Rate constants for hydrogen transfer

The observed behavior qualitatively follows the Marcus relationship for atom transfer, which predicts that the ratio of rate constants for hydrogen transfer from a common donor to two different acceptors will depend only on the relative energies of the bonds being formed, as described below. 

It is well known that kH is similar for all alkyl-substituted radicals but rate constants for reaction of tin hydride with carbonyl-substituted radicals are not known. Substituents can effect the rate constant for hydrogen transfer. For example, the benzyl radical is about 50 times less reactive than a primary alkyl radical. 

There is no doubt that free-radical chemistry has benefited enormously through the invention of tin-based chain-carrying reagents " . Of these, tributyltin hydride and, to a lesser extent, triphenyltin hydride have been the reagents of choice. Their ready availability and favourable rate constants for attack of the corresponding tin-centred radicals at a variety of radical precursors , coupled with useful rate constants for hydrogen transfer to aUtyl and other radicals, provide for reagents superior to their silicon counterparts only tris(trimethylsilyl)silane rivals tributyltin hydride in its synthetic utility . 

Nauser T, Felling J, Schoneich C. (2004) Thiyl radical reaction with amino acid side chains Rate constants for hydrogen transfer and relevance for posttransla-tional protein modification. Chem Res Toxicol 17 1323-1328. 

Reid and coworkers experimentally re-determined the rate constants for hydrogen transfer between a model thiol, 1,4-dithiothreitol, and several carbon-centered rad-... 

The kinetic NMR method permitted also the determination of rate constants for hydrogen transfer to cysteamine thiyi radicals from selected amino acids containing reactive side chains [91]. A summary of these rate constants is given in Table 3.2. Here, the rate constant represents the sum of the individual rate constants for hydrogen transfer from C-H [k o) and from the side chain C-H bonds (ksc), i.e., 1 32 = 1 30 T ksc-... 

Due to their weak P-H bonds (-370 kj mol"0 [2] and the high rate constants for the transfer of the P-H hydrogen [3] (/c=1.5 10 L rnoL s" for Ph2PH and k=5,0 10 L mol s for (c-hexyl)2PH), diaryl and dialkyl phosphines present a high interest as H-donors. Since the corresponding phosphinyl radicals are good chain carriers [4,5], diaryl and dialkyl phosphines can be added to olefinic or acetylenic compounds through radical chain reactions. Simpkins et al. [6] used... 

PEDRIELLI p, HOLKERi L M and SKIBSTED L H (2001b) Antioxidant activity of (+)-catechin. Rate constant for hydrogen atom transfer to peroxyl radicals, Eur Food Res Technol, 213, 405-8. 

Aryl substitution on germanium, whether single or multiple, has only a small effect on the rate constants for hydrogen atom transfer, whereas the rate constant increases substantially with substitution of an alkyl group on Ge by a silyl group, much as observed with the silanes. A strong substituent effect also was observed for germane 19. 

The rate constant for atom transfer between two radicals is about 30 times larger than the pre-exponential factor for hydrogen-atom transfer from a molecule to a radical. Some rate constant data are summarized in Tables VII and VIII. 

Rate and equilibrium constants have been reported for the reactions of butylamine, pyrrolidine, and piperidine with trinitrobenzene, ethyl 2,4,6-trinitrophenyl ether, and phenyl 2,4,6-trinitrophenyl ether in acetonitrile, hi these reactions, leading to cr-adduct formation and/or nucleophilic substitution, proton transfer may be rate limiting. Comparisons with data obtained in DMSO show that, while equilibrium constants for adduct formation are lower in acetonitrile, rate constants for proton transfer are higher. This probably reflects the stronger hydrogen bonding between DMSO and NH+ protons in ammonium ions and in zwitterions.113 Reaction of 1,3,5-trinitrobenzene with indole-3-carboxylate ions in methanol has been shown to yield the re-complex (26), which is the likely precursor of nitrogen- and carbon-bonded cr-adducts expected from the reaction.114 There is evidence for the intermediacy of adducts similar to (27) from the reaction of methyl 3,5-dinitrobenzoate with l,8-diazabicyclo[5.4.0]undec-8-ene (DBU) cyclization eventually yields 2-aminoindole derivatives.115... 

Dual fluorescence spectra of many species have been studied in detail (see a review by Barbara et al. [1989]) and allow the rate constants for hydrogen and deuterium transfer to be measured as a function of temperature. As a typical example, the spectrum of 2,5-bis(2-benzox-azonile)-4-methoxyphenone... 

The rate constant for hydrogen atom transfer (conversion II into III) spans six orders of magnitude in the range 290-80 K. The quantum limit of the rate constant and crossover temperature are 5xl0 3s 1 and 100 K, respectively. The ratio kH/ku increases from 10 to 5 x 103 as the temperature falls from 290 to 100 K. It is the H atom in position a that is transferred, since the substitution of deuterium atom at position b (R = H) does not change the rate constant. 

As = surface area of a semiconductor contact [A ] = concentration of the reduced form of a redox couple in solution [A] = concentration of the oxidized form of a redox couple in solution A" = effective Richardson constant (A/A ) = electrochemical potential of a solution cb = energy of the conduction band edge Ep = Fermi level EF,m = Fermi level of a metal f,sc = Fermi level of a semiconductor SjA/A") = redox potential of a solution ° (A/A ) = formal redox potential of a solution = electric field max = maximum electric field at a semiconductor interface e = number of electrons transferred per molecule oxidized or reduced F = Faraday constant / = current /o = exchange current k = Boltzmann constant = intrinsic rate constant for electron transfer at a semiconductor/liquid interface k = forward electron transfer rate constant = reverse electron transfer rate constant = concentration of donor atoms in an n-type semiconductor NHE = normal hydrogen electrode n = electron concentration b = electron concentration in the bulk of a semiconductor ... 

The rare gas excimers readily transfer energy to various additives. Rates for transfer to nitrogen and hydrogen in krypton are known at 1 atm. Because excimer species have strong absorptions in the visible region, it is necessary to quench them when studying reactions of other intermediates by absorption spectroscopy. Ethane has been shown to be convenient for this purpose. The rate constant for excitation transfer from excimers to ethane in xenon was measured by the pulse-probe technique to be 3.4 x 10 ° molal s at pressures near 50 bar. Thus, addition of a small concentration of ethane can be used to reduce the absorption due to excimers to a small level at nanosecond times. 

Figure 4.2 Rate constants for hydrogen atom transfer from a variety of hydrogen donor systems and primary radicals at 80 °C.

UV analysis can also be used to monitor loss of antioxidants or formation of antioxidant radicals or their product quinones , although there may be products other than just quinones vide supra). Absolute second-order rate constants for hydrogen atom transfer from antioxidants ( roo /AtOh) can be determined by following the rate of radical formation obs, and plotting obs vs [ArOH] (equation 25) . 

Quantitative kinetic studies of absolute rate constants for hydrogen atom transfer from substituted phenols to polystyrene peroxyl radicals by Howard and Ingold in the 1960s provided the first reliable data on substiment effects on antioxidant activities of phenols. Later, a very detailed report appeared providing data on substituent and structural effects on various classes of monohydroxy phenols. In addition, detailed reviews were given of substituent etfects ". These reports provide the basis for understanding how substituent and strucmral effects control the antioxidant activities of phenols and will be summarized in part below. 

The proportion of the desired rearranged product (BX) relative to unrearranged product (CX) depends, to a first approximation, on the rate constant of the rearrangement (k,), the rate constant for hydrogen or halogen atom transfer (kj and the concentration of the reaction partner YX ... 

The rate constants for hydrogen atom transfer can be measured by Stem-Volmer quenching and have been studied for a wide range of substrates, such as alcohols, hydrocarbons, and tin hydrides 134). The rate constants fall in the range 10 -10 s and qualitatively... 

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