Termolecular mechanism

Even though the rearrangements suggest that discrete carbocation intermediates are involved, these reactions frequently show kinetics consistent with the presence of at least two hydrogen chloride molecules in the rate-determining transition state. A termolecular mechanism in which the second Itydrogen chloride molecule assists in the ionization of the electrophile has been suggested. ... 

The stereochemistry of addition depends on the details of the mechanism. The addition can proceed through an ion pair intermediate formed by an initial protonation step. Most alkenes, however, react via a complex that involves the alkene, hydrogen halide, and a third species that delivers the nucleophilic halide. This termolecular mechanism is generally pictured as a nucleophilic attack on an alkene-hydrogen halide complex. This mechanism bypasses a discrete carbocation and exhibits a preference for anti addition. 

The major factor in determining which mechanism is followed is the stability of the carbocation intermediate. Alkenes that can give rise to a particularly stable carbocation are likely to react via the ion-pair mechanism. The ion-pair mechanism would not be expected to be stereospecific, because the carbocation intermediate permits loss of stereochemistry relative to the reactant alkene. It might be expected that the ion-pair mechanism would lead to a preference for syn addition, since at the instant of formation of the ion pair, the halide is on the same side of the alkene as the proton being added. Rapid collapse of the ion-pair intermediate leads to syn addition. If the lifetime of the ion pair is longer and the ion pair dissociates, a mixture of syn and anti addition products is formed. The termolecular mechanism is expected to give anti addition. Attack by the nucleophile occurs at the opposite side of the double bond from proton addition. 

SCHEME 2. A termolecular mechanism for Grignard reagent addition to carbonyl compounds via... 

Dichloracetamide (pAT, = 13.55) also shows evidence of ionisation but mono-chloracetamide (pXi = 14.17) does not moreover Bruylants and Kezdy also found evidence for a termolecular mechanism in the hydrolysis of chlor-acetamide. 

Why does hydrochlorination of /-butylethylene not also proceed in part by a termolecular mechanism The apparent reason is shown in Table 7.3 The carbocation formed from /-butylethylene is more stable than the cyclohexyl cation, and therefore kn of Equation 7.8 is larger for /-butylethylene. Furthermore, /-butylethylene has a small k2 because of steric interference of the bulky t-butyl group in a termolecular transition state. Table 7.3 gives the estimated rate constants, klt k2, and k3 of Equation 7.8 for four olefins. The rate constant, kx, decreases with the ability of the substrate to stabilize a positive charge. The larger value of k2 for 1,2-dimethylcyclohexene than for cyclohexene means that the j8 carbon in the transition state of the Ad3 mechanism has some cationic character... 

Electrophilic addition of HC1 to triple bonds can apparently also go by bi-or termolecular mechanisms. Thus in acetic acid 3-hexyne (14) gives predominantly anti addition through an Ad3 pathway, but 1-phenylpropyne (15), which can form the resonance-stabilized vinyl cation (16), gives predominantly syn addition through an ion pair Ad 2 mechanism.27... 

Ashby et al.1, , however, suggest that the addition of methylmagnesium bromide to benzophenone, again in solvent ether, proceeds by a termolecular mechanism, written as... 

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. 

X 109 and 8. 9 x 109 s 1 for Ndm and Ybm, respectively. Both the above-described in-termolecular mechanism, as well as an intramolecular pathway in the ternary complex with aad which forms in solution, are responsible for the observation of NIR luminescence in these systems. Addition of water to the toluene solutions quenches the NIR luminescence, while it enhances the visible CL emission of the corresponding solution of Eum and Tbm (Voloshin et al., 2000c). Neodymium and ytterbium tris(benzoyltrifluoroacetonates) display the same CL as tta complexes, although for Ybm its intensity is about 2.5 times lower than for the tta chelate. On the other hand, almost no CL is detected for acetylacetonate complexes (Voloshin et al., 2000a). Thermal or photochemical decomposition of aad also triggers CL from [Pr(dpm)3] and Pr(fod)3], both in the visible (from the 3Pi, 3Po, and 1D2 levels) and in the NIR at 850 nm ( Do -> 3F2 transition) and 1100 nm ( D2 3F4 transition). The excited... 

While this termolecular mechanism of acid-base catalysis seems very appealing and demonstrated in a number of cases, it does not nebessarily hold in all cases, and in fact Bell and coworkers have shown a number of specific examples in which it does not apply. 

The low overall activation energy of this reaction renders it considerably different from the reaction of NO with Brj or CI2 where the low temperature rate is dominated by a termolecular mechanism. 

A number of studies of the kinetics and mechanism of the base catalysed reaction of epoxides with phenolic alcohols have served as background for the polymerization studies. These studies [14] showed that both the alcohol and the alkoxide participate in the rate determining step and subsequently a termolecular mechanism was proposed. 

However, Ishii et al. [14] feel that a termolecular mechanism is a better explanation of the data and they show that the expression (2) applied for EO polymerizations in the presence of a large excess of alcohol, viz. 

The presence of hydroxyl groups during the reaction should be included within the rate expressions. Schecter et al (1 ) first proposed a termolecular mechanism to describe the effect of hydroxyl groups on the reaction, reactions (5) and (6). 

The parent propane ion C3H8 had been thought to be completely unreactive towards propane until Sieck et al. [265] recently found dimer ion formation via a termolecular mechanism... 

A cationic hydrido-methyl complex, HPtCHs ", is formed upon the almost barrierless insertion of a Pt ion into a C-H bond of methane and then 1,2-migration of a hydrogen atom and subsequent elimination of H2 from the transition-metal center gives the platinum carbene cation PtCHi"" with an apparent rate constant of k - 8.2 x 10" cm molecule" s. The reaction of ground-state platinum atoms produced by the photodissociation of [PtMe3-(CsHiMe)] follows a termolecular mechanism [4c] ... 

Equation 14.27), which involves two steps but only unimolecular and bimolecular processes, is far more probable than the single-step termolecular mechanism of Equation 14.26. 

The methylolation of NH-group-containing compounds with formaldehyde is first order with respect to the NH compound, formaldehyde, and catalyst (for example, H3O , H Me3, H3PO4), that is, the equation i = fc [> NH] [HCHO] [Cat] holds. Since a termolecular mechanism is improbable, it must be assumed that an associate is first formed, for example, in the bicarbonate-ion-catalyzed reaction ... 

The stereochemistry of addition is usualy anti for alkyl-substituted alkynes, whereas the addition to aryl-substituted compounds is not stereospecific. This suggests a termolecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation intermediate in the aryl case. Aryl-substituted alkynes can be shifted... 

At about the same time the possibility of a termolecular mechanism for the reaction of H2 with I2 was reintroduced by Semenov and others [13]. Since then the question of the bimolecular mechanism versus the termolecular mechanism has been investigated in several studies, both experimental and theoretical, but it has not been completely resolved. Nevertheless, it is now clear that the two mechanisms are really very similar. It is likely that both mechanisms contribute to the overall reaction. 

Our recent electronic structure calculations 3deld a potential energy surface adequate to explain, at least qualitatively and within the uncertainties due to an incomplete knowledge of relaxation rates, the available experimental observations for the hydrogen-iodine reaction. The rate expressions, the rate constants, their temperature dependence, the vibrational excitation of HI products, the excitation and/or dissociation of reactant I2, the photochemical rates - all are compatible with the recent ab initio potential energy surface and with the classical trajectory calculations carried out with a similar surface. And all are compatible with either the bimolecular or termolecular mechanisms. It appears most likely that both mechanisms contribute, but the matter is not resolved as yet. 

Since the concentration power is two for NO and one for 0" we might suggest a one-step termolecular mechanism involving two molecules of NO and one 0, molecule. This termolecular mechanism satisfies the observed overall reaction and the observed rate equation. Since termolecular reactions are rare it is more likely that the reaction proceeds through bimolecular and unimolecular steps. We may then assume that the first step is the fast reversible formation of NO. (intermediate). The NO. then reacts with a second NO molecule in the slow step ... 

Scheme 3.5 Carbamate formation from amines and carbon dioxide termolecular mechanism...

Crooks and Donnellan [30] investigated the kinetics of the reaction between CO2 and tertiary amines, such as MDEA and TEA (2,2, 2"-nitrilotriethanol triethanolamine) in water. A single step termolecular mechanism was proposed (Scheme 3.6) to be operative in this case, as suggested by the reaction order and the large negative activation entropy. 

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