Reactions in the gas phase

In the previous chapters, we have considered reactions on an empirical basis in terms of several concentration-time relationships that apply to many types of chemical systems. Our intuition indicates that while the overall reaction may be described in this way, on a molecular level individual reacting units must on some microscopic scale collide or make contact in some way. These units (molecules, ions, atoms, radicals, and electrons) must be involved in some simple step at the instant of reaction. These steps through which individual units pass are called elementary reactions. The sequence of these elementary reactions constitutes the mechanism of the reaction. 

In many cases, there must be energy transfer between the reacting molecules. For reactions that take place in the gas phase, molecular collisions constitute the vehicle for energy transfer, and our description of gas phase reactions begins with a kinetic theory approach to collisions of gaseous molecules. In simplest terms, the two requirements that must be met for a reaction to occur are (1) a collision must occur and (2) the molecules must possess sufficient energy to cause a reaction to occur. It will be shown that this treatment is not sufficient to explain reactions in the gas phase, but it is the starting point for the theory. 

5 Reactions with inorganic and Organoelement Compounds 2.1.11.4.5.1 Reactions in the Gas Phase 

The ion-molecule reactions of collisionally relaxed NH2 at 297 to 300 K in the gas phase with inorganic compounds are summarized in Table 20 and those with organoelement compounds in Table 21. The reactions were analyzed with a mass spectrometer [1, 2] in earlier experiments and later by Fourier transform (ion cyclotron resonance) mass spectrometry [3 to 7]. In other, more recent investigations, the flowing afterglow technique was applied and occasionally the newer selected-ion flow tube (SIFT) technique [8, 9], where the NHJ formed is separated from the other ions before the reaction. The methods allow the mass spectrometric identification of the anions only the other products have to be deduced from the mass balance. 

The products form by charge exchange, proton abstraction, or elimination. Reaction mechanisms were frequently discussed see for example [4, 8, 9]. Multiple H-D exchange, as in the formation of OH from D2O, was occasionally observed, but its minor importance indicates a short lifetime of the reactive intermediate complex [19]. The average dipole orientation (ADO) theory usually predicts the rate constants satisfactorily an exception is the value calculated for the reaction with N2O which is much too small [8]. Calculated structural parameters of the adduct of NHJ with NH3 are given on p. 269. 

The primary products from the reactions of thermally equilibrated amide ions with organoelement compounds form via nucleophilic substitution or proton abstraction by amide, or by competition between both reaction channels. Elimination reactions and other secondary processes are observable in some cases see for example [20 to 22]. 

P(CH3)3 P(CH3)2CH2 forms exclusively and irreversibly rapid and quantitative reaction [6] [25] 

The transition-state theory (TST) provides the framework to derive accurate relationships between kinetic and thermochemical parameters. Consider the common case of the gas-phase bimolecular reaction 3.1, where the transient activated complex C is considered to be in equilibrium with the reactants and the products  

From the rate law of this reaction in the forward direction, defined in terms of 

Here kn and h are the Boltzmann and the Planck constants, respectively, and p° is the standard pressure (1 bar). Now consider reaction 3.1 in the reverse direction. The rate law 

That is, the ratio between the forward and the reverse rate constants is equal to the equilibrium constant Kc. It can be easily shown that the perfect gas model implies 

For the reverse reaction, which involves the equilibrium between the species D, E, and the activated complex, equation 3.5 remains valid. Consequently, 

Heterogeneous catalytic gas-phase reactions are most important in industrial processes, especially in petrochemistry and related fields most petrochemical and chemical products are manufactured by this method. These reactions are currently being studied in many laboratories, and the results of this research can be also used for synthetic purposes. The reactions are usually performed [85] in a continuous system on a fixed catalyst bed (exceptionally a fluidized bed). 

In catalytic reactions, sufficient heat is usually required to overcome the activation energy barrier. In kinetic terms, the activation energy is the minimum energy required to form an activated complex undergoing transformation to the reaction products. Microwaves can be used as the source of thermal energy to induce catalytic reactions. The advantage of microwaves is that they can heat microwave-absorptive catalysts selectively to a temperature well above the bulk temperature 

The activation of methane by microwaves has long been a goal of scientists in attempts to convert this natural gas component into higher hydrocarbons valuable in the petrochemistry and chemical industry. Two pathways are being extensively investigated by research groups all over the world  

Oxidative coupling of methane to yield C2 and higher hydrocarbons The oxidative coupling of methane has been studied by several authors. The most elusive transformation has been the oxidative coupling of methane into C2 hydrocarbons (ethene, ethane), because the reaction is more endothermic than other transformations [2]. The application of rapid and efficient MW heating to endothermic reactions is particularly interesting. 

The most efficient catalysts for the desired transformation are metal oxides such 

Chemical reactions between stable chemical entities are rarely simple, and in passing from reactants to final products complex rearrangements of chemical bonds are required, which lead to intermediate particles or reactive centres. In general, a reaction usually involves a sequence of elementary changes, even though the ensemble can be still described by a single kinetic parameter (the extent of reaction) and can be stoichiometrically simple. 

The form of the rate expression for an actual reaction depends on the rates of the elementary steps, but it suffices for one step to be noticeably slower than the others for it to impose its rate on the system (to be the rate-determining step). 

In general, the experimental study of reactions in the gas phase is rather difficult, but the theoretical interpretation is easier than in solution because of the simpler environment of the molecules. 

Two principle types of reaction are found in gas phase reactions  

Open sequences the different elementary steps always take place in the same order from reactant to product. 

As noted in Section 4.2.1, the gas phase has proven to be a useful medium for probing the physical properties of carbanions, specifically, their basicity. In addition, the gas phase allows chemists to study organic reaction mechanisms in the absence of solvation and ion-pairing effects. This environment provides valuable data on the intrinsic, or baseline, reactivity of these systems and gives useful clues as to the roles that solvent and counterions play in the mechanisms. Although a variety of carbanion reactions have been explored in the gas phase, two will be considered here (1) Sn2 substitutions and (2) nucleophilic acyl substitutions. Both of these reactions highlight some of the characteristic features of gas-phase carbanion chemistry. 

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It was pointed out that a bimolecular reaction can be accelerated by a catalyst just from a concentration effect. As an illustrative calculation, assume that A and B react in the gas phase with 1 1 stoichiometry and according to a bimolecular rate law, with the second-order rate constant k equal to 10 1 mol" see" at 0°C. Now, assuming that an equimolar mixture of the gases is condensed to a liquid film on a catalyst surface and the rate constant in the condensed liquid solution is taken to be the same as for the gas phase reaction, calculate the ratio of half times for reaction in the gas phase and on the catalyst surface at 0°C. Assume further that the density of the liquid phase is 1000 times that of the gas phase. 

Yamamoto T 1960 Quantum statistical mechanical theory of the rate of exchange chemical reactions in the gas phase J. Chem. Phys. 33 281... 

Moore C B and Smith I W M 1996 State-resolved studies of reactions in the gas-phase J. Phys. Chem. 100 12 848... 

The method for calculating effective polarizabilitie.s wa.s developed primarily to obtain values that reflect the stabilizing effect of polarizability on introduction of a charge into a molecule. That this goal was reached was proven by a variety of correlations of data on chemical reactivity in the gas phase with effective polarizability values. We have intentionally chosen reactions in the gas phase as these show the predominant effect of polarizability, uncorrupted by solvent effects. 

In a recent experimental study of the femtosecond dynamics of a Diels-Alder reaction in the gas phase it has been suggested that both concerted and stepwise trajectories are present simultaneously It is interesting to read the heated debates between Houk and Dewar on the... 

Other conventions for treating equiUbrium exist and, in fact, a rigorous thermodynamic treatment differs in important ways. Eor reactions in the gas phase, partial pressures of components are related to molar concentrations, and an equilibrium constant i, expressed directiy in terms of pressures, is convenient. If the ideal gas law appHes, the partial pressure is related to the molar concentration by a factor of RT, the gas constant times temperature, raised to the power of the reaction coefficients. 

The hydrocarbon is carried in a stream of H2 or Ar, and P-SiC is formed or P-SiC is formed by reaction in the gas phase, under static conditions, of compounds such as SiO or CO formed in situ during the process. In this latter case the important reaction appears to be SiO + 3CO — SiC + 2CO2. This... 

Computer simulation techniques offer the ability to study the potential energy surfaces of chemical reactions to a high degree of quantitative accuracy [4]. Theoretical studies of chemical reactions in the gas phase are a major field and can provide detailed insights into a variety of processes of fundamental interest in atmospheric and combustion chemistry. In the past decade theoretical methods were extended to the study of reaction processes in mesoscopic systems such as enzymatic reactions in solution, albeit to a more approximate level than the most accurate gas-phase studies. 

Figure 11 Potential energy and potential of mean force of the Menshutkin reaction. The dashed line is for reaction in the gas phase, and the solid line for reaction in aqueous solution.

A simple model for the formation and growth of an aerosol at ambient conditions involves the formation of a gas product by the appropriate chemical oxidation reactions in the gas phase. This product must have a... 

It is possible to measure equilibrium constants and heats of reaction in the gas phase by using mass spectrometers of special configuration. With proton-transfer reactions, for example, the equilibrium constant can be determined by measuring the ratio of two reactant species competing for protons. Table 4.13 compares of phenol ionizations. 

BrCl can be prepared by the reaction in the gas phase or in aqueous hydrochloric acid solution. In the laboratory, BrCl is prepared by oxidizing bromide salt in a solution containing hydrochloric acid. 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

Pertiaps the most obvious experiment is to compare the rate of a reaction in the presence of a solvent and in the absence of the solvent (i.e., in the gas phase). This has long been possible for reactions proceeding homolytically, in which little charge separation occurs in the transition state for such reactions the rates in the gas phase and in the solution phase are similar. Very recently it has become possible to examine polar reactions in the gas phase, and the outcome is greatly different, with the gas-phase reactivity being as much as 10 greater than the reactivity in polar solvents. This reduced reactivity in solvents is ascribed to inhibition by solvation in such reactions the role of the solvent clearly overwhelms the intrinsic reactivity of the reactants. Gas-phase kinetic studies are a powerful means for interpreting the reaction coordinate at a molecular level. 

For the reaction in the gas phase, Emmons proposes a liomolytic O—N fission and (possibly simultaneous) alkyl shift [Eq. (28)]. The... 

CHEMICAL REACTIONS IN THE GAS PHASE AND IN SIMPLE SOLVENT MODELS... 

We are now ready to build a model of how chemical reactions take place at the molecular level. Specifically, our model must account for the temperature dependence of rate constants, as expressed by the Arrhenius equation it should also reveal the significance of the Arrhenius parameters A and Ea. Reactions in the gas phase are conceptually simpler than those in solution, and so we begin with them. 

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