Preferential breakage

While dioxolanes are generally inert, sodium in liquid ammonia opens 1,3-oxathiolanes by preferential breakage of the C—S bond, since the developing negative charge is accommo-... 

Many authors have assumed that breakage occurs preferentially along grain boundaries, but there is scant evidence for this. On the contraiy, Gorski [Bull. Acad. Pol. Sci. Ser Sci. Tech., 20(12), 929 (1972) CA 79, 20828k], from analysis of microscope sections, finds an intercrystalhne character of comminution of dolomite regardless of the type of crusher used. 

Co-adsorption experiments show a complex role of the nature and concentration of chemisorbed ammonia species. Ammonia is not only one of the reactants for the synthesis of acrylonitrile, but also reaction with Br()>nsted sites inhibits their reactivity. In particular, IR experiments show that two pathways of reaction are possible from chemisorbed propylene (i) to acetone via isopropoxylate intermediate or (ii) to acrolein via allyl alcoholate intermediate. The first reaction occurs preferentially at lower temperatures and in the presence of hydroxyl groups. When their reactivity is blocked by the faster reaction with ammonia, the second pathway of reaction becomes preferential. The first pathway of reaction is responsible for a degradative pathway, because acetone further transform to an acetate species with carbon chain breakage. Ammonia as NH4 reacts faster with acrylate species (formed by transformation of the acrolein intermediate) to give an acrylamide intermediate. At higher temperatures the amide may be transformed to acrylonitrile, but when Brreform ammonia and free, weakly bonded, acrylic acid. The latter easily decarboxylate forming carbon oxides. 

Fig. 5. Fragmentation nomenclature of peptides. Bond breakages of all bonds of the peptide backbone have a systematic name (I). When fragmenting multiply charged peptide ions the peptide bond breaks preferentially since it is among the most labile bonds and only relatively low collision energies are involved (II).

This may seem paradoxical, as the kinetic isotope effect induced by S-O bond breakage still exists. How can the overall reaction have little isotopic fractionation when one step within it has a large kinetic isotope effect The key to understanding this is in the isotopic composition of the intermediate species in the reaction chain. An intermediate may become enriched in heavier isotopes if the next step in the reaction chain preferentially consumes lighter isotopes. In the hypothetical case described above, at steady state the sulfate within the cell is enriched in the heavy isotope by an amount equal to the kinetic isotope effect occurring at step 2. Thus, the isotopic composition of the flux of S through step 2 is the same as that of the flux of S into the cell and the kinetic isotope effect occurring at step 2 has no effect on the overall isotopic fractionation. 

Backbone chain scission degradation can be divided as occurring via depolymerization, random chain breakage, weak-link or preferential site degradation, or some combination of these general routes. In depolymerization, monomer is split off from an activated end group. This is the opposite of the addition polymerization and is often referred to as unzipping. ... 

The use of single crystals allows specific reactions to be associated with specific crystal faces. Somorjai and coworkers showed that aromatization of n-hexane to benzene occurs over Pt(lll) faces, whereas isomerization and hydrogenolysis are not specifically structure-dependent. However, hydrogenolysis products from (100) terraces are those from internal C-C bond breakage, while terminal bonds are preferentially broken over (111) terraces. Hence, if specific crystal faces are covered by coke or poisons, a shift in selectivity can be expected. 

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