Antioxidant Activity of Adamantylphenols

Antioxidant properties of phenols with ortho- and para-adamantyl substituents are studied by microcalorimetry using cumene oxidation as a model reaction. The experimental rate constants for inhibition by adamantylphenols (k ₇) are comparable to the corresponding values for sterically hindered phenols, and the stoichiometric inhibition coefficient is higher than the calculated one. Adamantylphenols with adamantyl and methyl or methylene bridge groups in the ortho positions with respect to the functional group exhibit the maximal activity. The inhibition rate constant takes a maximal value of k ₇ = 3.3 × 10⁴ l mol^–1 s^–1 for 2,4-di-(adamantyl-1)-6-methylphenol and a minimal value of k ₇ = 1.4 × 10³ l mol^–1 s^–1 for 2,4,6-triadamantylphenol. The difference in the rate constants is due to steric hindrances in the reaction of a peroxy radical with the functional group of a phenol surrounded by bulky adamantyl groups.