Abstraction, hydrogen reactivity trends

The relative amounts of double bond addition, hydrogen abstraction and 13-scission observed are dependent on the reactivity and concentration of the particular monomer(s) employed and the reaction conditions. Higher reaction temperatures are reported to favor abstraction over addition in the reaction of t-butoxy radicals with AMS413 and cyclopentadiene 417 However, the opposite trend is seen with isobutylene.2 1 24... 

The trend of reactivities which is observed for hydrogen atom abstraction by partially fluorinated radicals is qualitatively similar to that for their addition to styrene. However, the absolute rates and the range of reactivities for each type of process can be seen to differ significantly. Thus, absolute rate constants for... 

As the title suggests, we chose a different classifying principle by defining the reaction centre instead of the radical. This limitation allows us to consider the particular problem of homolytic reactivity of the O—H group in satisfactory depth to discuss some kinetic problems of general interest. The field where the limitation has been felt to cause undesirable restrictions is that of Arrhenius parameters. Since a general trend is suspected, that discussion concerns hydrogen abstraction from other bonds as well. 

The trend of reactivity tert > sec > pri is consistently observed in various hydrogen atom abstraction reactions, but the range of reactivity is determined by the nature of the reacting radical. The relative reactivity of pri, sec, and tert positions toward hydrogen abstraction by methyl radicals is 1 4.8 61. An allylic or benzylic hydrogen is more reactive toward a methyl radical by a factor of about 9, compared to an unsubstituted C—H. The relative reactivity toward the t-butoxy radical is pri 1, sec 10, tert 50. In the gas phase, the bromine atom is much more selective, with relative reactivities of pri 1, sec 250, tert 6300. Data for other types of radicals have been obtained and tabulated. ... 

Table 11.11 provides further insight into the origin of the selectivity in free radical halo-genations. The is essentially zero for all hydrogen atom abstractions by fluorine atom, and the free energy barrier arises solely from the log A term of the Arrhenius equation, which is near 13 for all the halogenations given in Table 11.11. The activation energies for abstraction by chlorine atoms are also exceedingly small but in the direction of the trends discussed. Lastly, the activation energies for abstraction by bromine atoms are substantial, and they clearly produce the differential reactivities of 3°, 2°, and 1° C-H bonds. Because of these differences the relative selectivities of Table 11.10 are temperature dependent.

With the exception of implications regarding solubility, a feature not yet apparent is any recognized trend in the emissions from sulphur cures with variations in the base polymer. This is not the case with peroxide cures, where the reactivity of the polymer can influence both the quantity and type of emissions. A well-studied example is that of NR which carries an abundance of abstractable allylic hydrogens to favour alcohol formations (eqn (29)). Thus when DTOP (R = Me) is the peroxide, fert-butanol (BP 82°C) is obtained, whilst cumyl alcohol (2-phenyl-2-propanol BP 202°C) is obtained from Dicup (R = Ph). Ketone formation (eqn (30)) competes with hydrogen abstraction and can predominate in the presence of a different polymer emissions from formulations based on EPDM, silicone and a fluoroelastomer have been characterized. Other by-products include alkenes from alcohol dehydration, although numerous other reactions can occur. 

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