse rate constants (radical combination) are compared with existing data.     

Very low-pressure pyrolysis (VLPP) of penta-1,3-dienes. Kinetics of the unimolecular 1,4-hydrogen elimination from cis-penta-1,3-diene

The thermal unimolecular reactions of cis- and trans-penta-1,3-diene (c-PTD and t-PTD) have been studied over the temperature range of 1002–1235 K using the technique of very low-pressure pyrolysis (VLPP). c-PTD decomposes via 1,4-hydrogen elimination analogous to that previously reported for cis-but-2-ene. RRKM calculations incorporating a six-center transition state show that the experimental rate constants are consistent with the following high-pressure rate expression at 1100 K: where θ = 2.303RT kcal/mol, and the A factor was assumed to be the same as that for cis-but-2-ene. The activation energy is in excellent agreement with that obtained for cis-but-2-ene. t-PTD also undergoes decomposition by H₂ elimination presumably via the prior rapid isomerization to c-PTD the results are in exact agreement with those for c-PTD.     

The heat of formation of the acetonyl radical VLPP studies of acetonyl bromide,isopropenylmethylether, and hexanedione-2,5 in a search for acetonyl stabilization energy

The decomposition of acetonyl bromide, isopropenylmethylether, and hexanedione-2,5 was studied using the very-low-pressure pyrolysis (VLPP) technique. The acetonyl radical is a product of each reaction. Arrhenius parameters determined are or acetonyl bromide ← acetonyl + Br: and for isopropenylmethylether ← acetonyl + CH₃: These lead to values of acetonyl stabilization energy (SE) of 0.8 and ?4.0 kcal/mol, respectively. Comparison of the pyrolyses of hexanedione-2,5 and 2,5-dimethylhexane indicate a value of SE ～ 2 kcal/mol. The total of these results is taken, along with previous work, to indicate that 0 ? SE ? 2 kcal/mol.     

The very-low-pressure pyrolysis (VLPP) of n-propyl nitrate,tert-butyl nitrite,and methyl nitrite. Rate constants for some alkoxy radical reactions

The pyrolysis of n-propyl nitrate and tert-butyl nitrite at very low pressures (VLPP technique) is reported. For the reaction the high-pressure rate expression at 300°K, log k₁ (sec^?1) = 16.5 ? 40.0 kcal/mole/2.3 RT, is derived. The reaction was studied and the high-pressure parameters at 300°K are log k₂(sec^?1) = 15.8 ? 39.3 kcal/mole/2.3 RT. From ΔS_1,?1⁰ and ΔS_2,?2⁰ and the assumption E_?1 and E_?2 ? 0, we derive log k_?1(M^?1·sec^?1) (300°K) = 9.5 and log k_?2 (M^?1·sec^?1) (300°K) = 9.8. In contrast, the pyrolysis of methyl nitrite and methyl d₃ nitrite afford NO and HNO and DNO, respectively, in what appears to be a heterogeneous process. The values of k_?1 and k_?2 in conjunction with independent measurements imply a value at 300°K for of 3.5 × 10⁵ M^?1·sec^?1, which is two orders of magnitude greater than currently accepted values. In the high-pressure static pyrolysis of dimethyl peroxide in the presence of NO₂, the yield of methyl nitrate indicates that the combination of methoxy radicals with NO₂ is in the high-pressure limit at atmospheric pressure.     

Kinetics of the thermal unimolecular decomposition of hex-1-ene-3-yne. Heat of formation and resonance stabilization energy of the 3-ethenylpropargyl radical

The thermal unimolecular decomposition of hex-1-ene-3-yne (HEY) has been investigated over the temperature range 949–1230 K using the technique of very low-pressure pyrolysis (VLPP). One reaction pathway is the expected C₅? C₆ bond fission to form the resonance-stabilized 3-ethenylpropargyl radical. There is a concurrent process producing molecular hydrogen which probably occurs via the intermediate formation of hexatrienes and cyclohexa-1,3-diene. RRKM calculations yield the extrapolated high-pressure rate parameters at 1100 K given by the expressions 10^16.0±0.3 exp(?300.4 ± 12.6 kJ mol^?1/RT) s^?1 for bond fission and 10^13.2+0.4 exp(?247.7 ± 8.4 kJ mol^?1/RT) for the overall formation of hydrogen. The A factors were assigned from the results of previous studies of related alkynes, alkenes, and alkadienes. The activation energy for the bond fission reaction leads to ΔH [H₂CCHCC?H₂] = 391.9, DH [H₂CCHCCCH₂? H] = 363.3, and a resonance stabilization energy of 56.9 ± 14.0 kJ mol^?1 for the 3-ethenylpropargyl radical, based on a value of 420.2 kJ mol^?1 for the primary C? H bond dissociation energy in alkanes. Comparison with the revised value of 46.6 kJ mol^?1 for the resonance energy of the unsubstituted propargyl radical indicates that the ethenyl substituent (CH₂?CH) on the terminal carbon atom has only a small effect on the propargyl resonance energy. © John Wiley & Sons, Inc.     

Very low-pressure pyrolysis. IV. The decomposition of i-propyl iodide and n-propyl iodide

The decomposition rate constant of i-PrI under conditions of very low-pressure pyrolysis (VLPP) is completely consistent with the well-known high-pressure Arrhenius parameters and the RRK(M) theory. The decomposition of n-PrI under the same conditions proceeds via two pathways, the anti-Markownikoff dehydroiodination and C? I bond scission. The data, analyzed by taking into account the mutual interaction of the two pathways, is completely consistent with the known Arrhenius parameters for the bond scission step and, when combined with a reasonable A-factor, yields an activation energy for HI elimination which is as predicted for these semi-ion pair transition states.     

Chemistry of polycyclic aromatic hydrocarbons formation from phenyl radical pyrolysis and reaction of phenyl and acetylene

An experimental investigation of phenyl radical pyrolysis and the phenyl radical + acetylene reaction has been performed to clarify the role of different reaction mechanisms involved in the formation and growth of polycyclic aromatic hydrocarbons (PAHs) serving as precursors for soot formation. Experiments were conducted using GC/GC-MS diagnostics coupled to the high-pressure single-pulse shock tube present at the University of Illinois at Chicago. For the first time, comprehensive speciation of the major stable products, including small hydrocarbons and large PAH intermediates, was obtained over a wide range of pressures (25-60 atm) and temperatures (900-1800 K) which encompass the typical conditions in modern combustion devices. The experimental results were used to validate a comprehensive chemical kinetic model which provides relevant information on the chemistry associated with the formation of PAH compounds. In particular, the modeling results indicate that the o-benzyne chemistry is a key factor in the formation of multi-ring intermediates in phenyl radical pyrolysis. On the other hand, the PAHs from the phenyl + acetylene reaction are formed mainly through recombination between single-ring aromatics and through the hydrogen abstraction/acetylene addition mechanism. Polymerization is the common dominant process at high temperature conditions.     

Kinetics of the gas-phase reaction of allyl alcohol with iodine. The heat of formation and stabilization energy of the hydroxyallyl radical

The reaction of iodine with allyl alcohol has been studied in a static system, following the absorption of visible light by iodine, in the temperature range 150-190°C and in the pressure range 10-200 torr. The rate-determining step has been found to be and k₃ is consistent with the equation From the activation energy and the assumption E_-3 = 1 ± 1 kcal mol^?1, it has been calculated that kcal mol^?1. The stabilization energy of the hydroxyallyl radical has been found to be 11.4 ± 2.2 kcal mol^?1.