Transition State Type Of Mechanism Research Articles

The mechanism of the gas-phase elimination kinetics of the β,γ-unsaturated aldehyde 2,2–dimethyl-3-butenal: a theoretical study

ABSTRACTThe study on the mechanism of the gas-phase elimination or thermal decomposition kinetics of 2, 2-dimethyl-3-butenal has been carried out by using theoretical calculation at MP2, combined ab initio CBSQB3 and DFT (B3LYP, B3PW91, MPW1PW91, PBEPBE, PBE1PBE, CAMB3LYP, M06, B97d) levels of theory. A good reasonable agreement between experimental and calculated parameters was obtained by using CAMB3LYP/6-311G(d,pd) calculations. The contrasted calculated parameters against experimental values suggested decarbonylation reaction to proceed through a concerted five-membered cyclic transition state type of mechanism, involving the hydrogen transfer from the carbonyl carbon to the gamma carbon, consistent with observed kinetic isotope effect. The breaking of alpha carbon–carbonyl carbon bond to produce carbon monoxide is 50% advanced in the transition state. The reaction mechanism may be described as a concerted moderately non-synchronous process. Examination of the Atoms in Molecules (AIM) analysis of electron density supports the suggested mechanism.

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 k1 (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.

Kinetic and mechanism of the gas-phase pyrolysis of exo-2-norbornyl formate

The rate coefficient of the gas-phase pyrolysis or elimination of exo-2-norbornyl formate was determined in a deactivated, static reaction vessel over the temperature range 300–350°C and pressure range 41–105Torr. The substrate exo-2-norbornyl formate yielded surprisingly exo-2-norborneol and CO gas. This reaction, in the presence or absence of the free radical inhibitor toluene, proved to be homogeneous, unimolecular and follows a first-order rate law. The rate of equation is given by the following Arrhenius equation: log k1 (s−1)=(13.85±0.23)−(194.6±2.7)kJmol−1(2.303RT)−1, r=0.9998.Experimental data suggested a semi-polar concerted three-membered cyclic transition state type of mechanism.

Homogeneous and unimolecular gas‐phase thermal decomposition kinetics of methyl benzoylformate: experimental and theoretical study

The kinetics of the gas‐phase thermal decomposition of the α‐ketoester methyl benzoylformate was carried out in a static system with reaction vessel deactivated with allyl bromide, and in the presence of the free radical inhibitor propene. The rate coefficients were determined over the temperature range of 440–481 °C and pressures from 32 to 80 Torr. The reaction was found to be homogenous, unimolecular and obey a first‐order rate law. The products are methyl benzoate and CO. The temperature dependence of the rate coefficient gives the following Arrhenius parameters: log10 k (s−1) = 13.56 ± 0.31 and Ea (kJ mol−1) = 232.6 ± 4.4. Theoretical calculations of the kinetic and thermodynamic parameters are in good agreement with the experimental values using PBE1PBE/6‐311++g(d,p). A theoretical Arrhenius plot was constructed at this level of theory, and the good agreement with the experimental Arrhenius plot suggests that this model of transition state may describe reasonably the elimination process. These results suggest a concerted non‐synchronous semi‐polar three‐membered cyclic transition state type of mechanism. The most advanced coordinate is the bond breaking Cδ+‐‐‐δ‐OCH3 with an evolution of 66.7%, implying this as the limiting factor of the elimination process. Copyright © 2014 John Wiley & Sons, Ltd.

Kinetics and mechanisms of gas‐phase decarbonylation of α‐methyl‐trans‐cinamaldehyde and E‐2‐methyl‐2‐pentenal under homogeneous catalysis of hydrogen chloride

The kinetics of the gas‐phase elimination of α‐methyl‐trans‐cinamaldehyde catalyzed by HCl in the temperature range of 399.0–438.7 °C, and the pressure range of 38–165 Torr is a homogeneous, molecular, pseudo first‐order process and undergoing a parallel reaction to produce via (A) α‐methylstyrene and CO gas and via (B) β‐methylstyrene and CO gas. The decomposition of substrate E‐2‐methyl‐2‐pentenal was performed in the temperature range of 370.0–410.0 °C and the pressure range of 44–150 Torr also undergoing a molecular, pseudo first‐order reaction gives E‐2‐pentene and CO gas. These reactions were carried out in a static system seasoned reactions vessels and in the presence of toluene free radical inhibitor. The rate coefficients are given by the following Arrhenius expressions:Products formation from α‐methyl‐trans‐cinamaldehydeα‐methylstyrene:β‐methylstyrene:Products formation from E‐2‐methyl‐2‐pentenalE‐2‐pentene:The kinetic and thermodynamic parameters for the thermal decomposition of α‐methyl‐trans‐cinamaldehyde suggest that via (A) proceeds through a bicyclic transition state type of mechanism to yield α‐methylstyrene and carbon monoxide, whereas via (B) through a five‐membered cyclic transition state to give β‐methylstyrene and carbon monoxide. However, the elimination of E‐2‐methyl‐2‐pentenal occurs by way of a concerted cyclic five‐membered transition state mechanism producing E‐2‐pentene and carbon monoxide. The present results support that uncatalyzed α‐β‐unsaturated aldehydes decarbonylate through a three‐membered cyclic transition state type of mechanism. Copyright © 2014 John Wiley & Sons, Ltd.

Isomerisation Reactions of α-Methyl Allyl [Acetate, Trifluoroacetate]: Theoretical Study

The [3,3]-sigmatropic rearrangement reactions of α-methyl allyl acetate (1), and α-methyl allyl trifluoroacetate (2) have been studied using ab initio molecular orbital (MO) methods at the HF, B3LYP, B3PW91, and BHandHLYP levels with a 6-31++G** basis set. Among the methods used in this study, the values for activation parameters obtained with the BHandHLYP/6-31++G** method are in good agreement with experimental values. The calculated data demonstrate that in the rearrangement reactions of the compounds studied, the polarisation of the O3–C4 bond is rate determining. Optimised transition states at the BHandHLYP/6-31++G** level were used for calculating the nucleus-independent chemical shift (NICS) and also a natural bond orbital (NBO) analysis was carried out at the same level. Based on the optimised ground state geometries using the BHandHLYP/6-31++G** method, the NBO analysis of donor–acceptor (bonding–antibonding) interactions revealed that the stabilisation energies associated with electronic delocalisation from the σO(3)–C(4) bonding orbital to the π*C(5)=C(6) antibonding orbital, increase from compounds 1 to 2. The σO(3)–C(4) →π*C(5)=C(6) resonance energies for compounds 1 and 2 are 1.59 and 1.62 kcal mol−1, respectively. Also, analysis of the donor–acceptor interactions σO(3)–C(4) →π*C(5)=C(6) suggests that in these rearrangements, the TS structure in compound 2 has more aromatic character than for compound 1. The NBO results revealed that in these rearrangements, activation energies are not controlled by the σO(3)–C(4) →π*C(5)=C(6) resonance energy. NICS results indicate that the aromaticities of the transition states are controlled by the out-of-plane component. The predicted high-pressure-limit rate constants for the rearrangement reactions of compounds 1 and 2 are represented as 6.12 × 1012 exp(–21604/T), and 1.04 × 1013 exp(–17846/T) s−1, respectively. The fall-off pressures for the rearrangement reactions 1 and 2 are found to be 9.56 × 10−5 and 1.09 × 10−3 mmHg, respectively. The fall-off pressures of compounds 1 and 2 in both reactions are in the following order: P1/2(2) > P1/2(1) and the rates follow as: rate(2) > rate(1). Analysis of bond order, NBO charges, bond indices and synchronicity parameters suggest the Claisen rearrangement reactions of 1 and 2 occur through a concerted and slightly asynchronous six-membered cyclic transition state type of mechanism.

Kinetics and Mechanisms of the Thermal Decomposition of 2-Methyl-1,3-dioxolane, 2,2-Dimethyl-1,3-dioxolane, and Cyclopentanone Ethylene Ketal in the Gas Phase. Combined Experimental and DFT Study

The kinetics of the gas-phase thermal decomposition of 2-methyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, and cyclopentanone ethylene ketal were determined in a static system and the reaction vessel deactivated with allyl bromide. The decomposition reactions, in the presence of the free radical suppressor propene, are homogeneous, are unimolecular, and follow first-order law kinetics. The products of these reactions are acetaldehyde and the corresponding ketone. The working temperature range was 459-490 °C, and the pressure range was 46-113 Torr. The rate coefficients are given by the following Arrhenius equations: for 2-methyl-1,3-dioxolane, log k = (13.61 ± 0.12) - (242.1 ± 1.0)(2.303RT)(-1), r = 0.9997; for 2,2-dimethyl-1,3-dioxolane, log k = (14.16 ± 0.14) - (253.7 ± 2.0)(2.303RT)(-1), r = 0.9998; for cyclopentanone ethylene ketal, log k = (14.16 ± 0.14) - (253.7 ± 2.0)(2.303RT)(-1), r = 0.9998. Electronic structure calculations using DFT methods B3LYP and MPW1PW91 with 6-31G(d,p), and 6-31++G(d,p) basis sets suggest that the decomposition of these substrates takes place through a stepwise mechanism. The rate-determining step proceeds through a concerted nonsynchronous four-centered cyclic transition state, and the elongation of the C-OCH(3) bond in the direction C(α)(δ+)...OCH(3)(δ-) is predominant. The intermediate products of these decompositions are unstable, at the working temperatures, decomposing rapidly through a concerted cyclic six-centered cyclic transition state type of mechanism.

Ab Initio Study and Nbo Analysis of the Unimolecular Decomposition Kinetics of 2,2-Dimethyloxetane

Theoretical study of the thermal decomposition kinetics of 2,2-dimethyloxetane has been carried out at the B3LYP/6-311 + G**, B3PW91/6-311 + G**, and MPW1PW91/6-311 + G** levels. The MPW1PW91/6-311 + G** method was found to give a reasonably good agreement with the experimental kinetics and thermodynamic parameters. Two parallel unimolecular reactions occur to give either isobutene and formaldehyde [reaction (1)] or ethene and acetone [reaction (2)]. The calculated data demonstrate that the polarization of the O1-C2 bond in reaction (1) and C4-O1 bond in reaction (2), are rate determining. The analysis of bond order and NBO charges, bond indices, synchronicity parameters, and IRC calculations suggest reactions (1) and (2) occur through a concerted and asynchronous four-membered cyclic transition state type of mechanism.

DFT Calculations of the Elimination Kinetics of Silacyclobutanes and its Methyl Derivatives in the Gas-Phase

The gas-phase thermal decomposition kinetics of silacyclobutane (1), 1-methyl- silacyclobutane (2), and 1,1-dimethyl-1-silacyclobutane (3) has been theoretically studied at the B3LYP/6-311G**, B3PW91/6-311G**, and MPW1PW91/6-311G** levels. The B3LYP/6-311G** method was found to give a reasonable good agreement with the experimental kinetics and thermodynamic parameters. The decomposition reaction of compounds 1–3 yields ethylene and the corresponding silene. Based on the optimized ground state geometries using B3LYP/6-311G** method, the natural bond orbital (NBO) analysis of donor-acceptor (bonding–antibonding) interactions revealed that the perturbation energies (E2) associated with the electronic delocalization from σSi1–C2 to σ*C4–Si1 orbitals increase from compounds 1 to 3. The σSi1–C2→σ*C4–Si1 resonance energies for compounds 1–3 are 1.17, 1.26, and 1.43 kcal/mol, respectively. Also, the decomposition process in these compounds is controlled by σ→σ* resonance energies. Moreover, the obtained order of energy barriers could be explained by the number of electron-releasing methyl groups substituted to the Sisp2 atom. NBO analysis shows that the occupancies of σSi1–C2 bonds decrease for compounds 1–3 as 3 < 2 < 1, and the occupancies of σ*Si1–C2 bonds increase in the opposite order (3 > 2 > 1). Moreover, these results can fairly explain the decrease of the energy barriers (ΔEo) of the decomposition reaction of compounds 1 to 3. The calculated data demonstrate that in the decomposition process of the studied compounds, the polarization of the C3–C4 bond is the rate determining factor. Analysis of bond orders, NBO charges, bond indexes, synchronicity parameters, and IRC calculations indicate that these reactions are occurring through a concerted and asynchronous four-membered cyclic transition state type of mechanism.

Kinetics and Mechanisms of the Unimolecular Elimination of 2,2-Diethoxypropane and 1,1-Diethoxycyclohexane in the Gas Phase: Experimental and Theoretical Study

The gas-phase thermal elimination of 2,2-diethoxypropane was found to give ethanol, acetone, and ethylene, while 1,1-diethoxycyclohexane yielded 1-ethoxycyclohexene and ethanol. The kinetics determinations were carried out, with the reaction vessels deactivated with allyl bromide, and the presence of the free radical suppressor cyclohexene and toluene. Temperature and pressure ranges were 240.1-358.3 °C and 38-102 Torr. The elimination reactions are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are given by the following Arrhenius equations: for 2,2-diethoxypropane, log k(1) (s(-1)) = (13.04 ± 0.07) - (186.6 ± 0.8) kJ mol(-1) (2.303RT)(-1); for the intermediate 2-ethoxypropene, log k(1) (s(-1)) = (13.36 ± 0.33) - (188.8 ± 3.4) kJ mol(-1) (2.303RT)(-1); and for 1,1-diethoxycyclohexane, log k = (14.02 ± 0.11) - (176.6 ± 1.1) kJ mol(-1) (2.303RT)(-1). Theoretical calculations of these reactions using DFT methods B3LYP, MPW1PW91, and PBEPBE, with 6-31G(d,p) and 6-31++G(d,p) basis set, demonstrated that the elimination of 2,2-diethoxypropane and 1,1-diethoxycyclohexane proceeds through a concerted nonsynchronous four-membered cyclic transition state type of mechanism. The rate-determining factor in these reactions is the elongation of the C-O bond. The intermediate product of 2,2-diethoxypropane elimination, that is, 2-ethoxypropene, further decomposes through a concerted cyclic six-membered cyclic transition state mechanism.

Kinetic Study and NBO Analysis of the Dehydrogenation Mechanism of Five‐membered Ring Heterocyclic 2,5‐Dihydro‐[furan, thiophene, selenophene

AbstractThe theoretical study of the dehydrogenation of 2,5‐dihydro‐[furan (1), thiophene (2), and selenophene (3)] was carried out using ab initio molecular orbital (MO) and density functional theory (DFT) methods at the B3LYP/6‐311G**//B3LYP/6‐311G** and MP2/6‐311G**//B3LYP/6‐311G** levels of theory. Among the used methods in this study, the obtained results show that B3LYP/6‐311G** method is in good agreement with the available experimental values. Based on the optimized ground state geometries using B3LYP/6‐311G** method, the natural bond orbital (NBO) analysis of donor‐acceptor (bond‐antibond) interactions revealed that the stabilization energies associated with the electronic delocalization from non‐bonding lone‐pair orbitals [LP(e)X3] to δ*C(1)H(2) antibonding orbital, decrease from compounds 1 to 3. The LP(e)X3→δ*C(1)H(2) resonance energies for compounds 1–3 are 23.37, 16.05 and 12.46 kJ/mol, respectively. Also, the LP(e)X3→δ*C(1)H(2) delocalizations could fairly explain the decrease of occupancies of LP(e)X3 non‐bonding orbitals in ring of compounds 1–3 (3&gt;2&gt;1). The electronic delocalization from LP(e)X3 non‐bonding orbitals to δ*C(1)H(2) antibonding orbital increases the ground state structure stability, Therefore, the decrease of LP(e)X3→δ*C(1)H(2) delocalizations could fairly explain the kinetic of the dehydrogenation reactions of compounds 1–3 (k1&gt;k2&gt;k3). Also, the donor‐acceptor interactions, as obtained from NBO analysis, revealed that the (C(4)C(7)→δ*C(1)H(2) resonance energies decrease from compounds 1 to 3. Further, the results showed that the energy gaps between (C(4)C(7) bonding and δ*C(1)H(2) antibonding orbitals decrease from compounds 1 to 3. The results suggest also that in compounds 1–3, the hydrogen eliminations are controlled by LP(e)→δ* resonance energies. Analysis of bond order, natural bond orbital charges, bond indexes, synchronicity parameters, and IRC calculations indicate that these reactions are occurring through a concerted and synchronous six‐membered cyclic transition state type of mechanism.

Theoretical calculations on the mechanisms of the gas phase elimination kinetics of chlorocyclohexane, 3-chlorocyclohexene and 4-chlorocyclohexene

The kinetics of the thermal decomposition of the title compounds in the gas phase have been studied at the B3LYP/6-31G(d,p), B3LYP/6-31++G(d,p), MPW91PW91/6-31G(d,p), MPW91PW91/6-31++G(d,p), PBEPBE/6-31G(d,p), and PBEPBE/6-31++G(d,p) levels of theory. These halide substrates produce the corresponding cyclohexadiene and hydrogen chloride. The DFT calculations suggest a non-synchronous four-membered cyclic transition state type of mechanism. The elongation and subsequent polarization of the C–Cl bond, in the direction of C δ+…Cl δ−, is rate determining step in these elimination reactions. Differences in reactivity in these substrates are discussed in terms of the transition state structure and electron distribution.

Theoretical study of neighboring group participation of methyl ω‐chloroesters elimination kinetics in the gas phase

AbstractThe mechanisms of the homogeneous, unimolecular, gas‐phase elimination kinetics of several methyl ω‐chloroesters were examined by using the ‘ab initio’ and DFT level of theories. Theoretical calculations of dehydrochlorination of methyl 3‐chloropropionate suggest a planar concerted, non‐synchronous, four‐membered cyclic transition state to give methyl acrylate. However, the parallel competitive gas‐phase elimination of methyl 4‐chlorobutyrate and methyl 5‐chlrovalerate occurs through neighboring group participation to render methyl chloride and the corresponding lactone through a concerted, semi‐polar five‐ and six‐membered cyclic transition state type of mechanism. Calculated thermodynamic and kinetic parameters reasonably agree with the experimental values at DFT B3LYP/6‐31G* theory level. Geometrical parameters, NBO charges and bond indexes showed strong polarization at Cδ+···Clδ− bond in the transition state suggesting the breaking of CCl bond as rate‐determining factor for both dehydochlorination and lactone formation reactions. The synchronicity parameters suggest a concerted polar mechanism implying a TS which has ion‐pair character for lactone product formation. Copyright © 2008 John Wiley &amp; Sons, Ltd.

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.

MP2 study of the gas phase elimination mechanism of some neutral amino acids and their ethyl esters

AbstractThe mechanisms of the gas phase elimination of N,N‐dimethylglycine, picolinic acid, and N‐phenylglycine and their ethyl esters have been examined at Möller–Plesset MP2/6‐31G (d, p) level of theory. The ethyl esters of these 2‐amino carboxylic acids produce the corresponding amino carboxylic acid and ethylene in a rate‐determining step. However, the unstable intermediate amino carboxylic acid rapidly decarboxylate to give the corresponding amino compound. These calculations imply a concerted, semi‐polar six‐membered cyclic transition state type of mechanism for the ethyl esters, and a non‐synchronous five‐membered cyclic transition state for the amino acids decarboxylation. The present results support previous mechanistic consideration of the elimination of the above‐mentioned compounds in the gas phase. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Theoretical calculations of the thermal decomposition kinetics of several tert-nitroalkanes in the gas phase

The theoretical study of the gas-phase pyrolysis kinetics of several tert-nitroalkanes, 2-methyl-2-nitropropane, 2-methyl-2-nitrobutane, and 2, 3-dimethyl-2-nitrobutane, has been carried out at the MP2/6-31G(d), B3LYP/6-31G(d), B3PW91/6-31G(d), levels of theory. The nitroalkanes yield the corresponding alkene and HNO 2 gas in a rate determining step. The B3PW91/6-31G(d) method was found to give a reasonable good agreement with the experimental kinetic and thermodynamic parameters. The elimination of these reactions suggest a concerted non-synchronous five-membered cyclic transition state type of mechanism.

Theoretical studies on the gas-phase elimination kinetics of 2-arylethyl N, N-dimethylcarbamates [(CH 3) 2NCOOCH 2CH 2Z, Z = 4-CH 3C 6H 4, 4-CH 3OC 6H 4, 4-NO 2C 6H 4

The gas-phase elimination kinetics of 2-arylethyl N, N-dimethylcarbamates, (CH 3) 2NCOOCH 2CH 2Ar, has been studied at the ab initio MP2/6-31G and MPW1PW91/6-31G(d,p) levels of theory. These carbamates produce N, N-dimethylcarbamic acid and the corresponding 4-substituted styrene in a rate determining step. The unstable intermediate N, N-dimethylcarbamic acid rapidly decarboxylate through a four-membered cyclic transition state to give dimethylamine . These calculations imply a concerted, non-synchronous six-membered cyclic transition state type of mechanism. The present results support the fact that the acidity of the benzylic hydrogen as an important factor for faster elimination rates.

The unimolecular elimination kinetics of benzaldoxime in the gas phase

AbstractThe kinetics of the gas‐phase elimination of benzaldoxime was determined in a static reaction system over the temperature and pressure range 350°C–400°C and 56–140 Torr, respectively. The products obtained were benzonitrile and water. The reaction was found to be homogeneous, unimolecular, and tend to obey a first‐order rate law. The observed rate coefficient is represented by the following Arrhenius equation: equation image According to kinetic and thermodynamic parameters, the reaction proceeds through a concerted, semi‐polar, four‐membered cyclic transition state type of mechanism. © 2007 Wiley Periodicals, Inc. 39: 145–147, 2007

Catalysis by hydrogen chloride in the gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐buten‐3‐ol

AbstractA homogeneous, molecular, gas‐phase elimination kinetics of 2‐phenyl‐2‐propanol and 3‐methyl‐1‐ buten‐3‐ol catalyzed by hydrogen chloride in the temperature range 325–386 °C and pressure range 34–149 torr are described. The rate coefficients are given by the following Arrhenius equations: for 2‐phenyl‐2‐propanol log k1 (s−1) = (11.01 ± 0.31) − (109.5 ± 2.8) kJ mol−1 (2.303 RT)−1 and for 3‐methyl‐1‐buten‐3‐ol log k1 (s−1) = (11.50 ± 0.18) − (116.5 ± 1.4) kJ mol−1 (2.303 RT)−1. Electron delocalization of the CH2CH and C6H5 appears to be an important effect in the rate enhancement of acid catalyzed tertiary alcohols in the gas phase. A concerted six‐member cyclic transition state type of mechanism appears to be, as described before, a rational interpretation for the dehydration process of these substrates. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Theoretical study of the gas phase unimolecular elimination kinetics of 2-substituted electron-withdrawing groups of ethyl N, N-dimethylcarbamates

The gas-phase elimination of ethyl N, N-dimethylcarbamates substituted with electron-withdrawing groups at the 2-position of the ethyl were studied by using the Møller–Plesset MP2/6-31G method. Calculations suggest a concerted non-synchronous six-membered cyclic transition state type of mechanism of the above-mentioned carbamates. These substrates undergo thermal elimination to produce the N, N-dimethylcarbamic acid and the corresponding substituted olefin in the rate-determining step. The unstable intermediate, N, N-dimethylcarbamic acid rapidly decomposes through a four-membered cyclic transition state to dimethylamine and CO 2 gas. The logarithm of relative theoretical rate coefficients when plotted against original Taft's σ* values gave an approximate straight line ( ρ*=−0.137±0.023, r=0.9610 at 360 °C). In addition to this fact, plotting experimental log k rel. against the theoretical log k rel. for 2-substituted ethyl N, N-dimethylcarbamates an approximate straight line ( r=0.9578 at 360 °C) is obtained, which means similar mechanism.