Alkyl hydroxyanisoles as antioxidants

Abstract

Summary The antioxidant potency of a substituted hydroxyanisole greatly depends on the orientation of the substituents which make up the molecule. This fact is demonstrated by the synthesis and evaluation of a number of alkyl substituted hydroxyanisoles as to antioxidant effectiveness in the stabilization of lard. In the case of the derivatives of 4‐hydroxyanisole, maximum potency is realized by placement of a t‐butyl group in the number 3 position relative to the methoxy group. The substitution of other groups such as one or two methyl groups, a butyl group of normal, iso, or secondary configuration, or a t‐butyl group in the number 2 position leads to an antioxidant of lower potency. The potencies of the three t‐butylhydroxyphenetoles are comparable to those of the corresponding t‐butylhydroxyanisoles. The effectiveness of butylated 2‐ and 3‐hydroxyanisoles are all low; it is essential that the hydroxy group be in the 4‐position relative to the methoxy group. The stability of baked goods (soda crackers) is greater with lard inhibited with 3‐t‐butyl‐4‐hydroxyanisole than with 2‐t‐butyl‐4‐hydroxyanisole.

Dihydric Phenols as Antioxidants in Isotactic Polypropylene

“…Activities lower than those of pyrocatechol were found with all the observed substances of Type Ia, where R-= terf-alkyl group: (a) 1,1-dimethylethyl (denoted further as feri-butyl), (b) 1,1-dimethylpropyl (terr-amyl), (c) 1,1,3,3-tetramethylbutyl (terf-octyl), and (d) 1,1,3,3,5,5-hexamethylhexyl (terf-dodecyl); the relative activities (calculated from T0.D) are given in Table II. Contrary to our results, other workers ( 4,9,20,36 ) state that in the stabilization of carotene, paraffin wax, and lard the activity of pyro catechol is favorably affected by substitution at position 4, not only by normal but by tertiary alkyl groups as well. Disparate influences of substitution are not surprising when comparing the activity in different substrates owing to the possibility of directive influences in the process of inhibited oxidation.…”

“…Guaiacol and its alkyl derivatives show very little activity even in the stabilization of lard (36,39).…”

“…On the basis of these results it is possible to consider in the series of alkylpyrocatechols the effect of opti mum steric hindrance (taking into account the polar effects of the alkyl groups acting simultaneously). This effect can be observed by carefully analyzing the substitution effects on the activities of monohydric phenolic antioxidants in various substrates (30,36,43,48) as well as by studying the synergism (17). …”

The Mechanism of Antioxidant Action in Vitro

“…The activity of 2-methoxyphenols is much lower than that of the corresponding pyrocatechol possessing two free hydroxyl groups24 because 2-methoxyphenols are unable to stabilise the phenoxyl radical by hydrogen bonding as in (24). Phenolic antioxidants are effective in extending the induction period when added to an oil which has not deteriorated to a great extent, but they are virtually ineffective in retarding deterioration of lipids when added to severely deteriorated systems.…”

“…The introduction of a second hydroxyl group into the 2 or the 4 position of a phenol increases the antioxidant activity, The effectiveness of a 1,2-dihydroxybenzene derivative is increased by the stabilisation of the phenoxyl radical through an intramolecular hydrogen bond (24):22 (24) The antioxidant activity of dihydroxybenzene derivatives is partly due to the fact that the semiquinonoid radical produced initially can be further oxidised to a quinone by reaction with another lipid radical, or it may disproportionate into a quinone and a hydroquinone molecule (25 °UH) 00…”