Hydrogen abstraction by methyl

The evolution of carbon monoxide, carbon dioxide and methane must be ascribed to side-group scission according to reactions (35) and (39). Methane formation is due to hydrogen abstraction by methyl radicals but, since the yield of methane is lower than the yield of carbon monoxide plus carbon dioxide, only a fraction of the methyl radicals produced are able to escape the cage. Most of them recombine with macroradicals according to reactions (38) and (41). Chain scission occurs according to... 

The trend of reactivity tert > sec > pri is consistently observed in various hydrogen atom abstraction reactions, but the range of reactivity is determined by the nature of the reacting radical. The relative reactivity of pri, sec, and tert positions toward hydrogen abstraction by methyl radicals is 1 4.8 61. An allylic or benzylic hydrogen is more reactive toward a methyl radical by a factor of about 9, compared to an unsubstituted C—H. The relative reactivity toward the t-butoxy radical is pri 1, sec 10, tert 50. In the gas phase, the bromine atom is much more selective, with relative reactivities of pri 1, sec 250, tert 6300. Data for other types of radicals have been obtained and tabulated. ... 

Adams (52) has obtained a linear relation between the rate of oxidation of a-olefins over a bismuth molybdate catalyst at 460° and the rate of allyl hydrogen abstraction by methyl radicals in isooctane solution. 

Fig. 4. Comparison of the reactivity of olefins for oxidation over bismuth molybdate at 460° to that of allyl hydrogen abstraction by methyl radicals in isooetane solution at 65° (140). Triangles are for internal olefins.

Methyl ethyl ketone, a significant coproduct, seems likely to arise in large part from the termination reactions of j -butylperoxy radicals by the Russell mechanism (eq. 15, where R = CH and R = CH2CH2). Since alcohols oxidize rapidly vs paraffins, the j -butyl alcohol produced (eq. 15) is rapidly oxidized to methyl ethyl ketone. Some of the j -butyl alcohol probably arises from hydrogen abstraction by j -butoxy radicals, but the high efficiency to ethanol indicates this is a minor source. 

The formation of dimethyl sulfide, dimethyl sulfone, and methane (by H-abstraction) observed in these photolyses is thus accounted for. Hydrogen abstraction by the methylsulfinyl radical affords methanesulfenic acid, CH3SOH, a very reactive molecule, which rapidly undergoes a series of secondary reactions to produce the methanesulfonic acid, methyl methanethiolsulfonate (CH3S02SCH3), and dimethyl disulfide which were also observed during these photolyses. 

An example of radical coupling foUowing hydrogen abstraction by excited nitro-ethane from cyclohexane or diethyl ether in solution has also been reported Formation of -methyl-N-arylnitrones is observed during photoreduction (via electron transfer) of sterically hindered nitrobenzenes in triethylamine 39) ... 

Evidence for the polar character of the transition state is that electron-withdrawing groups in the para position of toluene (which would destabilize a positive charge) decrease the rate of hydrogen abstraction by bromine while electron-donating groups increase it,10 However, as we might expect, substituents have a smaller effect here (p -1,4) than they do in reactions where a completely ionic intermediate is involved, e.g., the SnI mechanism (see p. 344). Other evidence for polar transition states in radical abstraction reactions is mentioned on p. 685. For abstraction by radicals such as methyl or phenyl, polar effects are... 

Neither the relative number of benzylic hydrogens nor the base strength accounts for the slow oxidation rate of the methylnaphthalenes. Formation of radicals in the presence of aromatic hydrocarbons can lead to radical attack on the aromatic ring. Addition of phenyl or methyl radical to the ring gives a cyclohexadienyl radical that may disproportionate or dimerize, or undergo hydrogen abstraction by another radical (3, 9,13). 

Figure 1.1 Examples of temperature dependences of rate constants for the reactions in which the low-temperature rate constant limit has been observed 1, hydrogen transfer in excited singlet state of molecule (6.14) 2, molecular reorientation in methane crystal 3, internal rotation of CH3 group in radical (7.42) 4, inversion of oxyranyl radical (8.18) 5, hydrogen transfer in the excited triplet state of molecule (6.20) 6, isomerization in the excited triplet state of molecule (6.22) 7, tautomerization in the ground state of 7-azoindole dimer (6.15) 8, polymerization of formaldehyde 9, limiting stage of chain (a) hydrobromi-nation, (b) chlorination, and (c) bromination of ethylene 10, isomerization of sterically hindered aryl radical (6.44) 11, abstraction of a hydrogen atom by methyl radical from a methanol matrix in reaction (6.41) 12, radical pair isomerization in dimethylglyoxime crystal (Figure 6.25).

The photochemistry of carboxylic acid derivatives has been summarized by Coyle [20]. For arene carboxylic acid esters it has been shown that [2+2] cycloaddition competes with hydrogen abstraction by the excited ester from an allylic position of the alkene. The addition of methyl benzoate 17 to... 

In this way, the minor products (diacetyl peroxide and the oxidation products of methyl and formyl radicals) are explained by initiation and termination reactions, while the main product (peracetic acid) is given by the propogation reactions. Using this mechanism, a value for E 2 of 7 kcal. mole-1 can be obtained assuming Em is small. This is much the same as the value for abstraction of the same hydrogen atom by methyl radicals. 

A. T. Pudzianowski and G. H. Loew, /. Phys. Chem., 87, 1081 (1983). Hydrogen Abstractions from Methyl Groups by Atomic Oxygen. Kinetic Isotope Effects Calculated from MNDO/UHF Results and an Assessment of Their Applicability to Monooxygenase-Dependent Hydroxylations. 

Michael additions of 2-methyl-2-propanethiol onto 1,5-oxathiocinone 771 gave the 3-[(/-butylthio)methyl]-l,5-oxathiocan-2-one 772 in 82% yield. The reaction was performed in order to obtain model for chain-transfer products of hydrogen abstraction by carbon-centered radicals (see Section 14.07.6.4) (Equation 35) <2005ASC1811>. 

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