Autoxidation of 2-Naphthols

Brady, Paul Anthony (1974) Autoxidation of 2-Naphthols. PhD thesis, University of Glasgow.

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This thesis describes an investigation of the structural factors which influence the susceptibility of substituted 2-naphthols to autoxidation. The mechanism of these processes is discussed in detail in the light of the results obtained. Previous workers had found that 2-naphthols containing a-branched alkyl substituents at C-1 (ethyl, isopropyl, cyclohexyl) autoxidise readily under extremely mild conditions, the overall rate being greater for those containing bulkier substituents. 2-Naphthols containing small substituents at C-1 (H, methyl, benzyl) do not autoxidise at an appreciable rate. It is proposed that the major factor controlling the overall rate of autoxidation of substituted 2-naphthols is the degree of steric strain within the molecule, and especially the magnitude of the peri-interaction between substituents at C-1 and C-8. This peri-interaction is absent in the l-alkyl-l-hydroperoxy-2(1H)-naphthalenones (1) produced in the autoxidation reactions. The resulting strain relief is thought to be the factor controlling the overall autoxidation rates. I set out to prepare a range of 2-naphthols incorporating a variety of structural features which may elucidate the factors influencing the autoxidation rates, and to study the relative rates in a semi-quantitative manner. Several unknown naphthols which autoxidise rapidly under extremely mild conditions have been prepared, viz. 1-t-butyl- and l-t-pentyl-2-naphthol, 1-methyl-3,6-di-t-butyl-2-naphthol, 1,8-dimethyl-2-naphthol, and l-isopropyl-6-bromo-2-naphthol. Autoxidation of these compounds was found to give the corresponding hydroperoxides (1) in high yield. Several unknown naphthols which are stable to oxygen have also been prepared. A consideration of the results obtained shows conclusively that the autoxidation rates of 2-naphthols is indeed a function mainly of steric strain, electronic factors being of secondary importance. Two plausible mechanistic interpretations are proposed. It is thought that the reaction of the intermediate 2-naphthoxy radicals with molecular oxygen may be the rate-determining step, since this involves the removal of the unfavourable peri-interaction, or that the phenolic O-H bond may be weakened as a result of molecular distortions, thus facilitating the abstraction of hydrogen from the naphthol by the intermediate peroxy radical. Some of the anomalous spectroscopic and chemical behaviour of the compounds under investigation are also related to steric compression.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry
Date of Award: 1974
Depositing User: Enlighten Team
Unique ID: glathesis:1974-78668
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 28 Feb 2020 12:09
Last Modified: 28 Feb 2020 12:09

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