Kinetics of the iodine atom catalyzed gas phase geometrical isomerization of diiodoethylene. Rotation about a single bond

The spectrophotometric determination of the rate of iodine atom catalyzed geometrical isomerization of diiodoethylene in the gas phase from 502.8 to 609.1°K leads to a rate constant for the bimolecular reaction between I and trans-diiodoethylene of log k_t?c(M^?1 sec^?1) = 8.85 ± 0.12 ? (11.01 ± 0.30)/θ. Estimates of the entropy and enthalpy change for the addition of I atoms to trans-diiodoethylene (process a.b) lead to log K_a.b(M^?1) = ?2.99 ? 4.0/θ, and thus to log k_c (sec^?1) = log k_t?c – log K_ab = 11.8 ?7.0/θ for the rate constant for rotation about the single bond in the adduct radical. The theory for calculation of the rotation rate constant is presented and it is shown that while the exact value depends on the barrier height, a value of 6.8 kcal/mole for this quantity leads to log k (sec^?1) = 11.8 ?6.7/θ. The activation energy points to a better value of the group contribution to heat of formation of the group C -(I)₂(H)(C) than one based on bond additivity.     

Very low-pressure pyrolysis (VLPP) of 3,3-dimethylbut-1-yne (tert-butyl acetylene). The heat of formation and stabilization energy of the dimethylpropargyl radical

The unimolecular decomposition of 3,3-dimethylbut-1-yne has been investigated over the temperature range of 933°-1182°K using the technique of very low-pressure pyrolysis (VLPP). The primary process is C? C bond fission yielding the resonance stabilized dimethylpropargyl radical. Application of RRKM theory shows that the experimental unimolecular rate constants are consistent with the high-pressure Arrhenius parameters given by log (k/sec^?1) = (15.8 ± 0.3) - (70.8 ± 1.5)/θ where θ = 2.303RT kcal/mol. The activation energy leads to DH⁰[(CH₃)₂C(CCH)? CH₃] = 70.7 ± 1.5, θH⁰_f((CH₃)₂?CCH,g) = 61.5 ± 2.0, and DH⁰[(CH₃)₂C(CCH)? H] = 81.0 ± 2.3, all in kcal/mol at 298°K. The stabilization energy of the dimethylpropargyl radical has been found to be 11.0±2.5 kcal/mol.     

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.     

Homogeneous decomposition of vinyl ethers. The heat of formation of ethanal-2-yl

The thermal unimolecular decomposition of three vinylethers has been studied in a VLPP apparatus. The high-pressure rate constant for the retro-ene reaction of ethylvinylether was fit by log k (sec^?1) = (11.47 + 0.25) - (43.4 ± 1.0)/2.303 RT at <T> = 900 K and that of t - butylvinylether by log k (sec^?1) = (12.00 ± 0.27) - (38.4 ± 1.0)/2.303 RT at <T> = 800 K. No evidence for the competition of the higher energy homolytic bond-fission process could be obtained from the experimental data. The rate constant compatible with the C? O bond scission reaction in the case of benzylvinylether was log k (sec^?1) = (16.63 ± 0.30) - (53.74 ± 1.0)/2.303 RT at <T> = 750 K. Together with ΔH_f,300⁰(benzyl·) = 47.0 kcal/mol, the activation energy for this reaction results in ΔH_f,300⁰(CH₂CHO) = +3.0 ± 2.0 kcal/mol and a corresponding resonance stabilization energy of 3.2 ± 2.0 kcal/mol for 2-ethanalyl radical.     

Competitive unimolecular reactions at low pressures. The pyrolysis of cyclobutyl chloride

The thermal decomposition of cyclobutyl chloride has been investigated over the temperature range of 892–1150 K using the technique of very low-pressure pyrolysis (VLPP). The reaction proceeds via two competitive unimolecular channels, one to yield ethylene and vinyl chloride and the other to yield 1,3-butadiene and hydrogen chloride, with the latter being the major reaction under the experimental conditions. With the usual assumption that gas-wall collisions are «strong,» 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) = (14.8 ± 0.3) ? (61.1 ± 1.0)/Θ for vinyl chloride formation and log k₂(sec^?1) = (13.6 ± 0.3) ? (55.7 ± 1.0)/Θ for 1,3-butadiene formation, where Θ = 2.303 RT kcal/mol. The A factors were assigned from previous high-pressure low-temperature data of other workers assuming a four-center transition state for 1,2-HCl elimination and a chlorine-bridged biradical transition state for vinyl chloride formation. The activation energies are in good agreement with the high-pressure results which were obtained with a conventional static system. The difference in critical energies is 4.6 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.     

The carbon–hydrogen bond strength in dimethyl ether and some properties of iodomethyl methyl ether

The kinetics and mechanism of the reaction between iodine and dimethyl ether (DME) have been studied spectrophotometrically from 515–630°K over the pressure ranges, I₂ 3.8–18.9 torr and DME 39.6–592 torr in a static system. The rate-determining step is, where k₁ is given by log (k₁/M^?1 sec^?1) = 11.5 ± 0.3 – 23.2 ± 0.7/θ, with θ = 2.303RT in kcal/mole. The ratio k₂/k_?1, is given by log (k₂/k_?1) = ?0.05 ± 0.19 + (0.9 ± 0.45)/θ, whence the carbon-hydrogen bond dissociation energy, DH° (H? CH₂OCH₃) = 93.3 ± 1 kcal/mole. From this, ΔH°_f(CH₂OCH₃) = ?2.8 kcal and DH°(CH₃? OCH₂) = 9.1 kcal/mole. Some nmr and uv spectral features of iodomethyl ether are reported.     

The retro-aldol mechanism in the pyrolysis kinetics of primary,secondary, and tertiary β-hydroxy ketones in the gas phase

The pyrolysis kinetics of primary, secondary, and tertiary β-hydroxy ketones have been studied in static seasoned vessels over the pressure range of 21–152 torr and the temperature range of 190°–260°C. These eliminations are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are expressed by the following equations: for 1-hydroxy-3-butanone, log k₁(s^?1) = (12.18 ± 0.39) ? (150.0 ± 3.9) kJ mol^?1 (2.303RT)^?1; for 4-hydroxy-2-pentanone, log k₁(s^?1) = (11.64 ± 0.28) ? (142.1 ± 2.7) kJ mol^?1 (2.303RT)^?1; and for 4-hydroxy-4-methyl-2-pentanone, log k₁(s^?1) = (11.36 ± 0.52) ? (133.4 ± 4.9) kJ mol^?1 (2.303RT)^?1. The acid nature of the hydroxyl hydrogen is not determinant in rate enhancement, but important in assistance during elimination. However, methyl substitution at the hydroxyl carbon causes a small but significant increase in rates and, thus, appears to be the limiting factor in a retroaldol type of mechanism in these decompositions. © John Wiley & Sons, Inc.     

Linear correlation of polar substituents in the gas-phase pyrolyses of ethyl α-substituted acetates

The pyrolysis kinetics of several ethyl esters with polar substituents at the acyl carbon have been studied in the temperature range of 319.8–400.0°C and pressure range of 50.5–178.0 torr. These eliminations are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are given by the Arrhenius equations: for ethyl glycolate, log k₁ (s^?1) = (12.75 ± 0.30) – (201.4 ± 3.8) kJ/mol/2.303RT; for ethyl cyanoacetate, log k₁ (s^?1) = (12.19 ± 0.18) – (191.8 ± 2.1) kJ/mol/2.303RT; for ethyl dichloroacetate, log k₁ (s^?1) = (12.62 ± 0.36) – (193.9 ± 4.3) kJ/mol/2.303RT; for ethyl trichloroacetate, log k₁ (s^?1) = (12.27 ± 0.09) – (185.1 ± 1.0) kJ/mol/2.303RT. The results of the present work together with those reported recently in the literature give an approximate linear correlation when plotting log k/k₀ vs. σ* values (ρ* = 0.315 ± 0.004, r = 0.976, and intercept = 0.032 ± 0.006 at 400°C). This linear relationship indicates that the polar substituents affect the rate of elimination by electronic factors. The greater the electronegative nature of the polar substituent, the faster is the pyrolysis rate. The alkyl substituents yield, within experimental error, similar values in rates which makes difficult an adequate assessment of their real influence.     

Pyrolytic production of Se (3P) from carbon diselenide. II. Rate of CSe2 decay

Pyrolytic decay of carbon diselenide was monitored by ultraviolet absorption spectroscopy in reflected shock waves in the temperature range of 1600–2600°K. The temperature dependence of the absorption coefficient of CSe₂ at 2308 Å was determined and was used to provide kinetic information along with a deconvolution procedure which accounted for and removed systematic distortions of the fast time-resolved absorbance profile. For temperatures of 1600–2600°K and argon densities of 1.5–7.0 × 10^?5 mol/cm³ dilute (1.0–9.0 × 10^?9 mol/cm³) CSe₂ pyrolyzed with measured first-order decay rates in the range of log₁₀ k₁ (sec^?1) = 3.0?5.7; at midrange (2100°K and 4.3 × 10^?5 mol/cm³ in Ar) k₁ ≈ 3 × 10⁴ sec^?1. The decay probably occurs via a unimolecular low-pressure process, first order in both CSe₂ and Ar, for which k₂ ± 10⁹ cm³/mol·sec at 2100°K. The deconvoluted data yield Arrhenius activation energies of 53.2 kcal/mol under second-order treatment, but the activation energy is less reliable than the general magnitude of the rate constant. A comparison of CSe₂ with other molecules which are isoelectronic in their valence shells (CO₂, CS₂, OCS, and N₂O) is made.     

The elimination kinetics and mechanisms of ethyl piperidine‐3‐carboxylate,ethyl 1‐methylpiperidine‐3‐carboxylate,and ethyl 3‐(piperidin‐1‐yl)propionate in the gas phase

The gas‐phase elimination kinetics of the above‐mentioned compounds were determined in a static reaction system over the temperature range of 369–450.3°C and pressure range of 29–103.5 Torr. The reactions are homogeneous, unimolecular, and obey a first‐order rate law. The rate coefficients are given by the following Arrhenius expressions: ethyl 3‐(piperidin‐1‐yl) propionate, log k₁(s^?1) = (12.79 ± 0.16) ? (199.7 ± 2.0) kJ mol^?1 (2.303 RT)^?1; ethyl 1‐methylpiperidine‐3‐carboxylate, log k₁(s^?1) = (13.07 ± 0.12)–(212.8 ± 1.6) kJ mol^?1 (2.303 RT)^?1; ethyl piperidine‐3‐carboxylate, log k₁(s^?1) = (13.12 ± 0.13) ? (210.4 ± 1.7) kJ mol^?1 (2.303 RT)^?1; and 3‐piperidine carboxylic acid, log k₁(s^?1) = (14.24 ± 0.17) ? (234.4 ± 2.2) kJ mol^?1 (2.303 RT)^?1. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene through a concerted six‐membered cyclic transition state type of mechanism. The intermediate β‐amino acids decarboxylate as the α‐amino acids but in terms of a semipolar six‐membered cyclic transition state mechanism. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 106–114, 2006     

The pyrolysis kinetics of ethyl esters in the gas phase. The alkyl substituent effect at the acyl carbon

The gas-phase elimination of ethyl 3-methylbutanoate and ethyl 3,3-dimethylbutanoate has been studied, in a static system, over the temperature range of 360–420°C and in the pressure range of 71–286 torr. The reactions are homogeneous, unimolecular, and follow a first-order rate law. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for ethyl 3-methylbutanoate, log k₁ (s^?1) = (12.70 ± 0.36) – (202.5 ± 4.4) kJ/mol/2.303RT, and for ethyl 3,3-dimethylbutanoate, log k₁ (s^?1) = (13.04 ± 0.08) – (207.1 ± 1.0) kJ/mol/2.303RT. Alkyl substituents at the acyl carbon of ethyl esters yield very close values in rates. Consequently it is rather difficult to offer some conclusion concerning the effect of these substituents.     

The electronic effects of substituents at the acyl carbon in the gas-phase elimination kinetics of tert-butyl α-substituted acetates

The gas-phase eliminations of several tert-butyl esters, in a static system and in vessels seasoned with allyl bromide, have been studied in the temperature range of 171.5–280.1°C and the pressure range of 23–98 torr. The rate coefficients for the homogeneous unimolecular elimination of these esters are given by the following Arrhenius equations: for tert-butyl pivalate, log k₁(s^?1) = (13.44 ± 0.30) ? (169.1 ± 3.1) kJ · mol^?1 (2.303RT)^?1; for tert-butyl trichloroacetate, log k₁(s^?1) = (12.41 ± 0.08) ? (141.1 ± 0.7) kJ · mol^?1 (2.303RT)^?1; and for tert-butyl cyanoacetate log k₁(s^?1) = (11.31 ± 0.44) ? (137.8 ± 4.1) kJ · mol^?1 (2.303RT)^?1. The data of this work together with those reported in the literature yield a good linear relationship when plotting log k/k₀ vs. σ* values (ρ* = 0.635, correlation coefficient r = 0.972, and intercept = 0.048 at 250°C). The positive ρ* value suggests that the movement of negative charge to the acyl carbon in the transition state is rate determining. The present results along with previous investigations ratify the generalization that electron-withdrawing substituents at the acyl side of ethyl, isopropyl, and tert-butyl esters enhance the elimination rates, while electron-releasing groups tend to reduce them. The negative nature of the acyl carbon and the polarity in the transition state increases slightly from primary to tertiary esters.     

The kinetics and mechanisms of methyl 2-bromopropionate and 2-bromopropionic acid pyrolyses under maximum inhibition

The kinetics of the gas phase pyrolyses of methyl 2-bromopropionate and 2-bromopropionic acid were studied in a seasoned, static reaction vessel and under maximum inhibition of the free radical suppressor toluene. The working temperature and pressure range was 310–430°C and 26.5–201.5 torr, respectively. The reactions proved to be homogeneous, unimolecular, and obeys a first-order rate law. The rate coefficients are expressible by the following equations: for methyl 2-bromopropionate, log k₁(s^?1) = (13.10 ± 0.34) ? (211.4 ± 4.4)kJ mol^?1(2.303RT)^?1; for 2-bromopropionic acid, log k₁(s^?1) = (12.41 ± 0.29) ? (180.3 ± 3.4)kJ mol^?1(2.303RT)^?1. The bromoacid yields acetaldehyde, CO and HBr. Because of this result, the mechanism is believed to proceed via a polar five-membered cyclic transition state.     

The decomposition of dimethyl peroxide and the rate constant for CH3O + O2 → CH2O + HO2

The decomposition of dimethyl peroxide (DMP) was studied in the presence and absence of added NO₂ to determine rate constants k₁ and k₂ in the temperature range of 391–432°K: The results reconcile the studies by Takezaki and Takeuchi, Hanst and Calvert, and Batt and McCulloch, giving log k₁(sec^?1) = (15.7 ± 0.5) - (37.1 ± 0.9)/2.3 RT and k₂ ≈ 5 × 10⁴M^?1· sec^?1. The disproportionation/recombination ratio k_7b/k_7a = 0.30 ± 0.05 was also determined: When O₂ was added to DMP mixtures containing NO₂, relative rate constants k₁₂/k_7a were obtained over the temperature range of 396–442°K: A review of literature data produced k_7a = 10^9.8±0.5M^?1·sec^?1, giving log k₁₂(M^?1·sec^?1) = (8.5 ± 1.5) - (4.0 ± 2.8)/2.3 RT, where most of the uncertainty is due to the limited temperature range of the experiments.     

The gas-phase elimination kinetics of 3-buten-1-methanesulphonate and 3-methyl-3-buten-1-methanesulphonate

The elimination kinetics of the title compounds were carried out in a static system over the temperature range of 290–330°C and pressure range of 29.5–124 torr. The reactions, carried out in seasoned vessels with allyl bromide, obey first-order rate law, are homogeneous and unimolecular. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for 3-buten-1-methanesulphonate, log k₁(s^?1) = (12.95 ± 0.53) ? (175.3 ± 5.9)kJ mol^?1(2.303RT)^?1; and for 3-methyl-3-buten-1-methanesulphonate, log k₁(s^?1) = (12.98 ± 0.40) ? (174.7 ± 4.5)kJ mol^?1(2.303RT)^?1. The olefinic double bond appears to assist in the rate of pyrolysis. The mechanism is described in terms of an intimate ion-pair intermediate. © 1995 John Wiley & Sons, Inc.     

Thermal unimolecular isomerization of 1,4-dimethylbicyclo[2.2.0]hexane, 1,4-dimethylbicyclo[2.1.1]hexane,and 1,4-dimethylbicyclo[2.1.0]pentane

The thermal isomerization of the title compounds was studied in the vapor phase. Over the temperature range from 445.1 to 477.5°K, 1,4-dimethylbicyclo[2.2.0]hexane underwent a homogeneous unimolecular reaction to 2,5-dimethyl-1,5-hexadiene, the rate constants being represented by the equation: k = 1.86 × 10¹¹ exp (?31000 ± 1800/RT) sec^?1. Over the temperature range from 630.0 to 662.2°K, 1,4-dimethylbicyclo[2.1.1]-hexane also underwent a unimolecular isomerization to the same product, the rate constants being given by the equation: k = 8.91 × 10¹⁴ exp (?56000 ± 900/RT) sec^?1. The pyrolysis of 1,4-dimethylbicyclo[2.1.0]pentane gave 1,3-dimethylcyclopentene-1 and 2,4-dimethyl-1,4-pentadiene in the ratio of 9:1. The former reaction was influenced by surface effects but the latter was not. The rate constants for the formation of 2,4-dimethyl-1,4-pentadiene fitted the equation: k = 1.66 × 10¹⁷ exp (?57400 ± 3100/RT) sec^?1. The effect of the two methyl groups at the bridgehead positions in these molecules in influencing the rate of decomposition is discussed in terms of the non-bonded repulsive forces between the substituents.     

Neighboring olefinic double-bond participation in gas phase pyrolysis of some alkenyl chlorides

4-Chloro-1-butene, 5-chloro-1-pentene, and 6-chloro-1-hexene have been shown to decompose, in a static system, mainly to hydrogen chloride and the corresponding alkadienes. In packed and unpacked clean Pyrex vessels the reactions were significantly heterogeneous. However, in a vessel seasoned with allyl bromide these reactions were homogeneous, unimolecular, and follow a first-order law. The working temperature range was 389.6–480.0°C and with a pressure range of 53–221 Torr. The rate constants for the homogeneous reactions were expressed by the following Arrhenius equations: 4-chloro-1-butene: logk(sec^?1) = (13.79 ± 0.17) – (223.8 ± 2.1)kJ/mole/2.303RT; 5-chloro-1-pentene: logk(sec^?1) = (14.25 ± 1.20) – (238.4 ± 12.7)kJ/mole/2.303RT; and 6-chloro-1-hexene: logk(sec^?1) = (12.38 ± 0.22) – (209.6 ± 2.9)kJ/mole/2.303RT. The olefinic double bond has been found to participate in the rate of dehydrohalogenation of 4-chloro-1-butene. The insulation of the CH₂?CH in chlorobutene by one or two methylene chains to the reaction center does not indicate neighboring group participation. The three-membered conformation is the most favored structure for anchimeric assistance of the double bond in gas phase pyrolysis of alkenyl chlorides. The heterolytic nature of these eliminations is also supported by the present work.     

The mechanism of neutral amino acid decomposition in the gas phase. The elimination kinetics of N,N‐dimethylglycine ethyl ester,ethyl 1‐piperidineacetate,and N,N‐dimethylglycine

The gas‐phase elimination kinetics of the ethyl ester of two α‐amino acid type of molecules have been determined over the temperature range of 360–430°C and pressure range of 26–86 Torr. The reactions, in a static reaction system, are homogeneous and unimolecular and obey a first‐order rate law. The rate coefficients are given by the following equations. For N,N‐dimethylglycine ethyl ester: log k₁(s^?1) = (13.01 ± 3.70) ? (202.3 ± 0.3)kJ mol^?1 (2.303 RT)^?1 For ethyl 1‐piperidineacetate: log k₁(s^?1) = (12.91 ± 0.31) ? (204.4 ± 0.1)kJ mol^?1 (2.303 RT)^?1 The decompositon of these esters leads to the formation of the corresponding α‐amino acid type of compound and ethylene. However, the amino acid intermediate, under the condition of the experiments, undergoes an extremely rapid decarboxylation process. Attempts to pyrolyze pure N,N‐dimethylglycine, which is the intermediate of dimethylglycine ethyl ester pyrolysis, was possible at only two temperatures, 300 and 310°C. The products are trimethylamine and CO₂. Assuming log A = 13.0 for a five‐centered cyclic transition‐state type of mechanism in gas‐phase reactions, it gives the following expression: log k₁(s^?1) = (13.0) ? (176.6)kJ mol^?1 (2.303 RT)^?1. The mechanism of these α‐amino acids differs from the decarbonylation elimination of 2‐substituted halo, hydroxy, alkoxy, phenoxy, and acetoxy carboxylic acids in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33:465–471, 2001