Abstraction of H atoms

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Pulse radiolysis of N20-saturated aqueous solution of 1,4-cyclohexadiene leads to formation of three radicals, two by addition of either OH or H atoms to give the cyclohexenyl radicals 3 and 4 (equation 12 and 13) and one by abstraction of H atoms (equation 14). The last one, the cyclohexadienyl radical, can exist in two mesomeric forms (5a and 5b). Fessenden and Schuler16 found that the spin density of the cyclohexadienyl radical was highest at the central atom, i.e. form 5a is the predominant one. 

SECONDARY REACTIONS. The reactions of the free radicals include (1) abstractions (of H atoms, with preference for tertiary H, and of halogen atoms), (2) addition to double bonds, which are very efficient scavengers for radicals, (3) decompositions to give both small molecule products, such as CO2, and (4) chain scission and crosslinking of molecules. 

A very large kinetic isotope effect in the initiation stage is also suggested by the experimental results obtained with a 4a monodeuterated DHP. Denoting the rate constant for the abstraction of H atom 4a as k and that for the abstraction of H atom 4b as k we have... 

Table 7 Percentage Abstraction of H-Atom by OH from Various Positions in Simple Alcohols...

This reaction of 1 -heptene is dominated by addition of HO to the double bond (Table 16.4), but abstraction of H-atoms cannot be completely neglected (Table 16.3) ... 

Photoionization mass spectrometric studies, by Berkowitz and coworkers, of SiH2 formed by successive abstraction of H atoms from SiH4 by F atoms allowed determination of the ionization potentials of the two lowest states 1Ai(5o) = 9.15 0.02 or 9.02 0.02 eV and 3Bi(Ji) = 8.244 0.025 eV299. This important series of experiments300 yielded a singlet-triplet separation for SiH2 of either 0.78 0.03 or 0.91 0.03 eV. 

Atkinson (1987) developed very reliable fragment additivity SARs for estimating kHO(air) from molecular structure using more than 400 compounds in the database (Chapter 14 describes procedures for using these SARs). SARs for HO are based on the premise that rate constants for each of the several different classes of reactions of HO with organic compounds — abstraction of H- atom (kH), addition to double, triple or aromatic bonds (kE), and reaction with S or N atoms (kA) — can be estimated separately and then summed to give the total molecular rate constant, kHO ... 

Case Study of Tetrahydro-1,4-Oxazine (Morpholine). As in the case of aromatic pollutants, the oxidation of the N atom and the C atoms was observed with or without the destruction of the morpholine cycle (92). Also, the abstraction of H atoms to form methyl groups was illustrated by the formation of A-hydroxyamino-2-ethenyl ethyl ether, /V-formyl-formamine, and acetate ions. When the starting morpholine concentration was increased, intermediate products with mass peaks much greater than the molecular mass of morpholine were detected, showing that coupling reactions occurred the concentrations of these intermediates were very low. 

Quite different activation energies are involved in the different processes. The breaking of a C-H bond mag radical usually has an activation energy of about 40 kcal.mole while the value for the breaking of a C-C bond is somewhat less, some 30-35 kcal.mole The abstractions of H atoms by P radicals usually have an activation energy of about 10 kcal.mole . 

Free radicals—products of sequential abstraction of H-atoms from the above molecules (CH3, H, H02, OH, CH3OO, CH2OH, CH30, CHO, COOH, C2H5, C2H3). 

Since it is unanimously agreed that MTBE is eUminated via OH radicals, many studies have been performed in order to elucidate the mechanistic pathway of the conversion. OH radicals react with organic molecules by the abstraction of hydrogen from C - H or O - H bonds, via hydroxyl group addition to unsaturated carbon bonds or the interaction with N-, P-, and S-containing bonds [32]. Since in the MTBE molecule no unsaturated carbon bonds and no N-, P-, or S-atoms are present the initial reaction has to concentrate on the abstraction of H atoms or the cleavage of the C - 0 bond. 

The reaction of OH radicals with alkenes and aromatic compounds proceeds by addition to the double bond and to the benzene ring. In these cases the above formula is not applicable. This does not preclude the occurrence of abstraction of H atoms with a certain probability from long-chained alkenes and aromatic compounds with longer side chains. 

A few years later, a third type of reaction was added to the scheme, the isomerization of large radicals by internal abstraction of H atoms (9). This was shown (41) to account satisfactorily for the product distribution arising from the pyrolysis of long chain hydrocarbons (e.g., n-Ciel ). Very little has happened in the approximately 25 years since the last of these contributions to alter our conceptual understanding of the kinetics of hydrocarbon pyrolysis. Instead, the very extensive research done since then has generally been devoted to determining the quantitative kinetic parameters associated with the elementary step reactions of the pyrolysis chain. Much of this work has been summarized in some recent books (62) and reviews (26, 51). 

Recent measurements of the C-H bond strengths in benzene (55) and C2H4 (56) yield values of 112 kcal/mole for the former and a lower limit of 107 kcal/mole for the latter. These are so much higher than other C-H bond strengths that it suggests that abstraction of H atoms from an aromatic ring, or a double-bonded carbon atom, must be very rare. Instead, a much faster process is usually the addition of atoms or radicals to unsaturated carbon atoms to produce radicals. 

The rate of abstraction of H atoms from n-butane by lerl.-butoxy radicals was studied relative to the radical decomposition reaction... 

Finally, it should also be clear that ER reactions do not necessarily yield a gas-phase product. The new molecule may be trapped on the surface. There is evidence for an ER mechanism in the addition of incident H atoms to ethylene and benzene on Cu(l 11) [91], and in the abstraction of H atoms from cyclohexane by incident D atoms [92], and the direct addition of H atoms to CO on Ru(OOl) [93]. 

They have recently begun to be the object of some study (227,228). In a combined experimental and theoretical study, Williams et al. (229) demonstrated the direct pickup of relatively large adsorbed hydrocarbon species by polycyclic hydrocarbon ions. Chang et al. (230) have studied the abstraction of H atoms from diamond by H(g) H(g) + H(ad) - H2(g). 

We have already briefly discussed (Sect. 8.1) reactions other than cyclopropanation of alkenes that are typical for carbenes, namely insertion into C-H and O-H bonds, abstraction of H-atoms, and rearrangements. Except for ether formation (insertion into the OH bond) and rearrangement of diazocarbonyl compounds (Sect. 8.6) they are of little interest in organic synthesis. 

The abstraction of H atoms from molecules by alkoxyl radicals was reviewed by Gray and Williams (1959), by Ingold (1967), by Gray et al. (1967), by Heicklen (1968), and by Howard (1972). The studies discussed there will not... 

CH3O. The rate coefficients recommended by Gray et al. (1967) and by Heicklen (1968), based on their reviews of the early literature, are given in Table 18 for the abstraction of H atoms by CH3O. The rate-coefficient parameters reported by Gray et al. (1967) were obtained fixim the rate coefficients of the reverse reaction and the equilibrium constants. Those reported by Heicklen (1968) were obtained from the ratio k2sk 2lk -i for the reactions... 

C3H7O. Only very limited data exist on the abstraction of H atoms by i-C3H70 radicals. Batt and Milne (1977a), in their study of the thermal decomposition of 1-C3H7O radicals, found that at 160 C, 1-C4H10 at 10 M could remove —25% of the radicals. If we take our recommended value for the decomposition rate coefficient of 10 °exp —17.5// 7 sec , then the abstraction rate coefficient is 2.0 x lO Af -sec at 160°C. 

Stoddart et al. (1974) studied the gas-phase reactions of r-C4H90 radicals with 1-substituted butanes, the radicals being generated from the decomposition of t-butyl hypochlorite. The relative reactivities for abstraction of H atoms from the various positions are given in Table 22. 

Encina and Lissi (1978) studied the gas-phase abstraction of H atoms from various amines by t-C4H90 radicals in competition with r-C4H90 decay ... 

Quintet m-phenylenedinitrenes have been generated in low-temperature matrices by photolysis of the corresponding diazide precursors, and their secondary photochemical transformations studied. Two competing ring-opening pathways were identified, and it was also concluded that quintet dinitrenes are much more photochemically reactive than triplet nitrenes. An EPR study of radical intermediates formed in the photooxidation of 4,4 -diazidodiphenyl in benzene and toluene has also been published The radicals apparently arise by abstraction of H atoms from the solvent by a triplet nitrene-02 complexes. 

The main competing reactions are the abstraction of H atom from the solvent of neutral radicals, and further reduction in radical species. Liquid ammonia was used as a solvent to avoid the transfer of H atom [78-80]. 

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