Reactions of the Hydrogen Atom

Similar to OH, H is an electrophilic radical (Chap. 4.4), and in its additions to C-C double bonds it has a strong preference for electron-rich sites (Das et al. 1985 Table 10.18). Thus, the C(5)-position is the preferred site of addition in the pyrimidine series as depicted here for Ura [reaction (141)]. 

The C(6)-yl radical is also formed upon the reaction of OH with dihydropyrimidines [reaction (144) Schuchmann et al. 1984b], as can be seen from the data compiled in Table 10.19. For a quantum-mechanical study of their structure and EPR coupling constants see Jolibois et al. (1998). 

Due to the much lower rate of H-abstraction by H as compared to OH, H-ab-straction is generally of much lower if any importance (Table 10.18). 

6-dihydro-2 -deoxyuridin-6-yl was generated specifically by photolyzing the respective tertiary butyl ketone (Carter and Greenberg 2003). It reacts fast with 2-mercaptoethanol [reaction (150) k = 8.8 x 106dm3 mol-1 s 1] and noticeably slower with 2,5-dimethyltetrahydrofurane (31 dm3 mol1 s ). In the presence of 02, 6-hydroxy-5,6-dihydro-dUrd is the major product [reactions (151) and (152)]. Interestingly, 2-dRL is also formed under such conditions which is believed to be formed by an intramolecular C(l )-H abstraction via a six-membered transition state [reaction (153) for this type of reaction see also Chap. 8.6], 

In DNA, the C(l )-H is hidden in the minor groove and hence difficult to access by reactive free radicals such as OH (Chap. 12.2). If the above type of reaction would occur within DNA, it would lead to a tandem lesion (for their biological importance see Chap. 12.5). 

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Nguyen, M. T., Creve, S., Vanquickenborne, L. G., 1996, Difficulties of Density Functional Theory in Investigating Addition Reactions of the Hydrogen Atom , J. Phys. Chem., 100, 18422. 

The subsequent crosslinking probably occurs by reaction of the hydrogen atoms of the resulting urea groups with isocyanate groups still present in the starting polymer or the chain-extended polymer, with the formation of biuret groups ... 

Davies MJ, Gilbert BC (1984) Electron spin resonance studies, part 68. Addition versus overall one-electron abstraction in the oxidation of alkenes and dienes by S04, Cl2, and OH in acidic aqueous solution. J Chem Soc Perkin Trans 2 1809-1815 Deeble DJ, Parsons BJ, Johnson GRA (1990) Reactions of the hydrogen atom with azide, bromide and iodide ions in aqueous solution. Radiat Phys Chem 36 487-491 Eibenberger FI, Steenken S, O Neill P, Schulte-Frohlinde D (1978) Pulse radiolysis and E.S.R. studies concerning the reaction of S04" with alcohols and ethers in aqueous solution. J Phys Chem 82 749-750... 

However, it was subsequently shown9 that these observations could be more satisfactorily accounted for by the occurrence of free radical chain reactions. The free radicals might arise from ionic or excited species. The radiation chemistry of liquid water was claimed to be explicable10 in terms of the reactions of the hydrogen atom and the hydroxyl free radical. Ionic mechanisms consequently fell into disrepute, but it is interesting that in recent years ionic processes have been recognised as of increasing importance. The wheel has indeed turned full circle ... 

The subsequent reactions of the hydrogen atom and formyl radical are of considerable importance since they may arise in many systems where formaldehyde is given as a primary product. The ease with which hydrogen atoms may be abstracted from formaldehyde by radicals means that, even when a photooxidation reaction is taken to only 1% conversion, the formaldehyde produced may have reacted further.67... 

The conversion of hydrogen atoms to hydrated electrons is an exception to the rule of rapid reactivity in this case, the reaction is relatively slow, first order in [OH-], and has a second-order rate constant of 3 x 107 M-1 s-1.45 In less alkaline solutions, the direct reaction of the hydrogen atom with water is quite slow and yields H2 + OH. As a result of these low rate constants, the interconversion between hydrogen atoms and hydrated electrons can be catalyzed by weak bases such as F-and NH3. 

Reactions with the Primary Species. In aqueous solution the monomers are exposed to the action of the species formed from the water in the primary act. The rate constants published for the reactions of the hydroxyl radicals and hydrated electrons are included in Table I. Most of the hydroxyl rate constants were measured using thiocyanate and are therefore subject to the usual uncertainties of this method (5). No rate constants appear to have been published for the reactions of the hydrogen atoms. 

Table V. A Selectioii of Rate Constants for Reactions of the Hydrogen Atom...

Method A. The first successful system we developed for studying the reactions of the hydrogen atom involves the photolysis of deuterated thiols to produce deuterium atoms (11, 13, 14). The deuterium atoms then abstract either deuterium or hydrogen from the thiols or hydrogen from an organic hydrogen donor, QH. The reaction scheme is shown below. 

Indeed, the experimental and calculation data on the simplest reaction of the hydrogen atom abstraction H + H2- H corroborate the conclusions as to the preferability of the approach path of the type I [5]. 

The linear transition state structure II was revealed in the ab initio calculations [6], employing the IRC technique, on the reaction of the hydrogen atom abstraction from a methane molecule by the tritium atom ... 

Yet another inqtortant characteristic of radical abstraction reactions is the force constants of the ruptured and generated bonds. The depoidence of the activation energy for reactions of the type R- + r x - RX + r, where X = H, Cl, Br, or I, on the coefficients bi and b was demonstrated experimentally. It was found that parameter == const in these reactions, whilst the square root of activation energy for a thomally neutral reaction is directly proportional to the force constant of the ruptured bond. The smaller is the force constant of the C— X bond, the lower J eo nd that the relation to b(l + a)" is linear. The same result was obtained also for reactions of the hydrogen atoms with RCl, RBr, and Rl. This results confirm the important role of the force constant of the reacting bonds in the formation of the activation barrier. The activation energies for the R- + RX reactions can be easily estimated from the empirical formula... 

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