Zinc Dithiophosphates 55
from the intermediate zinc mercaptide (Zn[RS 2 ]), O,S,S-trialkyldithiophosphate will react with mer-
captide to produce alkyl mercaptan and results in the following structure:
RS P SRO−O(2.14)The nucleophilic phosphoryl oxygen (P=O) will then attack another phosphorus atom to produce a
P–O–P bond as in the following reaction:
P SR ++O OP P −SR
OP−
(2.15)A mercaptide anion subsequently cleaves the P–O–P bond at the original P–O site, giving rise to a
net exchange of one atom of oxygen for one atom of sulfur between the two phosphorus atoms:
P OPP +−SR O− + +P SR (2.16)This gives rise to a net reaction for conversion of Structure 2.14 to S,S,S-trialkyltetrathiophosphate,
dialkylsulfi de and S-alkylthiophosphate di-anion as shown in the following reaction:
3 RS P SR −SROO−RS P SRSSR2 RS P O−OO−+ ++R 2 S (2.17)The dialkylsulfi de and S,S,S-trialkyltetrathiophosphate decomposition products are soluble in oil.
The S-alkylthiophosphate decomposition product can also react with itself by way of a phos-
phoryl nucleophilic attack and elimination of mercaptide anion as in Reaction 2.15. This process
will continue until a zinc pyro- and polypyrophosphate molecule with low sulfur content is formed.
The chain will continue to extend until the product precipitates out of solution.
The decomposition of primary alkyl ZDDPs can be accurately described as discussed earlier.
ZDDPs made from branched primary alcohols will decompose in a similar fashion, although at a
much slower rate. This can be explained by the fact that the alpha carbon of the branched primary
alkyl group, being more sterically hindered than the unbranched primary alkyl group, will be less
susceptible to nucleophilic attack, as described in Reaction 2.9. The increased steric hindrance from
beta carbon branching will also decrease the amount of successful mercaptide anion attack on the
branched alkyl P–O–R bond, resulting in less dialkylsulfi de formation and a higher yield of mercap-
tan, an olefi n by-product (through a competing protonation or elimination reaction with mercaptide
anion). Lengthening the alkyl chain will have a much less pronounced effect on thermal stability
than branching at the beta carbon due to the greater steric hindrance derived from the latter.
The decomposition of secondary alkyl ZDDPs, although similar to primary decomposition,
shows that olefi n formation is much more pronounced. The increase in elimination over nucleo-
philic substitution in secondary ZDDPs over primary ZDDPs is easily explained by the fact that
elimination is accelerated by increasing the alkyl substitution around the double bond formed.
Thus, secondary alkyl groups will favor a thermal decomposition into olefi ns and phosphate
acids at the expense of the sulfur–oxygen interchange noted earlier. In a similar but much more
pronounced way, tertiary ZDDP decomposition will be dominated by facile production of olefi n
through elimination. This occurs at even moderate temperatures, making their use in commercial
applications prohibitive.