Absolute rate constants for

The absolute rate constants for the reaction of a variety of electrophilic free radicals with 4-(dimethylamino)-l,5-dimethyl-2-phenyl-l,2-dihydro-3//-pyrazol-3-one (aminopyrine) and l,5-dimefliyl-2-phenyl-l,2-dihydro-3//-pyrazol-3-one... 

The radicals formed by imimolecular rearrangement or fragmentation of the primary radicals arc often termed secondary radicals. Often the absolute rate constants for secondary radical formation are known or can be accurately determined. These reactions may then be used as radical clocks",R2° lo calibrate the absolute rate constants for the bimolecular reactions of the primary radicals (e.g. addition to monomers - see 3.4). However, care must be taken since the rate constants of some clock reactions (e.g. f-butoxy [3-scission21) are medium dependent (see 3.4.2.1.1). 

The electron transfer step is typically fast and efficient. Griller et a/.292 measured absolute rate constants for decay of benzophenone triplet in the presence of aliphatic tertiary amines in benzene as solvent. Values lie in die range 3-4x109 M 1 s 1 and quantum yields are close to unity. 

Rate constants tor reactions of carbon-centered radicals tor the period through 1982 have been compiled by Lorand340 and Asmus and Bonifacio- 50 and for 1982-1992 by Roduner and Crocket.3 1 The recent review of Fischer and Radom should also be consulted.j41 Absolute rate constants for reaction with most monomers lie in the range 105-106 M"1 s"1. Rate data for reaction of representative primary, secondary, and tertiary alkyl radicals with various monomers are summarized in Table 3.6. 

Absolute rate constants for addition reactions of cyanoalkyl radicals are significantly lower than for unsubstituted alkyl radicals falling in the range 103-104 M V1.341 The relative reactivity data demonstrate that they possess some electrophilic character. The more electron-rich VAc is very much less reactive than the electron-deficient AN or MA. The relative reactivity of styrene and acrylonitrile towards cyanoisopropyl radicals would seem to show a remarkable temperature dependence that must, from the data shown (Table 3.6), be attributed to a variation in the reactivity of acrylonitrile with temperature and/or other conditions. 

Absolute rate constants for the attack of aryl radicals on a variety of substrates have been reported by Scaiano and Stewart (Ph ) 7 and Citterio at al. (/j-CIPh-).379,384 The reactions are extremely facile in comparison with additions of other carbon-centered radicals [e.g. jfc(S) = 1.1x10s M"1 s"1 at 25 °C].3,7 Relative reactivities are available for a wider range of monomers and other substrates (Tabic 3.b). Phenyl radicals do not show clear cut electrophilic or... 

The general chemistry of acyl radicals has been recently reviewed/88 Acyl radicals have nucleophilic character. Absolute rate constants for substituted phenacyl radical addition to BA have been reported to be in the range 1.3-5.5xl05... 

Pioneering work by Wallingj94 established that the specificity shown by t-butoxy radical is solvent dependent. Work21 22396 on the reactions of /-butoxy radicals with a series of a-mcthylvinyl monomers has shown that polar and aromatic solvents favor abstraction over addition, and [3-scission over either addition or abstraction. Recently, Weber and Fischer418 and Tsentalovich at a/.410 reported absolute rate constants for [3-scission of r-butoxy radicals in various solvents. These studies indicate that p-scission is strongly solvent dependent while abstraction is relatively insensitive to solvent. 

Because of the importance of hydroperoxy radicals in autoxidation processes, their reactions with hydrocarbons arc well known. However, reactions with monomers have not been widely studied. Absolute rate constants for addition to common monomers are in the range 0.09-3 M"1 s"1 at 40 °C. These are substantially lower than kL for other oxygen-centered radicals (Table 3.7). 454... 

Time resolved EPR spectroscopy and UV-visible spectophotometry have proved invaluable in determining the absolute rate constants for radical-monomer reactions. The results of many of these studies are summarized in the Tables included in the previous section (3.4), Absolute rate constants for the reactions of carbon-centered radicals are reported in Table 3.6. These include t-butyl374 and cyanoisopropyP2 radicals. 

Pulsed laser photolysis (PLP) has emerged as the most reliable method for extracting absolute rate constants for the propagation step of radical polymerizations,343 The method can be traced to the work of Aleksandrov el al.370 PLP in its present form owes its existence to the extensive work of Olaj and eoworkers 71 and the efforts of an 1UPAC working party/45"351 The method has now been successfully applied to establish rate constants, /rp(overall), for many polymerizations and copolymerizations. 

Before any chemistry can take place the radical centers of the propagating species must conic into appropriate proximity and it is now generally accepted that the self-reaction of propagating radicals- is a diffusion-controlled process. For this reason there is no single rate constant for termination in radical polymerization. The average rate constant usually quoted is a composite term that depends on the nature of the medium and the chain lengths of the two propagating species. Diffusion mechanisms and other factors that affect the absolute rate constants for termination are discussed in Section 5.2.1.4. 

Even though the absolute rate constant for reactions between propagating species may be determined largely by diffusion, this does not mean that there is no specificity in the termination process or that the activation energies for combination and disproportionation are zero or the same. It simply means that this chemistry is not involved in the rate-determining step of the termination process. 

The absolute rate constants for attack of carbon-centered radicals on p-benzoquinone (38) and other quinones have been determined to be in the range I0M08 M 1 s 1.1 -04 This rate shows a strong dependence on the electrophilicity of the attacking radical and there is some correlation between the efficiency of various quinones as inhibitors of polymerization and the redox potential of the quinone. The complexity of the mechanism means that the stoichiometry of inhibition by these compounds is often not straightforward. Measurements of moles of inhibitor consumed for each chain terminated for common inhibitors of this class give values in the range 0.05-2.0.176... 

Absolute rate constants for the addition of a sulfonyl radical to multiple bonds are absent from the literature. However, there are some relative kinetic data. Correa and... 

The mechanism of this unusual process has been studied in some detail. Absolute rate constants for the spontaneous reaction of (TMSlgSiH with molecular oxygen (Reaction 33) has been determined to be at 70 °C and theoretical studies elucidate the reaction coordinates. ... 

For a review that lists many rate constants for abstraction of hydrogen at various positions of many molecules, see Hendry, D.G. Mill, T. Piszkiewicz, L. Howard, J.A. Eigenmann, H.K. J. Phys. Chem. Ref. Data, 1974, 3, 937 Roberts, B.P. Steel, A.J. Tetrahedron Lett., 1993, 34, 5167. See Tanko, J.M. Blackett, J.F. J. Chem. Soc., Perkin Trans. 2,1996, 1775 for the absolute rate constants for abstraction of chlorine by alkyl radicals. 

Carbene itself is extremely reactive and gives many side reactions, especially insertion reactions (12-19), which greatly reduce yields. When it is desired to add CH2 for preparative purposes, free carbene is not used, but the Simmons-Smith procedure (p. 1088) or some other method that does not involve free carbenes is employed instead. Halocarbenes are less active than carbenes, and this reaction proceeds quite well, since insertion reactions do not interfere.The absolute rate constant for addition of selected alkoxychlorocarbene to butenes has been measured to range from 330 to 1 x 10 A few of the many ways in... 

Measurements of absolute rate constants for the reduction and oxidation of metal ions by e, H- and OH- has been a prominent achievement of the technique of pulse radiolysis. This subject is too broad to be included in this review and is to be dealt with later in the series. A key reference is given, however, to help cover the interim period. 

Table XVIL —Absolute Rate Constants for Chain Propagation and...

Strand cleavage studies have provided relative rate constants for hole transport versus the rate constant for the initial chemical event leading to strand cleavage [18-20]. However, they do not provide absolute rate constants for hole transport processes. Several years ago we introduced a method based on femtosecond time-resolved transient-absorption spectroscopy for investigating the dynamics of charge separation and charge recombination in synthetic DNA hairpins [21, 22]. Recently, we have found that extensions of this method into the nanosecond and microsecond time domains permit investigation of the dynamics of hole transport from a primary hole... 

The determination of large values of the rate constant ratio ks/kpfrom the low yields of alkene product that forms by partitioning of carbocations in nucleophilic solvents. These rate constant ratios may then be combined with absolute rate constants for the overall decay of the carbocation to give absolute values of kp (s ).14 16 For example, the reaction of the l-(4-methylphenyl)ethyl carbocation in 50/50 (v/v) trifluoroethanol/water gives mainly the solvent adducts and a 0.07% yield of 4-methylstyrene from proton transfer to solvent, which corresponds to kjkp = 1400. This can be combined with ks = 6 x 109 s V4 to give kp = 4.2 x 106 s l (Table 1). 

Atkinson, R., Pitts, J.N., Jr. (1977a) Absolute rate constants for the reaction of oxygen (3P) atoms with a series of olefins over temperature range 293 -39 K. J. Phys. Chem. 67, 38 -3. 

TABLE 1. Absolute rate constants for the addition of methyl and trifluoromethyl radicals to simple alkenes at 164 °C3... 

Table 3. Absolute Rate Constants for Reactions of Singlet Carbenes with Proton...

One can correct the observed distribution of 1,2-C and 1,2-H products so as to reflect only the carbenic pathway.28 The corrected 1,2-C/1,2-H ratio (4.8), coupled with a pyridine ylide absolute rate constant for overall carbene rearrangement (kc + % = 6.8 x 107 s 1), gives the partitioned rate constants kc = 5.6 x 107 s 1 and ifeH = 1.2 x 107 s-1.28 The dominance of 1,2-C over 1,2-H in this system will be discussed below. 

Bystander effects are also known for a variety of alkychlorocarbenes.60,80,86-88 Absolute rate constants for the rearrangements of MeCCl (15), EtCCl (61), and i-PrCCl (62), as determined by photoacoustic calorimetry were reported by LaV-illa and Goodman.60... 

The intramolecular 1,2-H shifts of alkylchlorocarbenes are often very rapid making it difficult to relate structure with reactivity in terms of absolute rate constants. For example the ku values of Me2CHCCl, PhCHMeCCl, and EtCCl exceed 108 s 1 in hydrocarbon solvents at 25°C (Table 4).60 86 87 However, due to the stabilizing effect of the oxa spectator substituent, acetoxycarbenes react at much reduced rates relative to their chlorocarbene analogues,90,91 thus providing kinetically accessible results for a wide array of bystander-substituted alkylacetoxycarbenes.81 92... 

TABLE 3. Absolute rate constants for the reaction of chloroalkylperoxy radicals with chlorpromazine ... 

The Arrhenius expression for the reaction of the o(allyloxy)phenyl radical (9) with (Me3Si)3SiH relative to this unimolecular rearrangement [Eq. (4)] has been measured, v/z., og(kclku) (M) = 2.6 - 1.6/0.36 When the competition study was performed, however, reliable absolute rate constants for the cyclization of radical 9 to radical 10 were not available, although a... 

The reaction of thermally and photochemically generated tert-butoxyl radicals with trisubstituted silanes [Eqs. (6) and (7)] has been used extensively for the generation of silyl radicals in ESR studies, in time-resolved optical techniques, and in organic synthesis. Absolute rate constants for reaction (7) were measured directly by LFP techniques,56,62,63 whereas the gas phase kinetic values for reactions of Me3SiH were obtained by competition with decomposition of the tert-butoxyl radical.64,65... 

Absolute rate constants for the reaction of Et3SiH with aroyloxyl radicals, p-X-C6H4C(0)0, where X = MeO, Me, H, and Cl, were measured by... 

It is noteworthy that the absolute rate constants for the reaction of the benzophenone triplet with Et3SiH, n-C5HnSiH3, PhSiH3, and Cl3SiH have been measured by LFP,56 and comparison of the kinetic data with corresponding data for reactions of /-BuO radicals shows that these two transient species have a rather similar reactivity toward silanes. Furthermore, the xanthate and the p-methoxyacetophenone triplets were found to be more and less reactive, respectively, than the benzophenone triplet with Et3SiH.56 Similar behavior of excited states in reactions with tin hydrides is discussed in Section V. 

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