Reaction Pathways in Lubricant Degradation. 1. Analytical Characterization of n-Hexadecane Autoxidation Products

A new protocol is described for the quantitative analysis of mixtures of hydroperoxides, carboxylic acids, aldehydes, ketones, esters, and alcohols. Such complex mixtures arise during the autoxidation of lubricating oils and other hydrocarbons. The analytical protocol uses a novel combination of iodometric and potentiometric acid-base titrations and ¹H and ¹³C NMR spectroscopies. The techniques described were developed so that they would be suitable for the analysis of hydrocarbon samples that have undergone extensive oxidative degradation. The information obtained from these complementary analytical methods results in a set of six simultaneous algebraic equations that contains the six functional group concentrations as unknowns. Solution of these equations then provides the desired concentrations. The average difference between experimental and actual concentrations was less than 0.1 M for all compounds except aldehydes. The average difference for aldehydes was 0.5 M. This analytical scheme is a useful tool for resolving the reaction pathways responsible for hydrocarbon autoxidation.