Reaction Rates of t‐Butyl and Pivaloyl Radicals in Solution

Abstract

Summary. The rate constants of the unimolecular decomposition of the pivaloyl radical (AD) and of the bimolecular self terminations of pivaloyl (k,) and t-butyl radicals (k,) in liquid mcthylcyclopentane arc determined by ESR.-spectroscopy :The viscosity dependence of K, is analysed with respect to diffusion control of tlie reaction.Comparison of k D values of different acyl radicals reveals a strong dependence of the activation energies on radical structure. . The studies were limited to small temperature ranges and few activation parameters were reported.In this paper, we extend our previous studies of photochemically produced radicals and their reaction rates [lo-121 to a determination of the absolute rate constants of self termination of the pivaloyl (R1= (CH,),CCO) and t-butyl (R2-(CH,),C.) radicals and of decomposition of pivaloyl (R1 --f R2 + CO) in a wide temperature range. The radicals are produced by photocleavage of di-t-butylketone in methylcyclopentane solutions. Steady state and time resolved ESR.-spectroscopy is applied.The particular chemical system was chosen for several reasons: Firstly, the radical reactions are rather simple and well known at room temperature [13] [14]. Secondly, the high quantum yield of cleavage leads to strong ESR.-signals of R1and R2 [lo] [ll]. Thirdly, though the self termination of t-butyl (R2) has been reported for several systems at or near room temperature 1-31 [5] 1-12] [15] the data show a considerable scatter and no activation parameters have been given. These would be highly instructive with respect to the possibIe diffusion control of this reaction in solution and for a comparison with tlie recent gas phase values [16]. Finally, the rate constant for decomposition of pivaloyl, a reaction reported for liquids [17], is of interest because a comparison with the known constants of other l)

Rate constants for self‐ and cross terminations of transient radicals in solution by effect modulated electron spin resonance spectroscopy

“…Bimolecular terminations of small, sterically unhindered radicals have been found to occur near the diffusion-controlled limit, and their rate constants are commonly discussed in terms of a generalized Smoluchowski equation [14-161: where NA denotes Avogadro's number, u a spin statistical factor [16], p l 2 the reaction distance, and D1 and 0 2 the diffusion coefficients of radicals R 1 and R2. kT2 is the rate constant for infinitely fast diffusion [XI.…”

Effects of diffusion on the self‐termination kinetics of isopropylol radicals in solution

“…Under these conditions the spectrum of isopropylol did not exhibit CIDEP effects. Absolute isopropylol concentrations during steady-state photolysis were obtained in the standard manner described in [26] using the data system for numerical double integration. In kinetic runs with the photolysis interrupted by a sector (duty cycle 1:4) in the frequency range of 13 Hz < v 5 180 Hz, typically 80,000-120,000 individual amplitude versus time profiles were sampled for one line of the isopropylol radical (transitionK = 1, M = +1 or M = -1, CH3 coupling, M = -l/z, OH coupling).…”

“…In analogy to similar cases [26,27], the temperature dependence may be analyzed to yield estimates for the rate constant of decarbonylation. We assume that the termination rate constants of isopropylol and acyl radical are about equal.…”

TiO2 in Organic Photosynthesis: Sunlight Induced Functionalization of Heterocyclic Bases

“…For example, the decarbonylation rate for substituted acyl radicals with primary, secondary or tertiary alkyl group at 80 °C were calculated to be 1.9 10 4 , 3.9 10 5 , 1.0 10 7 s -1 respectively. In the case of the pivaloyl radical, the dependence of the rate of decarbonylation from the temperature was found out to follow the equation: log K d = 11.9 -9.3/2.3 RT [56]. While it is obvious that the rate of decarbonylation increases by increasing the temperature for entropic reasons, it is less obvious, although it has been calculated, the strong influence of the solvent polarity on the decarbonylation rate.…”