Novel antioxidant compounds

Parry, Adrian Trevor (1996). Novel antioxidant compounds. PhD thesis The Open University.



The autoxidation process, mechanism of action of radical-trapping antioxidants and the problem of textile yellowing are described and reviewed. Structures implicit in the function of an antioxidant by promotion of radical stabihsation are identified and textile yellowing is attributed largely to reactions of antioxidants with NO2.

The reaction of ethanolic solutions of 2,6-di-r-butyl-4-methylphenol (BHT) with several concentrations of gaseous NO2 are investigated. The main products are 2,6-dir-butyl-4-methyl-4-hydroxy-2,5-cyclohexadienone (MQOL) and 2,6-di-r-butyl-4-methyl-4-nitro-2,5-cyclohexadienone (MQN). Additional products are observed when lower- concentrations of BHT and NO2 are employed. The major product, MQN, is shown to be unstable in aqueous ethanohc solutions and the yellowing component of the reaction solutions is identified as 2,6-di-r-butyl-4-nitrophenol (NP).

Two methods for screening potential compounds for antioxidant activity involving the inhibition of BHT product formation and the NO2-/NO3-ratio are described. They indicate that kojic acid, maltol and 2,5-dimethyl-4-hydroxyfuranone possess similar degrees of antioxidant activity to BHT. This is shown to be consistent with antioxidant function being promoted by the structural features identified.

A quantitative NO2-yellowing test is described which allows rapid determination of the yellowing indices (OUYI) of test compounds. The results support the use of kojic acid and maltol as alternatives to BHT.

An investigation of kojic acid indicates that it is both nitrosated by gaseous NO2, and undergoes a complicated series of reactions. Investigations with 3-methylcyclopentane-1,2-dione confirm that cychc 1,2-diones both reduce and trap gaseous NO2, a possible strategy for reducing yellowing.

The foregoing studies indicate possible structural requirements for a NO2yellowing antioxidant and the synthesis of several N-(3,5-di-r-butyl-4-hydroxyphenyl) cyclic imides has been accomplished. This has provided a group of new compounds which show similar antioxidant activity to BHT, but are less prone to NO2-yellowing than 4-substituted hindered phenols possessing labile hydrogen atoms. Studies on the solvolysis of these cyclic imide compounds suggest that sufficient stability to obtain a commercially useful antioxidant may be imparted by the bulky alkyl substituents in the imide ring, as in the case of N-(3,5-di-f-butyl-4-hydroxyphenyl)-3,3-dimethylglutarimide and N-(3,5-di-tbutyl-4-hydroxyphenyl)-2,3-diethyl-2,3-dimethylsuccinimide which have been successfully prepared.

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