Dimethylpentane. Intramolecular oxidation

The unusual feature of the oxidation of 2,4-dimethylpentane is the formation, even at the lowest measurable conversions, of the dihydroperoxide in yields of over 90%, viz. 

Rust [37] showed that among several branched alkanes which gave difunctional products on oxidation at 120°C, 2,4-dimethylpentane gave the highest yield of the dihydroperoxide and on the basis of this selectivity he proposed that the key reaction involved intramolecular H-atom transfer from C-4 through a sterically favorable six-center transition state 

Mill and Montorsi [38], in a more detailed kinetic study, showed that, not only was intramolecular abstraction the dominant process, but the ratio of rates of intra- and intermolecular abstraction was almost unchanged with temperature indicating little ( 1 kcal mole-1), if any, difference in activation energy between the two steps. Moreover, at very low oxygen concentrations, some oxetane formed by ring closure of I in competition with (the much faster) addition of oxygen, viz. 

The rate law for the oxidation of 2,4-dimethylpentane (HRH) (at long chain lengths) obeys closely the relation 

Intramolecular oxidation proceeds with unusual facility in dimethyl-pentane compared with most other simple normal and branched alkanes because of structural and kinetic features that are particularly favorable. In the following reaction scheme (where HRH is dimethylpentane) 

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During PP oxidation, hydroxyl groups are formed by the intramolecular isomerization of alkyl radicals. Since PP oxidizes through an intense intramolecular chain transfer, many of the alkyl radicals containing hydroperoxy groups in the 0-position to an available bond can undergo this reaction. An isomerization reaction has also been demonstrated for the liquid-phase oxidation of 2,4-dimethylpentane [89], Oxidation products contain, in addition to hydroperoxides, oxide or diol. 

The comparable intramolecular oxidation is not observed in 2,3-di-methylbutane ([D]/[M] < 0.1), even though it oxidizes almost three times as fast as dimethylpentane [38,39] and the A-factor for such a process is at least a half-power of ten more favorable than for the comparable six-center process. We rationalize this result most readily in terms of an activation energy of nearly 7 kcal mole-1 of ring strain for the formation of the five-center transition state which reduces the rate of the intramolecular process to about one hundredth the rate of the bimolecular process at 100° C. 

The peroxyl radical of a hydrocarbon can attack the C—H bond of another hydrocarbon. In addition to this bimolecular abstraction, the reaction of intramolecular hydrogen atom abstraction is known when peroxyl radical attacks its own C—H bond to form as final product dihydroperoxide. This effect of intramolecular chain propagation was first observed by Rust in the 2,4-dimethylpentane oxidation experiments [130] ... 

It has been observed that the intramolecular propagation plays an important role in the oxidations of 2,4-dimethylpentane (32,33), 2,4,6-trimethylheptane (34), and polypropylene (35,36). However, this intramolecular propagation must be unimportant in PVA oxidation since 2,4-pentanediol did not give 2,4-pentanedione under similar conditions (Run 18). Rust (37) also found no 2,4-pentanedione in the thermal oxidation of 2,4-pentanediol. Table III shows that 4-hydroxy-2-pentanone is the major primary product from 2,4-pentanediol. Therefore, the primary products from PVA must be /3-hydroxyketone and hydrogen peroxide. 

Under oxidation conditions, this isomerization is a stage of intramolecular chain propagation. Competition between reactions (8) and (9) depends, naturally, on the structure of hydrocarbon and its concentration Vg/V9 = 8[RH]/ifc9. The ratio k%lk = 0.024 1/mol for 2,4-dimethylpentane and 0.71 1/mol for n-pentane (373 K), i.e., in hydrocarbons with two (or more) tertiary C—bonds in the P- and y-positions the intramolecular attack of RO-2 is more energetic. 

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