Primary alkylperoxy 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... 

Acylperoxy radicals arising from the oxidation of aldehydes apparently terminate via a tetroxide which then cleaves to form, first, primary carbon radicals and second, primary alkylperoxy radicals which terminate rapidly [130,131]. Termination rate coefficients for aliphatic aldehydes have values ranging from 0.7 X 107 to 10 X 1071 mole-1 s-1 [10]. 

The self reactions of the simplest primary alkylperoxy radicals have rate constant values at 298 K of about 10" cm molecule s" These rate constants for non-linear alkyl groups apparently increase with chain branching, e.g. 1.2 X 10" cm molecule s" for neopentylperoxy. These values seem to be the... 

A negative temperature dependence of the rate constant seems to be a general characteristic for self-reactions of primary alkylperoxy radicals, with most exponential factors being in the range 0-1000 K/7. 

X 10" cm molecule" s" for neopentylperoxy. Apparently, these values are roughly the lower and upper limits for primary alkylperoxy radicals and more data would be required to refine the relationship between structure and reactivity of such radicals. 

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. 

An important result of this work is that it demonstrates that the appearance of a cool flame during the oxidation of light alkanes is associated with reactions of the alkane itself and the primary alkylperoxy radical, not reactions of alkane oxidation intermediates, for example, the respective oxygenates, the oxidation of which was also observed to be accompanied by... 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

Thermally unstable cycHc trioxides, 1,2,3-trioxolanes or primary o2onides are prepared by reaction of olefins with o2one (64) (see Ozone). Dialkyl trioxides, ROOOR, have been obtained by coupling of alkoxy radicals, RO , with alkylperoxy radicals, ROO , at low temperatures. DiaLkyl trioxides are unstable above —30° C (63). Dialkyl tetraoxides, ROOOOR, have been similarly produced by coupling of two alkylperoxy radicals, ROO , at low temperatures. Dialkyl tetraoxides are unstable above —80°C (63). 

Oxygen-centered radicals are arguably the most common of initiator-derived species generated during initiation of polymerization and many studies have dealt with these species. The class includes alkoxy, hydroxy and aeyloxy radicals and tire sulfate radical anion (formed as primary radicals by homolysis of peroxides or hyponitrites) and alkylperoxy radicals (produced by the interaction of carbon-centered radicals with molecular oxygen or by the induced decomposition of hydroperoxides). 

Alkyl orthophosphate triesters, 79 41 terteAlkyl peroxycarbamates, decomposition of, 78 486 Alkyl peroxyesters, 78 478-487 chemical properties of, 78 480 487 physical properties of, 78 480 primary and secondary, 78 485 synthesis of, 78 478-480 synthetic routes to, 78 479 tert-Alkyl peroxyesters, 78 480 84, 485 as free-radical initiators, 74 284-286 properties of, 78 481-483t uses of, 78 487 Alkylperoxy radical, 74 291 Alkyl phenol ethoxylates, 8 678, 693 ... 

It appears, then, that alkylperoxy radical isomerization is capable of producing hydroperoxyalkyl radicals during the oxidation of all alkanes and that alkene-hydroperoxy radical addition will serve a similar function during the oxidation of those alkanes which contain a high proportion of primary C—H bonds. It remains to determine the proportion of hydroperoxy alkyl radicals arriving by each route as equilibrium is approached. 

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... 

This reaction is certainly exothermic and by analogy with similar hydrogen abstraction reactions might be expected to have an activation energy of anywhere from zero to 8 kcal. If it should be proved that this is the terminating interaction or a possible terminating interaction of primary and secondary alkylperoxy radicals, then we must begin to think anew about the entire mechanism for the over-all chain decomposition and oxidation of hydrocarbons. I believe it would also require us to review seriously our interpretation of the spin resonance work on alkylperoxy radicals at low temperatures. 

Alkylperoxy radicals play vital roles in both propagation and termination processes. Hydroperoxides, R02H, are usually the primary products of liquid phase autoxidations [reaction (4)] and may be isolated in high yields in many cases. Much of the present knowledge of autoxidation mechanisms has resulted from studies of the reactions of alkylperoxy radicals30-33 and the parent hydroperoxides,348-d independently of autoxidation. Thus, the various modes of reaction of organic peroxides are now well-characterized.35 -39... 

Co(acac)3 in combination with N-hydroxyphthalimide (NHPI) as cocatalyst mediates the aerobic oxidation of primary and secondary alcohols, to the corresponding carboxylic acids and ketones, respectively, e.g. Fig. 4.71 [205]. By analogy with other oxidations mediated by the Co/NHPI catalyst studied by Ishii and coworkers [206, 207], Fig. 4.71 probably involves a free radical mechanism. We attribute the promoting effect of NHPI to its ability to efficiently scavenge alkylperoxy radicals, suppressing the rate of termination by combination of al-kylperoxy radicals (see above for alkane oxidation). 

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. 

Kinetic, isotopic and product studies of autoxidation suggest that these reactions and reaction (42) proceed via a tetroxide intermediate (RO4R) which may be a transition state or a molecule of finite lifetime [151]. Russell [152] has suggested that the termination reactions of primary and secondary alkylperoxy radicals in fact involve the formation of a cyclic transition state, viz. 

Two criticisms of this mechanism can be made. First, these activation energies are overall activation energies for a two-step process for the decomposition of different alkylperoxy radicals [106] see opposite page. For the formation of 2-methyltetrahydrofuran both steps will involve cyclization and will have pre-exponential factors [104] of ca. lO sec", whereas the formation of pent-2-ene involves only one such step and a second step for which [39] A = 10 sec". Since the strain energy involved in the isomerizations of each of the alkylperoxy radicals is the same (ca. 6.5 kcal. mole" ) the activation energies of this step will only differ by the difference in primary and secondary C—H strengths (ca. 3.5 kcal. mole" ). It is difficult, therefore, to see how the overall activation energies for the formation of pent-2-ene and 2-methyltetrahydrofuran can be approximately equal. 

---