Primary Alkyl Radicals and Free-Radical Clock Methodology

1 PRIMARY ALKYL RADICALS AND FREE-RADICAL CLOCK METHODOLOGY 

The kinetic data for the reaction of primary alkyl radicals (RCH2 ) with a variety of silanes are numerous and were obtained by applying the free-radical clock methodology. The term free-radical clock or timing device is used to describe a unimolecular radical reaction in a competitive study [2-4]. Three types of unimolecular reactions are used as clocks for the determination of rate constants for this class of reactions. The neophyl radical rearrangement (Reaction 3.1) has been used for the majority of the kinetic data, but the ring expansion rearrangement (Reaction 3.2) and the cyclization of 5-hexenyl radical (Reaction 3.3) have also been employed. 

In Table 3.1 are collected the rate constants with various silanes and silan-thrane derivatives 4-7 and the available Arrhenius parameters for some of them. 

Intramolecular hydrogen abstraction by primary alkyl radicals from the Si—H moiety has been reported as a key step in several unimolecular chain transfer reactions [11]. In particular, the 1,5-hydrogen transfer of radicals 8-11 (Reaction 3.4), generated from the corresponding iodides, was studied in competition with the addition of primary alkyl radicals to the allyltributylstannane and approximate rate constants for the hydrogen transfer have been obtained. Values at 80 °C are in the range of (0.4-2) x 10 s, which correspond to effective molarities of about 12 M. 

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