Gas-phase elimination kinetics

Phenoxypropanoic acid, 3-(phenylthio)propanoic acid, 4-phenylbutanoic acid and the corresponding ethyl and methyl esters have been pyrolysed between 520 and 682 K.10 Analysis of the pyrolysates showed the elimination products to be acrylic acid and the corresponding arene. The thermal gas-phase elimination kinetics and product analysis have been found compatible with a thermal retro-Michael reaction pathway involving a four-membered cyclic transition state. 

The gas-phase elimination kinetics of ethyl oxamate, ethyl N,N-dimethyloxamate and ethyl oxanilate have been determined in a static reactor system, seasoned with allyl bromide and in the presence of a free radical inhibitor.12 These reactions are homogeneous, unimolecular and appear to proceed through moderately polar cyclic transition states. 

According to previous work166 and the gas-phase data for ethyl 4-bromobutyrate13, a reexamination of the gas-phase elimination kinetics of the latter was undertaken168,169. 

The gas-phase elimination kinetics of several ethyl esters of 2-oxocarboxylic acid have been found to be homogeneous, unimolecular, and follow a first-order rate law. ° Ethyl oxalyl chloride undergoes only decarboxylation, while both ethyl piperidinegly-oxilate and ethyl benzoyl formate exhibit parallel decarboxylation and decarbonylation reactions. The mechanisms of these decomposition reactions were described in terms of concerted discrete polar cyclic TS structures. 

The interconversion and gas-phase elimination kinetics of l-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-l-ene have been examined at MP2 and DFT levels... 

Gas-phase elimination kinetics of alkyl A,iV-diethylcarbamates (Et2NCOOR) are affected markedly by branching of R and fall in the rate order t-Bu > i-Pr > Et the effects of heteroatom substituents Z on pyrolysis of ZCOOEt have been correlated using the Taft-Topsom equation, log(i /i H) = —(0.68 0.12)cTa -l- (2.57 0.12)ctf — (1.18 0.27)ctr, which reveals that the field effect outweighs steric and resonance factors. ... 

The gas-phase elimination kinetics of 2-chloroethylsilane and derivatives has been studied at the ab initio B3LYP/6-311-K, B3PW91/6-31 1-Kj, and MPW1PW91/6-311-l-G levels of theory " the calculated data demonstrate that polarization of the C(1)-C1(3) bond is a determinating factor. 

The unimolecular gas-phase elimination kinetics of 2-methoxy-l-chloroethane, 3-methoxy-l-chloropropane, and 4-methoxy-l-chlorobutane has been studied using density functional theory (DFT) methods. Results calculated for 2-methoxy-l-chloroethane and 3-methoxy-l-chloropropane suggest that the corresponding olefin forms by dehydrochlorination through a concerted nonsynchronous four-centered cyclic transition state. In the case of 4-methoxy-l-chlorobutane, in addition to the 1,2-elimination mechanism, anchimeric assistance by the methoxy group, through a polar five-centered cyclic transition state, provides 4-methoxybutene, tetrahydrofuran, and chloromethane. Polarization of the C-Cl bond is rate limiting in these elimination reactions. 

The kinetics of concerted thermal elimination reactions of a series of ethyl (hetero) arylcarboxylate esters (2-thienyl-, 3-thienyl-, 2-furyl, 3-furyl, 4-pyridyl-, 3-pyridyl-, and 2 -pyridylcarbo x y I ate) in the gas phase seem to indicate that there is tittle charge separation in the transition state (83) this is in contrast with the behaviour of the corresponding /-butyl and isopropyl esters for which a semi-concerted transition state (82) was proposed previously.49 Results of a kinetic study of the gas-phase elimination reactions of methylbenzoyl fonnate (84) and 3-hydroxy-3-methylbutan-2-one (85) have been compared with those for pyruvic acid (87) and benzoylformic acid (86).50 The relative rates of reaction [(86)/(87) 46, (87)/(85) = 1.1 x 105 and (86)/(82) = 1 x 106] reveal that the acidity of the hydrogen atom involved in the elimination process, rather than the initial polarization of the C—C bond which undergoes cleavage, is the important rate-controlling factor. 

Kinetics of the gas-phase elimination of 2-hydroxynitroalkanes have been investigated at the MP2/6-31G level of theory.31 The thermal elimination of 2-hydroxynitroalkanes occurs in a retro-aldol type of mechanism with a six-membered transition state structure characterized by the transference of the hydroxyl hydrogen to the nitro group to give acetaldehyde and the corresponding nitroalkane for the secondary substrates and acetone and nitromethane for the tertiary substrate. 

The kinetics and mechanisms of gas-phase elimination of ethyl 1-piperidinecarboxyl-ate, ethyl pipecolinate, and ethyl 1-methylpipecolinate has been determined in a static reaction system.9 The reactions proved to be homogeneous, unimolecular, and obey a first-order rate law. The first step of decomposition of these esters is the formation of the corresponding carboxylic acids and ethylene. The acid intermediate undergoes a very fast decarboxylation process. The mechanism of these elimination reactions has been suggested on the basis of the kinetic and thermodynamic parameters. 

The kinetics of the gas-phase elimination of ethyl and /-butyl carbazates have been studied in a static reactor system over the temperature range 220.3-341.7 °C and pressure range 21.1-70.0 torr.13 Theoretical calculations on the thermal decomposition of ethyl carbazate (4) suggest that the reaction proceeds by a concerted non-synchronous mechanism, through a quasi-three-membered ring transition state (Scheme 4). In contrast, the transition state structure for the thermal decomposition of /-butyl carbazate is an almost planar six-membered ring. 

The kinetics of the gas-phase elimination of 3-hydroxy-3-methylbutan-2-one have been investigated in a static system, seasoned with allyl bromide, and in the presence of the free chain radical inhibitor toluene.14 The reaction was found to be homogeneous, unimolecular and to follow a first-order rate law. The products of elimination are acetone and acetaldehyde. Theoretical estimations suggest a molecular mechanism involving a concerted non-synchronous four-membered cyclic transition state process. 

It is a known fact that the gas-phase pyrolysis kinetics of alkyl bromides have not been extensively investigated due to the experimental difficulties as well as to the complexity of concurrent unimolecular and radical chain mechanisms. However, when these organic bromides are pyrolyzed under maximum inhibition, the reaction in the presence of a free radical suppressor is a molecular elimination. Sometimes, these organic bromides are pyrolyzed under maximum catalysis with HBr gas, and the process may proceed by an autocatalytic molecular mechanism. 

A number of kinetic isotope studies have been reported for gas-phase eliminations. Isopropyl bromide-dg decomposes more slowly than isopropyl bromide and the intramolecular isotope effects (A ,c2D4hx-c2D4)/ (C2D4hx-c2D.iH)) have been recorded for pyrolyses of ethyl acetate (2.0), chloride (2.20), and bromide (2.10) at 500°C. At the elevated reaction temperatures, these values correspond to the maximum predicted for complete loss of the C-H stretching vibration and they have been interpreted as indicating considerable weakening of the C-H bond in the transition state. Whether this is a homolytic or hetero-lytic bond fission, it is remarkably insensitive to beta substituent effects on rate. The intramolecular isotope effects in these cases could reflect predominantly a secondary isotope effect rather than the intended primary effect and dissection into an intermolecular and secondary isotope effect would prove more fruitful. (Section 2.2.1.)... 

In one case, pyrolysis of an ester led to anti elimination. The results were explained on the basis of a mechanism incorporating anchimeric assistance Smissman, E. E. Li, J. P. Creese, M. W. /. Org. Chem. 1970,35,1352. Anchimeric assistance was also proposed for a gas phase elimination of hydrogen chloride Hernandez,). A. Chuchani, G. Int. J. Chem. Kinet. 1978,10,923 Chuchani, G. Martin, I. /. Phys. Chem. 1986, 90, 431. 

The kinetics and mechanism of the unimolecular gas-phase elimination of 2-(dimethylamino)ethyl chloride have been examined by using DFT methods to explain the enhanced reactivity in gas-phase elimination compared to the parent compound ethyl chloride. The TS located on the minimum energy path had a four-centred cyclic configuration comprising chlorine, hydrogen, and two carbon atoms and benefited from electron delocalization involving the dimethylamino substituent. 

A stark difference exists in the potential surfaces for solution phase and gas phase eliminations, giving very different kinetic observations. The addition of hydroxide to alkyl halides in the gas phase leads to spontaneous formation of an ion-molecule complex. The hydroxide associates with the alkyl halide with no activation barrier, because it is the only way the hydroxide can achieve any solvation at all, albeit only by ion-dipole and ion-induced-dipole interactions (see Section 3.2.2). Once the complex has been formed, either substitution reactions or elimination reactions can occur, both of which have significant barriers (see the Going Deeper highlight of Section 11.5.4). Elimination commonly dominates over... 

Grain boundary phase elimination kinetics. Weight loss by grain boundary elimination was 1930°C in different settings under nitrogen gas flow. 

It is possible to eliminate the mass transfer resistances in Steps 2, 3, 7, and 8 by grinding the catalyst to a fine powder and exposing it to a high-velocity gas stream. The concentrations of reactants immediately adjacent to the catalytic surface are then equal to the concentrations in the bulk gas phase. The resulting kinetics are known as intrinsic kinetics since they are intrinsic to the catalyst surface and not to the design of the pores, or the pellets, or the reactor. 

When the mass transfer resistances are eliminated, the various gas-phase concentrations become equal a/(/, r, z) = j(r, z) = a(r, z). The very small particle size means that heat transfer resistances are minimized so that the catalyst particles are isothermal. The recycle reactor of Figure 4.2 is an excellent means for measuring the intrinsic kinetics of a finely ground catalyst. At high recycle rates, the system behaves as a CSTR. It is sometimes called a gradientless reactor since there are no composition and temperature gradients in the catalyst bed or in a catalyst particle. 

The gas phase decomposition of ozone is catalyzed by nitrogen pent oxide. Two mechanisms that purport to explain this process are given below. Can relatively simple kinetic measurements be used to eliminate one of the proposed mechanisms from further consideration Explain. 

The derivation of a rate law from a postulated mechanism is a useful application of reaction mechanisms. It shows how the kinetics of the elementary reaction steps are reflected in the kinetics of the overall reaction. The following example illustrates this for a simple, gas-phase reaction involving an open sequence. The derivations typically employ the stationary-state hypothesis (SSH) to eliminate unknown concentrations of reactive intermediates. 

The results reviewed above suggest that gas-phase diffusion can contribute significantly to polarization as O2 concentrations as high as a few percent and are not necessarily identifiable as a separate feature in the impedance. Workers studying the P02 -dependence of the electrode kinetics are therefore urged to eliminate as much external mass-transfer resistance in their experiments as possible and verify experimentally (using variations in balance gas or total pressure) that gas-phase effects are not obscuring their results. 

The apparatus s step change from ambient to desired reaction conditions eliminates transport effects between catalyst surface and gas phase reactants. Using catalytic reactors that are already used in industry enables easy transfer from the shock tube to a ffow reactor for practical performance evaluation and scale up. Moreover, it has capability to conduct temperature- and pressure-jump relaxation experiments, making this technique useful in studying reactions that operate near equilibrium. Currently there is no known experimental, gas-solid chemical kinetic method that can achieve this. 

Kinetics as a consequence of a reaction mechanism. The deduction of the kinetics from a proposed reaction mechanism generally consists in a reasonably straightforward transformation, where all the mechanistic details are eliminated until only the net gas-phase reaction and its rate remains. This approach may be used to investigate if a proposed mechanism consistent, what the reaction rate is and if it is consistent with available experimental data. 

Although as yet seemingly restricted to above ca. 1500 cm 1 by the limited availability of tuneable infrared detectors, this technique also virtually eliminates gas-phase contributions to spectra. The pulsed lasers used also open up the possibilities of fast (nanosecond or less) kinetic studies of catalytic reactions. 

Ab initio calculations at the MP2/6-31+ G level have been performed for gas-phase El elimination reactions of CH3CH2X (X = NH3"1", Br, Cl, F, SH) promoted by NH . OH-, F-, PH2. SH-, and Cl- in order to determine how changes in transition-state geometry, from reactant-like to product-like, influence kinetic isotope effects.9 Secondary isotope effects (a-H) on leaving group departure are correlated with the hybridization at C7 in the transition state, whereas there is no such correlation between secondary (/5-H) isotope effects and the transition state hybridization at C/ . The primary deuterium isotope effect is influenced markedly by the nucleophilic atom concerned but approach to a broad maximum for a symmetric transition structure can be discerned when due allowance is made for the element effect. 

Preliminary results on the enantioselective formation of sulfur and nitrogen mediumsized heterocycles by base-induced ring opening of hetero-oxabicyclic [3.2.1] and [3.3.1] systems have been reported.91 The reaction involves a deprotonation-C—O bond elimination sequence. The kinetics and mechanism of gas-phase unimolecular elimination reactions of some substituted aminoazoles have been studied as an aid to heterocycle synthesis.92... 

---