Ionic mechanisms continued

A homolytic cyclopropane C-C bond rupture that would furnish in turn a 1,3-diradical is also conceivable. However, it is always difficult to establish whether a purely diradical or ionic mechanism is in operation. Between these two extremes there exists a graded continuum of polarized diradicals of which the zwitterion represents the end of the spectrum. In addition, the continuous development of radical character during the formation of the transition state of a homolytic bond scission, called the continuous diradical, has been postulated to explain the behavior of some reactions. Alternatively, the contribution of a truly concerted transformation cannot be overlooked. ... 

Studies of the structure and reactivities of the three ions discussed in this article have continued to bear out the possibility of an ionic mechanism for soot formation. The reaction rate coefficients are certainly rapid and in general larger than those used by Olson and Calcote (10) in their model, at least for certain isomeric forms of the ions and at the temperatures of ca. 325 K used in our work. However, until more sophisticated flamesampling mass spectrometers are employed, the exact isomeric form of the many ions seen in flames cannot be known, and thus cannot be related directly to our work. As is to be expected, details of the reactivity or nonreactivity of various isomers vary as one moves from one ionic species to another. Thus in some cases (C5H5 ) acyclic forms are less reactive, while the cyclic isomers react rapidly, while in others (C H " , C Hc" ) the opposite is the case. Those channels where the cyclic ions are less reactive suggest an opportunity for the formation of cyclic neutrals in... 

The reduction of em-dihalogenocyclopropanes with tin hydrides continues to be a valuable source of information on the behaviour of cyclopropyl radicals. Breslow and Sugimoto have reduced tetrachlorocyclopropene by this method and find a change from radical to ionic mechanisms on going from non-polar to polar solvents. 

Extrusion of sulphur from disulphides " and trisulphides gives sulphides and disulphides, respectively, by treatment with phosphines (a polymeric aminophosphine has been advocated ), phosphites, and MeHgOAc. Ionic pathways continue to be preferred for these reactions, and concurrent electrophilic and nucleophilic mechanisms are in operation, according to interpretations of kinetic data for the reaction with dimethyl disulphide. A new view on nucleophilic attack is advanced in this work, where the encounter involves the o orbital of the S—S bond. Further support is given by c.i.d.n.p. to a radical mechanism that involves PhCHjSe for the reaction of dibenzyl selenide with PhjP. The process PhTeTePh + RN2 Cl - RTeCljPh in the presence of lCu(OAc)2l may be more easily understood on this basis. 

On the other hand Feniak et al. (1974) suggest that the continuous process now being operated by Polysar proceeds by the following ionic mechanism ... 

Although the alkylation of paraffins can be carried out thermally (3), catalytic alkylation is the basis of all processes in commercial use. Early studies of catalytic alkylation led to the formulation of a proposed mechanism based on a chain of ionic reactions (4—6). The reaction steps include the formation of a light tertiary cation, the addition of the cation to an olefin to form a heavier cation, and the production of a heavier paraffin (alkylate) by a hydride transfer from a light isoparaffin. This last step generates another light tertiary cation to continue the chain. 

The elementary reactions comprising the chain reaction mechanism are generally classified as initiation, propagation, or termination reactions. In the initiation reaction an active center or chain carrier is formed. Often these are atoms or free radicals, but ionic species or other intermediates can also serve as chain carriers. In the propagation steps the chain carriers interact with the reactant molecules to form product molecules and regenerate themselves so that the chain may continue. The termination steps consist of the various methods by which the chain can be broken. 

Many attempts have been made to calculate important macroscopic properties of ionic solutions without a detailed knowledge of their molecular structure. All models developed for this purpose can be classified into three categories according to the way the surrounding of the central ion is accounted for. Usually, continuous and discontinuous theories of ionic solvation are distinguished. Furthermore, it seems appropriate also to mention various attempts to apply statistical mechanics directly to ionic solutions. 

The mechanisms of ion formation in MALDI are a subject of continuing research. [30-34] The major concerns are the relationship between ion yield and laser flu-ence, [28,35] the temporal evolution of the desorption process and its implications upon ion formation, [36] the initial velocity of the desorbing ions, [29,37,38] and the question whether preformed ions or ions generated in the gas phase provide the major source of the ionic species detected in MALDI. [39,40]... 

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