The Chemistry of the Thermal Degradation of Zinc Dialkyldithiophosphate Additives

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A mechanism for the thermal decomposition of zinc dialkyldithiophosphate additives has been proposed that accounts for the major degradation products and their dependence on the nature of the alkyl group. The proposed mechanism generally follows well-known rules of phosphorus chemistry. The major points are as follows. (1) The decomposition is initiated by a general migration of alkyl groups from oxygen atoms to sulfur atoms. The rate of this reaction is governed by the ease with which nucleophilic substitution can take place at the α-carbon atom of the alkyl group. The well-established correlation between decreasing number of β-hydrogen atoms and increasing thermal stability is, therefore, explicable in terms of increasing steric hindrance, by the β-subtituents, of nucleophilic attack on the α-carbon atom. (2) Olefin elimination successfully competes with substitution where the alkyl group structure is favorable (i.e. secondary alkyl groups), leading to the formation of phosphorus acids. (3) Nucleophilic...