433. Alkylperoxy-radicals. Part II. Kinetics of autoxidations retarded by 2 : 4 : 6-trialkylphenols

Abstract

By use of a competitive method involving reactions of alkylperoxyradicals either with a 2 : 4 : 6-trialkylphenol (AH) or with a standard hydrocarbon (RH = 9 : 10-dihydroanthracene), the relative reactivities a t 60" of various phenols were determined by measuring initial rates of oxidation. Alkylperoxy-radicals were generated by the dissociation of 2 : 2 : 3 : 3-tetraphenylbutane and reaction of the resulting radicals with oxygen.Three types of kinetics were found to occur, depending on the phenol investigated, and were interpreted on the basis of three different reaction mechanisms.It appeared that phenolic antioxidants can be characterized (1) by the antioxidant efficiency, i.e., the ratio of the rates of hydrogen abstraction by alkylperoxy-radicals from the phenol and from the hydrocarbon respectively, and (2) by the rate of the chain-transfer reaction A* + RH --b AH + R-. All phenolic antioxidants reacted much faster than the standard hydrocarbon, differing among themselves by a factor of about 20. The rate of the chain-transfer reaction decreased with increasing size of orthosubstituents. The presence of two o-tert.-butyl groups completely prevented the starting of new oxidation chains by transfer.

Inhibition of the Autoxidation of Organic Substances in the Liquid Phase.

“…ROOH + R'SR" -> ROH + R'S(0)R" (34) ROOH + R'S(0)R" -> ROH + R'S(0)2R" (35) Monosulfides which contain at least one aliphatic or cycloaliphatic group attached to the sulfur atom are more effective antioxidants than mercaptans and disulfides, while diaryl sulfides and sulfones are inactive (87). The most active sulfides contain one ferf-butyl group attached to the sulfur and a second group which should be isopropyl, ferf-butyl, -CH (CHS) CH=CHR, or -CH2CH2COR (12a).…”

“…Inhibitors may therefore be classified into four groups according to whether they are strong or weak and effective or ineffective by measurements of the rates of the elementary reactions involved. Classifications of inhibitors by this method, particularly into strong and weak classes, have been made by a number of workers (34,35,47,82,89,142), but it must be remembered that because of the variety of other reactions in which the inhibitor and its free radical can be involved these classifications apply only to the particular reaction conditions used in each case.…”

“…Many of these secondary reactions can be suppressed if the reactive center of the inhibitor is sterically protected; moreover, the chances of observing an isotope effect may be improved at the same time (6). For example, 4-substituted 2,6-di-ieri-butylphenols react very slowly with benzoyl peroxide (322) and with hydroperoxides (166) and do not undergo chain-transfer reactions (34). For these reasons, a large number of 4-substituted 2,6-di-feri-butylphenols have been compared as inhibitors of the autoxidation of a white mineral oil at 160°C.…”

Absolute rate constants for hydrocarbon oxidation. XII. Rate constants for secondary peroxy radicals

“…Tetralin hydroperoxide and 9,10-dihydroanthracyl-9-hydroperoxide were prepared by oxidation of the appropriate hydrocarbon and purified by crystallization (9,10). n-Butyl hydroperoxide and sec-butyl hydroperoxide were prepared from the respective alkyl methane sulfonates (1 1).…”

Metal complexes as antioxidants. 7. Kinetics of inhibition of styrene autoxidation by zinc di-isopropyldithiophosphate

“…sulphur (2.19 -t O.ll%), and phosphorus (0.88 rt 0.04%). Zn[(iPrO)2PS2]2 would, therefore, appear to have been incorporated into the polymer by the inhibition reaction [12] and the chain transfer reaction [13].…”