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The mechanism of the antioxidant action of zinc dialkyl dithiophosphates
Tetd,c&c,n,
1966. Vol. 22. pa. 2153 to 2161. Pagunoa
Praa IA.
Rhcd
In Nonban
Irrlud
THE MECHANISM OF THE ANTIOXIDANT ACTION OF ZINC DIALKYL DITHIOPHOSPHATES A. J. BURN The British Petroleum Company Limited BP Research Centre. Chertsey Road, Sunbury-on-Thames, Middlesex (Receioed 15 October 1965; in reoisedfrom 7 Februury 1966) Abstract-Zinc dialkyl dithiophosphates have been shown to inhibit the axonitrileinitiated oxidation of cumene and squalane and the non-initiated oxidation of indene by acting as chain-breaking agents. Several related compounds have been tested as chain-breaking inhibitors and a mechanism is proposed to account for this behaviour. IT IS now generally accepted’ that the liquid phase a&oxidation of a hydrocarbon (RH) involves a free-radical chain mechanism in which the predominant chain-carrying species in solution is the peroxy radical (R02-)e-s Thermal or metal ion promoted decomposition of the initial hydroperoxide product (RO,H) results in autocatalysis. As a consequence of this mechanism two main types of antioxidant have been recognized;**& chain-breaking agents, which suppress propagation by removal of peroxy radicals in a reaction involving electron or hydrogen transfer, and peroxide-destroyers which remove hydroperoxides by causing their non-radical decomposition. Although zinc dialkyl dithiophosphates, [(RO),PS,], Zn, have been widely used as antioxidants for many years, no detailed mechanism of their action has been put forward. However, it is clear that they are efficient peroxide destroyers. This type of zinc salt has been shown to accelerate the decomposition of both t-buty16 and cumene6 hydroperoxide; in the latter case phenol, a product known’ to result from ionic decomposition, being formed. Related dithiophosphates, at a 1 molar % concentration, have also been showna to cause the decomposition of cumene hydroperoxide giving phenol and acetone. Larson9 has observed, by measurement of induction periods, that zinc dialkyl dithiophosphates are effective antioxidants when tested in a refined white mineral oil at 155”. In mosta*8*9of this work, antioxidant activity has been attributed to peroxide destruction by intermediates formed from the zinc dialkyl dithiophosphates. In contrast to the above approach the possibility that zinc dialkyl dithiophosphates
’ J. L. Bolland, Quart. Revs. 3, 1 (1949); L. Bateman, Ibid. 8, 147 (1954); G. A. Russell,J. Chem. Educ. 36, 111 (1959). ’ R. B. Mesrobian and A. V. Tobolsky, Autoxidation and Antioxidants (Edited by W. 0. Lundberg) Vol I; p. 1 IO. Interscience (1961). * G. Scott, Chem. & Ind. 271 (1963). ’ H. C. Bailey, The Indust. Chemist 38,215 (1962). L T. Colclough and J. I. Cunneen, J. Cfiem. Sot. 4790 (1964). ‘ G. W. Kennerly and W. L. Patterson, fnd Eirg. C/rem. 48, 1917 (1956). ’ E. G. E. Hawkins, Organic Peroxides p. 90. Sport, London (1961). (1J. D. Holdsworth, G. Scott and D. Williams, J. Chem. Sot. 4692 (1964). ’ R. Larson, Scientl$c Lubrication 10, 12 (August 1958). 2153
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might act as chain-breaking agents, by reaction with peroxy radicals, has been investigated since this point seemed to have been overlooked at the outset of our work. A test method4 was used involving measurement of the rate of oxidation of a suitable hydrocarbon containing a non-peroxidic initiator such as an azonitrile. By carrying out the reaction at a temperature low enough to avoid initiation by the hydroperoxide product, the azonitrile is the sole source of free radicals. Any antioxidant which lowers the rate of oxidation when added to such a system must act by removal of chain-propagating peroxy radicals. The remote possibility that addition of a zinc dialkyl dithiophosphate might also reduce the azonitrile initiation rate was however checked. The rate of decomposition of azobiscyclohexylnitrile for example, as determined by the rate of nitrogen evolution, was not found to be significantly affected by a zinc salt in squalane, in the absence of oxygen. Indene oxidation, without azonitrile initiation, was also chosen as a suitable system for this test for the following reasons. The oxidation of indene is readily initiated at 50” by direct reaction with oxygen.10 Propagation mainly involves addition of peroxy radicals to the indene double-bond rather than hydroperoxide formation, the product being a stable indene-oxygen copolymer” which does not initiate at 50”. Possible azonitrile-additive interactions are completely avoided in this system. The response of zinc dialkyl dithiophosphates to such a test is illustrated, for three different hydrocarbons, in Figs. 1, 2 and 3. These results clearly show that the zinc salts act as chain-breaking inhibitors. That inhibition occurs from the beginning of oxidation, under mild temperature conditions, indicates that the zinc salt itself, not a decomposition product, removes peroxy radicals. During the course of our work, Colclough and Cunneen5 have independently observed that three types of dithioate, including zinc diisopropyl dithiophosphate, inhibit the oxidation of squalene by acting as chain-breaking inhibitors as well as peroxide-destroyers. They suggested that electron-transfer from a sulphur atom to a
Fm. 1. Effect of Zinc Dialkyl Dithiophosphates (O*OlSSM) on the Oxidation of Squalane (25 ml) in the Presence of Az.obiscyclohexylnitrile (0.03 M) at 105” A No antioxidant B Zinc di-n-hexyl dithiophosphate C Zinc di-(4-methylpentyl-2) dithiophosphate lo G. A. Russell, J. Amer. Chem. SIC. 78,104l (1956). I1 G. A. Russell, J. Amer. Chem. SC. 78, 1035 (1956).
The mechanism of the antioxidant
action of zinc diahcyl dithiophosphates
20
40 TIME
60
60
(mLn)
FIG. 2. Effect of Zinc Dialkyl Ditbiophosphates (0.005 M) on the Oxidation of Cumene (5 ml) in the Presence of Arobisisobutyronitrile (O-06 M) at 60” A B C D E F G H
No antioxidant Zinc dicyclohexyl dithiophosphate Zinc di-isopropyl dithiophosphate Zinc di-+methylpenty1-2) dithiophosphate Zinc di-wbutyl dithiophosphate Zinc di-n-hexyl dithiophosphate Zinc di-(2,24imethylpentyl-1) dithiophosphate Zince di-n-butyl dithiophosphate
20 TIME
40
60
(min)
FIO. 3. Effect of Additives on the Oxidation of Indene (5 ml) at 50” A B C D E F G H I J
Ferric acetylacetonate (04OC036 M) Ferric di-isopropyl dithiophosphate (04OCKI33M) Nickel di-isopropyl dithiophosphate (OWJ5 M) No additive, or zinc dilauryl phosphate (O*OOOS M) or the disulphide [(pfO),P!U, (00005 M) Potassium di-isopropyl dithiophosphate (0401 M) Zinc di-sec-butyl dithiophosphate (04kW5 M) Zinc di-isopropyl dithiophosphate (04005 M) Zinc di-n-butyl dithiophosphate (04005 M) Potassium di+&nethylpentyl-2) dithiophosphate (0401 M) Cuprous di-isopropyl dithiophosphate (04005 M)
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A. J. BURN
peroxy radical would account for the chain-breaking activity but offered no discussion of the details of this mechanism. In the present work evidence concerning the mechanism by which zinc dialkyl dithiophosphates react with peroxy radicals has been obtained by studying the effect of variation of molecular structure on radical capture efficiency. Compounds examined include numerous metal dialkyl dithiophosphates (I), a xanthate (II), a zinc [Pr’DCs,], Zn
l(RW’S,I. M
II
I W1&I,?G,I,
tWW%I,
Zn
III
IV
dialkyl phosphate (III) and a disulphide (IV). The effect of these compounds on the rate of oxidation of indene and cumene is shown in Figs. 3 and 4. The pro-oxidant effect of the ferric and nickel salts in indene (Fig. 3) is due to their increasing the rate of initiation; ferric acetylacetonate has the same effect. The inhibitor test is thus invalid for the ferric and nickel salts when using indene. Use of cumene is valid however as metals do not change the azonitrile initiation rate; ferric acetylacetonate has a negligible effect on the rate of oxidation of cumene (Fig. 4).
50
100 TIME
FIG.
I50
(ran)
4. Effect of Additives on the Oxidation of Cumene (5 ml) in the Presence of Azobisisobutyronitrile (0.06 M) at 60”
A Ferric acetylacetonate (O+XtSM) B No additive, zinc dilauryl phosphate (0905 M) or the disulphide (0005 M) C Zinc di-isopropyl dithiophosphate (0905 M) D Nickel di-isopropyl dithiophosphate (OWS M) E Ferric di-isopropyl dithiophosphate (0005 M) F Potassium di-(4-methylpentyl-2) dithiophosphate (O-01 M) G Zinc isopropyl xanthate (0005 M) H Cuprous di-isopropyl dithiophosphate (O*OOSM)
[(Pr’O),PS,J,
Some preliminary attempts have been made to identify the products of the reaction between peroxy radicals and zinc di-isopropyl dithiophosphate. Thermal decomposition of azobisisobutyronitrile (AIBN) in benzene in the presence of oxygen was used as a source of peroxy radicals. This is not an ideal source as free radical production is inefficient and high yields of tetramethylsuccinonitrile are formed. In addition to
The mechanism of the antioxidant action of zinc dialkyl dithiophosphates
2157
this nitrile, Mahoney” has reported eight other unidentified products from the reaction of AIBN with oxygen in chlorobenzene or benzene and Boozer et uZ.~ have reported polymeric solid products. Separation of phosphorus-containing products from such a complex mixture has been difficult, however, yields of up to 10% of the disulphide (IV) have been isolated. Further work is in progress using different peroxy radical sources in the hope that more quantitative product analysis can be made and a mechanism more fully substantiated. It is interesting to note that the ferric salt (I, M = Fe; R = Pr’; x = 3) in benzene solution is oxidized to the disulphide (IV) almost quantitatively by molecular oxygen. Here it is probable that the oxygen diradical, in a ferric ion catalysed reaction, is acting as a type of peroxy radical by the same mechanism as that involved in chainbreaking inhibition of cumene oxidation by the ferric salt. Significantly, the results in Figs. 3 and 4 show that the zinc dialkyl phosphate (III), containing no sulphur, and the disulphide (IV), containing no metal, have no effect on the oxygen-uptake rate. Thus these compounds do not act as chain-breaking inhibitors. RO
S
5
RO’
’
RO
RO \p// / RO
S
‘S-Z”-S
'S&S' ..
“\ OR
Xp’ \
s-s
/
/p\oR
‘OR
S
OR \/
\P//
10R
t
Ro/ ‘S-&S .+
RO \pB
S
JW S
OR
\/ .
Ro/ B
‘OR
S
+
P
+a-s’’OR
VII SCHEME A These results, together with the isolation of the disulphide (IV) as a reaction product,
lend strong support to the proposal of a mechanism involving electron-transfer from an electron-rich sulphur atom to a peroxy radical by which zinc dialkyl dithiophosphates act as chain-breaking inhibitors. Logical expansion of Colclough and Cunneen’s equations for electron-transfer leads to the mechanism illustrated by Scheme A. This is now shown to be inadequate since the intermediate formation of a free dithiophosphate radical (VII) is incompatible with our observation that indene oxidation is a*L. R. Mahoney, J. Amer. Chem. Sot. 87,1089 (l%S). 1’ C. E. Boozer, G. S. Hammond, C. E. Hamilton and C. Peterson, 1. Amer. C/tern. Sot. 77, 3380 (1955).
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inhibited by zinc dialkyl dithiophosphates. Dialkyl dithiophosphoric acids have been found to undergo spontaneous radical addition to olefinP and to conjugated and nonconjugated diolefins. ls The dithiophosphate radical would therefore be expected to add readily to the indene double bond and the following rapid sequence, replacing the propagating peroxy radical by another peroxy radical (VIII), would exclude inhibition.
VIII
‘P’
RO’\s
RO
/\
S
Similar sequences have been proposed to account for the products obtained in the co-oxidation of thiols with a conjugated diene,” of thioacetic acid with indene,lr and of thiophenol with indene.18 It is therefore proposed that the el~~on-~ansfer mechanism involves a stabilized peroxy intermediate which, on attack of a second peroxy radical, leads to intramolecular dimerization of the incipient dithiophosphate radicals before addition to a double bond can occur. For example, in Scheme B the peroxy radical intermediate is stabilized RO
S NP/
; \P/
RO’
‘S.&n-s’
OR
‘OR
1
RO,.
R*
R_ S
RO’ L
1RO,,.
IX RO
~\p/
S-S
xp/
‘S-4-S
OR
‘OR
X
OR 2ROs- -I- ZIP+
RO’\ssH\OR-+ SCHEME B
I4 W. E. Bacon and W. M. LeSuer, J. Amer. Chem. Sot. 76,670 (1954). I6 A. A. Oswald, K. Grieabaum and B. E. Hudson, Division of Petroleum Chemistry Preprints 8 (1) 5, Amer. C/tern. Sue. Meeting Los Angek8,31st March-5th Aprii (1963). I* W. A. Thaier. A. A. Oswald and B. E. Hudson, J. Amer. Chem. Sot. 87,311 (1965). I’ A. A. Oswald, K. Gricabaum and W. Naegele, L Amer. Chem. Sot. 86.3791 (1964). I8 J. F. Ford, R. C. Pitkethly and V. 0. Young, Tetrahedron 4, 325 (1958).
The
mechanismof the antioxidant action of zinc dialkyl dithiophosphates
215s
by delocalization of the odd electron as illustrated by the structures IX and X. Similar reaction of another peroxy radical at the remaining thiono-sulphur atom leads to the formation of disulphide. It is not of course possible at present to rule out ion-p&s of the radical-ions (V and VI) with peroxy anions as stabilized peroxy intermediates which can similarly react with a second peroxy radical instead of decomposing to dithiophosphate radicals as shown in Scheme A. A similar stepwise type of mechanism, involving two peroxy radicals, has previously lB been discussed in connection with autoxidation inhibition by phenols and amines. This does not however imply that zinc dithiophosphate inhibition is kinetically identical with that of phenols and amines. Intramolecular dimerization would not of course be expected to be tenable for a monovalent metal dithiophosphate. It is therefore surprising to find that potassium and cuprous salts do inhibit indene oxidation (Fig. 3). Ebullioscopic mol. wt. measurements using benzene as solvent have however shown that the cuprous salt is tetrameric and that the potassium salts form aggregates of mol. wt. up to 10,000. These structures would also be expected to exist in the aromatic hydrocarbon solutions used for oxidation and the peroxy complex mechanism is therefore still an acceptable postulate; intermolecular dimerization occurring within a solvent cage with the monovalent metal salts. The function of the metal in Scheme B is to provide an easy route for heterolysis of the proposed radical intermediate. The absence of this route in the case of the disulphide (IV) being the reason for its inactivity although it is a dithioate. This mechanism is also consistent with the relatively small differences in chain-breaking activity (Figs. 2, 3 and 4) observed on variation of the alkyl group, the central metal atom (except cuprous see below), and on substitution of carbon for phosphorus. The high activity of the cuprous salt is most likely due to a complementary electron-transfer reaction of the type;20 Cu+ + RO,. + Cu’+ + RO,It is also noteworthy that cuprous dialkyl dithiophosphate, along with disulphide, results from the reaction of cupric ions with potassium dialkyl dithiophosphate in aqueous solution. 2Cu’++ WOM’S,- -+2CuS,P(OR),+ [(ROM’&], This reaction is, of course, analogous to the reaction of cupric ions with mercaptide, iodide or cyanide ions in aqueous solution. EXPERIMENTAL itfuterids. Cumene was boiled under reflux over Na in a N, atmosphere for 2 hr. Distillation under N, gave a fraction b.p. 152” which was 99.8 % pure (by GLC analysis). Squalane (ex Gattefosse, Lyons) was distilled under N, and had b.p. 205-206”/0*2 mm, n$’ 1.4524 (‘lit.” ng 1.4519). GLC analysis showed a single peak and no olelinic peaks were detected in the IR spectrum. Indene (ex Theodor Schuchardt. Munich) was acid and base washed in the sequence, 6NHCl (3 x 25Oml). distilled water (250 ml), 4N NaOH (6 x 250 ml) and water (2 x 250ml), It was then dried (@SO,) and percolated through silica gel (Grace, 50-100 meah). Distillation under N, gave a fraction b.p. 68”/15 mm which had an IR spectrum showing no impurities. These hydrocarbons were all stored I* C. E. Boozer and G. S. Hammond, J. Amer. Gem. Sot. 76, 3861 (1954); C. E. Boozer, G. S. Hammond, C. E. Hamilton and J. N. Sen, Ibid. 77, 3238 (1955). so K. U. Ingold, &em. Reus. 61,563 (1961). ‘I K. J. Sax and F. H. Stress, J. Org. Chem. 22, 1251 (1957).
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A. J. BURN
under N, in a refrigerator. Frequent tests for hydroperoxide wete made using Fe.SO,-KCNS solution and where this was positive the hydrocarbon was percolated through silica gel before use. Azobisisobutyronitrile (ex Whiffen and Sons Limited) was recrystallized from ether and had m.p. 102-104” (lit.” m.p. 103-104”). Azobiscyclohexylnitrile was prepared as described by Overberger et al.*’ and had m.p. 115” (ex ether). Dialkyl dithiophosphoric acids were prepared from P& and an alcohol and purified as described by Ashford et al. ‘* The acids were neutralized by KHCOgq and after evaporation of the water the K-salts were recrystallized to constant m.p. The disulphide [(PrQ),PS,], was prepared by I.-oxidation of potassium di-isopropyl dithiophosphate in aqueous solution and had m.p. 91-92” (ex heptane). Potassium dilauryl phosphate was prepared from dilauryl phosphate and KOH in EtOH. Potassium isopropyl xanthate was preparedly from isopropanol, KOH and CS,. Zinc and other metal salts were obtained from the above K-salts by double decomposition in aqueous solution, the precipitated product being separated by extraction with ether or benzene rather than by 6ltration. The product obtained by this method from CuSO, and potassium di-isopropyl dithiophosphate had m.p. 79-80” but on recrystallization from heptane, crystals of two distinctly different forms were obtained, one pale brown and the other yellow. These. crystals were collected by filtration and a selection of each carefully picked out by spatula. After recrystallization, the yellow crystals (055 g) were found to be identical with the disulphide described above m.p. and m.m.p. 92” with the correct IR spectrum. The brown crystals (3.45 g) were found to be cuprous di-isopropyl dithiophosphate m.p. 120”. (Found, C, 26.0; H, 5.23; P, 11.4; Cu. 22.7. C,H,,O,PS,Cu requites: C, 26.0; H, 5.10; P, 11.19; Cu, 22.96%; mol. wt. 1148, required: 277.) All the compounds tested and inhibitors had satisfactory elemental analyses, IR spectra, and where obtainable, NMR spectra. Oxidation rutes were obtained using two different pieces of apparatus to measure 0, uptake. The hrst, used in the case of the squalane data (Fig. 1) was simply a water-jacketed gas burette containing di-n-butyl phthalate, the level of which was adjusted by manual movement of a reservoir. The second, used in the case ofthe indene andcumenedata(Figs. 2,3 and4) has beenpreviouslydescribed.W In each case the apparatus was connected by a short glass capillary and neoprene tubing to a mechanically agitated reaction flask immersed in a thermostat bath (AO.1’). Where azonitriles were used, appropriate correction for N, evolution from these compounds during oxidation was made. Reaction of zinc di-isopropyl dithiophosphate withperoxy radicals. A solution of zinc di-isopropyl dithiophosphate (2.46 g, OGX moles) and azobisisobutyronitrile (1.64 g, O-01 moles) in benzene (50 ml, Na dried), through which a slow current of 0, was bubbled, was boiled under refhtx for 6&hr. Filtration of the product gave a very pale brown solid (O-75 g) m.p. > 300”. and a light brown filtrate. The solid was insoluble in water and contained 28.5% Zn. The IR spectrum contained absorptions due to three different types of C-N bond, and one due to a P-C-C group not attributable to the disulphide (IV). No carbonyl was detected. Removal of the solvent from the filtrate gave a viscous liquid which was extracted with pet. ether (b.p. 40-W’). A brown gum (0.5 g) remained undissolved and the petrol solution gave a pale yellow liquid (2.5 g) which crystallized on standing. The IR spectrum of the brown gum contained absorptions due to organic nitrile C%N, inorganic cyanide, cyanate or possibly thiocyanate, a P-O-C group (probably the initial Zn salt) and a carbonyl group. Neither this gum nor the solid precipitate have been further identified. The pale yellow crystals obtained by petrol extraction had m.p. 76-79” and crystallization from heptane gave yellow crystals (0.21 g) m.p. 91” identified as disulphide [(PrlO),PS,], (correct IR spectrum). The residue from crystallization was a viscous liquid, the IR spectrum of which indicated the presence of tetramethylsuccinonitrile, a carbonyl compound, and a P-O-C containing compound, probably the initial Zn salt. In a repeat experiment, on a larger scale, zinc di-isopropyl dithiophosphate (9.84 g, 0.02 moles) and azobisisobutyronitrile (8.20 g, O-05 moles) were similarly treated. The liquid product obtained from the filtrate was chromatographed on silica gel. Pet. ether (b.p. 60-80”, 8 x 500 ml) and pet. ether (60-80°) containing 5 y0 benzene (4 x 500 ml) gave fractions shown to contain the initial Zn salt and disulphide by IR spectra. These fractions were recombined (5.3 g) and recrystallized from heptane. The two different crystalline forms of Zn salt and disulphide are easily distinguished and W C. G. Overberger, M. T. O’Shaughnessy and H. Shalit, J. Amer. Chem. Sot. 71, 2661 (1949). u J. S. Ashford, L. Bmtherick and P. Gould, J. Appl. Chem. 15, 170 (1965). u A. I. Vogel, Practical Orgunic Chemistry (3rd Enlarged Edition) p. 499. Longmans, London. U L. Bateman and J. I. Cunneen, J. Chem. Sot. 1596 (1955).
The mechanism of the antioxidant
action of zinc dialkyl dithiophosphates
2161
filtration followed by careful manual selection and recrystallization gave the disulphide (046 g) m.p. 92’ (correct IR spectrum), the Zn salt (2.4 g). and a residue containing both. Further chromatographic fractions led only to the separation of tetramethylsuccinonitrile. Reaction of ferric di-fiopropyl dirhiophosphate with oxygen. A solution of ferric di-isopropyl dithiophosphate. (0.5 g) in benzene (50 ml, Na dried) through which a slow current of OS was bubbled, was boiled under reflux for 5 hr. Filtration of the product gave a brown solid residue (O-14 g) and a clear red filtrate. After decolourization with charcoal, evaporation of solvent from the filtrate followed by recrystallization of the residue from heptane gave the disulphide [(Pr~O),PS,J, as pale yellow crystals (0.25 g) m.p. 89-90”. correct IR spectrum. An identical reaction carried out at room temp took 2 weeks. This time the filtrate was not red and evaporation gave the disulphide (0.44 g) m.p. 89-W recrystallized from heptane (0.32 g) m.p. and m.m.p. 91”. correct IR spectrum. The brown residue from the latter reaction (0117 g) was found (IR spectrum) to contain some dithiophosphate groups. This was confirmed by elemental analysis. (Found, Fe, 53.3; P, 5.4; S, 7*5x.) The effect of a zinc dialkyl dithiophosphate on the decomposition rate of azobiscyciohexylnitrile (ADCN). Squalane (25 ml) containing ADCN (0.1835 g) was kept at 105”. The rate of N, evolution was measured using the simple gas-burette apparatus described above for oxidation measurements. A first order rate-constant of 1.46 x lo-’ se& was obtained. In a similar experiment in the presence of zinc d&(4-methyl-pentyl-2) dithiophosphate (0.2491 g) a rate constant of 1.33 x 10-4sec-’ was obtained. kcknow/e&ement-The author wishes to thank the Chairman and Directors of The British Petroleum Company Limited, for permission to publish this paper.
12
1966. Vol. 22. pa. 2153 to 2161. Pagunoa
Praa IA.
Rhcd
In Nonban
Irrlud
THE MECHANISM OF THE ANTIOXIDANT ACTION OF ZINC DIALKYL DITHIOPHOSPHATES A. J. BURN The British Petroleum Company Limited BP Research Centre. Chertsey Road, Sunbury-on-Thames, Middlesex (Receioed 15 October 1965; in reoisedfrom 7 Februury 1966) Abstract-Zinc dialkyl dithiophosphates have been shown to inhibit the axonitrileinitiated oxidation of cumene and squalane and the non-initiated oxidation of indene by acting as chain-breaking agents. Several related compounds have been tested as chain-breaking inhibitors and a mechanism is proposed to account for this behaviour. IT IS now generally accepted’ that the liquid phase a&oxidation of a hydrocarbon (RH) involves a free-radical chain mechanism in which the predominant chain-carrying species in solution is the peroxy radical (R02-)e-s Thermal or metal ion promoted decomposition of the initial hydroperoxide product (RO,H) results in autocatalysis. As a consequence of this mechanism two main types of antioxidant have been recognized;**& chain-breaking agents, which suppress propagation by removal of peroxy radicals in a reaction involving electron or hydrogen transfer, and peroxide-destroyers which remove hydroperoxides by causing their non-radical decomposition. Although zinc dialkyl dithiophosphates, [(RO),PS,], Zn, have been widely used as antioxidants for many years, no detailed mechanism of their action has been put forward. However, it is clear that they are efficient peroxide destroyers. This type of zinc salt has been shown to accelerate the decomposition of both t-buty16 and cumene6 hydroperoxide; in the latter case phenol, a product known’ to result from ionic decomposition, being formed. Related dithiophosphates, at a 1 molar % concentration, have also been showna to cause the decomposition of cumene hydroperoxide giving phenol and acetone. Larson9 has observed, by measurement of induction periods, that zinc dialkyl dithiophosphates are effective antioxidants when tested in a refined white mineral oil at 155”. In mosta*8*9of this work, antioxidant activity has been attributed to peroxide destruction by intermediates formed from the zinc dialkyl dithiophosphates. In contrast to the above approach the possibility that zinc dialkyl dithiophosphates
’ J. L. Bolland, Quart. Revs. 3, 1 (1949); L. Bateman, Ibid. 8, 147 (1954); G. A. Russell,J. Chem. Educ. 36, 111 (1959). ’ R. B. Mesrobian and A. V. Tobolsky, Autoxidation and Antioxidants (Edited by W. 0. Lundberg) Vol I; p. 1 IO. Interscience (1961). * G. Scott, Chem. & Ind. 271 (1963). ’ H. C. Bailey, The Indust. Chemist 38,215 (1962). L T. Colclough and J. I. Cunneen, J. Cfiem. Sot. 4790 (1964). ‘ G. W. Kennerly and W. L. Patterson, fnd Eirg. C/rem. 48, 1917 (1956). ’ E. G. E. Hawkins, Organic Peroxides p. 90. Sport, London (1961). (1J. D. Holdsworth, G. Scott and D. Williams, J. Chem. Sot. 4692 (1964). ’ R. Larson, Scientl$c Lubrication 10, 12 (August 1958). 2153
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A. J. BURN
might act as chain-breaking agents, by reaction with peroxy radicals, has been investigated since this point seemed to have been overlooked at the outset of our work. A test method4 was used involving measurement of the rate of oxidation of a suitable hydrocarbon containing a non-peroxidic initiator such as an azonitrile. By carrying out the reaction at a temperature low enough to avoid initiation by the hydroperoxide product, the azonitrile is the sole source of free radicals. Any antioxidant which lowers the rate of oxidation when added to such a system must act by removal of chain-propagating peroxy radicals. The remote possibility that addition of a zinc dialkyl dithiophosphate might also reduce the azonitrile initiation rate was however checked. The rate of decomposition of azobiscyclohexylnitrile for example, as determined by the rate of nitrogen evolution, was not found to be significantly affected by a zinc salt in squalane, in the absence of oxygen. Indene oxidation, without azonitrile initiation, was also chosen as a suitable system for this test for the following reasons. The oxidation of indene is readily initiated at 50” by direct reaction with oxygen.10 Propagation mainly involves addition of peroxy radicals to the indene double-bond rather than hydroperoxide formation, the product being a stable indene-oxygen copolymer” which does not initiate at 50”. Possible azonitrile-additive interactions are completely avoided in this system. The response of zinc dialkyl dithiophosphates to such a test is illustrated, for three different hydrocarbons, in Figs. 1, 2 and 3. These results clearly show that the zinc salts act as chain-breaking inhibitors. That inhibition occurs from the beginning of oxidation, under mild temperature conditions, indicates that the zinc salt itself, not a decomposition product, removes peroxy radicals. During the course of our work, Colclough and Cunneen5 have independently observed that three types of dithioate, including zinc diisopropyl dithiophosphate, inhibit the oxidation of squalene by acting as chain-breaking inhibitors as well as peroxide-destroyers. They suggested that electron-transfer from a sulphur atom to a
Fm. 1. Effect of Zinc Dialkyl Dithiophosphates (O*OlSSM) on the Oxidation of Squalane (25 ml) in the Presence of Az.obiscyclohexylnitrile (0.03 M) at 105” A No antioxidant B Zinc di-n-hexyl dithiophosphate C Zinc di-(4-methylpentyl-2) dithiophosphate lo G. A. Russell, J. Amer. Chem. SIC. 78,104l (1956). I1 G. A. Russell, J. Amer. Chem. SC. 78, 1035 (1956).
The mechanism of the antioxidant
action of zinc diahcyl dithiophosphates
20
40 TIME
60
60
(mLn)
FIG. 2. Effect of Zinc Dialkyl Ditbiophosphates (0.005 M) on the Oxidation of Cumene (5 ml) in the Presence of Arobisisobutyronitrile (O-06 M) at 60” A B C D E F G H
No antioxidant Zinc dicyclohexyl dithiophosphate Zinc di-isopropyl dithiophosphate Zinc di-+methylpenty1-2) dithiophosphate Zinc di-wbutyl dithiophosphate Zinc di-n-hexyl dithiophosphate Zinc di-(2,24imethylpentyl-1) dithiophosphate Zince di-n-butyl dithiophosphate
20 TIME
40
60
(min)
FIO. 3. Effect of Additives on the Oxidation of Indene (5 ml) at 50” A B C D E F G H I J
Ferric acetylacetonate (04OC036 M) Ferric di-isopropyl dithiophosphate (04OCKI33M) Nickel di-isopropyl dithiophosphate (OWJ5 M) No additive, or zinc dilauryl phosphate (O*OOOS M) or the disulphide [(pfO),P!U, (00005 M) Potassium di-isopropyl dithiophosphate (0401 M) Zinc di-sec-butyl dithiophosphate (04kW5 M) Zinc di-isopropyl dithiophosphate (04005 M) Zinc di-n-butyl dithiophosphate (04005 M) Potassium di+&nethylpentyl-2) dithiophosphate (0401 M) Cuprous di-isopropyl dithiophosphate (04005 M)
2155
2156
A. J. BURN
peroxy radical would account for the chain-breaking activity but offered no discussion of the details of this mechanism. In the present work evidence concerning the mechanism by which zinc dialkyl dithiophosphates react with peroxy radicals has been obtained by studying the effect of variation of molecular structure on radical capture efficiency. Compounds examined include numerous metal dialkyl dithiophosphates (I), a xanthate (II), a zinc [Pr’DCs,], Zn
l(RW’S,I. M
II
I W1&I,?G,I,
tWW%I,
Zn
III
IV
dialkyl phosphate (III) and a disulphide (IV). The effect of these compounds on the rate of oxidation of indene and cumene is shown in Figs. 3 and 4. The pro-oxidant effect of the ferric and nickel salts in indene (Fig. 3) is due to their increasing the rate of initiation; ferric acetylacetonate has the same effect. The inhibitor test is thus invalid for the ferric and nickel salts when using indene. Use of cumene is valid however as metals do not change the azonitrile initiation rate; ferric acetylacetonate has a negligible effect on the rate of oxidation of cumene (Fig. 4).
50
100 TIME
FIG.
I50
(ran)
4. Effect of Additives on the Oxidation of Cumene (5 ml) in the Presence of Azobisisobutyronitrile (0.06 M) at 60”
A Ferric acetylacetonate (O+XtSM) B No additive, zinc dilauryl phosphate (0905 M) or the disulphide (0005 M) C Zinc di-isopropyl dithiophosphate (0905 M) D Nickel di-isopropyl dithiophosphate (OWS M) E Ferric di-isopropyl dithiophosphate (0005 M) F Potassium di-(4-methylpentyl-2) dithiophosphate (O-01 M) G Zinc isopropyl xanthate (0005 M) H Cuprous di-isopropyl dithiophosphate (O*OOSM)
[(Pr’O),PS,J,
Some preliminary attempts have been made to identify the products of the reaction between peroxy radicals and zinc di-isopropyl dithiophosphate. Thermal decomposition of azobisisobutyronitrile (AIBN) in benzene in the presence of oxygen was used as a source of peroxy radicals. This is not an ideal source as free radical production is inefficient and high yields of tetramethylsuccinonitrile are formed. In addition to
The mechanism of the antioxidant action of zinc dialkyl dithiophosphates
2157
this nitrile, Mahoney” has reported eight other unidentified products from the reaction of AIBN with oxygen in chlorobenzene or benzene and Boozer et uZ.~ have reported polymeric solid products. Separation of phosphorus-containing products from such a complex mixture has been difficult, however, yields of up to 10% of the disulphide (IV) have been isolated. Further work is in progress using different peroxy radical sources in the hope that more quantitative product analysis can be made and a mechanism more fully substantiated. It is interesting to note that the ferric salt (I, M = Fe; R = Pr’; x = 3) in benzene solution is oxidized to the disulphide (IV) almost quantitatively by molecular oxygen. Here it is probable that the oxygen diradical, in a ferric ion catalysed reaction, is acting as a type of peroxy radical by the same mechanism as that involved in chainbreaking inhibition of cumene oxidation by the ferric salt. Significantly, the results in Figs. 3 and 4 show that the zinc dialkyl phosphate (III), containing no sulphur, and the disulphide (IV), containing no metal, have no effect on the oxygen-uptake rate. Thus these compounds do not act as chain-breaking inhibitors. RO
S
5
RO’
’
RO
RO \p// / RO
S
‘S-Z”-S
'S&S' ..
“\ OR
Xp’ \
s-s
/
/p\oR
‘OR
S
OR \/
\P//
10R
t
Ro/ ‘S-&S .+
RO \pB
S
JW S
OR
\/ .
Ro/ B
‘OR
S
+
P
+a-s’’OR
VII SCHEME A These results, together with the isolation of the disulphide (IV) as a reaction product,
lend strong support to the proposal of a mechanism involving electron-transfer from an electron-rich sulphur atom to a peroxy radical by which zinc dialkyl dithiophosphates act as chain-breaking inhibitors. Logical expansion of Colclough and Cunneen’s equations for electron-transfer leads to the mechanism illustrated by Scheme A. This is now shown to be inadequate since the intermediate formation of a free dithiophosphate radical (VII) is incompatible with our observation that indene oxidation is a*L. R. Mahoney, J. Amer. Chem. Sot. 87,1089 (l%S). 1’ C. E. Boozer, G. S. Hammond, C. E. Hamilton and C. Peterson, 1. Amer. C/tern. Sot. 77, 3380 (1955).
2158
A. J. BURN
inhibited by zinc dialkyl dithiophosphates. Dialkyl dithiophosphoric acids have been found to undergo spontaneous radical addition to olefinP and to conjugated and nonconjugated diolefins. ls The dithiophosphate radical would therefore be expected to add readily to the indene double bond and the following rapid sequence, replacing the propagating peroxy radical by another peroxy radical (VIII), would exclude inhibition.
VIII
‘P’
RO’\s
RO
/\
S
Similar sequences have been proposed to account for the products obtained in the co-oxidation of thiols with a conjugated diene,” of thioacetic acid with indene,lr and of thiophenol with indene.18 It is therefore proposed that the el~~on-~ansfer mechanism involves a stabilized peroxy intermediate which, on attack of a second peroxy radical, leads to intramolecular dimerization of the incipient dithiophosphate radicals before addition to a double bond can occur. For example, in Scheme B the peroxy radical intermediate is stabilized RO
S NP/
; \P/
RO’
‘S.&n-s’
OR
‘OR
1
RO,.
R*
R_ S
RO’ L
1RO,,.
IX RO
~\p/
S-S
xp/
‘S-4-S
OR
‘OR
X
OR 2ROs- -I- ZIP+
RO’\ssH\OR-+ SCHEME B
I4 W. E. Bacon and W. M. LeSuer, J. Amer. Chem. Sot. 76,670 (1954). I6 A. A. Oswald, K. Grieabaum and B. E. Hudson, Division of Petroleum Chemistry Preprints 8 (1) 5, Amer. C/tern. Sue. Meeting Los Angek8,31st March-5th Aprii (1963). I* W. A. Thaier. A. A. Oswald and B. E. Hudson, J. Amer. Chem. Sot. 87,311 (1965). I’ A. A. Oswald, K. Gricabaum and W. Naegele, L Amer. Chem. Sot. 86.3791 (1964). I8 J. F. Ford, R. C. Pitkethly and V. 0. Young, Tetrahedron 4, 325 (1958).
The
mechanismof the antioxidant action of zinc dialkyl dithiophosphates
215s
by delocalization of the odd electron as illustrated by the structures IX and X. Similar reaction of another peroxy radical at the remaining thiono-sulphur atom leads to the formation of disulphide. It is not of course possible at present to rule out ion-p&s of the radical-ions (V and VI) with peroxy anions as stabilized peroxy intermediates which can similarly react with a second peroxy radical instead of decomposing to dithiophosphate radicals as shown in Scheme A. A similar stepwise type of mechanism, involving two peroxy radicals, has previously lB been discussed in connection with autoxidation inhibition by phenols and amines. This does not however imply that zinc dithiophosphate inhibition is kinetically identical with that of phenols and amines. Intramolecular dimerization would not of course be expected to be tenable for a monovalent metal dithiophosphate. It is therefore surprising to find that potassium and cuprous salts do inhibit indene oxidation (Fig. 3). Ebullioscopic mol. wt. measurements using benzene as solvent have however shown that the cuprous salt is tetrameric and that the potassium salts form aggregates of mol. wt. up to 10,000. These structures would also be expected to exist in the aromatic hydrocarbon solutions used for oxidation and the peroxy complex mechanism is therefore still an acceptable postulate; intermolecular dimerization occurring within a solvent cage with the monovalent metal salts. The function of the metal in Scheme B is to provide an easy route for heterolysis of the proposed radical intermediate. The absence of this route in the case of the disulphide (IV) being the reason for its inactivity although it is a dithioate. This mechanism is also consistent with the relatively small differences in chain-breaking activity (Figs. 2, 3 and 4) observed on variation of the alkyl group, the central metal atom (except cuprous see below), and on substitution of carbon for phosphorus. The high activity of the cuprous salt is most likely due to a complementary electron-transfer reaction of the type;20 Cu+ + RO,. + Cu’+ + RO,It is also noteworthy that cuprous dialkyl dithiophosphate, along with disulphide, results from the reaction of cupric ions with potassium dialkyl dithiophosphate in aqueous solution. 2Cu’++ WOM’S,- -+2CuS,P(OR),+ [(ROM’&], This reaction is, of course, analogous to the reaction of cupric ions with mercaptide, iodide or cyanide ions in aqueous solution. EXPERIMENTAL itfuterids. Cumene was boiled under reflux over Na in a N, atmosphere for 2 hr. Distillation under N, gave a fraction b.p. 152” which was 99.8 % pure (by GLC analysis). Squalane (ex Gattefosse, Lyons) was distilled under N, and had b.p. 205-206”/0*2 mm, n$’ 1.4524 (‘lit.” ng 1.4519). GLC analysis showed a single peak and no olelinic peaks were detected in the IR spectrum. Indene (ex Theodor Schuchardt. Munich) was acid and base washed in the sequence, 6NHCl (3 x 25Oml). distilled water (250 ml), 4N NaOH (6 x 250 ml) and water (2 x 250ml), It was then dried (@SO,) and percolated through silica gel (Grace, 50-100 meah). Distillation under N, gave a fraction b.p. 68”/15 mm which had an IR spectrum showing no impurities. These hydrocarbons were all stored I* C. E. Boozer and G. S. Hammond, J. Amer. Gem. Sot. 76, 3861 (1954); C. E. Boozer, G. S. Hammond, C. E. Hamilton and J. N. Sen, Ibid. 77, 3238 (1955). so K. U. Ingold, &em. Reus. 61,563 (1961). ‘I K. J. Sax and F. H. Stress, J. Org. Chem. 22, 1251 (1957).
2160
A. J. BURN
under N, in a refrigerator. Frequent tests for hydroperoxide wete made using Fe.SO,-KCNS solution and where this was positive the hydrocarbon was percolated through silica gel before use. Azobisisobutyronitrile (ex Whiffen and Sons Limited) was recrystallized from ether and had m.p. 102-104” (lit.” m.p. 103-104”). Azobiscyclohexylnitrile was prepared as described by Overberger et al.*’ and had m.p. 115” (ex ether). Dialkyl dithiophosphoric acids were prepared from P& and an alcohol and purified as described by Ashford et al. ‘* The acids were neutralized by KHCOgq and after evaporation of the water the K-salts were recrystallized to constant m.p. The disulphide [(PrQ),PS,], was prepared by I.-oxidation of potassium di-isopropyl dithiophosphate in aqueous solution and had m.p. 91-92” (ex heptane). Potassium dilauryl phosphate was prepared from dilauryl phosphate and KOH in EtOH. Potassium isopropyl xanthate was preparedly from isopropanol, KOH and CS,. Zinc and other metal salts were obtained from the above K-salts by double decomposition in aqueous solution, the precipitated product being separated by extraction with ether or benzene rather than by 6ltration. The product obtained by this method from CuSO, and potassium di-isopropyl dithiophosphate had m.p. 79-80” but on recrystallization from heptane, crystals of two distinctly different forms were obtained, one pale brown and the other yellow. These. crystals were collected by filtration and a selection of each carefully picked out by spatula. After recrystallization, the yellow crystals (055 g) were found to be identical with the disulphide described above m.p. and m.m.p. 92” with the correct IR spectrum. The brown crystals (3.45 g) were found to be cuprous di-isopropyl dithiophosphate m.p. 120”. (Found, C, 26.0; H, 5.23; P, 11.4; Cu. 22.7. C,H,,O,PS,Cu requites: C, 26.0; H, 5.10; P, 11.19; Cu, 22.96%; mol. wt. 1148, required: 277.) All the compounds tested and inhibitors had satisfactory elemental analyses, IR spectra, and where obtainable, NMR spectra. Oxidation rutes were obtained using two different pieces of apparatus to measure 0, uptake. The hrst, used in the case of the squalane data (Fig. 1) was simply a water-jacketed gas burette containing di-n-butyl phthalate, the level of which was adjusted by manual movement of a reservoir. The second, used in the case ofthe indene andcumenedata(Figs. 2,3 and4) has beenpreviouslydescribed.W In each case the apparatus was connected by a short glass capillary and neoprene tubing to a mechanically agitated reaction flask immersed in a thermostat bath (AO.1’). Where azonitriles were used, appropriate correction for N, evolution from these compounds during oxidation was made. Reaction of zinc di-isopropyl dithiophosphate withperoxy radicals. A solution of zinc di-isopropyl dithiophosphate (2.46 g, OGX moles) and azobisisobutyronitrile (1.64 g, O-01 moles) in benzene (50 ml, Na dried), through which a slow current of 0, was bubbled, was boiled under refhtx for 6&hr. Filtration of the product gave a very pale brown solid (O-75 g) m.p. > 300”. and a light brown filtrate. The solid was insoluble in water and contained 28.5% Zn. The IR spectrum contained absorptions due to three different types of C-N bond, and one due to a P-C-C group not attributable to the disulphide (IV). No carbonyl was detected. Removal of the solvent from the filtrate gave a viscous liquid which was extracted with pet. ether (b.p. 40-W’). A brown gum (0.5 g) remained undissolved and the petrol solution gave a pale yellow liquid (2.5 g) which crystallized on standing. The IR spectrum of the brown gum contained absorptions due to organic nitrile C%N, inorganic cyanide, cyanate or possibly thiocyanate, a P-O-C group (probably the initial Zn salt) and a carbonyl group. Neither this gum nor the solid precipitate have been further identified. The pale yellow crystals obtained by petrol extraction had m.p. 76-79” and crystallization from heptane gave yellow crystals (0.21 g) m.p. 91” identified as disulphide [(PrlO),PS,], (correct IR spectrum). The residue from crystallization was a viscous liquid, the IR spectrum of which indicated the presence of tetramethylsuccinonitrile, a carbonyl compound, and a P-O-C containing compound, probably the initial Zn salt. In a repeat experiment, on a larger scale, zinc di-isopropyl dithiophosphate (9.84 g, 0.02 moles) and azobisisobutyronitrile (8.20 g, O-05 moles) were similarly treated. The liquid product obtained from the filtrate was chromatographed on silica gel. Pet. ether (b.p. 60-80”, 8 x 500 ml) and pet. ether (60-80°) containing 5 y0 benzene (4 x 500 ml) gave fractions shown to contain the initial Zn salt and disulphide by IR spectra. These fractions were recombined (5.3 g) and recrystallized from heptane. The two different crystalline forms of Zn salt and disulphide are easily distinguished and W C. G. Overberger, M. T. O’Shaughnessy and H. Shalit, J. Amer. Chem. Sot. 71, 2661 (1949). u J. S. Ashford, L. Bmtherick and P. Gould, J. Appl. Chem. 15, 170 (1965). u A. I. Vogel, Practical Orgunic Chemistry (3rd Enlarged Edition) p. 499. Longmans, London. U L. Bateman and J. I. Cunneen, J. Chem. Sot. 1596 (1955).
The mechanism of the antioxidant
action of zinc dialkyl dithiophosphates
2161
filtration followed by careful manual selection and recrystallization gave the disulphide (046 g) m.p. 92’ (correct IR spectrum), the Zn salt (2.4 g). and a residue containing both. Further chromatographic fractions led only to the separation of tetramethylsuccinonitrile. Reaction of ferric di-fiopropyl dirhiophosphate with oxygen. A solution of ferric di-isopropyl dithiophosphate. (0.5 g) in benzene (50 ml, Na dried) through which a slow current of OS was bubbled, was boiled under reflux for 5 hr. Filtration of the product gave a brown solid residue (O-14 g) and a clear red filtrate. After decolourization with charcoal, evaporation of solvent from the filtrate followed by recrystallization of the residue from heptane gave the disulphide [(Pr~O),PS,J, as pale yellow crystals (0.25 g) m.p. 89-90”. correct IR spectrum. An identical reaction carried out at room temp took 2 weeks. This time the filtrate was not red and evaporation gave the disulphide (0.44 g) m.p. 89-W recrystallized from heptane (0.32 g) m.p. and m.m.p. 91”. correct IR spectrum. The brown residue from the latter reaction (0117 g) was found (IR spectrum) to contain some dithiophosphate groups. This was confirmed by elemental analysis. (Found, Fe, 53.3; P, 5.4; S, 7*5x.) The effect of a zinc dialkyl dithiophosphate on the decomposition rate of azobiscyciohexylnitrile (ADCN). Squalane (25 ml) containing ADCN (0.1835 g) was kept at 105”. The rate of N, evolution was measured using the simple gas-burette apparatus described above for oxidation measurements. A first order rate-constant of 1.46 x lo-’ se& was obtained. In a similar experiment in the presence of zinc d&(4-methyl-pentyl-2) dithiophosphate (0.2491 g) a rate constant of 1.33 x 10-4sec-’ was obtained. kcknow/e&ement-The author wishes to thank the Chairman and Directors of The British Petroleum Company Limited, for permission to publish this paper.
12