Kinetics and mechanisms of the homogeneous, unimolecular gas-phase elimination of trimethyl orthoacetate and trimethyl orthobutyrate

The gas-phase elimination kinetics of the title compounds have been examined over the temperature range of 310-369 degrees C and pressure range of 50-130 Torr. The reactions, in seasoned vessels, are homogeneous, unimolecular, and follow a first-order rate law. The products are methanol and the corresponding methyl ketene acetal. The rate coefficients are expressed by the Arrhenius equation: for trimethyl orthoacetate, log k1 (s(-1)) = [(13.58 +/- 0.10) - (194.7 +/- 1.2) (kJ mol(-1))](2.303RT)(-1)r = 0.9998; and for trimethyl orthobutyrate, log k1(s(-1)) = [(13.97 +/- 0.37) - (195.3 +/- 1.6) (kJ mol(-1))](2.303RT)(-1)r = 0.9997. These reactions are believed to proceed through a polar concerted four-membered cyclic transition state type of mechanism.     

Rate constant and activation energy measurement for the reaction of atomic hydrogen with thiocyanate and azide in aqueous solution

Arrhenius parameters for the reaction of hydrogen atoms with azide and thiocyanate in aqueous solution have been determined using electron pulse radiolysis and electron paramagnetic resonance free induction decay attenuation measurements. Absolute values for SCN-, N3(-), and HN3 were well-described over the temperature range of 9-81 degrees C by the equations log k5 = (12.03 +/- 0.12) - [(21.05 +/- 0.66 kJ mol(-1))/2.303RT], log k10 = (12.75 +/- 0.21) - [(18.43 +/- 1.22 kJ mol(-1))/2.303RT], and log k15 = (11.59 +/- 0.12) - [(21.44 +/- 0.69 kJ mol(-1))/2.303RT], corresponding to room temperature (22 degrees C) rate constants of (2.07 +/- 0.03) x 10(8), (3.15 +/- 0.08) x 10(9), and (6.31 +/- 0.05) x 10(7) M(-1) s(-1) and activation energies for these chemicals of 21.05 +/- 0.66, 18.4 +/- 1.2, and 21.44 +/- 0.69 kJ mol(-1), respectively. The similarity of these three measured activation energies, taken together with the available information on reaction products, suggests a similar reaction mechanism, which is proposed to be an initial hydrogen atom adduct formation in these molecules, followed by single bond breakage.     

Kinetic Determination of the Gas‐Phase Decarbonylation of Butyraldehyde in the Presence of HCl Catalyst

The gas‐phase kinetics and mechanism of the homogeneous elimination of CO from butyraldehyde in the presence of HCl has been experimentally studied. The reaction is homogeneous and follows the second‐order kinetics with the following rate expression: log k ₁ (s⁻¹ L mol⁻¹) = (13.27 ± 0.36) – (173.2 ± 4.4) kJ mol⁻¹(2.303RT )⁻¹. Experimental data suggested a concerted four‐membered cyclic transition state type of mechanism. The first and rate‐determining step occurs through a four‐membered cyclic transition state to produce propane and formyl chloride. The formyl chloride intermediate rapidly decomposes to CO and HCl gases.     

The kinetics and mechanisms of the gas phase elimination of 4-(methylthio)-1-butyl acetate and 1-chloro-4-(methylthio)-butane

The gas phase elimination of 4-(methylthio)-1-butyl acetate and 1-chloro-4-(methylthio)-butane has been investigated in a seasoned, static reaction vessel over the temperature range of 310–410°C and the pressure range of 46–193 Torr. The presence of the inhibitors propene, cyclohexene, and/or toluene had no effect on the rates. The reactions are homogeneous, unimolecular, and obey a first-order rate law. The rate coefficients are given by the following Arrhenius equations: for 4-(methylthio)-1-butyl acetate, log k₁(s^?1) = (12.32 ± 0.29) ? (192.1 ± 3.6) kJ/mol/2.303RT; for 1-chloro-4-(methylthio)-butane, log k₁(s^?1) = (12.23 ± 0.59) ? (175.7 ± 6.8) kJ/mol/2.303RT. The CH₃S substituent in 1-chloro-4-(methylthio)-butane has been found to participate in the elimination reaction, where tetrahydrothiophene and methyl chloride formation may result from an intimate ion-pair type of mechanism. The yield of a cyclic product in gas phase reactions provides additional evidence of an intimate ion pair mechanism through neighboring group participation in gas phase elimination of special types of organic halides.     

Gas-phase elimination kinetics of 1-substituted ethyl acetates. Effect of polar substituents at the α carbon of secondary acetates

The gas-phase elimination of several polar substituents at the α carbon of ethyl acetates has been studied in a static system over the temperature range of 310–410°C and the pressure range of 39–313 torr. These reactions are homogeneous in both clean and seasoned vessels, follow a first-order rate law, and are unimolecular. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: 2-acetoxypropionitrile, log k₁ (s^?1) = (12.88 ± 0.29) – (203.3 ± 2.6) kJ/mol (2.303RT)^?1; for 3-acetoxy-2-butanone, log ±₁(s^?1) = (13.40 ± 0.20) – (202.8 ± 2.4) kJ/mol (2.303RT)^?1; for 1,1,1-trichloro-2-acetoxypropane, log ?₁ (s^?1) = (12.12 ± 0.50) – (193.7 ± 6.0) kJ/mol (2.303RT)^?; for methyl 2-acetoxypropionate, log ?₁ (s^?1) = (13.45 ± 0.05) – (209.5 ± 0.5) kJ/mol (2.303RT)^?1; for 1-chloro-2-acetoxypropane, log ?₁ (s^?1) = (12.95 ± 0.15) – (197.5 ± 1.8) kJ/mol (2.303RT)^?1; for 1-fluoro-2-acetoxypropane, log ?₁ (s^?1) = (12.83 ± 0.15)– (197.8 ± 1.8) kJ/mol (2.303RT)^?1; for 1-dimethylamino-2-acetoxypropane, log ?₁ (s^?1) = (12.66 ± 0.22) –(185.9 ± 2.5) kJ/mol (2.303RT)^?1; for 1-phenyl-2-acetoxypropane, log ?₁ (s^?1) = (12.53 ± 0.20) – (180.1 ± 2.3) kJ/mol (2.303RT)^?1; and for 1-phenyl?3?acetoxybutane, log ?₁ (s^?1) = (12.33 ± 0.25) – (179.8 ± 2.9) kJ/mol (2.303RT)^?1. The C_α? O bond polarization appears to be the rate-determining process in the transmition state of these pyrolysis reactions. Linear correlations of electron-releasing and electron-withdrawing groups along strong σ bonds have been projected and discussed. The present work may provide a general view on the effect of alkyl and polar substituents at the C_α? O bond in the gas-phase elimination of secondary acetates.     

Activation of the sulfhydryl group by Mo centers: kinetics of reaction of benzyl radical with a binuclear Mo(micro-SH)Mo complex and with arene and alkane thiols

This paper provides evidence from kinetic experiments and electronic structure calculations of a significantly reduced S-H bond strength in the Mo(micro-SH)Mo function in the homogeneous catalyst model, CpMo(micro-S)(2)(micro-SH)(2)MoCp (1, Cp = eta(5)-cyclopentadienyl). The reactivity of 1 was explored by determination of a rate expression for hydrogen atom abstraction by benzyl radical from 1 (log(k(abs)/M(-)(1) s(-)(1)) = (9.07 +/- 0.38) - (3.62 +/- 0.58)/theta) for comparison with expressions for CH(3)(CH(2))(7)SH, log(k(abs)/M(-)(1) s(-)(1)) = (7.88 +/- 0.35) - (4.64 +/- 0.54)/theta, and for 2-mercaptonaphthalene, log(k(abs)/M(-)(1) s(-)(1)) = (8.21 +/- 0.17) - (4.24 +/- 0.26)/theta (theta = 2.303RT kcal/mol, 2sigma error). The rate constant for hydrogen atom abstraction at 298 K by benzyl radical from 1 is 2 orders of magnitude greater than that from 1-octanethiol, resulting from the predicted (DFT) S-H bond strength of 1 of 73 kcal/mol. The radical CpMo(micro-S)(3)(micro-SH)MoCp, 2, is revealed, from the properties of slow self-reaction, and exclusive cross-combination with reactive benzyl radical, to be a persistent free radical.     

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     

Steric factors in the gas phase elimination kinetics of ethyl N‐benzyl‐N‐cyclopropylcarbamate and ethyl diphenylcarbamate

The elimination kinetics of ethyl N‐benzyl‐N‐cyclopropylcarbamate and ethyl diphenylcarbamate were investigated over the temperature range of 349.9–440.0°C and the pressure range of 31–106 Torr. These reactions have been found to be homogeneous, unimolecular, and obey a first‐order rate law. The products are ethylene, carbon monoxide, and the corresponding secondary amine. The rate coefficient is expressed by the following Arrhenius equations: For ethyl N‐benzyl‐N‐cyclopropylcarbamate log k₁ (s^?1) = (12.94 ± 0.09) ? (198.5 ± 0.9) kJ mol^?1 (2.303RT)^?1 For ethyl diphenylcarbamate log k₁ (s^?1) = (12.91 ± 0.18) ? (208.2 ± 2.4) kJ mol^?1 (2.303RT)^?1 The presence of phenyl and bulky groups at the nitrogen atom of the ethylcarbamate showed a decrease in the rate of elimination. Steric factor may be operating during the process of decomposition of these substrates. These reactions appear to undergo a semipolar six‐membered cyclic transition type of mechanism.© 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 67–71, 2002     

The kinetics and mechanisms of the gas-phase unimolecular eliminations of halogeno esters. Participation of the carbomethoxy group

The kinetics of the gas-phase elimination of several chloroesters were determined in a static system over the temperature range of 410–490°C and the pressure range of 47–236 torr. The reactions in seasoned vessels, and in the presence of a free-radical inhibitor, are homogeneous, unimolecular, and follow a first-order law. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for methyl 3-chloropropionate, log k₁(s^?1) = (13.22 ± 0.07) - (231.5 ± 1.0) kJ/mol/2.303RT; for methyl 4-chlorobutyrate, log k₁(s^?1) = (13.31 ± 0.25) - (221.5 ± 3.4) kJ/mol/2.303RT; and for methyl 5-chlorovalerate, log k₁(s^?1) = (13.12 ± 0.25) - (221.7 ± 3.2) kJ/mol/2.303RT. Rate enhancements and lactone formation reveal the participation of carbonyl oxygen of the carbomethoxy group. The order COOCH₃-5 > COOCH₃-6 > COOCH₃-4 in assistance is similar to the sequence of group participation in solvolysis reactions. The partial rates for the parallel eliminations to normal dehydrohalogenation products and lactones have been estimated and reported. The present results lead us to consider that an intimate ion-pair mechanism through participation of the carbomethoxy group may well be operating in some of these reactions.     

Pyrolysis of 5-chloropentan-2-one and 4-chloro-1-phenylbutan-1-one. Participation of the carbonyl group

The rates of elimination of 5-chloropentan-2-one and 4-chloro-1-phenylbutan-1-one in the gas phase have been determined in a static system, seasoned with allyl bromide, and in the presence of the chain inhibitor propene. The reactions are unimolecular and follow a first-order rate law. The working temperature and pressure ranges were 339.4–401.1°C and 46–117 torr, respectively. The rate coefficients for the homogeneous reactions are given by the following Arrhenius equations: for 5-chloropentan-2-one, log k₁(s^?1) = (13.12 ± 0.88) - (207.8 ± 11.0)kJ/mol/2.303RT; and for 4-chloro-1-phenylbutan-1-one, log k₁(s^?1) = (12.28 ± 1.09) - (185.2 ± 12.0)kJ/mol/2.303RT. The carbonyl group at the γ position of the C? Cl bond of haloketones apparently participates in the rate of pyrolysis. The five-membered conformation appears to be a favorable structure for anchimeric assistance of the C?O group in the gas-phase elimination of chloroketones.     

Kinetics of radical heterolysis reactions forming alkene radical cations

Rate constants for heterolytic fragmentation of beta-(ester)alkyl radicals were determined by a combination of direct laser flash photolysis studies and indirect kinetic studies. The 1,1-dimethyl-2-mesyloxyhexyl radical (4a) fragments in acetonitrile at ambient temperature with a rate constant of k(het) > 5 x 10(9) s(-1) to give the radical cation from 2-methyl-2-heptene (6), which reacts with acetonitrile with a pseudo-first-order rate constant of k = 1 x 10(6) s(-1) and is trapped by methanol in acetonitrile in a reversible reaction. The 1,1-dimethyl-2-(diphenylphosphatoxy)hexyl radical (4b) heterolyzes in acetonitrile to give radical cation 6 in an ion pair with a rate constant of k(het) = 4 x 10(6) s(-1), and the ion pair collapses with a rate constant of k < or = 1 x 10(9) s(-1). Rate constants for heterolysis of the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(diphenylphosphatoxy)ethyl radical (5a) and the 1,1-dimethyl-2-(2,2-diphenylcyclopropyl)-2-(trifluoroacetoxy)ethyl radical (5b) were measured in various solvents, and an Arrhenius function for reaction of 5a in THF was determined (log k = 11.16-5.39/2.3RT in kcal/mol). The cyclopropyl reporter group imparts a 35-fold acceleration in the rate of heterolysis of 5a in comparison to 4b. The combined results were used to generate a predictive scale for heterolysis reactions of alkyl radicals containing beta-mesyloxy, beta-diphenylphosphatoxy, and beta-trifluoroacetoxy groups as a function of solvent polarity as determined on the E(T)(30) solvent polarity scale.     

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     

Gas phase thermolysis of allylphosphines,kinetic study

Diallylphenyl, allylbenzylphenyl and allylmethylphenyl phosphines were pyrolized in a stirred-flow reactor at 380–429°C/7-20 torr, using toluene as carrier gas. The reaction products were propene, 1-phospha-1,3-butadiene, 1-phospha-1,2-diphenylethylene and 1-phosphaethylene. The phospha-alkenes formed evolve into cyclo addition products. The propene elimination reaction showed first-order kinetics with rate coefficients following the Arrhenius equations: Diallylphenylphosphine: k(s⁻¹) = 10^{10.57 ± 0.31} exp(-143 ± 4 kJ/mol.RT) Allylbenzylphenylphosphine: k(s⁻¹) = 10^{9.71 ± 0.47} exp(-135 ± 6 kJ/mol.RT) Allylbenzylphenylphosphine: k(s⁻¹) = 10^{9.61 ± 0.61} exp(-144 ± 9 kJ/mol.RT) A six-center cyclic transition state unimolecular reaction mechanism, consistent with the above Arrhenius parameters, is proposed for the propene elimination reaction.     

Studies of the kinetics and thermochemistry of the forward and reverse reaction Cl + C6H6 = HCl + C6H5

The laser flash photolysis resonance fluorescence technique was used to monitor atomic Cl kinetics. Loss of Cl following photolysis of CCl4 and NaCl was used to determine k(Cl + C6H6) = 6.4 x 10(-12) exp(-18.1 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 578-922 K and k(Cl + C6D6) = 6.2 x 10(-12) exp(-22.8 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 635-922 K. Inclusion of literature data at room temperature leads to a recommendation of k(Cl + C6H6) = 6.1 x 10(-11) exp(-31.6 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) for 296-922 K. Monitoring growth of Cl during the reaction of phenyl with HCl led to k(C6H5 + HCl) = 1.14 x 10(-12) exp(+5.2 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 294-748 K, k(C6H5 + DCl) = 7.7 x 10(-13) exp(+4.9 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 292-546 K, an approximate k(C6H5 + C6H5I) = 2 x 10(-11) cm(3) molecule(-1) s(-1) over 300-750 K, and an upper limit k(Cl + C6H5I) < or = 5.3 x 10(-12) exp(+2.8 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 300-750 K. Confidence limits are discussed in the text. Third-law analysis of the equilibrium constant yields the bond dissociation enthalpy D(298)(C6H5-H) = 472.1 +/- 2.5 kJ mol(-1) and thus the enthalpy of formation Delta(f)H(298)(C6H5) = 337.0 +/- 2.5 kJ mol(-1).     

The intimate ion pair mechanism in the maximally inhibited elimination kinetics of methyl 4-chlorobutyrate and methyl 5-chlorovalerate in the gas phase

The gas phase elimination of methyl 4-chlorobutyrate and methyl 5-chlorovalerate has been reexamined, in a static system and seasoned vessel, over the temperature range of 419.6–472.1°C and pressure range of 45–108 torr. The reactions, under maximum inhibition with propene, are homogeneous, unimolecular, and obey a first-order rate law. The rate coefficients are given by the following Arrhenius equations: for methyl 4-chlorobutyrate, log (k₁(s^?1) = (13.41 ± 0.60) - (226.8 ± 8.2) kJ/mol/2.303RT; and for methyl 5-chlorovalerate, log k₁(s^?1) = (13.20 ± 0.02) - (227.6 ± 0.3) kJ / mol / 2.303RT. The pyrolysis rates are found to be about a half of the rates reported in a previous work. As already advanced, the carbomethoxy substituent appears to provide anchimeric assistance in the elimination process, where normal dehydrochlorination and lactone formation arise from an intimate ion pair type mechanism. The partial rates towards each of these products have been determined and reported.     

Direct measurements of the rate constants of the reactions NCN + NO and NCN + NO2 behind shock waves

The high-temperature rate constants of the reactions NCN + NO and NCN + NO(2) have been directly measured behind shock waves under pseudo-first-order conditions. NCN has been generated by the pyrolysis of cyanogen azide (NCN(3)) and quantitatively detected by sensitive difference amplification laser absorption spectroscopy at a wavelength of 329.1302 nm. The NCN(3) decomposition initially yields electronically excited (1)NCN radicals, which are subsequently transformed to the triplet ground state by collision-induced intersystem crossing (CIISC). CIISC efficiencies were found to increase in the order of Ar < NO(2) < NO as the collision gases. The rate constants of the NCN + NO/NO(2) reactions can be expressed as k(NCN+NO)/(cm(3) mol(-1)s(-1)) = 1.9 × 10(12) exp[-26.3 (kJ/mol)/RT] (±7%,ΔE(a) = ± 1.6 kJ/mol, 764 K < T < 1944 K) and k(NCN+NO(2))/(cm(3) mol(-1)s(-1)) = 4.7 × 10(12) exp[-38.0(kJ/mol)/RT] (±19%,ΔE(a) = ± 3.8 kJ/mol, 704 K < T < 1659 K). In striking contrast to reported low-temperature measurements, which are dominated by recombination processes, both reaction rates show a positive temperature dependence and are independent of the total density (1.7 × 10(-6) mol/cm(3) < ρ < 7.6 × 10(-6) mol/cm(3)). For both reactions, the minima of the total rate constants occur at temperatures below 700 K, showing that, at combustion-relevant temperatures, the overall reactions are dominated by direct or indirect abstraction pathways according to NCN + NO → CN + N(2)O and NCN + NO(2) → NCNO + NO.     

Kinetic and Mechanisms of the Homogeneous,Unimolecular Elimination of Phenyl Chloroformate and p‐Tolyl Chloroformate in the Gas Phase

The gas‐phase elimination of phenyl chloroformate gives chlorobenzene, 2‐chlorophenol, CO₂, and CO, whereasp‐tolyl chloroformate produces p‐chlorotoluene and 2‐chloro‐4‐methylphenol CO₂ and CO. The kinetic determination of phenyl chloroformate (440–480^oC, 60–110 Torr) and p‐tolyl chloroformate (430–480°C, 60–137 Torr) carried out in a deactivated static vessel, with the free radical inhibitor toluene always present, is homogeneous, unimolecular and follows a first‐order rate law. The rate coefficient is expressed by the following Arrhenius equations: Phenyl chloroformate: Formation of chlorobenzene, log k_I = (14.85 ± 0.38) – (260.4 ± 5.4) kJ mol^?1 (2.303RT)^?1; r = 0.9993 Formation of 2‐chlorophenol, log k_II = (12.76 ± 0.40) – (237.4 ± 5.6) kJ mol^?1(2.303RT)^?1; r = 0.9993 p‐Tolyl chloroformate: Formation of p‐chlorotoluene: log k_I = (14.35 ± 0.28) – (252.0 ± 1.5) kJ mol^–1 (2.303RT)^?1; r = 0.9993 Formation of 2‐chloro‐4‐methylphenol, log k_II = (12.81 ± 0.16) – (222.2 ± 0.9) kJ mol^?1(2.303RT)^–1; r = 0.9995 The estimation of the k_I values, which is the decarboxylation process in both substrates, suggests a mechanism involving an intramolecular nucleophilic displacement of the chlorine atom through a semipolar, concerted four‐membered cyclic transition state structure; whereas the k_II values, the decarbonylation in both substrates, imply an unusual migration of the chlorine atom to the aromatic ring through a semipolar, concerted five‐membered cyclic transition state type of mechanism. The bond polarization of the C–Cl, in the sense C^{δ+ …} Cl^δ?, appears to be the rate‐determining step of these elimination reactions.     

The gas‐phase elimination kinetics of ethyl 2‐furoate and ethyl 2‐thiophenecarboxylate

The gas‐phase elimination kinetics of ethyl 2‐furoate and 2‐ethyl 2‐thiophenecarboxylate was carried out in a static reaction system over the temperature range of 623.15–683.15 K (350–410°C) and pressure range of 30–113 Torr. The reactions proved to be homogeneous, unimolecular, and obey a first‐order rate law. The rate coefficients are expressed by the following Arrhenius equations: ethyl 2‐furoate, log k₁ (s^?1) = (11.51 ± 0.17)–(185.6 ± 2.2) kJ mol^?1 (2.303 RT)^?1; ethyl 2‐thiophenecarboxylate, log k₁ (s^?1) = (11.59 ± 0.19)–(183.8 ± 2.4) kJ mol^?1 (2.303 RT)^?1. The elimination products are ethylene and the corresponding heteroaromatic 2‐carboxylic acid. However, as the reaction temperature increases, the intermediate heteroaromatic carboxylic acid products slowly decarboxylate to give the corresponding heteroaromatic furan and thiophene, respectively. The mechanisms of these reactions are suggested and described. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 145–152, 2009     

Kinetics of the gas‐phase elimination of α‐Bromophenylacetic acid under maximum inhibition

The gas phase elimination kinetics of the title compound was studied over the temperature range of 260.1–315.0°C and pressure range of 20–70 Torr. This elimination, in seasoned static reaction system and in the presence of at least fourfold of the free radical inhibitor toluene, is homogeneous, unimolecular and follows a first‐order rate law. The reaction yielded mainly benzaldehyde, CO, and HBr, and small amounts of benzylbromide and CO₂. The observed rate coefficients are expressed by the following Arrhenius equations: For benzaldehyde formation: log k₁ (s⁻¹) = (12.23 ± 0.26) − (164.9 ± 2.7) kJ mol⁻¹ (2.303 RT)⁻¹ For benzylbromide formation: log k₁ (s⁻¹) = (13.82 ± 0.50) − (192.8 ± 5.5) kJ mol⁻¹ (2.303 RT)⁻¹ The mechanisms are believed to proceed through a semi‐polar five‐membered cyclic transition state for the benzaldehyde formation, while a four‐centered cyclic transition state for benzylbromide formation. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 725–728, 1999     

Rate constants for cyclizations of α-hydroxy radical clocks

The 1-hydroxy-1-methyl-6,6-diphenyl-5-hexenyl radical (4a) and the 1-hydroxy-1-methyl-7,7-diphenyl-6-heptenyl radical (4b) were prepared from the corresponding PTOC esters (anhydrides of a carboxylic acid and N-hydroxypyridine-2-thione). The key step in the synthetic method for the precursors was a coupling reaction of the respective carboxylic acids with the thiohydroxamic acid, which was conducted for ca. 5 min and followed rapidly by chromatography. Rate constants for cyclizations of radicals 4a and 4b in acetonitrile and in THF were measured directly between -30 and 60 °C by laser flash photolysis methods. The Arrhenius functions in acetonitrile are log k = 9.9-2.6/2.303RT and log k = 8.9-4.4/2.303RT (kcal mol(-1)) for 4a and 4b, respectively. Rate constants for cyclizations at room temperature of 9 × 10(7) s(-1) and 4 × 10(5) s(-1) are somewhat larger than the rate constants for cyclizations of analogous alkyl radicals. Crude rate constants at room temperature for H-atom trapping of 4a by thiophenol and 4b by t-butylthiol were k(T) = 1.2 × 10(9) M(-1) s(-1) and k(T) = 2 × 10(7) M(-1) s(-1), respectively, which are modestly larger than rate constants for reactions of alkyl radicals with the same trapping agents.