The Kinetics of the Inhibited Autoxidation of Tetralin

Abstract

The kinetics of the inhibition of the autoxidation of tctralin by 2,6-cli-l-b~it~~l-4-rneth>~l-phenol, phenol, and 4-methoxyphenol have been investigated a t 65 OC. The highly hindered 2,6-di-t-butyl-4-methylphenol follows simple hrst order kinetics and exhibits a normal tleuteriunl isotope effect ( k n / k~ = 10). 'The lcinetics with phcnol arc complicatetl by the fact that the phenoxy radical can abstract a hydrogen, ator11 from both tetralin alltl its hydroperoxide. This leads to oxidation rates which are ~nversely proportional to the square root of the phenol concentration. The tleuteri~~rn isotope efiect has about the value to be expected in view of this square root relatio~l. The Icinctics with 4-methoxyphenol result from chain transfer and from chain terminatio~l by the coupling of 4-methoxyphenoxy radicals. The isotope effect varies between zero and a value that approaches the upper limit of about 10 a t low inhibitor concentratio~~s.The inhibition of the autoxidation of styrene by phenols has been described in considerable detail in a recent series of papers from this laboratory (1). The overall process can be represented by the follo\ving simplified reaction scheme:[7 IRoc. + A' + inactive products.12 represents c~,c~'-azo-bis-isobutyronitrile (AIBN) ~v l~i c h was used to initiate the oxidations.RH represents the styrene and AH the phenolic inhibitor. The radical R' in reaction [2] represents the poly(peroxy-styryl) radical. Our experimental conditions were such that chain termination occurred predominantly by reaction [7], so that the bimolecular selftermination reaction could be ignored. Under these conditions, the theoretical rate of oxidation of the styrene is given by where p is the measured inhibited rate and e is the efficiency with which the AIBN initiates reaction chains. The correction factor (2e -l)kl[Iz] allows for oxygen absorption and nitrogen evolution by the AIBN. This equation describes the kinetics of the inhibition process with a variety of phenols a t low concentrations where self-association of the phenol by hydrogen bonding and hydrogen bonding between the phenol and the initiator were unimportant. These results in styrene differ in two important respects froin the results reported by several other worliers using basically the same technique but employing different hydrocarbon substrates. Firstly, a deuterium isotope effect ( k & / ( k~)~ = 10.6 a t 65 OC was lIsst~ed as 1V.R.C. No. 8097.

Absolute rate constants for hydrocarbon autoxidation. XVII. The oxidation of some cyclic ethers

“…Although this is the order usually observed for the abstraction of secondary and tertiary hydrogens from similar molecules by free radicals, exceptions have been noted in hydrocarbon oxidations. For example, the propagation rate constant for the oxidation of cumene is lower than that for the oxidation of ethylbenzene (18).…”

Arrhenius Parameters for Reaction of tert-Butylperoxy Radicals with some Hindered Phenols and Aromatic Amines

“…Conventional kinetic studies have shown that overall rates of hydrocarbon autoxidation inhibited by 2,6-di-tert-butyl-4-substit~1ted phenols are proportional to the rate of chain initiation and inversely proportional to the phenol concentration (8, [10][11][12][13][14]. This implies that reaction of a chain carrying alkylperoxy radical and a phenol is bimolecular.…”

Metal Complexes as Antioxidants. I. The Reaction of Zinc Dialkyldithiophosphates and Related Compounds with Peroxy Radicals

“…The autoxidation apparatus and general experimental techniques have been adequately described in our earlier papers on inhibition by phenols (16) and amines (17) and on the determination of absolute rate constants for hydrocarbon autoxidation (18). At 50" autoxidations were initiated by the thermal deconlposition of a,af-azobis-isobutyronitrile (AIBN) and at 30" they were initiated by the photochemical deconlposition of a,a'-azo-biscyclohexanecarbonitrile (ACHN).…”

“…(or, for a polymerizable olefin such as styrene (16) k" can be neglected. The initial rate of oxidation (corrected for oxygen absorption by, and nitrogen evolution from, the initiator) according to this scheme will be independent of the oxygen pressure.…”