Hydrocarbons hydrogen abstraction from

For this reaction, the relative rates for different R s are often assumed to be directly related to the bond strengths of the R—H bonds. Many workers (20, 217, 220, 229) have postulated that, during oxidation, hydrogen abstraction from hydrocarbon molecules by radicals should occur at rates determined by the numbers and strengths of C—H bonds present. According to Rice (178), the relative rates at 300° C. should be primary secondary-.tertiary = 1 3 33. 

Extensive information exists in the literature on the Arrhenius parameters for hydrogen abstraction from hydrocarbons and related com-... 

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

Combination of BP with 2-propanol or amines induces homopolymerization alone. The rate constants of BP 3 - isopropylamine and triethylamine are 2.95 10 and 2.42 1()9m-1s-1, respectively(22) whereas that of BP 3 - isooctane as a model of OPP is 1.0 lO M s l (24). Also hydrogen abstraction from 2-propanol(k=1.0 106 M s"1) (25) is much more efficient than that from aliphatic hydrocarbons. Even methanol is more reactive (k=2.8 10% - s - -) (25) than OPP towards BP 3. The aforementioned results and the finding that surface grafting does not occur in methanol are well interpreted by the following elementary reactions. 

These arguments support the conclusion that these species are benzyl and methyl-benzyl species produced by hydrogen abstraction from one of the methyl groups of the starting hydrocarbons. 

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

If hydrogen abstraction from the zinc salt is fast enough to compete with that from the hydrocarbon substrate, then Scheme 2 amounts to a chain-transfer reaction similar to that illustrated in Reaction 9 for phenolic inhibition. Also, an alternative termination via the peroxyalkyl-ester radical (X) is conceivable since this radical might be expected to cyclize... 

As seen in Table 6.1, the reactions of the nitrate radical with the simple aromatic hydrocarbons are generally too slow to be important in the tropospheric decay of the organic. However, one of the products of the aromatic reactions, the cresols, reacts quite rapidly with NO,. o-Cresol, for example, reacts with N03 with a room temperature rate constant of 1.4 X 10 " cm3 molecule-1 s-1, giving a lifetime for the cresol of only 1 min at 50 ppt N03. This rapid reaction is effectively an overall hydrogen abstraction from the pheno-... 

Singlet NH inserts into the CH bonds of hydrocarbons, much like singlet methylene (see Chapter 7 in this volume). Triplet NH abstracts hydrogen atoms from hydrocarbons to form aminyl (NHp radicals and alkyl radicals in the same manner as triplet methylene, in spite of the fact that the reactions of CH2 are exothermic, whereas some reactions of NH are endothermic, depending on the alkane Absolute rate constants for many of these processes have been measured in the gas phase. However, the gas-phase chemistry of methylene is much more developed than that of imidogen. ... 

Cox (246) estimates the concentration of HN02 to be 109 molec cm"3 in the daytime natural troposphere. The photolysis of HN02 may be an important source of OH in the troposphere, since HN02 absorbs the sun s radiation above 3000 A. The reactions of OH with hydrocarbons (either hydrogen abstraction from paraffins or addition to the double bond in olefins) in the troposphere are known to be the initial steps for photochemical smog formation [see Section VIII-2, p. 333],... 

The cyclopropane synthesis is also suitable for the preparation of highly strained bicyclic hydrocarbons such as [2.1.0]bicyclopentanes (14) and spiropentanes (16) [14a,b]. The formation of the spiropentane 16 is particularly remarkable as it is the result of a homolytic hydrogen abstraction from a cyclopropane ring. Those processes are very rarely observed due to the relatively high C-H-bond energies of cyclopropanes (Sch. 8). 

Cr porphyrin complexes react with O2 to form stable oxochromium(IV) species. Por example, the red [(TPP)CrO] contains a five-coordinate Cr displaced out of the porphyrin plane toward the oxygen atom (Cr-O, 157.2pm). There are also a few Cr scorpionates featuring terminally bound 0x0 ligands. The most recent of these is paramagnetic [Tp Cr(pz H)(0)]BARP (Cr-O, 160.2pm), which abstracts weakly bound hydrogen atoms from hydrocarbons. ... 

Initial formation of radicals by photoinduced hydrogen attraction from the substrate is not in dispute and, as noted in Section 1, hydrogen abstraction from alcohols, ethers and hydrocarbons is effected with high quantum efficiencies by aromatic carbonyl compounds in which the lowest lying triplet level is of n—n type, e.g. benzophenone, anthra-quinone (42 -44). 

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