Rate constants for hydrogen

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. 

Table 11.1. First-order rate constants for hydrogenation of pentenes.

The electrochemical rate constants for hydrogen peroxide reduction have been found to be dependent on the amount of Prussian blue deposited, confirming that H202 penetrates the films, and the inner layers of the polycrystal take part in the catalysis. For 4-6 nmol cm 2 of Prussian blue the electrochemical rate constant exceeds 0.01cm s-1 [12], which corresponds to the bi-molecular rate constant of kcat = 3 X 103 L mol 1s 1 [114], The rate constant of hydrogen peroxide reduction by ferrocyanide catalyzed by enzyme peroxidase was 2 X 104 L mol 1 s 1 [116]. Thus, the activity of the natural enzyme peroxidase is of a similar order of magnitude as the catalytic activity of our Prussian blue-based electrocatalyst. Due to the high catalytic activity and selectivity, which are comparable with biocatalysis, we were able to denote the specially deposited Prussian blue as an artificial peroxidase [114, 117]. 

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. 

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... 

Table VIII. Absolute (and Relative) Rate Constants for Hydrogen Atom Abstraction from Hydrocarbons at 30°C.

Bunce, N. J., K. U. Ingold, J. P. Landers, J. Lusztyk, and J. C. Scaiano, Kinetic Study of the Photochlorination of 2,3-Dimethyl-butane and Other Alkanes in Solution in the Presence of Benzene. First Measurements of the Absolute Rate Constants for Hydrogen Abstraction by the Free Chlorine Atom and the Chlorine Atom-Benzene tr -Complex. Identification of These Two Species as the Only Hydrogen Abstractors in These Systems, . /. Am. Chem. Soc., 107, 5464-5472 (1985). 

The rate constants for hydrogen abstraction from Rh H, O H, and C-H bonds by chromyl ions and Craq002+ are summarized in Table VI. Also listed in the table are selected relative rate constants for hydrogen abstraction by tert-butoxyl and tert-butylperoxyl radicals, expressed as .buo/AbuOO- The difference between the two sets of data is striking in that alkoxyl radicals are 105-107 times more reactive than alkylperoxyl radicals, but in the chromium series the ratio kcrolkcrOO is only about 102 for all the reactions studied. This ratio is preserved over about 103-fold change in absolute rate constants within each series. 

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. 

Figure 7. Temperature dependence on the rate constant for hydrogen release from the Dy Co -H system on the 0 + y...

The exact values for the rate constants for hydrogen abstraction by triplet benzophenone are not yet entirely certain. Three groups338-338 have reported a value of 108M-1 sec-1 for abstraction from 2-propanol in concentrated 2-propanol, while the combination of the data of three other groups333,338 339 for dilute benzene solutions yields a value of only 105M-1 sec-1. This discrepancy could well reflect a solvent effect such as that found in studies of the reactivity of alkoxy radicals.340 However, the hundredfold difference between the reported rates for attack of triplet benzophenone on toluene338,338 undoubtedly reflects experimental problems, because both values were measured in aromatic solvents. 

A quenching study at one concentration of 2-propanol (2.6Af) in benzene indicates that the rate constant for hydrogen abstraction is about 1 x 106Af 1 sec-1,451 similar to that for benzophenone. A fact long known is that sulphonate substituents in the 2 and 6 positions do not affect quantum yields of photoreduction, whereas in the 1 and 5 positions they cut O from 1.0 down to 0.01-0.05.216... 

Figure 3. Relative rate constants for hydrogenation of -substituted benzaldehydes vs. Hammett p+ values...

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. 

FIGURE 1. Rate constants for hydrogen abstraction (k, M 1 s 1) from a variety of group 14 reducing agents by primary alkyl radicals at 25 °C... 

The first-order ft-scission of the ferf-butoxy radical is one of the oldest radical clock reactions and has been used for over 50 years for the measurement of the relative rates of hydrogen abstraction from organic compounds (AH) in solution (Scheme 10.14). At low conversions, when the concentration of AH has not appreciably changed, the ratio of the rate constants for hydrogen atom abstraction, kAH> and /3-scission, kp, can be determined simply by analysis for acetone and ferf-butyl alcohol formation in the reaction. This is most conveniently achieved by gas chromatography ... 

If the clock reaction has been calibrated, i.e. if kp is known under the precise reaction conditions employed, then the absolute values of kAH (rate constant for hydrogen abstraction from the substrate AH) can be obtained using Equation 10.12. The relative rates of hydrogen abstraction for two different substrates AH and BH in separate reactions can also be determined using this simple reaction as long as the temperature, solvent system and concentrations remain the same for both reactions. It is then possible to compare kAH and ksH since kp remains constant throughout and can be eliminated from the expression. 

Rate Constants for Hydrogen Exchange with Liquid Ammonia and a Solution of Potassium Ethoxide in Ethanol... 

Alkoxides are very much stronger bases in anhydrous ethylenediamine than in alcohols (ethanol and ethanolamine). This can be seen from a comparison of the rate constants for hydrogen exchange in the CD group of triphenylmethane (Table 3) (Shatenshtein and Yakovleva, 1958). 

Table 4 shows the rate constants for hydrogen exchange in these solvents (Shatenshtein and Yakovleva, 1958, 1955). 

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