Analysis of the hydrolytic stability of zinc(II)O,O-dialkyl dithiophosphates as a function of the nature of the alkyl groups by 31P NMR spectroscopy

Abstract

Zinc(II) bis(O,O-dialkyl dithiophosphates) Zn[S₂P(OR)₂]₂1 where R = ethyl 1a, 2-propyl 1b, 2-butyl 1c, hexyl 1d, 4-methyl-2-pentyl 1e, and 2-ethylhexyl 1f, are hydrolysed to phosphoric acid 5 and other by-products O-alkyl O,O-dihydrogen thiophosphate 6, O,O-dialkyl O-hydrogen thiophosphate 7, and alkyl dihydrogen phosphate 8via the corresponding O,O-dialkyl S-hydrogen dithiophosphate 3 and thiophosphoric acid 4. Contrary to expectation, the rates of hydrolysis of the zinc(II)O,O-dialkyl dithiophosphates are found to be independent of the alkyl substituent present and fall within the range k_obs=(2.34–2.96)× 10^–4 s^–1. The null effect of the alkyl groupings is attributed to hydrophobic interactions that leave the central zinc atom exposed to attack by water irrespective of their size.