Ashless Antiwear and Extreme-Pressure Additives 223
P(OR) 3 + H 2 O ⇒ (RO) 2 P(=O)H + ROH (8.10)
P(OR) 3 + HCl ⇒ (RO) 2 P(=O)H + RCl (8.11)
2P(OR) 3 + HP(=O)(OH) 2 ⇒ 3(RO) 2 P(=O)H (8.12)
By carrying out the preceding reactions in the presence of hydrogen chloride acceptors such as
pyridine, the isolation of mono, di, and trialkyl phosphites is feasible. However, with alcohols of
normal reactivity, the product is often mainly dialkyl hydrogen phosphite. This can be made in
up to 85% yield, by adding PCl 3 to a mixture of methanol and a higher alcohol at low tempera-
ture. The methyl and hydrogen chlorides are then removed by heating under reduced pressure on a
steambath.
PCl 3 + 2ROH + CH 3 OH ⇒ (RO) 2 P(=O)H + CH 3 Cl + 2HCl (8.13)
The commonly used phosphites available in the marketplace are dimethyl hydrogen phosphite,
diethyl hydrogen phosphite, diisopropyl hydrogen phosphite, dibutyl hydrogen phosphite, bis(2-
ethylhexyl) hydrogen phosphite, dilauryl hydrogen phosphite, bis(tridecyl) hydrogen phosphite,
dioleyl hydrogen phosphite, trisnonylphenyl phosphite, triphenyl phosphite, triisopropyl phosphite,
tributyl phosphite, triisooctyl phosphite, tris(2-ethylhexyl) phosphite, trilauryl phosphite, triisodecyl
phosphite, diphenylisodecyl phosphite, diphenylisooctyl phosphite, phenyldiisodecyl phosphite,
ethylhexyl diphenyl phosphite, and diisodecyl pentaerythritol diphosphite.
8.2.2.2.2 Chemical and Physical Properties
Phosphites tend to hydrolyze when exposed to humidity in the air or moisture in the lubricant.
The extent of hydrolysis depends on the moisture content of the ambient atmosphere, the tempera-
ture, and the duration of exposure. Generally, liquid phosphites are more stable than solids because
of the reduced surface area available for moisture pickup. But hydrolysis can be minimized if proper
precautions, such as dry nitrogen atmosphere, cool storage, and use of tight seals, are observed. The
lower dialkyl hydrogen phosphites hydrolyze in both acidic and alkaline solutions to monoalkyl
esters and phosphorus acid. Rates of hydrolysis normally decrease with increasing molecular
weight. The lower esters of trialkyl phosphites are rapidly hydrolyzed by acids; however, they are
relatively stable in neutral or alkaline solutions. In general, the hydrolytic stability of the trialkyl
phosphites increases with molecular weight.
Since the dialkyl hydrogen phosphites are predominately in the keto form, they are somewhat
resistant to oxidation and do not complex with cuprous halides. Both of these reactions are charac-
teristic of trivalent organic phosphorus compounds [28–30]. These esters are relatively resistant to
reaction with oxygen and sulfur, but react quite readily with chlorine and bromine giving the cor-
responding dialkyl phosphorohalidates ((RO) 2 P(=O)X where X = Cl or Br) [27].
The hydrogen atom of the dialkyl hydrogen phosphites is replaceable by alkali but is not
acidic in the usual sense. The alkali salts are readily obtainable by reaction of ester with metals.
In contrast with the parent compound, these salts readily add sulfur to form the corresponding
phosphorothioates. Sodium salts of phosphites can be reacted with alkyl chlorides to produce alkyl
phosphonates. These salts react with halophosphites to produce pyrophosphites and with chlorine
or bromine to yield the corresponding hypophosphates. Dialkyl hydrogen phosphites add readily
to ketones, aldehydes, olefi ns, and anhydrides, and these reactions are catalyzed by bases and free
radicals. This type of reaction provides an excellent method for preparing phosphonates.
Sulfur reacts readily with trialkyl or triaryl phosphites to form corresponding trialkyl or triaryl
phosphorothioates, which are also ver y useful antiwear additives. The reaction of trialkyl phosphites
with halogens is an excellent method for preparing dialkyl phosphorohalidates. Acyl halides and
most polyfunctional primary aliphatic halides can be used. Triisopropyl phosphite provides a unique
means for preparation of unsymmetrical phosphonates and diphosphonates because the by-product
isopropyl halide reacts very slowly and thereby does not compete with the primary reaction.