Mechanisms breaking single bonds

When we learned how to draw resonance structures, we saw two conunand-ments that we must not violate (1) never break a single bond, and (2) never exceed an octet for second-row elements. When drawing mechanisms, we are trying to understand where the electrons actually moved to break and form bonds. Therefore, it is OK to break single bonds. In fact, it happens in almost every reaction. So when drawing mechanisms there is only one commandment to follow never exceed an octet for second-row elements. 

The coupling of single electron transfer, bond breaking and bond formation has received a great deal of attention over the past decade both from experimental and theoretical points of view, resulting in substantial progress in the understanding of dynamics and mechanisms. 

An alternative mechanism occurs for concerted photoreactions in which the bond-breaking and bond-forming processes occur together. Such reactions proceed via a single transition state ... 

In a similar manner may proceed reactions in which the number of single bonds is increased, as a result not of the breaking of a double bond, but of a valency change of one of the atoms. Thus, for example, the mechanism of the reaction... 

In resonance, you should never, ever break a single bond however, many mechanisms involve the breaking of a single bond. Nonetheless, you should never, ever exceed an octet of electrons for any atom in the second period. 

C—N bonds are breaking, and force constants of the C—H bonds on both sides are altered at the transition state.69 Pryor and Smith have found independent evidence favoring the single bond cleavage mechanism for unsymmetrical azo compounds.70... 

We apply the concept of catalytic commitment, as proposed by Northrop, O Leary, and Cleland for multistep enzyme-catalyzed processes, to nonenzy-mic decarboxylation for comparison.52 The interpretation of CKIEs for decarboxylation reactions is dependent upon whether the process is viewed as a single-step or multi-step process. In a single-step mechanism, carbon-carbon bond-breaking is not affected by any other rate-limiting process. In this case, the CKIE for a particular compound will be constant under a standard set of conditions. Substantial changes in bond order must occur in the... 

Consider the first step of the mechanism above, where we are breaking a single bond ... 

So far, two different mechanisms of single strand break formation based on adiabatically stable anions have been proposed. The first mechanism, suggested by the Leszczynski group, assumes the formation of stable anions of 3 - and 5 -phosphates of thymidine and cytidine in which the cleavage of the C-O bond take place via the SN2-type process. The second reaction sequence, proposed by us, starts from the electron induced BFPT process followed by the second electron attachment to the pyrimidine nucleobase radical, intramolecular proton transfer, and the C-O bond dissociation. In both mechanisms the bottleneck step is associated with very low kinetic barrier which enables the SSB formation to be completed in a time period much shorter than that required for the assay of damage. 

If, whatever the interest of conceiving electron-pair transfer reactions such as Sn2 substitution as an inner sphere electron-transfer process, single electron transfer is intended to qualify reactions in which the rate-determining step is an outer sphere, non-dissociative or dissociative electron-transfer step preceding the bond-formation step, then the answer is no. There are a number of cases where true SN2 mechanisms (in which the bond-breaking and bond-formation steps are concerted) do occur, even with nucleophiles that are members of reversible one-electron reversible redox couples. In terms of activation energy, the SN2 mechanism merges with the outer sphere, dissociative electron-transfer mechanism when the bonded interactions in the transition state vanish. Steric constraints at the electro-... 

The Hammond postulate is often accepted as a general principle an increase in reactivity is accompanied by a decrease in selectivity because the transition structure becomes closer to that of the reactant state as the energy barrier decreases. This idea has some truth for a hypothetical A to B reaction model where it is implicit that only a single bond change occurs moreover the Hammond postulate is predicted by the Marcus theory (above). The postulate often breaks down for reactions where more than one major bond change results in product formation. It should be emphasised that any discussion of the reactivity-selectivity relationship has to be confined to those reactions where there is no change in rate-limiting step or mechanism. 

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