Ionic Mechanism Hypothesis

Increasing KC1 concentration lowers inhibition as shown in Table II. The fact that damage increased with KC1 concentration is consistant with the ionic ratio hypothesis and suggests a base exchange mechanism whereby calcium ions are more easily extracted from the clay and replaced by potassium ions as the potassium concentration increases. 

Consideration of reasonable mechanisms for producing formic acid from an aldose led to the hypothesis that the sugar forms an addition product with the hydroperoxide anion, comparable with an aldehyde sulfite or the addition product of aldoses with chlorous acid (52). The intermediate product (12) could decompose by a free-radical or an ionic mechanism. In the absence of a free-radical catalyst, the ionic mechanism of Scheme VIII seems probable. By either mechanism the products are formic acid and the next lower sugar. The lower sugar then repeats the process, with the result that the aldose is degraded stepwise to formic acid. Addition of the hydroperoxide anion to the carbonyl carbon is in accord with its strong nucleophilic character (53) and with certain reaction mechanisms suggested in the literature (54) for related substances. 

Mack, J. W., Steven, A. C., and Steinert, P. M. (1993). The mechanism of interaction of filaggrin with intermediate filaments. The ionic zipper hypothesis./. Mol. Biol. 232, 50-66. 

The hypothesis of the existence of a pure ionic mechanism, involving species such as CgHjO or CgHjCOO in the reaction of epoxy with either phenol or benzoic acid, has been mentioned in the earliest publications relating to this reaction Shechter et al. studying the reaction between l,2-epoxy-3-phenoxypropane and octanoic acid in the presence of a basic catalyst (KOH or benzyldimethylamine), proposed an ionic mechanism with the carboxylate ion as active species. However, their hypothesis has no fundamental kinetic grounds. 

As already stated, Kakiuchi and Tanaka criticized the hypothesis of a pure ionic mechanism that they did not find consistent with the dependence of the stem on the temperature and solvent. Their kinetic study provides supplementary clues to the critics kinetic relations resultii from the hypothesis of a pure ionic mechanism do not fit experimental values. On the other hand, if the reaction proceeds as shown in (44), (45), and (46), the rate of formation of IV is given by ... 

In fact, the present available data on aromatic halide anion radicals reactivity seem to support the second hypothesis, since their life time is generally much greater than that corresponding to that of a solvent cage. On the other hand, it must be realized that the known reactivity on aromatic halide anion radicals rests only on cases where the anion radical is a free species, whereas, in the above ionic mechanism, the anion radical is considered to be coordinate at least partially by the metallic moiety. This may induce several bond distortions vis a vis a free anion radical, which may help to decrease considerably the life time of the... 

It appeared to us that the only reasonable non-ionic reaction product of an acid and an olefin would be an ester, and for this reason we put forward the idea that this is the active species in the pseudo-cationic polymerizations. Of course, the idea of an ester in this role has a respectable ancestry which has been discussed in this new context [6]. The ester mechanism of polymerization will be discussed in sub-section 3.3. It must be understood that our conclusion concerning the non-ionic nature of the chain-carriers in the pseudocationic polymerizations is quite independent of our view that the chain-carriers are esters this is at present merely an hypothesis to explain our factual conclusion. 

Stereoselective polymerization may proceed by ionic or coordination mechanisms. In many cases one admits that in the counterion or in the catalytic complex enantiomeric active centers exist, which give rise to predominantly (R) or (S) chains, respectively. Such centers may exist prior to polymerization or may be formed by reaction of a nonchiral precursor with the enantiomeric mixture of the monomers. Alternatively, one can think that the stereoselectivity depends mainly on the interaction between the entering monomer molecule (which is chiral) and the last unit in the chain (also chiral) according to this hypothesis, the enantiomeric excess inside each chain is generally low, because the occurrence of an accidental error brings about an inversion of the sense of stereoselection. 

The experimental evidence for the second hypothesis was the observed increase of the cracking rate of alkanes after addition of small amounts of alkenes to the feed. Both early theories assumed the continuation of the cracking reaction by intermolecular transfer of the charge from the products to fresh starting molecules, that is like an ionic chain mechanism with the catalyst acting only as an initiator. The problem was further clouded by the fact that two types of acid centres exist on the surface, the Br0n-... 

The second view stipulates an ion-exchange mechanism (21, 16, 19, 22). In this hypothesis, it is the unpaired hydrophobic alkyl ions that adsorb onto the non-polar surface and cause the column to behave as an ion-exchanger. As the chain length of the ion-pairing reagent increases, the surface coverage of the stationary phase increases, with a concomitant increase in retention of the ionic sample. 

Based on this postulate and the pronounced effect of agitation on particle incorporation Buelens et al7X77 proposed a five-step mechanism for composite deposition. In the first step particles in the bulk of the electrolyte obtain an ionic cloud by adsorbing ions from the electrolyte. In the second and third step the particles are transported by bath agitation to the hydrodynamic boundary layer and by diffusion through the diffusion layer to the cathode surface. Finally, the particles adsorb on the cathode surface still surrounded by their ionic cloud and are incorporated by the reduction of some of the adsorbed ions. A model for the calculation of the weight percent of incorporated particles was developed consistent with this mechanism. The basic hypothesis of the model is that a certain amount, x, out ofX ions adsorbed on a particle must be reduced at the... 

It has been suggested that active sites in heterogeneous Heck reactions are actually dissolved palladium species.1901 Such a hypothesis is in agreement with a mechanism where palladium nanoparticles suspended in an ionic liquid are suggested to act as reservoir to a molecular catalyst, which is formed upon oxidative addition, as shown in Figure 6.4.1311... 

The currently favoured working hypothesis for the stabilization mechanism of the particles is to claim an ampholytic surface structure, whereas the cationic surfactant only stabilizes the hydrophobic "patches" of the non-ionic methylsilane group, the remaining surface of the particles is stabilized by the anionic Si-0 groups. 

---