The mechanism of the homogeneous, unimolecular gas-phase elimination kinetic of 1,1-dimethoxycyclohexane: experimental and theoretical studies (original) (raw)
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The journal of physical chemistry. A, 2012
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-...
Journal of Molecular Structure-theochem, 2009
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 d+ . . .Cl dÀ , 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.
The Journal of Physical Chemistry, 1979
analogous to those shown in Figure 3 for the decomposition of the hydroxylamines formed in the reactions involving primary and secondary amines. This heuristic model offers explanations for both the unusually great importance of the R-loss route in the 0 + TMA reaction and the fact that the H20 loss route was not observed in the same reaction. The latter route would require the loss of two primary hydrogens in sequential steps, each of which involves a competition with a second pathway which is probably energetically favored. These six studies have begun to reveal details of the mechanism of 0 + amine reactions under essentially collision-free conditions following the formation of an energy-rich adduct. Recognizing that an excited amine N-oxide is the first intermediate in this reaction, we have shown in this study that amine N-oxides with 60-70 kcal/mol of internal energy decompose not only along the path of lowest free-energy increase, but to a very great extent by other routes which have not been observed before. Acknowledgment. The authors gratefully acknowledge the financial support of the National Science Foundation. References and Notes
International Journal of Chemical Kinetics, 2007
The pyrolysis kinetics of 2-chloro-2-methylbutane and 2-chloro-2,3-dimethylbutane have been investigated, in a static system and seasoned vessel, over the pressure range of 50-280 torr and the temperature range of 260-320 "C. The reactions are homogeneous and unimolecular, follow a first-order law, and are invariable to the presence of a cyclohexene inhibitor. The temperature dependence of the rate coefficients is given by the following Arrhenius equations: for 2-chloro-2-methylbutane, log k1 (s-l) = (13.77 f 0.25)-(184.1 f 2.6) kJ-mol-' (2.303R7')-1; for 2-chloro-2,3-dimethylbutane, log k1 (9-I) = (13.33 f 0.18)-(175.3 f 1.9) kJ-mol-' (2.303RT)-'. The distribution of the olefin products from these reactions has been quantitatively determined and reported in details. The alkyl series ((CH3),C, (CH3)&H, CH3CH2, CH3, and H) in the tertiary halides, 2-chloro-2alkylpropanes, influence the rate of elimination by electronic effect. This is similar to those obtained with a-and P-alkyl-substituted ethyl chlorides. The plot of log k/ko vs. "*(R) gives a very good straight line with p* =-4.75, r = 0.994, and intercept = 0.048 at 300 "C. The previous and present results reveal that, if a reaction center at the transition state of an organic molecule is markedly polar, the +I inductive electron release of alkyl substituents may affect gas-phase elimination processes.
The Journal of Organic Chemistry, 1993
Elimination reactions are one of the most studied transformations in all of chemistry. Numerous aspects of the mechanism have been probed, and a tremendous wealth of information has been 0btained.l The selectivity in these processes i s often influenced by solvation, aggregation, and counterion effects. The intrinsic reactivity, therefore, is of special interest. Ab initio molecular orbital calculations and gas-phase ion molecule investigations are noteworthy in this regard. In this paper the first stereochemical information on 1,4-eliminations in the gas phase is presented. Strong bases (amide and hydroxide) are found to be relatively nonselective whereas weaker bases (tert-butoxide and fluoride) display a strong preference for the syn pathway. Elimination reactions, somewhat surprisingly, have only recently been examined with high-level computations,2 but they have been the subject of numerous gas-phase studies.3 Many questions remain unanswered, however, in part because substitutions and eliminations both afford the same ionic products (which are what is detected). One method for overcoming this difficult is to design substrates so that the ions "tell" how they are formed. For example, 1-methoxy-2-cyclohexene (1) reacts with a number of bases (B-) to afford cyclohexadienide (21, methoxide clusters (CHBO-~BH, 3), and free methoxide (4, eq 1).4 The former two species must result from an elimination reaction and cannot be due to substitution. We have previously examined the regiochemistry in this system, 1,2-vs 1,4elimination, by labeling 1 with deuterium at either C4 or C6. Strong bases were found to induce l,4-eliminations, ~~ (1) (a) Gandler,
International Journal of Quantum Chemistry, 2012
The unimolecular gas-phase elimination kinetics of 2-methoxy-1chloroethane, 3-methoxy-1-chloropropane, and 4-methoxyl-1chloroburane has been studied by using density functional theory (DFT) methods to propose the most reasonable mechanisms of decomposition of the aforementioned compounds. Calculation results of 2-methoxy-1-chloroethane and 3-methoxy-1-chloropropane suggest dehydrochlorination through a concerted nonsynchronous four-centered cyclic transition state (TS) to give the corresponding olefin. In the case of 4-methoxyl-1-chloroburane, in addition to the 1,2-elimination mechanism, the anchimeric assistance by the methoxy group, through a polar five-centered cyclic TS, provides additional pathways to give 4-methoxy-butene, tetrahydrofuran and chloromethane. The bond polarization of the CACl, in the direction of C dþ ÁÁÁCl dÀ , is the limiting step of these elimination reactions. The significant increase in rate together with the formation of a cyclic product tetrahydrofuran in the gas-phase elimination of 4-methoxyl-1-chloroburane is attributed to neighboring group participation of the oxygen of the methoxy group in the TS. The theoretical calculations show a good agreement with the reported experimental results.
Journal of Physical Organic Chemistry, 2003
The gas-phase elimination kinetics of 2-substituted ethyl methylcarbonates were determined in a static reaction system over the temperature range of 323–435°C and pressure range 28.5–242 Torr. The reactions are homogeneous, unimolecular and follow a first-order rate law. The kinetic and thermodynamic parameters are reported. The 2-substituents of the ethyl methylcarbonate (CH3OCOOCH2CH2Z, Z=substituent) give an approximate linear correlation when using the Taft–Topsom method, log(kZ/kH)=−(0.57±0.19)σα+(1.34±0.49)σR− (r=0.9256; SD=0.16) at 400°C. This result implies the elimination process to be sensitive to steric factors, while the electronic effect is unimportant. However, the resonance factor has the greatest influence for a favorable abstraction of the β-hydrogen of the Cβ—H bond by the oxygen carbonyl. Because ρα is significant, a good correlation of the alkyl substituents of carbonates with Hancock's steric parameters was obtained: log(kR/kH) versus ESC for CH3OCOOCH2CH2R at 400°C, R=alkyl, δ=−0.17 (r=0.9993, SD=0.01). An approximate straight line was obtained on plotting these data with the reported Hancock's correlation of 2-alkyl ethylacetates. This result leads to evidence for the β-hydrogen abstraction by the oxygen carbonyl and not by the alkoxy oxygen at the opposite side of the carbonate. The carbonate decompostion is best described in terms of a concerted six-membered cyclic transition state type of mechanism. Copyright © 2003 John Wiley & Sons, Ltd.
The Journal of Physical Chemistry, 1986
K gave rise to the following approximate rate constant for the unimolecular decomposition process: k3 = (5.5 f 2.0) X 10" cm3.mol-'.s-' for the unimolecular decomposition of C6Hs0 and the reaction of CH3 with C6H50, respectively. The relatively low values of the A factor and activation energy for the C6H50 decomposition reaction favor the mechanism that involves a bicyclic radical intermediate. Additionally, the modeling of CO yields in the very early stage of anisole decomposition at temperatures below 1200 kl (1.2 f 0.3) X 1OI6 exp(-33 10O/T) s-' The rate constant determined hereon for this important process to be useful for the interpretation of the complex C6H.5 is combustion chemistry.
Journal of Physical …
The gas-phase elimination of kinetics 4-chlorobutan-2-one, 5-chloropentan-2-one, and 4-chloro-1-phenylbutan-1-one has been studied using electronic structure methods: B3LYP/6-31G(d,p), B3LYP/6-31RRG(d,p), MPW91PW91/6-31G(d,p), MPW91PW91/6-31RRG(d,p), PBEPBE/6-31G(d,p), PBEPBE /6-31RRG(d,p), and MP2/6-31RRG(d,p). The abovementioned substrates produce hydrogen chloride and the corresponding unsaturated ketone. Calculation results of 4-chlorobutan-2-one suggest a non-synchronous four-membered cyclic transition state (TS) type of mechanism. However, in the case of 5-chloropentan-2-one and 4-chloro-1-phenylbutan-1-one, the carbonyl group assists anchimerically through a polar five-membered cyclic TS mechanism. The polarization of the C-Cl bond, in the sense of C dR . . .Cl dS , is a rate-determining step in these elimination reactions. The significant increase in rates in the elimination of 5-chloropentan-2-one and 4-chloro-1-phenylbutan-1-one is attributed to neighboring group participation due to the oxygen of the carbonyl group assisting the C-Cl bond polarization in the TS.
Journal of Physical Organic Chemistry, 2004
The gas-phase elimination kinetics of several arylethyl N,N-dimethylcarbamates and o-phenylalkyl N,N-dimethylcarbamates were determined in the temperature range 299.6-399.9°C and pressure range 18-95 Torr. The reactions in a static system, seasoned with allyl bromide, and in the presence of a free radical suppressor are homogeneous and unimolecular and follow a first-order rate law. The rate coefficients are given by the Arrhenius equations: for 4-phenethyl N,N-dimethylcarbamate, log[k 1 (s À1)] = (11.32 AE 0.22) À(166.9 AE 2.5) kJ mol À1 (2.303RT) À1 ; for methylphenethyl N,N-dimethylcarbamate, log[k 1 (s À1)] = (12.07 AE 0.36) À(178.6 AE 4.3) kJ mol À1 (2.303RT) À1 ; for 4-methoxyphenethyl N,N-dimethylcarbamate, log[k 1 (s À1)] = (11.03 AE 0.60) À(167.3 AE 7.1) kJ mol À1 (2.303RT) À1 ; for 4-nitrophenethyl N,N-dimethylcarbamate, log[k 1 (s À1)] = (11.31 AE 0.54) À(163.7 AE 6.1) kJ mol À1 (2.303RT) À1 ; for 3-(4-methoxyphenyl)propyl N,N-dimethylcarbamate, log[k 1 (s À1)] = (13.52 AE 0.54) À(208.4 AE 6.8) kJ mol À1 (2.303 RT) À1 ; for 4-phenyl-1-butyl N,N-dimethylcarbamate, log[k 1 (s À1)] = (12.00 AE 0.34) À(185.2 AE 4.2) kJ mol À1 (2.303 RT) À1 ; and for 5-phenyl-1-pentyl N,N-dimethyl carbamate, log[k 1 (s À1)] = (11.79 AE 0.31) À(182.2 AE 3.9) kJ mol À1 (2.303RT) À1. The results imply the absence of anchimeric assistance of the phenyl group, while the acidity of the benzylic b-hydrogen appears to be responsible for a small but significant rate augmentation in these eliminations.