Radical clock methodology

The kinetic data reported in this chapter have been determined either by direct measurements, using for example kinetic EPR spectroscopy and laser flash photolysis techniques or by competitive kinetics like the radical clock methodology (see below). The method for each given rate constant will be indicated as well as the solvent used. An extensive compilation of the kinetics of reaction of Group 14 hydrides (RsSiH, RsGeH and RsSnH) with radicals is available [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. 

The free-radical clock methodology has been also applied to calibrate unim-olecular radical reactions based on the A h values of Table 3.2 and, when avail-... 

Scheme 6.9 shows the radical clock methodology approach used for obtaining the rate constant of the ring expansion (k e)- Radical 37 was obtained by the reaction of the corresponding phenylseleno derivative with (TMS)3SiH. A relative rate constant of = 20.2 was obtained at 80 °C under first-... 

Scheme 6.9 Radical clock methodology approach for measuring the rate constant of ring expansion (cf. Section 3.1.1)...

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