The main inhibiting action of dialkyl sulfoxides in the autoxidation of hydrocarbons derives from their ability to form sulfenic acids on thermal decomposition. The sulfenic acids are extremely active radical scavengers, the rate constant of the reaction of sulfenic acid and peroxy radicals being at least 107 l.mole−1 · S−1 at 60°C. In the AIBN‐initiated oxidation of tetralin in the presence of sulfoxides one peroxy radical is scavenged per molecule of sulfenic acid. This inhibition can be described kinetically as a reduction of the rate of initiation by an amount equal to the rate of decomposition of the sulfoxide.
KONINKLIJKE/SHELL-LABORATORIUM, AMSTERDAMWe have studied the reactions of alkylperoxy radicals with olefins in an inert solvent and atmosphere. The peroxy radicals react by addition to the double bond, mainly leading to epoxide formation, and by abstraction of allylic hydrogen, followed by termination between ally1 and peroxy radicals. Relative and absolute rate constants at 120°C have been determined of the reactions of terr-butylperoxy radicals with 2,3-dimethyl-2-butene, 1 -hexene, a-methylstyrene, propene, 3.3-dimethyl-1-butene and styrene, and of a-cumylperoxy radicals with 3,3-dimethyl-1-butene.We have found that for olefins having allylic hydrogen the abstractionladdition ratio determines the efficiency as well as the rate of epoxide formation, whereas substantial epoxide formation from olefins lacking easily abstractable hydrogen atoms can be expected only if the double bond is sufficiently reactive to suppress the reactions between the peroxy radicals.
Abstract. This paper analyses several complicating factors which can play a role in the inhibition of autoxidation by dialkyl awlfoxides. the uncomplicated case having been dealt with in the first paper'. It was found that in those cases where the autoxidation proceeds via tertiary peroxy radicals. sulfoxides yielding tertiary (or, possibly, also secondary) alkylsulfenic acids show poor antioxidant activity. This is attributed to the generation of radicals from the sulfinic acid perester formed by the reaction of peroxy radicals with the sulfinic radicals produced in the inhibition reaction. A type of inhibition of autoxidation which does not involve radical scavenging arises from the ionic decomposition of hydroperoxides by strong acids formeci from sulfoxides; this type of inhibition is most effective with those sulfoxides which are also active as inhibitors via the formation of sulfenic acid. Sulfoxides also display a pro-oxidant action, which is explained as a catalysis of the radical decomposition of hydroperoxides via the formation of a compound or complex from hydroperoxide and sulfoxide.
tert.‐Butylperoxyradikale reagieren mit Olefinen, die eine Allyl‐Gruppierung aufweisen (2,3‐Dimethyl‐2‐buten, l‐Hexen, α‐Methylstyrol, Propen) sowohl unter Abstraktion des allylisch gebundenen Hund Bildung von Hydroperoxid als auch unter Anlagerung an die Doppelbindung und Epoxid‐Bildung.
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