Primary processes in the 147-nm photolysis of 1,1-dichloroethane

The room temperature photolysis of 1,1-dichloroethane at 147 nm in the pressure range of 1.34-196.2 torr is characterized almost entirely by the molecular elimination of HCl, Cl₂, and small quantities of H₂. Acetylene is also produced. While it is possible that the C₂H₂ arises, in part, from the decomposition of vibrationally excited ground states of C₂H₃Cl and/or C₂H₄, in this particlar case serious consideration has to be given to alternative explanations where the products of the primary processes are formed in electronically excited states. The ±, elimination of molecular chlorine is not inconsistent with an increased degree of Cl? Cl interaction predicted for a «Rydberg «state of 1,1-C₂H₄Cl₂. Varying small yields of CH₄ are observed in the presence and absence of NO. The effect of large pressures of CF₄ on the quantum yields of the major products is extremely small. The extinction coefficient for 1,1-C₂H₄Cl₂ at 147 nm and 296°K is 246 ± 29 cm^?1 ± atm^?1.     

Vacuum ultraviolet (147 nm) photolysis of 1,2-fluorochloroethane

1,2-Fluorochloroethane was photolyzed at 147 nm in the pressure range of 3.8–20.9 torr. The effects of added NO, H₂S, and large pressures of CF₄ were also investigated. The exponential extinction coefficient at 147 nm and 296°K was found to be 147 ± 4 atm^?1 · cm^?1. The photochemistry in some respects is similar to that of ethyl chloride. The primary processes again appear to involve at least two excited states. One of these yields ethylene by FCl elimination (Φ ? 0.3) and has a lifetime of ～3.2 × 10^?10 sec, with respect to an internal conversion to the vibrationally excited ground state or, more probably, a collisionally induced crossover to a state decomposing mainly by carbon? halogen bond fission. The molecular elimination of HCl, H₂, and small amounts of HF also occurs but not apparently from the same state as does FCl. The quantum yields of products with radical precursors, however, are not large, and hence little, if any, of the FCl and probably none of HCl, H₂, and HF subsequently dissociates. The vinyl fluoride and vinyl chloride, accompanying the elimination of HF and HCl, are postulated as possible sources of the secondary production of acetylene.     

The mechanisms of the homogeneus,unimolecular elimination kinetics of ethyl 4-chlorobutyrate and 4-chlorobutyric acid in the gas phase

Ethyl 4-chlorobutyrate, which is reexamined, pyrolyzes at 350–410°C to ethylene, butyrolactone, and HCl. Under the reaction conditions, the primary product 4-chlorobutyric acid is responsible for the formation of γ-butyrolactone and HCl. In seasoned vessels, and in the presence of a free-radical inhibitor, the ester elimination is homogeneous, unimolecular, and follows a first-order rate law. For initial pressures from 69–147 Torr, the rate is given by the following Arrhenius expression: log k₁(s^?1) = (12.21 ± 0.26) ? (197.6 ± 3.3) kJ mol^?1 (2.303RT)^?1. The rates and product formation differ from the previous work on the chloroester pyrolysis. 4-Chlorobutyric acid, an intermediate product of the above substrate, was also pyrolyzed at 279–330°C with initial pressure within the range of 78–187 Torr. This reaction, which yields γ-butyrolactone and HCl, is also homogeneous, unimolecular, and obeys a first-order rate law. The rate coefficient, is given by the following Arrhenius equation: log k₁(s^?1) = (12.28 ± 0.41) ? (172.0 ± 4.6) kJ mol^?1 (2.303RT)^?1. The pyrolysis of ethyl chlorobutyrate proceeds by the normal mechanism of ester elimination. However, the intermediate 4-chlorobutyric acid was found to yield butyrolactone through anchimeric assistance of the COOH group and by an intimate ion pair-type of mechanism. Additional evidence of cyclic product and neighboring group participation is described and presented.     

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.     

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     

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.     

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.     

Vapor pressure measurements of ferrocene,mono- and 1,1′-di-acetyl ferrocene

By using different techniques the vapor pressure of ferrocene, mono-acetyl ferrocene and 1,1′-di-acetyl ferrocene was measured. The following pressure—temperature equations were derived ferrocene log P(kPa)= 9.78 ± 0.14 ? (3805 ± 46)/T mono-acetyl ferrocene log P(kPa) = 14.83 ± 0.14 ? (5916 ± 48)/T 1,1′-di-acetyl ferrocene log P(kPa) = 8.82 ± 0.11 ? (4289 ± 44)/T By second- and third-law treatment of the vapor data the ΔH⁰_sub,298 = 74.0 ± 2.0 kJ mole^?1 for the sublimation process of ferrocene was calculated and compared with the literature data. For the sublimation enthalpy of mono- and 1,1′-di-acetyl ferrocene the values ΔH⁰_sub,298 = 115.6 ± 2.5 kJ mole^?1 and ΔH⁰_sub,298 = 91.9 ± 2.5 kJ mole^?1 were derived by second-law treatment. Thermal functions of these compounds were also estimated.     

Experimental and theoretical study of the homogeneous,unimolecular gas‐phase elimination kinetics of 2‐furoic acid

The kinetics of the gas‐phase elimination kinetics of CO₂ from furoic acid was determined in a static system over the temperature range 415–455°C and pressure range 20–50 Torr. The products are furan and carbon dioxide. The reaction, which is carried out in vessels seasoned with allyl bromide and in the presence of the free‐radical suppressor toluene and/or propene, is homogeneous, unimolecular, and follows a first‐order rate law. The observed rate coefficient is expressed by the following Arrhenius equation: log k₁(s^?1) = (13.28 ± 0.16) ? (220.5 ± 2.1) kJ mol^?1 (2.303 RT)^?1. Theoretical studies carried out at the B3LYP/6‐31++G** computational level suggest two possible mechanisms according to the kinetics and thermodynamic parameters calculated compared with experimental values. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 298–306, 2007     

The elimination kinetics of secondary Alkyl Bromides in the gas phase

Elimination kinetics of 2-bromohexane and 2-bromo-4-methylpentane in the gas phase were examined over the temperature range of 310–360°C and pressure range of 46–213 torr. The reactionsin seasoned, static reaction vessels, and in the presence of the free radical inhibitor cyclohexene, are homogeneous, unimolecular, and follow first order rate laws. The overall rate coefficients are described by the following Arrhenius equations: For 2-bromohexane, log??₁(s^?1) = (13.08 ± 0.70) ? (185.7 ± 8.2) kJ mol^?1 (2.303RT)^?1; for 2-bromo-4-methylpentane, log??₁(s^?1) = (13.08 ± 0.33) ? (183.4 ± 3.8) kJ mol^?1 (2.303RT)^?1. The electron releasing effect of alkyl groups influences the overall elimination rates. The olefin products isomerize in the presence of HBr gas until an equilibrium mixture is reached.     

Steric influence in the direction of elimination of gas-phase pyrolyses of several highly branched secondary acetates

The rate coefficients for the gas-phase pyrolyses of a series of structurally related secondary acetates have been measured in a static system over the temperature range of 289.1–359.5°C and the pressure range 50.0–203.0 torr. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for 3-hexyl acetate, log k₁ (s^?) = (12.12 ± 0.33) ? (176.1 ± 3.9)kJ/mol/2.203RT; for 5-methyl-3-hexyl acetate, log k₁ (s^?) = (13.17 ± 0.20) ? (186.2 ± 2.3)kJ/mol/2.303RT; and for 5,5-dimethyl-3-hexyl acetate, log k₁ (s^?) = (12.70 ± 0.19) ? (177.4 ± 2.2)kJ/mol/2.303RT. The direction of elimination of these esters has shown from the invariability of olefin distributions at different temperatures and percentages of decomposition that steric hindrance is a determining factor in the eclipsed cis conformation. Moreover, a more detailed analysis indicates that the greater the alkyl–alkyl interaction, the less favored the elimination tends to be. Otherwise, an increase of alkyl–hydrogen interaction caused steric acceleration to be the determining factor.     

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.     

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.     

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     

Kinetics of thermal gas‐phase decomposition of 2‐bromopropene using static system

The gas phase elimination kinetics of 2‐bromopropene was studied over the temperature range of 571–654 K and pressure range of 12–46 Torr using the seasoned static reaction system. Propyne was the only olefinic product formed and accounted for >98% of the reaction. This product was formed by homogeneous, unimolecular pathways with high‐pressure first‐order rate constant k_∞ given by the equation k_∞ = 10^{13.47 ± 0.6} exp^{?208.2 ± 6.7} (kJ mol^?1)/RT. The error limits are 95% certainty limits. The observed Arrhenius parameters are consistent with the four centered activated complex. The presence of methyl group on α‐carbon lowers the activation energy by 41 kJ mol^?1. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 1–5, 2007     

The mechanism of the elimination kinetics of 2-bromo-2-butene in the gas phase

The kinetics of the gas phase elimination of 2-bromo-2-butene were determined in a static system over the temperature range of 340–380°C and pressure range of 37–134 torr. The reaction in seasoned vessels, even in the presence of a free radical inhibitor, is catalyzed by hydrogen bromide. Under maximum catalysis of HBr, the kinetics were found to be of order 1.0. The reaction, when maximally catalyzed with HBr, appears to undergo a molecular elimination of HBr which follows first-order kinetics. The products are 1,2-butadiene and hydrogen bromide. The rate coefficients. under maximum catalysis, are given by the Arrhenius equation log ??₁(s^?1) = (13.57 ± 0.56) ? (200.4 ± 6.8) kJ mol^?1 (2.303RT)^?1. The catalyzed pyrolysis of 2-bromo-2-butene appears to proceed through a six-membered cyclic transition-state type of mechanism.     

The mechanism in the elimination kinetics of 2-chloropropionic acid in the gas phase

The elimination kinetics of 2-chloropropionic acid have been studied over the temperature range of 320–370.2°C and pressure range of 79–218.5 torr. The reaction in seasoned vessel and in the presence of the free radical suppressor cyclohexene, is homogeneous, unimolecular, and obeys a first-order rate law. The dehydrochlorination products are acetaldehyde and carbon monoxide. The rate coefficient is expressed by the following Arrhenius equation: log k₁(s^?1) = (12.53 ± 0.43) – (186.9 ± 5.1) kJ mol^?1 (2.303RT)^?1. The hydrogen atom of the carboxylic COOH appears to assist readily the leaving chloride ion in the transition state, suggesting an intimate ion pair mechanism operating in this reaction.     

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 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     

The gas-phase elimination kinetics of 2-hydroxy-2-methylbutyric acid and 2-ethyl-2-hydroxybutyric acid

The elimination kinetics of the title compounds have been examined over the temperature range of 270–320°C and pressure range of 19–117 torr. The reactions, carried out in seasoned vessels, with the free-radical suppressor toluene always present, are homogeneous, unimolecular, and follow a first-order rate law. The products of 2-hydroxy-2-methylbutyric acid are 2-butanone, CO, and H₂O; while of 2-ethyl-2-hydroxybutyric acid are 3-pentanone, CO, and H₂O. The rate coefficient is expressed by the following Arrhenius equation: for 2-hydroxy-2-methylbutyric acid, log k₁(s^?1 = (12.87 ± 0.19) ? (171.2 ± 2.1) kJ mol^?1 (2.303 RT)^?1; and for 2-ethyl 2-hydroxybutyric acid, log k₁s^?1) = (12.13 ± 0.34) ? (159.4 ± 3.7) kJ mol^?1 (2.303 RT)^?1. Augmentation of alkyl bulkiness at the 2-position of the 2-hydroxycarboxylic acids showed an increase in the rate of dehydration. The electron release of alkyl groups, rather than steric acceleration, appears to enhance the pyrolysis decomposition of these substrates. These reactions are believed to proceed through a semi-polar five-membered cyclic transition type of mechanism. © 1995 John Wiley & Sons, Inc.