Borderline states

In the reactions shown in Eqs. 7(a) and 7(b), we tried to symbolize this dynamic behavior in a simplified manner. Certainly, the rotation motion around the Ar-O-OH axis generates a diversity of momentary electron distributions and thus very mixed conformer states. This diversity can hardly be introduced into an all-comprising chemical model. Hence, for simplification we define and further consider only two borderline states the planar state stands for the donor molecules with low angle of twist, whereas the perpendieular state includes the molecules with higher deformation angle. 

Electroless reactions must be autocatalytic. Some metals are autocatalytic, such as iron, in electroless nickel. The initial deposition site on other surfaces serves as a catalyst, usually palladium on noncatalytic metals or a palladium—tin mixture on dielectrics, which is a good hydrogenation catalyst (20,21). The catalyst is quickly covered by a monolayer of electroless metal film which as a fresh, continuously renewed clean metal surface continues to function as a dehydrogenation catalyst. Silver is a borderline material, being so weakly catalytic that only very thin films form unless the surface is repeatedly cataly2ed newly developed baths are truly autocatalytic (22). In contrast, electroless copper is relatively easy to maintain in an active state commercial film thicknesses vary from <0.25 to 35 p.m or more. 

The main features of the effect of structure on the site of attack are summarized in Table 3, and can be understood in.terms of a borderline 5n2 (59CRV737) transition state (48) which somewhat resembles an 5n1 transition state in charge distribution because C—O bond breaking runs ahead of Nu—C bond making. 

Alkyl groups under nonacidic conditions sterically deflect nucleophiles from C, but under acidic conditions this steric effect is to some extent offset by an electronic one the protonated oxirane opens by transition states (Scheme 40) which are even more 5Nl-like than the borderline Sn2 one of the unprotonated oxirane. Thus electronic factors favor cleavage at the more substituted carbon, which can better support a partial positive charge the steric factor is still operative, however, and even under acidic conditions the major product usually results from Cp attack. 

Short-time tests also can give misleading results on alloys that form passive films, such as stainless steels. With Borderline conditions, a prolonged test may be needed to permit breakdown of the passive film and subsequently more rapid attack. Consequently, tests run for long periods are considerably more reahstic than those conducted for short durations. This statement must be quahfied by stating that corrosion should not proceed to the point at which the original specimen size or the exposed area is drastic y reduced or the metal is perforated. 

The ionization and direct displacement mechanisms can be viewed as the extremes of a mechanistic continuum. At the 8 1 extreme, there is no covalent interaction between the reactant and the nucleophile in the transition state for cleavage of the bond to the leaving group. At the 8 2 extreme, the bond formation to the nucleophile is concerted with the bondbreaking step. In between these two limiting cases lies the borderline area, in which the degree of covalent interaction between the nucleophile and the reactant is intermediate between the two limiting cases. The concept of ion pairs is important in the consideration of... 

Several proposals have been made to fit the borderline reactions into a well-defined mechanistic scheme. Most of these adopt one of two viewpoints either (1) borderline substrates undergo concurrent SnI and Sn2 processes, with the particular system determining which mechanism, if either, predominates or (2) all Sn reactions are related by essentially the same mechanism, which differs from case to case in the detailed disposition of electrons in the transition state. In this view pure SnI and Sn2 processes are merely the extreme limiting forms of a single mechanism, and the borderline mechanism is a merged process having some features of both. 

The notion of concurrent SnI and Sn2 reactions has been invoked to account for kinetic observations in the presence of an added nucleophile and for heat capacities of activation,but the hypothesis is not strongly supported. Interpretations of borderline reactions in terms of one mechanism rather than two have been more widely accepted. Winstein et al. have proposed a classification of mechanisms according to the covalent participation by the solvent in the transition state of the rate-determining step. If such covalent interaction occurs, the reaction is assigned to the nucleophilic (N) class if covalent interaction is absent, the reaction is in the limiting (Lim) class. At their extremes these categories become equivalent to Sn and Sn , respectively, but the dividing line between Sn and Sn does not coincide with that between N and Lim. For example, a mass-law effect, which is evidence of an intermediate and therefore of the SnI mechanism, can be observed for some isopropyl compounds, but these appear to be in the N class in aqueous media. 

The N-Lim classification does not eliminate the possibility of borderline cases between these two categories, but it leads to the suggestion that no sharp distinction can be made between the possible intermediates in these mechanisms and that perhaps all solvolyses proceed via an intermediate. The mechanistic category of a particular solvolysis then depends upon the relative weights of the canonical structures 3, 4, and 5 to the transition state resonance hybrid. 

In reactions in which separated ion pairs are involved, e.g., R4N+, K or Na +, and as a borderline case, Li +, the cation does not contribute to the adjustment of the reaction partners in a dense, well-ordered transition state poor selcctivities arc usually the result of these carbanionic carbonyl additions. Further, the high basicity of such carbanionic species may cause decomposition or racemization of sensitive reactions partners. 

The reactions of some aromatic metal carboxylates are on the borderline of classification as solid-state processes. While there is no evidence of liquefaction, rates of decomposition in the poorly crystallized or vitreous reactant obey kinetic expressions more characteristic of reactions proceeding in a homogeneous phase. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

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