Termolecular Reactions Mechanism

We can define a termolecular reaction as one that requires the participation of three individual particles in a single kinetic process. Thus we might think of the reaction of NO with CI2 as proceeding through the simultaneous interaction of two NO molecules with CI2 to form the transient reaction complex (NO)2Cl2, which then decomposes in a single step into two molecules of NOCl. Or in more general terms, for the reaction of A + B + C — products we may write 

An alternative formulation which is in many cases more useful is that termolecular reactions are in reality the result of two successive bimolecular processes involving the participation of a transient complex formed from any pair of the reacting species. Thus for the reaction of A + B + C we can write 

If we apply the stationary-state hypothesis to Eq. (XII.15.1), we have for the rate of formation of products 

There are two distinct types of processes which are kinetically of third order. One of these is the association of atoms or simple molecular species in which a third molecule is required to remove the excess energy of the association. In this case the third body is a true catalyst and acts essentially to transport energy and momentum. The second case is one which involves the chemical reaction of three species. 

By using the hard sphere collision model we can compute a collision frequency for three molecules A, B, and C by first computing the stationary concentration of the three possible binary complexes AB, BC, and CA. If we call tab, tbc, and tca the mean lifetime of these binary complexes/ their stationary concentrations are approximately given by 

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Modern theoretical and experimental studies have revealed additional details of the reaction kinetics. The results of these studies surest that the low temperature thermal reaction proceeds by both the direct bimolecular reaction of hydrogen molecules with vibrationally excited iodine molecules H2 + l2(hi v) —> HI + HI and the termolecular reaction of hydrogen molecules with iodine atoms H2 + I + I—>HI- - HI. The direct bimolecular reaction mechanism and the termolecular reaction mechanism were among those suggested by Bodenstein one hundred years ago. 

In examples 6-8, the increase in n above the value of unity associated with a termolecular reaction mechanism was ascribed to quater-molecular reaction in which the reactive complex has the formula A W W B, where W represents a water molecule and each dot represents a hydrogen bond. However, this interpretation is not unique. 

In Table 1 (pp. 251-254), IM rate constants for reaction systems that have been measured at both atmospheric pressure and in the HP or LP range are listed. Also provided are the expected IM collision rate constants calculated from either Langevin or ADO theory. (Note that the rate constants of several IM reactions that have been studied at atmospheric pressure" are not included in Table I because these systems have not been studied in the LP or HP ranges.) In general, it is noted that pressure-related differences in these data sets are not usually large. Where significant differences are noted, the suspected causes have been previously discussed in Section IIB. These include the reactions of Hcj and Ne with NO , for which pressure-enhanced reaction rates have been attributed to the onset of a termolecular collision mechanism at atmospheric pressure and the reactions of Atj with NO and Cl with CHjBr , for which pressure-enhanced rate constants have been attributed to the approach of the high-pressure limit of kinetic behavior for these reaction systems. 

SCHEME 12. Termolecular polar mechanism proposed by Swain and Boyles and Ashby and coworkers . The first reaction in Scheme 2... 

Having established that 1 catalyzes the hydrolysis of orthoformates in basic solution, the reaction mechanism was probed. Mechanistic studies were performed using triethyl orthoformate (70) as the substrate at pH 11.0 and 50 °C. First-order substrate consumption was observed under stoichiometric conditions. Working under saturation conditions (pseudo-0 order in substrate), kinetic studies revealed that the reaction is also first order in [H+] and in [1]. When combined, these mechanistic studies establish that the rate law for this catalytic hydrolysis of ortho-formates by host 1 obeys the overall termolecular rate law rate = k[H+][Substrate][l], which reduces to rate = k [H ][l] at saturation. 

This result is of a very rough character, but it shows that termolecular reactions can reasonably be interpreted in terms of simple kinetic mechanisms. 

The simple intermediate steps that make up a reaction mechanism invariably involve (a) spontaneous decomposition of one molecule, (b) most commonly a bimolecular collision between two molecules, or (c) an unlikely termolecular collision between three molecules. From a practical standpoint, nothing more complicated is ever observed. 

Of particular interest in the present context is the mechanism by which nitric oxide becomes oxidized to nitrogen dioxide. The termolecular reaction... 

Most asymmetric catalyses are termolecular reactions. To obtain a sufficient asymmetric bias, the reactant and/or substrate must be placed in a chiral environment induced by the catalyst. Perhaps one of the most reliable mechanisms for transmitting stereochemical information is the in situ formation of reactive intermediates in which the chiral catalyst and reactant are covalently bound. Under some conditions, the inter-... 

The first mechanism implies k19 = k5AK11A 11A the second leads to ki9 = k2iKlm- . Independent evidence suggests the existence of both intermediate species in the nitric oxide-oxygen system, and both mechanisms involve entirely reasonable collision complexes. In both, the equilibrium step is rapid, and the overall kinetics are third order. Theoretical calculations based on the activated complex theory were made by assuming a true termolecular reaction the predicted rates agree well with experiment.161 The experimental rate constants are summarized in Tables 4-3 and 4-4. 

The first studies of the kinetics of the NO-F2 reaction were reported by Johnston and Herschbach229 at the 1954 American Chemical Society (ACS) meeting. Rapp and Johnston355 examined the reaction by Polanyi s dilute diffusion flame technique. They found the free-radical mechanism, reactions (4)-(7), predominated assuming reaction (4) to be rate determining, they found logfc4 = 8.78 — 1.5/0. From semi-quantitative estimates of the emission intensity, they estimated 6//t7[M] to be 10-5 with [M] = [N2] = 10 4M. Using the method of Herschbach, Johnston, and Rapp,200 they calculated the preexponential factors for the bimolecular and termolecular reactions with activated complexes... 

A reaction mechanism is the sequence of elementary reactions, or elementary steps, that defines the pathway from reactants to products. Elementary reactions are classified as unimolecular, bimolecular, or termolecular, depending on whether one, two, or three reactant molecules are... 

A further possibility is that the reaction occurs as an elementary termolecular reaction which mechanism(s) is the more likely ... 

Either of the two complex mechanisms would be more likely than the termolecular reaction, since the former only require two molecules to come together simultaneously in any given step. The termolecular mechanism would require the much more unlikely situation of a three body collision. 

It means that we consider only mono-, bi- and (rarely) termolecular reactions. The coefficients stoichiometric coefficients and stoichiometric numbers observed in the Horiuti-Temkin theory of steady-state reactions. The latter indicate the number by which the elementary step must be multiplied so that the addition of steps involved in one mechanism will provide a stoichiometric (brutto) equation containing no intermediates (they have been discussed in Chap. 2). 

NO Formation from Dinitrogen Oxide (Nitrous Oxide) takes place during combustion of gaseous hydrocarbons of volatiles in the case of lean mixtures. In accordance with this mechanism at first the dinitrogen oxide N2O is formed by the termolecular reaction ... 

There are relatively few systems in either category of termolecular reactions which have been studied in any great detail, and the data for these are presented in Table XII.9. Only three wholly chemical processes are included, and all involve the reaction of NO. The data for the reaction of NO with H2 which has been studied above lOOO K, appear to be third-order, but the mechanism is probably not simple. 

An alternative mechanism which was felt to be more useful includes a key role for vdW syst ns. Specifically, a termolecular reaction is considered as the ovecall result of two successive bimolecular processes. In the first step, a vdW system is formed (in general between any pair of reactants, c.g., A. .. B, B. .. C, A. .. Q and in the second step the vdW system reacts with the remaining component and forms the activated complex which decomposes to give the products. 

One possible mechanism would be a single-step termolecular reaction of two NO molecules with one O2 molecule. This would be consistent with the form of the rate expression, but termolecular collisions are rare, and if there is an alternative pathway it is usually followed. 

The examples of reversible and consecutive reactions presented here give a very modest introduction to the subject of reaction mechanisms. Most reactions are complex, consisting of more than one elementary step. An elementary step is a unimolecular or bimolecular process which is assumed to describe what happens in the reaction on a molecular level. In the gas phase there are some examples of termolecular processes in which three particles meet simultaneously to undergo a reaction but the probability of such an event in a liquid solution is virtually zero. A detailed list of the elementary steps involved in a reaction is called the reaction mechanism. 

The phase-space model has been applied to triple collisions by F. T. Smith (1969) in a detailed study of termolecular reaction rates. He classified 3-body entry or exit channels into two classes, of pure and indirect triple collisions, and introduced kinematic variables appropriate to each class. These variables were then used to develop a statistical theory of break-up cross-sections. A recent contribution (Rebick and Levine, 1973) has dealt with collision induced dissociation (C1D) along similar lines. Two mechanisms were distinguished in the process A + BC->A + B + C. Direct CID, where the three particles are unbound in the final state, and indirect CID, where two of the particles emerge in a quasi-bound state. Furthermore, a distinction was made in indirect CID, depending on whether the quasi-bound pair is the initial BC or not. Enumeration of the product (three-body) states was made in terms of quantum numbers appropriate to three free bodies (see e.g. Delves and Phillips, 1969) the vibrational quantum number of a product... 

Correlations found for the six-coordinate complexes (SnR2X2Y2) reveal a possible mechanism for an (unprecedented) termolecular reaction involving a concerted double addition and elimination process in which the R ligands adopt transoid positions, and the X and Y groups are mutually cis in the intermediate (or transitional) octahedral conformer Britton and Dunitz dubbed this an Sn3 reaction ... 

Some termolecular elementary steps occur, but they are extremely rare because the probability of three particles colliding simultaneously with enough energy and with an effective orientation is very small. Higher molecularities are not known. Unless evidence exists to the contrary, it makes good chemical sense to propose only unimolecular or bimolecular reactions as the elementary steps in a reaction mechanism. 

The mechanism of free radical formation in cyclohexanol was studied by the inhibitor technique [54]. The termolecular reaction was found to be predominant with a rate... 

The latter step in this mechanism, in fact, represents a combination of two steps. The first of these steps involves the formation of a weakly bonded complex, which is often represented as H2 , and the second step involves the interaction of this complex with another iodine atom. For the purposes of the question, it is acceptable to join these steps together to give m apparently termolecular reaction.)... 

The concentration of OH radicals as a function of time has been observed directly in a shock tube (see Chapter 6 for details of this technique) under conditions where the reaction between H2 and O2 proceeds smoothly (1100-2600°K, in the presence of excess argon) [28-30]. An induction period approximately 10 to 100 /isec long is followed by a rapid rise of the OH concentration to a maximum value and finally a slow approach to equilibrium. The rise of the OH concentration over its equilibrium value (a phenomenon also observed for H and O) results from a partial approach to equilibrium in the three propagation steps in the mechanism before a significant amount of termolecular reaction occurs in the gas phase termination step. A detailed analysis shows that this overshoot can occur only if there is a decrease in the number of moles in the overall reaction, since it is only in this case that the termolecular termination step may become rate limiting. 

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