Alkyl radical scavenging

Fig. 24. Reductive dehalogenation of a chlorinated hydrocarbon in the presence of a metal forming an alkyl radical, showing (Pathway (I)) the alkyl radical scavenging a hydrogen atom, and (Pathway (II)) the alkyl radical losing a second halogen to form an alkene...

Scheme 1.65. Alkyl-radical scavenging by benzoquinone. After Al-Malaika (1989).

The slope of this line is a measure of the alkyl radical scavenging efficiency, ks, for a given temperature and is also an indication of the efficiency of the nitroxide radical in competing with oxygen for these radicals. There is clear... 

The use of a profluorescent nitroxide, in this case the phenanthrene analog TMDBIO, offers the prospect of not only quantifying the alkyl radical scavenging efficiency in the earliest stages of polyolefin oxidation (in the retardation period) as demonstrated here, but also integration over time of the total alkyl radical population formed on oxidation. Future prospects include imaging of the zones of degradation by fluorescence microscopy. 

Both 102 and 108 were confirmed as (macro)alkyl radical scavengers in an oxygen deficient environment. 102 traps 1-cyano-l-methylethyl used in excess [107]. The respective thermolabile JV-alkylate 110 (R = EtO, 20% yield) and a stable 8-alkylate 111 (R = EtO, 50% yield) are formed. 6-Ethoxy-8-alkoxy-2,2,4-trimethyl-l,2-dihydroquinoline 112 (R=EtO, R1 = 1-cyano-l-methylethyl) is formed as the main product from 108, probably by isomerization of the O-substituted hydroxylamine generated in the first stage from 108. 

Scheme 18.8 Alkyl radical scavenging involving intramolecular hydrogen shift.

Hydroxylamines, besides acting as alkyl radical scavenger, can also act as hydroperoxide decomposer (Scheme 18.12) [95]. As alkyl radical chemistry is most important under oxygen-deficient conditions and at high-temperatures, these stabilizer types are used only as processing stabilizer—they are not effective as long-term heat stabilizers. 

Since the advent of NMP, at least 80 different nitroxides have been reported as mediators. The majority of these is listed in the outstanding review by Bagryanskaya and Marque on alkyl radical scavenging by nitroxides. The most relevant and efficient nitroxides are shown in Scheme 4.12, along with the most important alkyl radicals derived from the related alkoxyamines (see Section 4.6). 

The ultimate fate of the oxygen-centered radicals generated from alkyl hydroperoxides depends on the decomposition environment. In vinyl monomers, hydroperoxides can be used as efficient sources of free radicals because vinyl monomers generally are efficient radical scavengers which effectively suppress induced decomposition. When induced decomposition occurs, the hydroperoxide is decomposed with no net increase of radicals in the system (see eqs. 8, 9, and 10). Hydroperoxides usually are not effective free-radical initiators since radical-induced decompositions significantly decrease the efficiency of radical generation. Thermal decomposition-rate studies in dilute solutions show that alkyl hydroperoxides have 10-h HLTs of 133—172°C. 

Stabilization of Fuels and Lubricants. Gasoline and jet engine fuels contain unsaturated compounds that oxidize on storage, darken, and form gums and deposits. Radical scavengers such as 2,4-dimethyl-6-/ f2 butylphenol [1879-09-0] 2,6-di-/ f2 -butyl-/)-cresol (1), 2,6-di-/ f2 -butylphenol [128-39-2], and alkylated paraphenylene diamines ate used in concentrations of about 5—10 ppm as stabilizers. 

The hexabutyldistannane used in this reaction is not involved in the propagation sequence but may be involved in initiation or scavenging of potential chain-termination radicals. Intermolecular additions of alkyl radicals to alkynes have also been observed. 

With a radical-scavenging compound present in the reaction mixture, an alkyl radical species like 5 can be trapped, thus suggesting a fast conversion of the alkoxy radical 3 by intramolecular hydrogen abstraction, followed by a slow intermolecular reaction with nitrous oxide. 

This sequence of formation of radical cation which is followed by a C—S bond scission into alkyl radical and alkyl sulfonyl cation was previously suggested by the same authors for the radiolysis of polyfolefin sulfonefs in the solid state72 and was confirmed by scavenger studies73. Scavengers are ineffective in crystalline solids such as dialkyl sulfones and hence could not be used in this study. 

Anti-oxidants can be divided into two classes depending on which part of the radical chain they quench. Primary anti-oxidants are radical scavengers and will react with alkyl chain radicals (R ) or hydroperoxides (ROOH). Secondary antioxidants work in combination with primary anti-oxidants and principally act by converting peroxide radicals (ROO ) into non-radical stable products. Synergism often works when both classes are used together. 

Since autoxidation is mainly initiated by hydroperoxide, it can be inhibited not only by scavengers of peroxyl and alkyl radicals, but also by compounds reactive to hydroperoxide (see Chapter 12). 

The reaction of trialkylboranes with 1,4-benzoquinones to give in quantitative yield 2-alkylhydroquinones was the first reaction of this type occurring without the assistance of a metal mediator [81,82], An ionic mechanism was originally proposed but rapidly refuted since the reaction is inhibited by radical scavengers such as galvinoxyl and iodine [83]. This procedure is in many cases superior to the more widely use organometallic additions. For instance, when primary and secondary alkyl radicals have been used and afford the addition products in high yield (Scheme 33) [84],... 

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