Butane. Oxidation at secondary and primary C—H bonds

The detailed mechanism of oxidation of butane [35] is well accounted for by the same elementary steps as for isobutane but with two important differences. First, the rate coefficient for propagation at 100°C in n-butane is only 1/10 as large as for isobutane, as expected for abstraction of a sec-C—H bond stronger by 3.5 kcal mole 1 than the f-C—H bond. The second, more important, difference arises in the self-reaction of sec-R02 radicals, viz. 

Unlike the corresponding self-reaction of t-R02 radicals [steps (29) and (30)] where only one in two to twenty interactions gives termination at 100°C, almost every self-reaction of sec-R02 leads to termination by disproportionation (a 1.0) that is, k2g 0. This shift in termination mechanism has two results one is that few alkoxy radicals are formed as chain carriers at temperatures below 120—130° C and the second is that the rate of termination is nearly 100 times as fast as for the f-Bu02 radicals at 100° C. 

The great commercial utility of n-butane for producing acetic acid rests on the fact that as the temperature increases to 160—200° C, a high proportion of alkoxy radicals is formed both in the self-reaction of sec-Bu02 and by homolysis of initially formed sec-Bu02H these in turn lead to two-carbon fragment precursors of acetic acid, viz. 

Mayo [36] has shown that all of the products from the oxidation of 

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