Alkanes kinetics

From the viewpoint of the combustion chemist, mechanistic and theoretical studies of abstraction reactions serve two purposes. First, they can determine the overall rate coefficient for an abstraction over a range of temperatures, especially when there are limited experimental data. Second, the combustion modeller wishes to know the rate of abstraction at any particular site on a hydrocarbon molecule. For reaction (10) this is trivial as there is only one type of site a primary C—H bond. However, for more complex fuels there will be a variety of different sites which to a first-order approximation can be considered as primary, secondary and tertiary C—H bonds. As mentioned in the introduction to this section, Atkinson et al. [10] and Walker [11] have attempted to describe radical/ alkane kinetics with the following simple model based on equation (2.4)... 

J. Liu, M. J. Laster, S. Taheri, E. 1. et al. Effect of n-alkane kinetics in rats on potency estimations and the Meyer-Overton hypothesis. Anesth. Analg., 79, 1049-55 (1994)... 

Ligand substitutions of bound fluoroalkanes and noble gases have also been studied. In perfluorocyclohexane, the reaction of Cr(CO)5 (CgFj2) with CO yields Cr(CO)j with a rate constant that is 10 times larger than that for recombination in cyclohexane. Similar fast kinetic studies have been conducted on 16-electron complexes bound by noble gases. The reactions of Cp Rh(CO)(Kr) with alkanes forms Cp rih(CO)(alkane) prior to oxidative addition of the alkane. Kinetic studies on these processes have suggested that the... 

On the fundamental side, several questions are emerging. It is not clear why some metal complexes favour intermolecular reactions, whereas many other systems undergo intramolecular cyclometallation or do not react at all with alkanes. Kinetics and thermodynamics of inter and intramolecular CH bond activation were determined by Jones comparing the reaction of the 16-e intermediate 27 with propane solvent or with the phosphine propyl group (reaction 21) [48]. 

Sinev MY Free radicals in catalytic oxidation of light alkanes kinetic and thertnochetnical aspects,/ Catal 216(1—2) 468—476, 2003. 

Work continues on improving the efficiency of this process, such as for freeing the alkan olamine from heat-stable salts that can form (125). Formulations have been developed which inhibit degradation of mono- and diethanolamine in processing (126). Models (127), computer programs (128), and kinetics and enthalpies (129—136) have been developed to help determine equiUbria of the acid gas—alkanolamine—water system. Additional references relate to the use of tertiary alkan olamines, such as triethanolamine, for gas conditioning (137—139). 

Kinetic Models Used for Designs. Numerous free-radical reactions occur during cracking therefore, many simplified models have been used. For example, the reaction order for overall feed decomposition based on simple reactions for alkanes has been generalized (37). 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

The Lead-Off Reaction Addition of HBr to Alkenes Students usually attach great-importance to a text s lead-off reaction because it is the first reaction they see and is discussed in such detail. 1 use the addition of HBr to an alkene as the lead-off to illustrate general principles of organic chemistry for several reasons the reaction is relatively straightforward it involves a common but important functional group no prior knowledge of stereochemistry or kinetics in needed to understand it and, most important, it is a polar reaction. As such, 1 believe that electrophilic addition reactions represent a much more useful and realistic introduction to functional-group chemistry than a lead-off such as radical alkane chlorination. 

The radical chain mechanism of the sulfochlorination is very similar to that of the chlorination. Accordingly, in normal cases the regioselectivities of the sulfochlorination and the chlorination are equal. For example, (-1) substituents decrease the reactivities of the adjacent C-H bond. This influence can even be observed at the y position. Thus, the consecutive second sulfochlorination affords no geminal or vicinal disulfochlorides in the product. Where there are differences between the regioselectivities of sulfochlorination and chlorination (as in the case of isoalkanes), it is because under the conditions of sulfochlorination, chlorination also takes place to a considerable extent. Figure 6 shows the main components of a sulfochlorination mixture. Today the kinetics and the regioselectivity of the sulfochlorination of /z-alkanes are so well known that the kinetic modeling of the concentration-conversion curves is possible for all partners of the reaction [12]. 

The results of kinetic studies suggest that alkane substitution reactions typically proceed by a radical chain mechanism (Section 13.9). The initiation step in the chlorination of methane is the dissociation of chlorine ... 

Thermal rearrangement of trans-l,2-dibromo compounds is known in the literature (refs. 6-10). In all case studies only one pair of bromine in each organic molecular was studied. Bellucci (ref. 10), for example, studied the kinetics of such trans-l,2-cyclo alkanes as cyclopentane, hexane, octane, etc. The intermediates suggested as an explanation for the experimental results are bromonium bromide I in polar solvents and four center transition state II in non-polar solvents. 

To illustrate how a bifunctional catalyst operates, we discuss the kinetic scheme of the isomerization of pentane [R.A. van Santen and J.W. Niemantsverdriet, Chemical Kinetics and Catalysis (1995), Plenum, New York]. The first step is the dehydrogenation of the alkane on the metal ... 

While alkane metathesis is noteworthy, it affords lower homologues and especially methane, which cannot be used easily as a building block for basic chemicals. The reverse reaction, however, which would incorporate methane, would be much more valuable. Nonetheless, the free energy of this reaction is positive, and it is 8.2 kj/mol at 150 °C, which corresponds to an equihbrium conversion of 13%. On the other hand, thermodynamic calculation predicts that the conversion can be increased to 98% for a methane/propane ratio of 1250. The temperature and the contact time are also important parameters (kinetic), and optimal experimental conditions for a reaction carried in a continuous flow tubiflar reactor are as follows 300 mg of [(= SiO)2Ta - H], 1250/1 methane/propane mixture. Flow =1.5 mL/min, P = 50 bars and T = 250 °C [105]. After 1000 min, the steady state is reached, and 1.88 moles of ethane are produced per mole of propane consmned, which corresponds to a selectivity of 96% selectivity in the cross-metathesis reaction (Fig. 4). The overall reaction provides a route to the direct transformation of methane into more valuable hydrocarbon materials. 

Secondly, this mechanism (1,3-carbon-carbon bond activation) applies to both acyclic and cychc paraffins such as hexane and cyclohexane (Scheme 40 and Table 8). Kinetic studies on the hydrogenolysis of these alkanes are note-... 

Figure 6.2. Typical ignition delay of an alkane fuel as a function of the initial mixture s temperature. Three different kinetic models are shown (a) High temperature chemistry only that is, no peroxy radical chemistry, (b) Same as (a), but the Q OOH chain-branching channel of the peroxy radicals has been considered, (c) Same as (b), bnt the concerted elimination of RO2 to alkene + HO2 has been considered. (Figure courtesy of Timothy Barckholtz, ExxonMobil Research and Engineering.)...

For transition-metal catalyzed hydroxylation of alkane C-H bonds, the reactions of alkanes with platinum(II) complexes were the most successful. In an aqueous solution of hexachloroplatinic acid and Na2PtCl4, alkanes were converted into a mixture of isomeric alkyl chlorides, alcohols, and ketones, and the platinum(IV) is reduced to platinum(II).7 The kinetics of the reaction with methane as the alkane have been described in detail.8... 

The three cycles have to turn over in the same range of temperature. This catalytic approach of the DeNOx reaction is not new. There is the same process for isomerization of alkanes, where there are also 3 catalytic cycles which have to turn over simultaneously (bifunctional catalysis). The kinetics of isomerization is given by only one cycle, the other two turning over very rapidly and are near equilibrium [13]. 

R,5,R,5-[Ni(937)]+ reacts with a series of alkyl halides in aqueous alkaline solution to form alkylnickel(II) complexes of the type [RNi(937)]+. Kinetic data indicate that the reaction occurs in two steps, the first being a one-electron transfer from [Ni(937)]+ to RX (X = halide), yielding an alkyl radical R. The second step involves rapid capture of the alkyl radical by [Ni(937)]+.2324 [Ni(937)]+ has also been reacted with a number of variously disubstituted alkanes, including... 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

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