Trimolecular mechanism

However, the postulated trimolecular mechanism is highly questionable. The third-order rate law would also be consistent with mechanisms arising from consecutive bimolecular elementary reactions, such as... 

A kinetic study of the reaction between phosphine and formaldehyde showed that it is of the first order with respect to phosphine and to the aldehyde. It is catalysed by HCl. At hydrogen chloride concentrations of less than 0.2 mol/1 the rate of reaction is proportional to the HCl-content of the solution, at higher concentrations the rate is independent of the HCl-content The dependencies found can be accounted for by both bi- and trimolecular mechanisms. In the latter case, a simultaneous interaction between one molecule of aldehyde, one of phosphine and a proton must be assumed ... 

Thus, the above findings149) indicate that the first stage of the initiation of the epoxy compound polymerization under the action of the TA even in precision-cleaned systems may take place by the trimolecular mechanism to give quaternary ammonium alcoholates owing to impurity quantities of the proton donor or other electrophilic particles in the system. This reaction will not occur in the absence of such impurities. Upon appearance of the hydroxyl groups in the system due to the chain transfer to the counterion (see Sect. 3.3.3) the reaction according to Scheme (33) with participation of the unsaturated alcohol becomes the main source of the active growing sites. 

Table 11 presents one more result important for the chemistry of epoxy compounds, namely within the experimental error the rate constant of the free ion is the same for all counterions. This means that such strong nucleophilic particles as carbanions (and evidently alkoxy anions) are capable of opening the epoxy ring without additional electrophilic activation. This result explains the apparently contradictory results that, depending on the reaction conditions, either tri-140 144,166-I71) or bimolecular kinetics 175-I79> is observed. The bimolecular kinetics also can be explained in terms of the trimolecular mechanism, since proton-donor additives play a dual role. 

Since the Mannich reaction is a condensation involving three reactants (substrate, aldehyde, and amine), pathways a or b (Fig. 24) can be followed, if one excludes a rather unlikely trimolecular mechanism ... 

Shimasaki, K., Aida, T., and Inoue, S., 1987, Living Polymerization of 5-Valerolactone with Aluminium Porphyrin. Trimolecular Mechanism by the Participation of Two Aluminium Porphyrin Molecules. Macromolecules, 26 3076... 

Then, the resulting tin(II) mono- and/or dialkoxide initiates polymerization in the same manner as the other metal alkox-ides. However, there was, at that time, no direct proof of such a mechanism and several other mechanisms have been pro-posed. " The most often cited was the trimolecular mechanism in which first the catalyst-monomer complex is formed. This mechanism has conclusively been shown not to operate since it excludes the presence of Sn atoms covalently bonded to the growing macromolecules. The matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectral measurements of the cyclic ester/ROH/Sn(Oct)2 system revealed the presence of tin(II) alkoxides in the growing polyester chains. Moreover, the kinetic studies also clearly supported this sequence of the exchange reactions. ... 

A concerted mechanism has also been discussed [29,30], involving either a 2+2+1 or 3+2 mechanism. To avoid trimolecular reactions this requires an interaction between Rh(I) and silanes prior to the reaction with a ketone. Interaction of silanes not leading to oxidative addition usually requires high-valent metals as we have seen in Chapter 2. The model is shown in Figure 18.16 it proved useful for the explanation of the enantiomers formed in different instances. The formation of a rhodium-carbon bond is included and thus formation of silyl enol ethers remains a viable side-path. 

On one hand, they increase the reaction rate due to an electrophilic assistance for the epoxy ring opening and, on the other, lower the reactivity of the alcoxy anion owing to its solvation and the decrease of its nucleophility. Positive, neutral or even negative effects of the alcohol additives on the reaction rate are governed by the relationship between these two factors. The chain propagation reaction mechanism itself remains trimolecular. 

The existence of trimolecular reactions is sometimes suggested. For example, H + OH + M —> H2O+M, where M is a third body. However, the reaction probably proceeds by a two-step mechanism, i.e., (1) H + OH — H2O, and (2) H2O + M — H2O + M, where H2O is an energy-rich water molecule with an energy that exceeds the dissociation limit, and the function of M is to take away the energy. That is, the reaction actually proceeds via bimolecular collisions. 

Even in the days of van t Hoff many such cases were well known— multimolecular reactions which were kinetically unimolecular or bimolecu-lar or (rarely) trimolecular. But it was relatively recently, mainly in the second decade of this century, that chemists began to realize that some reactions may very well be kinetically of a mixed type. To quote a case where the mechanism was complete, mention may be made of the... 

Molecularities. An equally elementary criterion is the fact that a great majority of reaction steps are uni- or bimolecular trimolecular steps are rare and slow, and steps of still higher molecularities are unheard of (see Section 2.1). A trimolecular forward or reverse step in a postulated mechanism calls for an explanation why its reactants are not consumed by bimolecular steps before they have a chance to undergo the trimolecular one (e.g., see the Example 7.8 farther below). No mechanism involving a forward or reverse step of even higher molecularity should ever be considered. 

Hint 2.10 Avoid formulating mechanisms involving trimolecular steps. Instead, try to break a... 

A part of the evidence for the mechanisms given in equations (40) and (42) is provided by the work of Lowry and co-workers (Lowry and Richards, 113 Lowry and Faulkner, 24) on the mutarotation of tetra methylglucose. In water the reaction proceeds at a measurable rate, and it is clearly catalyzed by both acids and bases. In aqueous solution pyridine is a powerful catalyst but in pure dry pyridine no reaction occurs, and likewise in pure dry re-cresol there is no reaction. Upon investigating the reaction in a mixture of pyridine and n-cresol, Lowry and Faulkner (24) found it to take place very rapidly. From these experiments Lowry drew the important conclusion that a proton cannot by itself wander from one part of the molecule to another. The transformation can occur only if the medium in which the molecule is placed has both acidic and basic properties, so that a proton can be removed from the molecule at one place and a proton added to the molecule at another place. Now these experiments furnish strong support to the mechanism of reactions (40) and (42) whereby both members of the conjugate acid-base pair play a part in the reaction. Instead of representing this mutual dependence by means of consecutive bimolecular reactions, Lowry chose to represent it by means of one trimolecular reaction... 

Armstrong using a mechanism similar to that of hydroxylation has considered the presence of water essential and formulated a medianism in which oxygen and water acted as a unit to form dihydroxy derivatives of the hydrocarbons and hydrogen peroxide. The formation of the dihydroxy derivatives or of hydrogen peroxide was not shown. The validity of this theory has been questioned on the basis of the necessity for a trimolecular reaction and also on the basis of some work on the... 

In (92), there is a four-center exchange of a bonds. In principle, six-center trimolecular reactions involving three diatomic molecules could be in this group, providing only o bonds are permitted. Although several elementary reactions of the type (92) have been studied (Glasstone et al., 1941), there appear to be few gas-phase processes which follow this indicated mechanism. The once-classical example of a bimolecular reaction, i.e. hydrogen iodide decomposition (93), is actually a multistep... 

Its formation during roasting could result from a trimolecular reaction between ethylglyoxal, ammonia and acetaldehyde resulting from the Strecker degradation of alanine. The mechanism of chain elongation reactions of glyoxal has been elucidated by Yaylayan and Keyhani (1998). 

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