Abstract
Functionalized 4,5-diphenyl-imidazoles, 4,5-diphenyl-1,2,4-triazoles and 5-(o-hydroxyphenyl)-4-phenyl-1,2,4-triazoles at the 2-position with thiol, thiomethyl and thiobenzyl groups, have been tested as antioxidant additives for lubricating oils. Whereas the thiomethyl groups in such compounds increased the antioxidant property than the thiol group, the corresponding thiobenzyl groups did the reverse. The results can be explained, based on correlating the electron donating and withdrawing abilities of the substituents with the oxidation stability. The triazoles carrying a phenolic hydroxyl group have more antioxidant power than those without such a group. The imidazoles gave the oils more oxidation stabilities than the two types of triazoles with the same functionalities. The 4,5-diphenyl-2-thiomethyl-imidazole (2), as an additive, has the highest antioxidant property, reaching the level of standard one when its concentration is 1.0% wt. instead of the 0.8% wt. of the standard. The correlation of the antioxidant character of the heterocyclic additives with their structures has been investigated using the semiempirical gas phase AM1 calculations for the studied heterocycles. The relative stability of the imidazoles 1 and 3 compared to 2 were in the order 2>1>3. Similarly, the relative stability of the triazoles are in the same order where 5>4>6 and 8>7>9.