Rates of fragmentation

In this reaction, too, the stability of the radical being eliminated is the major factor in determining the rate of fragmentation. 

These fragmentations are sufficiently slow that the initial radicals can undergo reactions such as addition to alkenes at rates which are competitive with the rate of fragmentation. 

Collisions between NO2 molecules produce N2 O4 and consume NO2. At the same time, fragmentation of N2 O4 produces NO2 and consumes N2 O4. When the concentration of N2 O4 is veiy low, the first reaction occurs more often than the second. As the N2 O4 concentration increases, however, the rate of fragmentation increases. Eventually, the rate of N2 O4 production equals the rate of its decomposition. Even though individual molecules continue to combine and decompose, the rate of one reaction exactly balances the rate of the other. This is a dynamic equilibrium. At dynamic equilibrium, the rates of the forward and reverse reactions are equal. The system is dynamic because individual molecules react continuously. It is at equilibrium because there is no net change in the system. 

Cooks, R.G. Williams, D.H. The Relative Rates of Fragmentation of Berrzoyl Ions Generated Upon Electron Impact from Different Precursors. Chem. Common. 1968, 627-629. 

The present experiments may be subject to some uncertainty in terms of molecular weight estimates and diffusion effects that could affect the exactness of these interpretations. The conclusion is reached that the experiments strongly suggest evidence for discontinuous topological quantification. In other words, the order and rate of fragment release is consistent with expectations based on fragment topology. 

This simple model predicts that the observed kinetics is determined by the rate of fragmentation only when the reverse process is much slower than counterdiffusion (i.e. when k f under activation control. On the other hand, for an endergonic fragmentation it is expected that k f k and /Cobs = The reaction now is described as a pre-equilibrium... 

More recently, Mies and Kraus have presented a quantum mechanical theory of the unimolecular decay of activated molecules.13 Because of the similarity between this process and autoionization they used the Fano theory of resonant scattering.2 Their theory provides a detailed description of the relationships between level widths, matrix elements coupling discrete levels to the translational continuum, and the rate of fragmentation of the molecule. 

Nitro-substituted phenyl halides produce radical anions that fragment with a rather low rate (=10 2-102/s) [28]. For this reason the nitro group is not a suitable substituent for most aromatic SrnI reactions. However, exceptions are found with o-iodonitrobenzene [29] whose radical anion has a relatively high rate of fragmentation, and nitroaryldiazo phenyl or tert-butyl sulfides [30]. 

With the substrate 4-bromobenzophenone, whose radical anion has a very slow rate of fragmentation (6 x 102 s 1 in liquid ammonia)171, the trend changes, possibly because of an equilibrium with the radical anion of bromobenzene which, although thermodynamically less stable, fragments very rapidly. 

With PhTe" ions the symmetrical telluride (/ -NCC6H4)2Te was formed. With p-chlorobenzonitrile, whose radical anion has a slow rate of fragmentation, the yield of 254 was 70%309a. However, with o- and m-chlorobenzonitriles the yields of the products were low, but in the presence of l,2-di(4-pyridyl)ethylene as redox catalyst, the yields could be improved310. 

The stereoelectronic effects discussed in Sect. 3.3 are also illustrated by the data in Table 1. Thus, the meta derivatives fragment slower than para isomers an increase in the spatial separation between the n system and the scissile bond lowers the rate of fragmentation and substituents increasing the extent of charge delocalization in the transition state accelerate the cleavage [99, 106]. 

The rates associated with ion-pair dynamics are thus competitive with the rates of BET as well as with the rates of fragmentation. The quantum yield for fragmentation, therefore, usually represents a composite of all these rate constants ([Pg.29]

Rates of Fragmentation. The solvent dependence was studied first (Table IV). Solutions of styrene ozonide were stored at 50°C. At various intervals, samples were quenched with ice water, and the acid formed was titrated with 0.051V KOH, using phenolphthalein indicator. While the ozonide was stable in benzene, carbon tetrachloride, and chloroform... 

Rule 5 the release of strain by the fragmentation of the ozonide is very important to the rate of fragmentation. Table XII gives three pairs of ozonides. Each pair is chemically similar but differs in the Baeyer strain. The strained ozonides are cleaved up to more than 100 times faster than the unstrained ozonides. 

Attempts have been made to estimate the rates of fragmentation processes, correlate them with cr and deduce mechanistic information from the slope of the regression line. After making a number of assumptions, the rate constants (ky, ko) were related to the concentration ratio (Z s [A]/[M]) of fragment ion (A) and molecular ion (M) kyjko — [AJ/ [My] / [Ao]/[Mo] = Zy/Zo (Bursey and McLafferty, 1966). The ratio Z was assumed to be equal to the relative intensities of the relevant peaks, measured at the collector (but see Howe and Williams, 1968b, and Bursey and Rosenthal, 1968). 

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