Olefin addition reaction absolute rate

Table VII. Absolute Rate Constants for Selected Thermal Hydrogen Abstraction and Olefinic Addition Reactions by Atomic Fluorine"...

A comprehensive listing of absolute thermal rate coefficients for hydrogen abstraction and olefinic addition reactions by atomic fluorine has been given in Table X. 

No absolute rate data exist for fluorine atom additions to olefins. This is not at all surprising in view of the tremendous experimental difficulties in carrying out reactions between fluorine and organic compounds of almost any sort. Rowland and co-workers [45] have been studying the interaction of fluorine atoms with a variety of compounds, however, and these include olefins. Relative rate data are given in Table 11 in a form such that the inter- and intramolecular reactivity of various unsaturated carbons is normalized against ethylene. The total reactivity is, of course, the summation of the positional reactivities of the individual carbons for any given compound. Rowland and co-workers [45a] have also measured the relative reactivity of the terminal versus the central carbon of propene towards fluorine atoms, and find the terminal position to be 1.35 times more reactive. 

The present author, over several years, has published a number of papers on the relative reactivity of olefins towards addition of HBr, using competition methods, and has derived some mechanisms without obtaining absolute rates [65, 66]. The reaction is a complex one. The initial radical is in an excited state, and is deactivated most effectively by... 

The methyl radical is not very discriminating, and shows none of the electrophilic character of most of the other radicals discussed earlier in this chapter. To what extent steric effects enter into the determination of the rate is hard to decide, but they probably are not pronounced, since the relative reactivity series is not greatly different from that of hydrogen atom additions to the same series of olefins. The relative rates are given in Table 50. It should be noted that these are derived from the absolute rates, and are not the results of competition reactions. Accordingly, there may be some substantial errors in the individual values. A very large number of miscellaneous olefinic compounds have been studied and the rate parameters for the addition are given in Table 51. In many of these cases, the methyl radicals were produced by photochemical decarbonyla-tion of acetaldehyde. After the methyl radical adds to the olefin, the new... 

No attempt has been made to provide the intensive detail found in Howard s review of oxyradicals [10], the review of Hendry et al. [43] of H-atom transfer to several radicals or Anbar and Neta s review of HO-radical reactions [44]. Instead, we have attempted to extend the scope of those reviews in two ways (i) rate coefficients are provided for R02-radical addition to many olefins, for ring closures to form cyclic ethers, and for intramolecular abstraction (ii) for each reaction, we have estimated the best value Arrhenius parameters (A-factor and E) and, where such values have been measured they are also listed. We believe the value of absolute rate coefficients is improved substantially by the availability of reliable Arrhenius parameters, with which one can calculate the values of rate coefficients at other temperatures for use in experimental or modelling studies. 

Finally, what of my early idea of reacting a series of arylcarbenes (X-CgFl4CH) with a fixed pair of olefins, and correlating log (kjk ) vs. Gx (see Section 1.1) I actually carried out such experiments during my doctoral research. For example with carbene substituents X = p-MeO, p-Me, FI, p-C, and m-C, and the olefin pair isobutene/iraws-butene, the log ( 1) data is correlated by a constants to give p = +0.3. [7] We can interpret this result as the difference of the p values for carbene addition to isobutene vs. traws-butene, where the p for isobutene is more positive than that for traws-butene. It is difficult to interpret this data further. A clearer picture emerges with the use of absolute rate constants for carbene/aUcene addition reactions. Those experiments will be described below in Section 2.2. 

To deterimine the absolute rate for addition of PhCCl to Me2C=CMe2, Eq. 9, one generates the carbene by LFP of the diazirine in isooctane that contains a specific concentration of the substrate olefin. A pseudo-first-order rate constant can then be determined for the decay of the carbene by following the time-dependent loss of its UV absorbance at 320 nm. (Conventional photolysis and product analysis establishes that the reaction gives the expected cyclopropane... 

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