<SUP>31</SUP>P NMR spectroscopy was used to investigate the mechanism and kinetics of the hydrolysis of the metal dithiophosphate class of lubricant additives. The homogeneous hydrolysis of zinc(II) bis (<I>O,O</I>-dialkyl dithiophosphates), [(RO)<SUB>2</SUB>PS<SUB>2</SUB>]<SUB>2</SUB>Zn, (ZDTPs), where R= ethyl, 2-propyl, 2-butyl, n-hexyl, 4-methyl-2-pentyl and 2-ethylhexyl, at 85°C in 1,2-dimethoxyethane solution proceeded to give phosphoric acid, <I>O</I>-alkyl thiophosphoric acid, <I>O,O</I>-dialkyl dithiophosphoric acid and <I>O</I>-alkyl phosphoric acid via the corresponding intermediary <I>O,O</I>-dialkyl dithiophosphoric acid and thiophosphoric acid. The rates of hydrolysis were found to be independent of the alkyl substituent and fall within the range k<SUB>obs</SUB>=(2.34-2.96)x10<SUP>-4</SUP>s<SUP>-1</SUP>. The null effect of the alkyl groups upon the rate of hydrolysis is attributed to hydrophobic interactions that leave the central zinc atom exposed to attack by water irrespective of their size. A comparative study of the hydrolysis of the related cadmium(II) and nickel(II) bis(<I>O,O</I>-diethyl dithiophosphates) CdDTP and NiDTP) showed that both give rise to hydrolysis products identical to those observed for ZDTPs. Diethyl CdDTP is hydrolysed at a comparable rate to diethyl ZDTP (k<SUB>obs</SUB>=2.28x10<SUP>-4</SUP>s<SUP>-1</SUP> <I>νs</I> 2.34x10<SUP>-4</SUP>s<SUP>-1</SUP>). Diethyl NiDTP on the other hand hydrolyses at a rate that is some one hundred-fold slower. These results are explained in terms of the respective metal-sulphur bond strengths which are 205kJmol<SUP>-1</SUP> and 208.5kJmol<SUP>-1</SUP> for Zn-S and Cd-S, but 344kJmol<SUP>-1</SUP> for the Ni-S bond. The stronger Ni-S bond also results in the observation of <SUP>31</SUP>P NMR resonances for two hydrated species of the NiDTP whose participation in the hydrolytic process had previously been the subject of speculation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:663503 |
| Date | January 1993 |
| Creators | Wastle, John Patrick |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/13214 |
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