Hydrogen abstraction rates

Future work will be concerned with quantitative measurement of hydrogen abstraction rates of labile hydrogens in the carbamate moieties of several aromatic diisocyanate based polyurethanes. It is expected that experimental conditions will alter significantly the hydrogen abstraction rate. Emphasis will also be placed on measurement of transient intermediates in polyurethane films. Finally, extensive laser flash photolysis experiments will be conducted on polyurethanes based on both 2,A-toluenediisocyanate and 2,6-toluenediisocyanate. Preliminary data suggest that the placement of the methyl substituent can alter the nature of the transient intermediates formed. 

Upon irradiation (366 nm) in 2-propanol, 1-nitronaphthalene is inefficiently photoreduced (Ah<10 1 moles i s i) despite its high triplet yield 3). With tributylstannane, a marked increase of the hydrogen abstraction rate (<3 x 10 ... 

Although a slower hydrogen abstraction rate is anticipated for 3(jc,7i )-states compared to 3(n,7r )-states, 2-nitronaphthalene is more efficiently ( =0.037) photoreduced than nitrobenzene ( = (1.14 0.08) X 10 2 iD). The longer triplet lifetime obviously overcompensates a slower reaction rate. 

From Table 14-1, the following results could be found for the kinetic parameters of various radical pairs (1) The hydrogen abstraction rate (ka) had already been measured from the decay of triplet precursors with transient optical absorption methods. Because the obtained ko values listed in Table 14-1 were found to be similar to those measured with optical absorption methods, the present simulation in PYDESR was found to be correct. (2) The cage product formation rate (A p) had not been determined with other methods including optical absorption ones before this PYDESR one. Thus, it is noteworthy that this was the first method to determine the kp values. (3) The escape and the relaxation rates (fe and r+ r ) had already been measured with transient optical absorption methods, but the... 

TABLE 2. Lifetimes and Hydrogen Abstraction Rates of Various Radicals that Initiate Lipid Oxidation. 

The configuration and reactivity of methyl radical-cyanide ion pairs produced by dissociative electron capture in the two solid phases of CH3CN have been studied by e.s.r. techniques using CD313CN.103 The results indicate that the radical configuration is planar and that the reactivity of the radical (as estimated from hydrogen-abstraction rates) in crystal I is at least 10 times greater than in crystal II. 

In contrast to photochlorination, the corresponding hydrogen abstraction or addition steps in photobromination are usually reversible, endothermic and more selective.155 The hydrogen abstraction rates in the allylic or benzylic positions can be relatively high, however. Table 6.19 shows the relative reactivities of the benzylic C—H bonds in a series of alkylbenzenes in the photobromination reaction carried out in CCI4.1304 It is apparent that not only electronic but also steric effects control the reaction kinetics. 

Fig. 11.17. Correlation of experimental hydrogen abstraction rates widi rates calculated by Equation (11.7). Reproduced from Combust. Set and Tech., 95, 1 (1994) by permission of Taylor and Francis...

Hydrogen abstraction rate constants of only a few hundred dm moF s were derived assuming the pH-dependent equation (26) was the single chainterminating reaction ... 

The proposed reaction scheme was employed to successfully model the kinetics of the DTBA-mediated MMA polymerization in the early reaction phase via PREDICI simulations (see Figure 6), resulting in a hydrogen abstraction rate coefficient of fetr = 3.0xl0 L-moE -s and a reinitiation rate of the alkylsulfanyl radical of fejeini = 7.8 L moP The... 

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]. 

A thermochemical scale of hydrogen bond basicity has been proposed based on the differences in the heats of solution, 5(A, of pyrrole, 7V-methylpyrrole, benzene and toluene. The thermochemical scale correlates well with the parameter of Kamlet and Taft, with the exception of dioxane and especially of triethylamine. The correlation gives a value for water of Bj 0.2, while interpolation of hydrogen abstraction rates has given a value of 0.31. 

Hydrogen abstraction from tertiary carbon has an activation energy lower by approximately 15 kj mol than an abstraction reaction on secondary carbon (Ref. 2, p. 386), and is the predominant mode of formation of hydroperoxides in PP. In addition to energy considerations, the hydrogen abstraction rate constant is dependent on steric factors and polymer conformation. It is found, for instance, that fes (Scheme 15.1) in solution is lower in a theta solvent than in a good solvent, owing to increased steric repulsion in a contracted molecular coil [7]. 

The reaction of NH2 with CH3OH is much faster than expected for a hydrogen abstraction reaction. A room-temperature value of k(293 K) = 9.0xio cm mor s" was measured [185] which is more than two orders of magnitude higher than the k value of the reaction with e.g. C2H6. Probably a different mechanism is valid for methanol, and thus the rate constant can not be compared to hydrogen abstraction rate constants. 

Figure 6.18 shows the temperature dependence of the H+Hj, F+H2 and CHj+Hj hydrogen abstraction rates. The classical rates tend to underestimate the lower temperature experimental rates, but the general trends are reproduced very well. The onpirical ISM has an interesting pedagogical value in chemical kinetics because it makes the calculation of classical atom-transfer rate constants possible with a pocket calculator and provides basic information on the reactants and products. 

Figure 6.18 Temperature dependence of the H+Hj, F+Hj and CH3+H2 hydrogen abstraction rates, and their comparison with the classical ISM rates.

Lewis and Hirsch studied four a-cycloalkoxyacetophenones, in which the ring is even farther away from the carbonyl and the y-hydrogen is tertiary.Table 58.3 lists the results, which consist of oxas-piro[3,n]alkanol and cycloalkanone formation, via normal cyclization and cleavage of the 1,4-biradicals. Simple isopropoxyacetophenone is included for the sake of comparison to acyclic compounds. As with other a-alkoxy ketones, overall quantum yields are quite high and cyclization yields are reasonable, except for the cyclopropyloxy ketone. Hydrogen abstraction rate constants are very high and decrease as expected as the rings get smaller. 

Wang, G., Winnik, M. A., Schaefer, H. J., and Schmidt, W, Intramolecular photochemical hydrogen abstraction rate and its temperature dependence in doubly anchored alkane chains. Chin. Scl Bull, 38, 615,1993 (Chem. Abstr., 119,159622, 1993). 

Fig. 5.1 Reaction scheme for the photolytic dissociation of hydrogen peroxide. knA is the hydrogen abstraction rate constant...

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