Break, take

CN/CC replacement has also been observed on treatment of pteridine with malonitrile or cyanoacetamide 6-amino-7-R-pyrido[2,3,-h]pyrazine (R = CN, CONH2) beingformed (73JCSP(1)1615) (Scheme 15). The reaction involves initial addition of the reagent to the N-3-C-4 bond, scission of the dihydro bond between N-3 and C-4 in the covalent adduct, and recycli-zation. This mechanism is fundamentally different from the mechanism mentioned in Scheme 14, where two molecules of the reagent were used for addition and where the bond breaking takes place between N-1 and C-2. 

Figure 8.8 Advective propagation of a chemical wave of tracer i moving with a velocity v in a wetted porous solid at times t= 1,2, 3 for different values of d2Cwl7(dCUq )2. Breaking takes place at t = 3 in cases b and c.

When bond-formation and -breaking take place simultaneously... 

Reverse reactions, in which the disulfides are reduced and S—S bond breaking takes place, are also possible. So thiuram disulfide reacts with elemental Cu or Ag yielding the metal dithiocarbamates, and (109) can be oxidized with thiuram disulfide to (110) and (111) to (112). 

Pericyclic reactions are the third distinct class. They have cyclic transition structures in which all bond-forming and bond-breaking takes place in concert, without the formation of an intermediate. The Diels-Alder reaction and the Alder ene reaction are venerable examples. The curly arrows can be drawn in either direction—clockwise, as here, but equally well anti- clockwise. They could even be drawn with fishhook arrows, and would still... 

The anows can be written in either directional sense since these reactions are concerted rearrangements with all bond making-bond breaking taking place at die same time. This example emphasizes the fact that curved-arrow notation is merely an electron bookkeeping method. 

C-C bond making or breaking takes place, is only possible because of the acidity of C-2 (the C), which allows the formation of the nucleophilic carbanion. This acidity can be explained by the resonance structures which may be drawn for the alkyl-carbonyl "group" ... 

As described in Chapter 3, the reasons given to explain this dilution effect of copper on nickel were that the hydrogenolysis reaction required a group or ensemble of nickel atoms on the catalyst surface and the presence of copper prevented the formation of the appropriately sized ensembles and the reaction was inhibited. On the other hand, the reactions involving C-H bond breaking take place on single atom sites so the surface dilution by copper has no effect until the surface is almost completely covered by the copper. The rate increase observed in the deuterium exchange on cyclopentane (Fig. 12.7b), however, is not easily rationalized by this surface ensemble effect alone. 

Likewise, in the cationic [1,2] alkyl shift, both components must be suprafacial for there to be positive overlap in the TS between orbitals where bondmaking and bond-breaking take place. The migrating group retains its configuration because of the requirement for suprafaciality. 

By contrast, in the [1,2] anionic H shift, the dominant FMO interaction is between the Is orbital of the one-atom component and t/q of the two-atom component. For there to be positive overlap between orbitals where bond-making and bond-breaking take place, the two-atom component must react antarafacially. Thus, for this shift to be a thermally allowed process, the H atom must have partial bonds to the top and bottom faces of the C2 unit simultaneously. Because this arrangement is geometrically impossible, [1,2] anionic H shifts are thermally disallowed reactions. 

On the basis of mass - spectroscopic method there was made a supposition that the break takes place mainly in carboxyl groups in three states [203] ... 

Bimolecular reactions involve a collision between two molecules, with enough energy to overcome the activation barrier. These processes are usually concerted, with bond formation and breaking taking place simultaneously. The relative orientation is important, so that the new bonds can be formed between atoms that are near enough to each other. Reactions (1.1a) and the first step of (1.7b) are examples of bimolecular reactions. Reaction (1.10) is an example of a bimolecular reaction that does not involve ions. Three covalent bonds are broken and formed synchronously, and Diels Alder reactions of this type are very useful... 

These models are often oversimplified by neglecting the movement of the solid-gas interface and therefore result in too high temperatures [38,42] In this model by Arnold and Bityurin [55], a thermal surface model and a photochemical volume model have been combined. In this model, it is assumed that photothermal bond breaking takes place within the bulk polymer. When a density of broken bonds reaches a critical value, ablation begins. This model can account for sharp ablation thresholds and Arrhenius tails. 

The dominant FMO interaction in the TS of the [1,5] sigmatropic rearrangement, a six-electron reaction, is between the H(ls) orbital and ifc of the five-atom component, no matter how the six electrons are divided among the two components. (An interaction between two half-filled orbitals (singly occupied MOs. or SOMOs) is shown.) Again, positive overlap in the TS between orbitals where bond-making and bond-breaking take place is produced when the five-atom component reacts suprafacially. 

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