Tertiary alkylperoxy radicals

TABLE 9. Rate constants for the formation of di-tertiary alkyl peroxides from tertiary alkylperoxy radicals ... 

Bennett et al. [151] have been able to measure the equilibrium constants for reaction (51) at —120 °C for several tertiary alkylperoxy radicals, but whether tetroxides are also formed as intermediates at high temperature remains a problem for conjecture. 

The available evidence suggests, therefore, that alkylperoxy radical disproportionation reactions are important for low molecular weight alkanes, but reliable values of the Arrhenius parameters for reactions (42) and (14a ) are urgently needed to confirm this. However, since is greater than fei4a by ca. 10 this will not be the case for alkanes which can form secondary or tertiary alkylperoxy radicals capable of undergoing extensive isomerization involving 1 5 or 1 6 H-transfer from further secondary or tertiary carbon atoms. 

No new data have been reported for tertiary alkylperoxy radicals. It has been confirmed that the rate constant of the t-butylperoxy radical self-reaction is very... 

Tertiary alkylperoxy radicals will be discussed first because the mechanism for these radicals is reasonably well understood. On the other hand the mechanism for self-reaction of secondary and primary alkylperoxy radicals is still in doubt and these radicals are considered in the second part of this review. 

Table II Overall and self-termination rate constants for some tertiary alkylperoxy radicals (IT).

The hydroxyl radical will be the predominant entity which attacks the alkane to regenerate an alkyl radical (Reaction 10) under conditions where isomerization and decomposition are the usual fate of alkylperoxy radicals. The activation energy for attack on an alkane molecule by OH, although difficult to determine accurately (30), is low (I, 3) (1-2 kcal. per mole). This has an important consequence. The reaction will be unselective, being insensitive to C—H bond strength. Each and every alkyl radical derived from the alkane skeleton will therefore be formed. To describe the chain-propagation steps under conditions where isomerization is a frequent fate of alkylperoxy radicals it is necessary, then, to consider each and every alkylperoxy radical derived from the alkane and not just the tertiary radicals. 

With smaller alkylperoxy radicals, however, fewer / -, y-, and 8-carbon atoms are available. Thus, for example, the 2-methylprop-l-ylperoxy radical, OOCH2CH(CH8)o, has no y- or 8-C atoms, and the / -C atoms carry primary hydrogen only. Isomerization of this alkylperoxy radical by 1,5 transfer of primary H competes only moderately successfully with isomerization by 1,4 transfer of tertiary H (Table III). In the ethylperoxy radical, only 1,4 H-transfer is possible. For these cases, then, hydrogen abstraction will be a more frequent mode of oxidation of the alkyl radical than for larger radicals, but the calculation suggests that it will account... 

In recent years much emphasis has been placed on studies of co-oxidations, since they can provide quantitative data about fundamental processes (such as the relative reactivities of peroxy radicals toward various hydrocarbons48-50), which are difficult to obtain by other methods. Co-oxidations are also quite important from a practical viewpoint since it is possible to utilize the alkylperoxy intermediates for additional oxidation processes instead of wasting this active oxygen. That the addition of a second substrate to an autoxidation reaction can produce dramatic effects is illustrated by Russell s observation51 that the presence of 3 mole % of tetralin reduced the rate of cumene oxidation by two-thirds, despite the fact that tetralin itself is oxidized 10 times faster than cumene. The retardation is due to the higher rate of termination of the secondary tetralyl-peroxy radicals compared to the tertiary cumylperoxy radicals (see above). 

Contrary to our results, other workers (4, 9, 20, 36) state that in the stabilization of carotene, paraffin wax, and lard the activity of pyrocatechol is favorably affected by substitution at position 4, not only by normal but by tertiary alkyl groups as well. Disparate influences of substitution are not surprising when comparing the activity in different substrates owing to the possibility of directive influences in the process of inhibited oxidation. The participation of phenolic antioxidants in the inhibition of autoxidation can be demonstrated (1, 2, 3) simply as a reaction between the molecule of antioxidant AH and the alkylperoxy radical ROO formed duririg the autoxidation of the substrate RH. During this process, an aryloxy radical (A ) is first generated. 

As the rate constants for the termination reaction of alkylperoxy radicals are normally in the sequence primary > secondary P tertiary, termination will be predominantly between the R OO radicals. 

Thus, in the case of 3-ethylpentane initial attack at a secondary C—H bond may always be followed by oxygen addition and 1 5 H-transfer involving another secondary C—H bond. Furthermore, since the initial attack is unselective during cool-flame oxidation a considerable proportion of primary alkylperoxy radicals will be formed from this alkane and these may all undergo the relatively easy isomerization involving 1 5-hydrogen transfer from a tertiary C—H bond... 

This is not a termination reaction. It is one means of converting alkylperoxy radicals to alkoxy radicals. It is the dominant reaction when neither peroxy radical contains an a-hydrogen, but it even occurs to a significant extent (in one report about 40% of the time [17]) with peroxy radicals that do contain a-hydrogens. Alkoxy radicals are vigorous hydrogen abstractors [12]. This appears to be the main reaction for primary alkoxy radicals the products are primary alcohols. Secondary and tertiary alkoxys, however, tend to undergo a competitive 6-scission reaction to a major extent [18] ... 

Alkylperoxy radicals participate in the chain propagation step of oxidation. In photo-oxidized PP, most ROO terminate after a few propagation steps [190]. The few ROO that escape from this recombination propagate with a great rate. At the same time, they are easily scavenged by chain-breaking phenolic and aromatic aminic AO or HAS derived NOH. Secondary and tertiary HAS associate in... 

The abstraction of a hydrogen atom from an alkane first produces an alkyl radical. In the atmosphere, however, alkyl radicals have but little choice other than to combine with oxygen to yield an alkylperoxy radical. As mentioned previously, tertiary hydrogen atoms are abstracted more easily than secondary H atoms, and their abstraction, in turn, is more facile than that of primary H atoms. In the higher hydrocarbons the number of secondary H atoms usually exceeds that of primary or tertiary ones, so that secondary alkyl and alkylperoxy radicals are most frequently formed ... 

The rate of reaction of oxygen with most radicals is very rapid because of the triplet character of molecular oxygen. The ease of the autoxidation is therefore largely governed by the ease of hydrogen abstraction in the second step of the propagation sequence. The alkylperoxy radicals that act as the abstracting species are fairly selective. Substrates that are relatively electron-rich or that provide particularly stable radicals are the most easily oxidized. Benzylic, allylic, and tertiary positions are most susceptible to oxidation. This selectivity makes radical-chain oxidation a suitable preparative reaction in some cases. 

Kinetic results were consistent with a bimolecular termination reaction whereas reaction products and mechanisms were something of a mystery. At that time it was known that the termination rate constant for autoxidation of cumene ( ) is about three orders of magnitude smaller than the termination rate constant for autoxidation of tetralin (7.). It was, however, generally accepted that the tennination rate constants for tertiary ( ) and secondary (9 ) alkylperoxy radicals are insensitive to the structure of the hydrocarbon residue in the radical. 

The propagation reactions by peroxy radicals should generate mostly radical (3.56), since a tertiary C—H bond is about 25 times more reactive than a secondary C—H bond toward alkylperoxy radicals at 25°C [771, 2182] so that the main oxidation products of poly(vinyl chloride) are assumed... 

---