Hydrogen donors rate constants

Based on a competition technique, using hydrogen donors as probes (cyclopentane, anisole, methyl f-butyl ether or methanol). The value reported is consistent with the hydrogen abstraction rate constants reported in [79 Won 1]. 

Table 1.2. Relative rate constants of some selected Diels-Alder reactions in water compared to organic solvents of different hydrogen bond donor capacities.

The intermediate diphenylhydroxymethyl radical has been detected after generation by flash photolysis. Photolysis of benzophenone in benzene solution containing potential hydrogen donors results in the formation of two intermediates that are detectable, and their rates of decay have been measured. One intermediate is the PhjCOH radical. It disappears by combination with another radical in a second-order process. A much shorter-lived species disappears with first-order kinetics in the presence of excess amounts of various hydrogen donors. The pseudo-first-order rate constants vary with the structure of the donor with 2,2-diphenylethanol, for example, k = 2 x 10 s . The rate is much less with poorer hydrogen-atom donors. The rapidly reacting intermediate is the triplet excited state of benzophenone. 

The reaction between the photoexcited carbonyl compound and an amine occurs with substantially greater facility than that with most other hydrogen donors. The rate constants for triplet quenching by amines show little dependence on the amine a-C-H bond strength. However, the ability of the amine to release an electron is important.- - This is in keeping with a mechanism of radical generation which involves initial electron (or charge) transfer from the amine to the photoexcited carbonyl compound. Loss of a proton from the resultant complex (exciplex) results in an a-aminoalkyl radical which initiates polymerization. The... 

A fast, unimolecular reaction can be used to excellent advantage. The rm-butoxyl radical offers the advantage that /3-scission occurs with a known rate constant. For Eq. (5-31), ki = 1.4 X 106 s-1 in water.8 In the presence of a hydrogen donor, AH, the competition is... 

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... 

The microautoclave solvent activity tests measure coal conversion in a small batch reactor under carefully controlled conditions. The tests are described as Kinetic, Equilibrium and SRT. The Kinetic and Equilibrium Tests measure coal conversion to tetrahydrofuran solubles at conditions where conversion should be monotonically related to hydrogen transfer. The Kinetic Test is performed at 399°C for 10 minutes at an 8 to 1 solvent to coal ratio. The combination of high solvent ratio and low time provide a measure of performance at essentially constant solvent composition. The measured conversion is thus related to the rate of hydrogen donation from solvent of roughly a single composition. In contrast, the Equilibrium Test is performed at 399°C for 30 minutes at a 2 to 1 solvent to coal ratio. At these conditions, hydrogen donors can be substantially depleted. Thus performance is related to hydrogen donor... 

In the reverse WGS reaction, hydrogen promoted both decomposition paths of the formate to H2+C02 and H20+CO, and the decomposition selectivity did not change. Thus, the mechanism of hydrogen promotion is different from that of electron donors in the WGS reaction. COz not only blocks the adsorption sites of H2 but also suppresses the decomposition of the formate intermediate. The rate constant for the steady-state reaction is higher than that obtained from the formate decomposition in vacuum, but it is smaller than that for the formate decomposition under the ambient H2. As a result, the reverse WGS reaction proceeds with a balance of H2 promotion and C02 suppression. 

With good hydrogen donors photoreduction of nitrobenzene becomes more efficient the rate constant for hydrogen abstraction from tributylstannane by (n, 7i )-nitrobenzene has been determined as 4 x 10 1 mole s-i... 

Quantitative treatment of rate constants for the hydride attack, / hp, the metal protonation, A fpm. and the exchange process in the framework of Scheme 10.7 have resulted in A hp = 2.7 x 10 M Vs and A fpm = 2.8 x 10 M /s. Thus, the hydride protonation occurs faster by a factor of 10. In earlier chapters we have shown that transition metal hydrides form dihydrogen bonds in the presence of proton donors. Now, based on the principle of microscopic reversibility, one can suggest that proton transfer to a hydridic hydrogen actually occurs via a dihydrogen bond. 

Eq. 1 log Vmax, rate constant for CYP2E mediated metabolism AE,. E umo - EHomo-Eq. 2 AGbind, free energy of binding HB, total number of hydrogen bond acceptors and donors Mr, relative molecular mass Nm, number of active site hydrogen bonds formed between substrate and human CYP2E1. 

Radical chain processes break down whenever the velocity of a termination reaction is comparable to the velocity of the rate-controlling step in a chain reaction. This situation would occur, for example, if one attempted to use EtsSiH as the hydrogen atom donor in the alkyl halide reduction sequence in Figure 4.6 and employed typical tin-hydride reaction conditions because the rate constant for reaction of the silane with an alkyl radical is 4 orders of magnitude smaller than that for reaction of Bu3SnH. Such a slow reaction would not lead to a synthetically useful nonchain sequence, however, because no radical is persistent in this case. In fact, a silane-based radical chain reduction of an alkyl halide could be accomplished successfully if the velocity of the initiation reaction was reduced enough such that it (and, hence, also the velocity of alkyl radical termination... 

Figure 4.12. Second-order rate constants for reactions of hydrogen atom donors with various radical types at ambient temperature. Data sources group 14 (IV A) hydrides (15) aminyl radicals (69) amidyl radicals (70) alkyl radials with group 16 (VI A) hydrides (71) acyl radical with PhSeH (72).

In many cases, rate constants for the fragmentation reactions are not available, but the reactions can be deduced to be quite fast because intermediate radicals are not trapped efficiently by reactive hydrogen atom donors. 

C. Chatgihaloglu and M. Newcomb, Hydrogen donor abilities of the group 14 hydrides, Adv. Organometal Chem. 1999, 44, 67 (rate constants for radical reactions with Group 14 (IV A) metal hydrides). 

TABLE 9.8. Rate Constants (at 300 K) and Arrhenius Parameters for the Reaction of DPC (14a) with Various Hydrogen Donor Substrates... 

TABLE 9.9a. Rate Constants for the Reaction of Triplet Carbenes with Hydrogen Donor Snbstrates... 

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. 

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