Termolecular process

Numerous other reactions of this type, including a sequence involving HO2, exist and the relative importance of each reaction depends on the mixture composition. The rates of these termolecular processes increase with increasing pressure but have Httle or no temperature dependence. 

The three basic mechanisms that have been considered to be involved in electrophilic additions to alkynes are shown below. The first involves a discrete vinyl cation. In general, it can lead to either of the two stereoisomeric addition products. The second mechanism is a termolecular process which would be expected to lead to stereospecific anti addition. The... 

Swain and Eddy have queried the wide applicability of the S l and Sif2 mechanisms and favored a push-pull termolecular process for the reaction of pyridine with methyl bromide in benzene solution for example, they have suggested that the effects observed on the addition of methanol, phenol, p-nitrophenol, and mercuric bromide to the reaction mixture can be explained by an intermediate of type 168. ... 

Termolecular processes are common when two reactant molecules combine to form a single small molecule. 

In the atmosphere, [AT] is usually assumed to be atmospheric pressure at the altitude of interest. Therefore, unlike unimolecular and bimolecular processes, termolecular processes are pressure dependent. The units for the termolecular rate constant are cm /molecule s. 

If this is so, the protonated derivatives B and C would not appear at all. This conclusion stems from a value of w (see p. 335) of 5, indicating that water acts as a proton donor here as well as a nucleophile." " Termolecular processes are rare, but in this case the two water molecules are already connected by a hydrogen bond. (A similar mechanism, called Bac3, also involving two molecules of water, has been found for esters that hydrolyze without a catalyst. Such esters are mostly those containing halogen atoms in the R group.)... 

Since an elementary reaction occurs on a molecular level exactly as it is written, its rate expression can be determined by inspection. A unimolecular reaction is first-order process, bimolecular reactions are second-order, and termolecular processes are third-order. However, the converse statement is not true. Second-order rate expressions are not necessarily the result of an elementary bimolecular reaction. While a... 

Termolecular Reactions. If one attempts to extend the collision theory from the treatment of bimolecular gas phase reactions to termolecular processes, the problem of how to define a termolecular collision immediately arises. If such a collision is defined as the simultaneous contact of the spherical surfaces of all three molecules, one must recognize that two hard spheres will be in contact for only a very short time and that the probability that a third molecule would strike the other two during this period is vanishingly small. 

Another cyclotrimerization was observed in the Pd°-catalyzed transformation of substrates of type 6/1-51 which led to annulated naphthalenes 6/1-52, as described by Grigg and coworkers [37] (Scheme 6/1.11). The reaction can also be performed as a two-component transformation involving a combination of an intra- and an in-termolecular process. 

The low-pressure third-order result can also be written as a termolecular process ... 

The measured order of the overall reaction varies between 2.60 and 2.89. However, Pannetier and Souchayl l suggested that the above reaction could not be expected to occur owing to the improbable nature of the termolecular process. They proposed... 

However, if it occurs, it appears to be minor. Thus, based on a review of the relevant studies reported in the literature, DeMore et al. (1997) suggest that k l0 = 4.5 X 10-l4e-1260/7 = 6.6 X 10 16 cm3 molecule 1 s 1 at 298 K. This can be compared to an effective second-order rate constant for reaction (9) at 1 atm of 1.3 X 10 12 cm3 molecule-1 s-1. In short, the two-body reaction is more than three orders of magnitude slower than the termolecular process at 1 atm pressure. 

This is a termolecular process and would be expected to be entropically highly unfavourable, as indeed reactions of this sort generally are. A considerable amount of data is now available, (some of it is given in the appropriate sections below) and intermolecular reactions thought for other reasons to involve general base catalysis commonly have entropies of activation in the region — 40 to —50 eu. 

From the point of view of the transition-state theory we can write for the specific rate constant of a termolecular process... 

The need for an oxidant is shown by the failure of the colour to develop if insufficient m-dinitrobenzene is used [202], although addition of a substance like lead dioxide causes rapid appearance of colour in such a mixture [igg]- w-Nitro-aniline is always formed in reactions using excess of m dinitrobenzene as the oxidant [203]. It is uncertain at present whether the abstraction of hydride ion and proton occurs as a synchronous termolecular process, or as two discrete steps in one order or the other. 

The reaction is first order in both reactants which rules out the termolecular process... 

The results of earlier workers were critically received, re-calculated and extended by Taylor and Wilensky °. These workers felt there was no basis for any mechanism other than the simple termolecular process (98), and (99). The activation energy was calculated to vary from 4 kcal.mole" at 273-323 °K to 986 kcal.mole in the range 523-573 °K. Trotman-Dickenson represented the low-temperature data by the expression. 

Any detailed description of the mechanism of an octahedral substitution must also account for the stereochemical changes that accompany reaction. Werner recognized this and made use of it in his discussions of the stereochemistry of reactions of cobalt(III) complexes. The available experimental results can be explained on the basis of possible molecular rearrangements and some cautious predictions can even be made. The base hydrolysis of cobalt III)ammines appears to be unique in that it often occurs with rearrangement it also affords the few known examples of optical inversion. These results can be explained by formation of a 5-coordinated species with a trigonal bipyramidal structure. Optically active metal complexes racemize by either an intramolecular or an in-termolecular process. Substitution reactions of platinum metal complexes often occur with retention of configuration. 

The following termolecular process is the only one in the discharge which does not use 0 atoms to make O3 ... 

An experimental illustration for the approach of the low pressure limit k kg, where is proportional to the bath gas concentration [M] and the association reaction is a termolecular process, is shown in figure 2 for the reaction... 

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. 

Estimates of effective molarities in general acid catalysis of acetals require the analogous intermolecular reaction to be observable and therefore, as we have seen in the previous section, for the oxocarbenium ion to be particularly stable. The termolecular processes corresponding to the hydrolysis of salicyl... 

The first system we consider is the isotope exchange reaction for H2 and Dj. Experimental studies of this reaction are very difficult to perform since impurities allow the formation of H atoms, which then results in a radical exchange reaction. The early theoretical studies were flawed by the assumption that the reaction proceeded by a four-center symmetry-forbidden process. While the later theoretical studies focused on the symmetry-allowed termolecular process... 

This third-order rate dependence suggests a termolecular transition state, one that involves two molecules of the hydrogen halide. Figure 9.5 depicts such a termolecular process using curved arrow notation to show the flow of electrons, and dashed-line notation to... 

Termolecular processes are common when two reactant molecules combine to form a single small molecule. Such reactions are often exothermic and the role of the third body is to carry away some of the energy released and thus stabilize the product molecule ... 

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