Bond breaking synchronous

In a perfect world of balanced checkbooks, the bond breaking and bond forming processes are synchronized, as it happens (albeit not always perfectly) in concerted pericyclic reactions. Even when such synchronization is not perfect, unimolecular... 

When the two o bonds break simultaneously and at an exactly the same rate, it is called a synchronous process. If, however the reactant is unsymmetrical, the two o bonds do not break at the same rate. 

Finally, in the presence of halide salts (bromide or chloride, which in low polarity non-protic solvents bind to Br2 to give a stable trihalide species), the addition reaction proceeds through a rate- and product-determining nucleophilic attack of Br anion on the 1 1 it complex, Scheme 2 path c. No intermediate is formed in this latter reaction the nucleophilic attack of halide (X ) and the Br-Br bond breaking are indeed concerted, although not necessarily synchronous. 

Along with a very wide synthetic application the Cope rearrangement continues to be a subject of intense debates. The key mechanistic question is whether the rearrangement of 1,5-hexadiene derivatives is concerted and passes via a six-electron aromatic transition state, or whether it involves the formation of a diradical intermediate, i.e. a cyclization-cleavage mechanism. In the former case, bond making and bond breaking occur synchronously (a survey of this question has been published210). 

A condition arising when reaction parameters for different bond-breaking or bond-making processes have developed synchronously as the transition state is approached. Imbalance is common in elimination and addition reactions as well as in proton transfer reactions. See also Synchronous Synchronization... 

Hammond behaviour may be observed if the intersecting parabolae are of different curvature. The physical interpretation of this difference in curvature is that bond-making and bond-breaking processes are not necessarily synchronous as is assumed in conventional Marcus theory. While such a modification in the theory may overcome the inherent problem of treating anti-Hammond effects it does make application of the Marcus theory more difficult by the introduction of additional unknowns into the free energy relationship of (112). 

The reactions of 0-naphthol and 4-methoxyphenol with acetyl, propionyl, butyryl, 0-chloropropionyl and chloracetyl chlorides in acetonitrile produce some striking kinetic results109. The behaviour of acetyl, propionyl and n-butyryl chlorides fit reasonably well into the pattern for acetyl chloride in nitromethane and acetyl bromide in acetonitrile. However, with chloracetyl chloride the mechanism is essentially a synchronous displacement of covalently bound chlorine by the phenol and this process is powerfully catalysed by added salt with bond breaking being kinetically dominant. When no added salt is present the rate of hydrolysis of chloracetyl chloride is ca. 8000 times slower than that of acetyl chloride. Although, normally, in second-order acylation reactions, substituents with the greatest electron demand have been found to have the fastest rates, the reverse is true in this system. Satchell proposes that a route such as... 

Again a distinction is possible between associative, dissociative and interchange mechanisms. In the dissociative mechanism at least one bond is broken. Dissociative reactions thus have higher AH A values than associative mechanisms. Interchange mechanisms, where synchronous bond breaking and bond forming occur, have values in between. 

The stereospecific cis-addition of diboron tetrachloride to alkynes and alkenes (37) may be interpreted as an interaction of the empty 7r-orbitals of the boron atoms with the 7r-orbital of the organic species. According to this picture, boron-boron bond breaking would lag behind boron-carbon bond formation. The transition state is a 4 + 2 Hiickel aromatic ( .=0), and thermal addition is allowed. If bond making and breaking were synchronous, this four-center reaction would be more like the <7-77 exchange reactions, which we shall discuss later. With regard to (37), there is a discrepant case in which an apparent trans addition of diboron tetrachloride to cyclopentadiene has been found (Saha et al., 1967). 

The name Houk has become synonymous with calculations on the transition states of pericyclic reactions. For two decades, as increasingly sophisticated types of electronic structure calculations became feasible for such reactions, Ken s group used these methods to investigate the geometries and energies of the transition structures. Ken s calculations showed that, in the absence of unsymmetrical substitution, bond making and bond breaking occur synchronously in pericyclic reactions. 

These features are thus characteristic of a rearrangement in which bond-breaking and bond-making are essentially synchronous processes, and no charge separation develops between the migrating carbon atom and the N=C residue. 

CHj- H bond requires 104 kcal/mole. We might have expected that only 1 kcal/ mole additional energy would be needed for feaction to occur however, this is not so. Bond-breaking and bond-making evidently are not perfectly synchronized, and the energy liberated by the one process is not completely available for the other. Experiment has shown that if reaction is io occur, an additional 4 kcal/mole of energy must be supplied. 

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