Free radical chain processes

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

Note, Added in Proof-. In their study of the autoxidation of 2-butyl-isoindoline, Kochi and Singleton showed that 2-butylisoindole is formed and is converted by further oxidation to 2-butylphthalimide and 2-butylphthalimidine. The rate of oxidation of 2-butylisoindoline to the isoindole was found to be markedly dependent on hydrogen donor ability of the solvent and was shoivn to involve a free radical chain process. Autoxidation of 2-butylisoindole also appears to be a radical process since it can initiate autoxidation of 2-butylisoindoline. 

Scheme 1 Free radical chain process involved in polymer oxidation.

If the loss of solubility of these initially linear polymers takes place through a free-radical-chain process in which the cross-linking reaction represents the termination step, one may be hesitant at first to explain the fact that a free radical, generated at one point on a relatively sluggish polymer chain, can find a radical on a neighboring chain with which to terminate. However, perhaps reactions of the type R + R H --- RH + R or R00 + R H-- ... 

Electron-transfer reaction, free radical chain processes in aliphatic systems involving an, 23, 271 Electron-transfer reactions, in organic chemistry, 18,79 Electronically excited molecules, structure of, 1, 365... 

Force-field methods, calculation of molecular structure and energy by, 13,1 Free radical chain processes in aliphatic systems involving an electron-transfer reaction, 23, 271 Free radicals, and their reactions at low temperature using a rotating cryostat, study of, 8. I Free radicals, identification by electron spin resonance, 1, 284... 

Electron-transfer reaction, free radical chain processes in aliphatic systems involving an, 23, 271... 

The use of zeolites can overcome many of these limitations and provide new controlled entries into these oxidized hydrocarbons and new materials. For example, some of the most valuable industrial intermediates are terminally oxidized hydrocarbons, snch as n-hexanol or adipic acid, that are not readily available in free-radical chain processes. The ability of zeolites to function as shape-selective catalysts can, in principle, be used to restrict access, by reactant or transition state selectivity, to sites not normally attacked by oxidants [3]. 

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

The chlorination of polyethylene, poly(vinyl chloride), and other saturated polymers has been studied [Favre et al., 1978 Lukas et al., 1978 McGuchan and McNeil, 1968 Stoeva and Vlaev, 2000]. The reaction is a free-radical chain process catalyzed by radical initiators. 

The more acidic fluorene in tert-butyl alcohol solution, or in DMSO solution, reacts by a process that involves the carbanion in equilibrium with hydrocarbon. Thus, fluorene and 9,9-dideuteriofluorene oxidize at identical rates. We have established that the oxidation of the anion of fluorene can be catalyzed by a variety of electron acceptors (v), including various nitroaromatics (18). The catalyzed oxidation rates were found to follow the rates of electron transfer measured by ESR spectroscopy in the absence of oxygen. These results established the catalyzed reaction as a free radical chain process without shedding light upon the mechanism of the uncatalyzed reaction. 

The oxidation of benzhydrol and 9-fluorenol in basic solution again shows a difference in regard to mechanism that can be primarily attributed to a difference in acidity as carbon acids. In tert-butyl alcohol benzhydrol enters into an oxidation scheme as the mono (oxy) anion. The data strongly suggest a free radical chain. Under these conditions the more acidic fluorenol or xanthenol oxidizes via carbanions or dianions. These oxidations can be catalyzed to occur via a free radical chain process by one-electron acceptors, such as nitrobenzene, and a free radical chain process may well be involved in the absence of the catalyst. 

Diphenyl-4//-pyran (151a R = Ph) undergoes a free-radical chain process with trichloromethyl radicals generated from carbon tetrachloride, affording pyrylium radical cation 378a353 (see Eq. 19). 

Primary aliphatic alcohols are air-oxidized to the corresponding esters in the presence of a bromine-nitric acid catalyst. Evidence supporting a mechanism, which is not a free radical chain process, is presented. 

C>2 is known to react with olefins to form allylic hydroperoxides via the Schenck reaction. Even if cyclohexene has a rather low reactivity towards 02 [19], it is likely that at least part of the allylic oxidation products (enylOOH, enol, enone) arise from a 02 reaction, rather than from a free radical chain process. 

Pyrolysis. Vinyl chloride is more stable than saturated ehloroalkanes to thermal pyrolysis. That is why nearly all vmyl chlonde made commercially comes from thermal clehydrochlorination of ethylene dichloride (EDC). When vinyl chloride is heated to 450°C, only small amounts of acetylene form. Decomposition of vinyl chlonde via a free-radical chain process begins at approximately 550°C, and increases with increasing temperature. Acetylene, HC1. chloropiene, and vinylacetylene are formed in about 35% total yield at 680°C. At higher temperatures, tar and soot formation becomes increasingly important. When dry and in contact with metals, vinyl chloride does not decompose below 450°C. However, if water is present, vinyl chloride can corrode iron, steel, and alum in 11m because ofthe presence of trace amounts of HC1. This HC1 may result from the hydrolysis of the peroxide formed between oxygen and vinyl chlonde. 

The reactions of 1,3-dihaloadamantanes with various carbanions in DMSO have been studied.18 For example, potassium enolates of acetophenone and pinacolone and the anion of nitromethane react with 1,3-diiodoadamantane (19) under photo-stimulation a free-radical chain process forms a 1-iodo monosubstitution product (20) as an intermediate, which undergoes concerted fragmentation to yield derivatives of 7-methylidenebicyclo[3.3.1]nonene (21). These and other results were interpreted in terms of the Srn1 mechanism. The work has been extended to the reactions of 1- and 2-halo- and 1,2-dichloro-adamantanes, examples of the SrnI mechanism again being found.19... 

Although less common, ketyl anions can also be generated by removal of an a-hydrogen from an alkoxide (Figure 1, reaction 3). An interesting example where a ketyl anion is formed as an intermediate in this manner is provided by the electrochemically-initiated reduction of an aryl halide by an alkoxide anion via the free radical chain process illustrated in Scheme l6. 

Reductions of a-bromo ketones and esters can also be accomplished in a free radical chain process (Scheme 11) using 2-propanol or 2-methyldioxolane45. The 2-hydroxypropyl radical is an effective reducing agent with E =—1.11 V vs Ag/AgCl. An application of this chemistry to yield spiro-y-lactones has been reported (Scheme 12)46. 

Therefore, it seems reasonable to suppose that in the semiconductor catalyzed oxidation of DPE, the resulting radical cation of the olefin could react with the superoxide anion to give a peroxy-ethy1-1,4-diradical which subsequently reacts with the olefin and oxygen through the free radical chain processes. 

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