Termolecular addition mechanism

This mechanism, called the AdnS mechanism (termolecular addition, lUPAC AnAe)," has the disadvantage that three molecules must come together in the transition state. However, it is the reverse of the E2 mechanism for elimination, for which the transition state is known to possess this geometry (p. 1300). 

The basic mechanisms that are considered to be involved in electrophilic additions to alkynes are outlined below. The first involves a discrete vinyl cation. In general, this reaction will lead to a mixture of the two stereoisomeric addition products. Mechanisms B and C depict bridged intermediates formed without (B) or with (C) participation of a second electrophilic molecule. Mechanisms B and C should lead to anti addition. Mechanism D is a termolecular process that would be expected to be a stereospecific anti addition. Mechanisms A and B are of the Adg2 type, whereas C and D are classified as Ad S. Each of these mechanisms may involve a prior complex formation between the alkyne and an electrophile. 

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. 

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. ... 

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. 

As indicated by the involvement of water vapor and an inert third body, this reaction has several channels (see DeMore et al., 1997, for a review). There is both a bimolecular channel, which is pressure independent, and a termolecular channel, which is pressure dependent. In addition, the rate constant increases in the presence of gaseous water, suggesting that the reaction proceeds through a mechanism such as... 

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

The case for a two-step mechanism for the Aac2 reaction involving a tetrahedral addition intermediate, has already been discussed (p. 104), and has been widely accepted. It is not possible, however, to exclude completely the possibility that a concerted nucleophilic displacement is involved, and it is possible to write such mechanisms involving transition states of lhe correct composition. All such mechanisms necessarily involve a termolecular collision and are therefore not readily reconciled with the observed entropies of activation. 

Schechter 55) proposed that the catalytic effect of hydroxyl groups on the epoxide-amine addition reaction involved a termolecular activated complex formed in the concerted reaction of amine, epoxide and hydroxyl. Smith 57) suggested a modified mechanism in which the same activated complex is formed in a bimolecular reaction between an adduct formed from epoxide (E) and the proton donor (HX), and the amine ... 

Such mechanisms are called Ad3 (addition-termolecular). Ad3 transition states analogous to 12 and 13 but leading to syn adductsHare, precluded by the steric requirements of the addends.22 Thus increased chloride ion concentration in-creases the contribution of the second term of the rate equation relative to the other two, and an/z-HCl adduct is formed more rapidly than jyn-HCl or -HOAc adducts. 

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... 

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). 

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... 

Gordon and Knipe (ioc. cit) present further data in support of such a mechanism, including an additional termination step for O atoms at the walls and a termolecular termination step CO + O + M CO2 + M. Lewis and Von Elbe have objected to such a mechanism on the grounds that O + CO must produce an excited triplet state of CO2 with a very short lifetime for dissociation. Such an argument is by no means decisive, since a lifetime of even 10 sec for CO2 would be sufficient to account for the branching rates observed. ... 

In addition to these steps, the mechanism must include radical-radical termolecular recombination reactions. The key feature of this mechanism is the competition between H and N for C2H5 radicals as indicated in steps (8) and (9). The addition of hydrogen atom reactions to the mechanism brings the NO titration and C2H4 reaction estimates of N atom concentrations into fair agreement. 

Chart 2.1. The negative-tone resists that were first used in semiconductor manufacturing were based on a matrix resin of synthetic rubber prepared by Ziegler-Natta polymerization of isoprene followed by acid-catalyzed cycliza-tion to improve the mechanical properties. This cyclized rubber was rendered photosensitive by addition of a bisarylazide that undergoes photolysis to produce a bisnitrene. The nitrene reacts with the cyclized rubber to create in-termolecular cross-links that render the exposed areas insoluble. 

The mechanisms of reactions such as (e) are difficult to elucidate. Reaction via a direct termolecular step, H2 addition to undetectable amounts of dimer, or the process outlined in equation (i) have all been considered for cobalt(II) systems. 

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 ... 

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