The Role of Certain Organic Sulfur Compounds as Preventive

19 The Role of Certain Organic Sulfur

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Compounds as Preventive Antioxidants III. Reactions of tert-Butyl tert-Butanethiolsulfinate and Hydroperoxide D O N A L D M . K U L I C H and J. R E I D S H E L T O N Department of Chemistry, Case Western Reserve University, Cleveland, O H 44106

Thiolsulfinates and their reaction products play an important role in the preventive antioxidant activity observed with organic sulfides and disulfides. An investigation of the decomposition of cumene hydroperoxide in benzene at 25°C in the presence of tert-butyl tert-butanethiolsulfinate h shown that the actual peroxide decomposer is an acidic species whose activity is affected by the basic character of the S-O group in the parent thiolsulfinate and in the sulfoxides. Alternative mechanisms for generating the acidic species are discussed. Although hydroperoxide decomposition occurs primarily by a polar process, the results also indicate the involvement of radical generating processes. ^ d d i t i v e s that destroy intermediate hydroperoxides provide an effective means of stabilization of polymers against oxidative degradation. A wide variety of sulfur compounds, including sulfides and disulfides, can be effective stabilizers. Pro-oxidant as well as antioxidant behavior is observed, indicating the involvement of reactions generating radicals and peroxide decomposition. Oxidation studies show that preventive antioxidant behavior by disulfides is exhibited only after the absorption of oxygen, resulting in the formation of the corresponding thiolsulfinate. Thiolsulfinates are also thermolysis products of sulfoxides. Recent work has shown that thiolsulfinate itself does not catalytically decompose hydroperoxide ( I ) . Thus, the oxidation of disulfides to thiolsulfinates is only the first i n a series of reactions leading to the formation of the active preventive antioxidant. A study of cumene hydroperoxide decomposition © 1978 American Chemical Society 0-8412-0381-4/78/33-169-226$05.00/l Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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Preventive Antioxidants

K U L I C H A N D SHELTON

227

has indicated that this induced decomposition of the hydroperoxide is primarily a polar process ( 1 , 2 ) . This chapter presents the intial results of a systematic investigation of the hydroperoxide decomposing activity exhibited by tert-butyl terf-butanethiolsulfinate. Various alternative mechanisms for generating active species from the thiolsulfinate i n the presence of cumene hydroperoxide are examined. Another aspect of the reaction mechanism considered is the ability of the highly polar S - O group i n the thiolsulfinate and sulfoxide to complex and deactivate the acidic peroxide decomposing agent. This proposal was tested by examin ing the effects of various sulfoxides on the decomposition of cumene hydroperoxide by strong acids. These results are compared with a deter mination of the relative hydrogen bonding ability of thiolsulfinate and sulfoxides. Experimental Unless otherwise indicated, hydroperoxide decomposition studies were carried out i n spectral grade benzene (Mallinckrodt). Preparation and purification of the sulfoxides, tert-butyl feri-butanethiolsulfinate, and cumene hydroperoxide were discussed previously ( 1 , 2 ) . Reagent grade phenol was purified by vacuum sublimation. The carbon tetrachloride was spectroscopic grade dried over Ρ Ο . Decomposition studies were carried out under nitrogen i n screw-top vials stored i n a constant tem perature bath at 25 ± 0.05°C (J, 2 ) . Cumene hydroperoxide concentra tions were iodometrically determined by the Hercules Method I reviewed by M a i r and Graupner (3). Blanks were 0.001-0.003 meq of iodine. 2

δ

Possible Mechanisms The polar nature of the catalytic decomposition of cumene hydro peroxide with added tert-butyl terf-butanethiolsulfinate has been clearly established ( J ) . The thiolsulfinate is converted into an active peroxide decomposer capable of destroying many moles of hydroperoxide per mole of sulfur compound. The acidic character of the active species was demonstrated by its effective neutralization w i t h the added base calcium carbonate. Formation of the active peroxide decomposer may be en visaged as involving one or more of the following three reaction types: concerted process, ionic processes, and free-radical processes. Concerted Process. Thiolsulfinates are thermally labile, and those with appropriate alkyl groups are known to undergo an intramolecular cycloelimination reaction on heating (4): H--^0 H

^ H

y

C

CH3

lL \

JR

->

(CH ) C=CH 3

2

2

+ RSSOH

S

CH3

Allara and Hawkins; Stabilization and Degradation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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STABILIZATION A N D DEGRADATION OF P O L Y M E R S

Formation of the thiosulfoxylic acid intermediate has been demonstrated b y mass spectroscopic studies and trapping experiments. The thiosulfoxylic acid formed may then react with hydroperoxide through a mechnism that destroys many moles of hydroperoxide per mole of sulfur compound ( J ) . Alternatively, the formation of thiosulfoxylic acid may be only the first in a series of consecutive reactions leading to the effective peroxide decomposing species. Polar Processes. Kice (5) has presented extensive evidence that cleavage of the S - S bond can be catalyzed by electrophilic and nucleophilic assistance. Heterolytic cleavage would generate sulfenic acid as the active peroxide decomposing species. Traces of water and sulfurcontaining compounds may be considered as potential nucleophiles. W A T E R AS A N U C L E O P H I L E . Trace sulfenic acid formation may occur by thiolsulfinate reacting with water. Ο RSSR + H 0 ^ 2RSOH

(2)

2

It has been estimated that the reverse of the above equihbrium is favored by 10 for R — aryl, and to an even greater extent for R = alkyl (4). However, i n the presence of hydroperoxide sulfenic acid would be readily oxidized to sulfonic acid, a catalytic peroxide decomposer ( I ) . S U L F U R NUCLEOPHTLES. Thiolsulfinates are known to readily dispro portionate with the generation of intermediate sulfenic and sulfinic acids as i n reactions 3, 4, and 5 ( 4 ) . 6

Ο RSSR Η 0 0 RSSR + [RSSR]

H

+

+

Η Ο > [RSSR]

(3)

+

0 [RSSSR] R

RSOH +

0 [RSSSR]* + H 0 -> R S 0 H + R S S R + H R 2

RS0 H 2

2

ROOH

(4)

+

+

* RS0 H 3

(5)

(6)

Initially the acid catalyst for Reaction 3 w o u l d be supplied by decompo sition of the thiosulfinate to thiosulfoxylic acid as an induction period.


19.


229


Reactions 3, 4, 5, and 6 would then ensue with sulfonic acid being the catalytic peroxide decomposer, and the major source of H for the induced decomposition of the thiolsulfinate. The disproportionation of thiolsulfinates is accelerated markedly by the addition of substances such as alkyl sulfides that contain a more nucleophilic sulfur atom than found i n thiolsulfinates. Homolytic processes: Evidence also has been presented for the radi cal-induced decomposition of thiolsulfinates ( 6 , 7 ) . Homolytic cleavage is facilitated b y the weak S-S bond ( ^ 4 0 k c a l ) . The availability of sulfidic sulfur for radical attack is indicated by the observation that thiolsulfinates strongly retard the free radical polymerization of vinyl monomers (8). +

Results and

Discussion

Information regarding the importance of the above processes i n the formation of the active antioxidant from thiolsulfinate i n the presence of hydroperoxide can be provided by examining the effects of added water, organosulfur nucleophiles, radical trapping agents, and determining the nature of the sulfur-containing products. The following results were obtained using cumene hydroperoxide and tert-butyl terf-butanethiolsulfinate.

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Figure 1. Effect of water on the decomposition of cumene hydroper oxide in the presence of tert-butyl tert-butanethiolsulfinate at 25°C: • 0.20M CHP and 0.020M tert-BuSS(0)tert-Bu in spec, benzene (0.05% H 0); Ο 0.20U CHP and 0.021M tert-BuSS(0)tert-Bu in spec, benzene dried over CaH and distilled; • 0.20M CHP and 0.020M tert-BuSS(O)tert-Bu in spec, benzene and ~ 1 % water added 2

t


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STABILIZATION

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A N D DEGRADATION O F P O L Y M E R S

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Figure 2. Effect of xi-butyl sulfide on the decomposition of cumene hydro peroxide in the presence of text-butyl text-butanethiolsvlfinate at 25° C: Μ 0.20M CHP; Ο 0.19M CHP and 0.020U text-BuSS(0)text-Bu; Δ 0.20M CHP, 0.020M text-BuSS(0)text-Bu, and 0.018M (n-Bu) S; A 0.21M CHP and 0.020M (n-Bu) S s

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Figure 3. Effect of vanous sulfoxides on the decomposition of cumene hydroperoxide in the presence of text-butyl text-butanethiolsulfinate at 25°C (from Ref. 2): O 0.21U CHP and 0.02U text-BuSS(0)text-Bu; • 0.20M CHP, 0.02M C H S(0)C H , and 0.02M text-BuSS(0)text-Bu; • 0.20M CHP, 0.02M CH S(0)CH , and 0.02M text-BuSS(0)text-Bu; Φ 0.20M CHP, 0.02M text-BuS(Q)text-Bu, and 0.02M text-BuSS(Q)text-Bu 6

5

6

5

S

s


19.


231


The effect of water on the rate of cumene hydroperoxide decompo sition i n the presence of thiolsulfinate is shown i n Figure 1. The displace ment of thiolsulfinate by water is not the primary means of generating the active peroxide decomposer. T o the contrary, water inhibits peroxide decomposition. Water may be expected to hydrate the thiolsulfinate (9) and the acidic decomposing species thereby decreasing the observed hydroperoxide decomposition. Since thiolsulfinates are very weakly nucleophilic, cleavage of the S-S bond should be subject to nucleophilic assistance i f Reaction 4 operates. Figure 2 shows the effect of added η-butyl sulfide on the thiol sulfinate hydroperoxide reaction. F r o m Figure 2, the addition of sulfide —whether thiolsulfinate is present or absent—results i n the consumption of one mole of hydroperoxide per mole of sulfide presumably forming the sulfoxide and thereby preventing further hydroperoxide decomposition. The inhibitory effect of sulfoxides on the thiolsulfinate hydroperoxide reaction has been noted previously (see Figure 3) ( 2 ) . Table I.

Spectral Shifts

Proton Acceptor

0

tert-BuS (0) tert-Bu

1.5 6.2 1.9 2.1 8.7 9.4

CH S(0)CH tert-BuSS(0) tert-Bu 3

3

C H S(0)C H e

5

e

5

Solutions are in CC1 , 4.3 X 10 * Shift relative to ρ "free" at 3613 Value of 360 cm" reported by Chim. Pays-Bas (1970) 89, 1202. Value of 294 cm" reported by β

4

9

1

d

1

Δ / (cm' )

Molarity

1

Χ 10" Χ ΙΟ" X10" Χ 10" Χ 10" Χ 10"

2 3

2

2

3 3

413 413 368° 292 289 285*

Af in phenol. cm" at 20°C. Engberts, J. B. R. N., Zuidema, G., Rec. Trav.

_3

1

Gramstad, T., Spectrochim. Acta (1963) 89, 829.

The relative ability of tert-butyl terf-butanethiolsulfinate, methyl sulfoxide, tert-butyl sulfoxide, and phenyl sulfoxide to participate as pro ton acceptors was studied by ir spectroscopy. Phenol i n CCI4 was used as the proton donor. A dilute solution of the phenol i n C C 1 displayed a single absorption band because of the O - H stretch (see Table I ) . T h e addition of the proton acceptor gave rise to a new, broad, and intense band at a lower frequency, but the free peak position and appearance changed little except for a decrease in intensity. Variation of the con centrations resulted in no significant changes i n Δγ. Therefore, the Δγ values should be considered reasonable approximations of 1:1 complexes at infinite dilution. The spectral shifts are indicative of ground state electron availability and basicity. Table I shows that terf-butyl tertbutanethiolsulfinate is comparable to phenyl sulfoxide and is not as effi cient as alkyl sulfoxides in its ability to bind to hydrogen. 4


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STABILIZATION A N D DEGRADATION O F P O L Y M E R S

I n Figure 3 phenyl sulfoxide has a small but distinct retarding effect on the decomposition of hydroperoxides by thiolsulfinate solutions. Since the thiolsulfinate is comparable i n basicity to the phenyl sulfoxide, the thiolsulfinate should exert a similar effect, and no decomposition should occur until sufficient acid is generated. The addition of the more basic methyl sulfoxide or f erf-butyl sulfoxide prevents hydroperoxide decom position b y effectively complexing the active acidic species. The inability of complexed acid to decompose hydroperoxide was verified b y adding sulfoxide to a solution of cumene hydroperoxide de composing under acid catalysis. I n Figure 4 the addition of tert-butyl sulfoxide to a solution of cumene hydroperoxide at 122 hr halted the ability of either sulfuric or tert-butyl sulfonic acid to further decompose hydroperoxide. A t elevated temperatures tert-butyl sulfoxide functions as a peroxide decomposer. W e have found that the ability of the sulfoxide to complex the acidic species formed is significant under these conditions also. A t elevated temperatures the decomposition of tert-butyl sulfoxide to acidic species is rapid. Thus, the concentration of the acid species soon exceeds the capacity of the remaining sulfoxide to effectively complex it. There fore, the basic influence of the sulfoxide is indicated by a pronounced induction period. In contrast to η-butyl sulfide, tert-butyl sulfide is not oxidized b y cumene hydroperoxide at 25°C. The addition of tert-butyl disulfide d i d

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Figure 4. Effect of sulfoxide on the decomposition of cumene hydro peroxide in the presence of acids at 25°C: Ο 0.20M CHP and 0.0005M H SO with 0.042M tert-BuS(0)tert-Bu added at 122 hr; · 0.20M CHP and 0.001M tert-BuSO H with 0.041M tert-BuS(0)tert-Bu added at 122 hr 2

h

s


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Figure 5. Effect of radical trapping agents on the decomposition of cumene hydroperoxide in the presence of tert-butyl tert-butanethiolsulfinate at 25°C: Δ 0.20M CHP, 0.020U tert-BuSS(0)tert-Bu and 0.019U β-naphthol; A 0.20M CHP, 0.020M tert-BuSS(0)tert-Bu, and 0.006M β-naphthol; • 0.20U CHP, 0.022M tert-BuSS(0)tert-Bu, and 0.011U 2,6-di-tevt-butyl-4-methylphenol; % 0.20M CHP, 0.022M tert-BuSS(0)teit-Bu, and 0.024M cyclohexanol; Ο 0.20M CHP and 0.020M tert-BuSS(0)tert-Bu 9

not accelerate the decomposition of cumene hydroperoxide i n the pres ence of thiolsulfinate. Similarly, no accelerative effect was observed when the nucleophile was added along with benzoic acid. Thus, a car boxylic acid is not strong enough to catalyze the decomposition under the conditions indicated. The induction period remained at ~ 20 hr for the thiolsulfinate hydroperoxide reaction i n the presence of added benzoic acid. It has been established that the decomposition of cumene hydro peroxide i n the presence of thiolsulfinate occurs primarily via a polar process ( I ) . However, a homolytic process may be involved in the con version of the thiolsulfinate to the active peroxide decomposer. This was probed by adding the radical inhibitors β-naphthol and 2,6-ài-tertbutyl-4-methylphenol (see Figure 5 ) . The inhibitors totally suppressed the decomposition of hydroperoxide by thiolsulfinate. I n contrast to β-naphthol and 2,6-di-ter£-butyl-4-methylphenol, the addition of cyclohexanol had no significant effect. Similarly, the addition of methanol only reduced the decomposition of hydroperoxide by the thiolsulfinate by ~ 1 % after 186 hr. T o insure that the phenolic inhibitor was not simply interfering with the acid-catalyzed hydroperoxide decomposition, 2,6-di-ter£-butyl-4-methylphenol was added after hydroperoxide decomposition commenced. I n


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STABILIZATION A N D DEGRADATION O F P O L Y M E R S

contrast to initial addition, the phenolic inhibitor had no detectable effect once the active peroxide decomposing agent had been generated. The following reaction sequence is suggested. Initially the thiolsul finate undergoes an intramolecular cycloelimination reaction as i n Reac tion 1. Extensive supporting evidence for this postulate has been presented by Block (4) from studies on the decomposition of thiolsulfinate under m i l d conditions ( < 1 0 0 ° C ) . Since Reaction 1 is favored over homolysis and a reactive species, thiosulfoxylic acid, is formed, Reaction 1 is prob ably the initial step. The thiosulfoxylic acid may be expected to undergo oxidation to the corresponding thiosulfurous and thiosulfuric acid (2). ROOH

RSSOH

> RSS0 H 2

ROOH

> RSS0 H 3

(7)

The analogous sulfenic acid has been shown to undergo rapid oxidation consuming 2 mol of cumene hydroperoxide at 2 5 ° C ( J ) . The thiosulfuric acids expected from Reaction 7 are unstable com pounds. Acidified solutions of their salts are reported to undergo oxida tion forming the corresponding disulfide (10). [0]

2RSS0 H + H 0 3

2

> RSSR + 2 H S 0 2

4

(8)

The sulfuric acid, if formed i n our reactions, would be a particularly effective peroxide decomposer. The parent thiolsulfinate w i l l complex with the acidic species formed: 0 R S S R + H A ^ [RSSO- · · H A ] complex R

(9)

W h e n sufficient acid ( H A ) is generated, hydroperoxide decomposition ensues:

ROOH

HA

> nonradical products

(10)

Accordingly, if a more basic species (i.e., an alkyl sulfoxide) is added, the acidic species w i l l be complexed effectively preventing the decom position of cumene hydroperoxide by thiolsulfinate solutions at 25°C. Radical involvement is indicated also by the inhibitory effect of radical trapping agents and product analysis, since 10-15% of the prod ucts from cumene hydroperoxide decomposition induced by the organic sulfur compounds result from free-radical processes (2). Acids w i l l


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235


hydrogen bond to hydroperoxides and can influence rupture of the O - O bond i n the hydroperoxide leading to formation of free radicals ( I I ) . R 0 0 - · · ·HA - » radicals ( R 0 - , etc.) H

(11)

Thus, it has been proposed that the homolytic decomposition of hydro peroxides can be induced b y sulfenic acid (12,13). There is evidence that various carboxylic acids can promote radical formation from hydro peroxides at elevated temperatures (11,14). The intermediate thiosulfurous acid (Reaction 7) itself may function as the source of radicals, since sulfinic acid is known to initiate the radical polymerization of vinyl monomers at 2 0 ° C (15). Based o n the AIBN-initiated oxidation of cumene, Koelewijn and Berger (16) proposed that pro-oxidant effects arise from catalysis of the radical decomposition of hydroperoxides b y intermediate compound formation between the hydroperoxide and sulf oxide. However, under our conditions hydroperoxide was stable i n the presence of sulfoxide alone. The radicals generated may induce decomposition of the thiolsul finate as proposed by Barnard and Percy (6). A n alternative fate of generated radicals w i l l be an attack on thiosulfoxylic acid: R S S O H + R 0 - - * RSSO- + R 0 H 2

(12)

2

This reaction is facilitated b y formation of the stabilized RSSO · radical that is isoelectronic with the stabilized polysulfide radical, R S - . The analogous sulfenic acids are effective radical scavengers reacting with peroxy radicals with a rate constant of 1 0 M sec" at 60°C (16). T h e S-S bond i n the thiolsulfinate is weak, and the corresponding bond i n the thiosulfoxyl radical should be considerably less stable. Thus, the thiosulfoxyl radical may function as a source of sulfur oxides: 3

7

- 1

1

R S S O - -> R S - + [SO]

(13)

Thermolysis of the analogous butyl sulfoxylate produces sulfur and sulfur dioxide presumably via sulfur monoxide (17). Further studies regarding the participation of free radicals are i n progress. In conclusion, several alternative mechanisms for the thiolsulfinate hydroperoxide reaction have been suggested. T h e results indicate that nucleophilic displacement on terf-butyl teri-butanethiolsulfinate b y water, sulfides, and disulfide does not play a significant role under the conditions cited. T h e possibility of a homolytic process has been considered, and evidence indicates that radicals play an essential role i n formation of the active peroxide decomposing species, that then function primarily v i a a


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STABILIZATION A N D DEGRADATION OF P O L Y M E R S

polar mechanism. Since the active peroxide decomposer is an acidic species, its activity can be affected significantly by the basic character of the parent thiolsulfinate or sulfoxide. Acknowledgment The authors wish to thank the Goodyear Tire and Rubber Company for financial support of this research project. Literature Cited

1. Shelton, J. R., Harrington, E. R., Rubber Chem.Technol.(1976) 49, 147. 2. Harrington, E.R., PhD dissertation, Case Western Reserve University (1976). 3. Mair, R. D., Graupner, A. J., Anal. Chem. (1964) 36, 1944. 4. Block, E., O'Connor, J.,J.Am. Chem. Soc. (1974) 96, 3929. 5. Kice, J. L., Cleveland, J.P.,J.Am. Chem. Soc. (1973) 95, 109. 6. Barnard, D., Percy, E. J., Chem. Ind. (1960) 1332. 7. Koch, P., Ciuffarin, E., Fava, Α.,J.Am. Chem. Soc. (1970) 92, 5971. 8. Barnard, D., Bateman, L., Cole, E. R., Cunneen, J. I., Chem. Ind. (1958) 918 9. Barnard, D.,J.Chem. Soc. (1957) 4675. 10. Milligan, B., Swan, J., Rev. Pure Appl. Chem. (1962) 12, 72. 11. Emanuel, Ν. M., Denisov, E. T., Maizus, Ζ. K., "Liquid-Phase Oxidation of Hydrocarbons," p. 81, Plenum, New York, 1967. 12. Shelton, J. R., Rubber Chem.Technol.(1974) 47, 949. 13. Scott, G., Mech. React. Sulfur Compd. (1969) 4, 99. 14. Privalova, L.G.,Maizus, Z. K., Izv. Akad. Nauk SSSR (1964) 281; Chem. Abstr. (1964) 60, 11867. 15. Overberger, C. G., Godfrey, J. J.,J.Polym. Sci. (1959) 40, 179. 16. Koelewijn, P., Berger, H., Rec. Trav. Chim. Pays-Bos (1974) 93, 63. 17. Mathey, F., Lampin, J. P., Tetrahedron Lett. (1972) 3121. RECEIVED May 12, 1977.


The Role of Certain Organic Sulfur Compounds as Preventive

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