Sulfur Research Trends

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The Reaction of Mercaptans with Liquid Sulfur H. J. LANGER and J. Β. HYNE a

Downloaded by UNIV OF NEW SOUTH WALES on August 26, 2015 | http://pubs.acs.org Publication Date: December 1, 1972 | doi: 10.1021/ba-1972-0110.ch008

Alberta Sulphur Research Ltd., University of Calgary, Calgary, Canada

The kinetics of the reactions of various substituted thiophenols with liquid sulfur have been investigated, and intermediates of the general form RS H and HS H have been identified. The studies suggest that the reaction is usually free radical involving an initiation period in which a steady state concentration of sulfur radical species is estab lished. The reaction appears to be second order in mercaptan and third order in sulfur. These findings have been interpreted using a complex sulfur radical species involving several S molecules. X

X

8

Recent studies of the reaction of hydrogen sulfide with liquid sulfur (1,2,3) have indicated that the reaction is probably free radical in nature and yields a variety of hydrogen polysulfides or sulfanes. No com parable study of mercaptans with liquid sulfur has been reported, how ever, despite interest in this system in the vulcanization, petrochemical, and geochemical fields. The RSH-sulfur reaction offers advantages over the H S - S system since the effect of varying the R group in the mercaptan is used as an additional probe in elucidating the reaction charac teristics. The results of kinetic and product analysis studies of the reaction of a range of p-substituted aromatic mercaptans (R = X - C H - ) with liquid sulfur are presented. Experimental details will be published in a separate more extensive paper (4). 2

e

4

Kinetic Studies Order of Reaction. Typical second order rate plots for the reaction of the parent thiophenol with liquid sulfur at 130 °C are shown in Figure a

Present address: Ashland Oil Inc., Columbus, Ohio. 113

In Sulfur Research Trends; Miller, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

SULFUR

RESEARCH

TRENDS


114

0

I 0

1

ι

ι

ι

ι

ι

1

10

20

30

40

50

60

70

TIME

(min.)

1

Figure I. Second order (with respect to thiol) rate plots for the reac tion of thiophenol with sulfur (130° C). Ratios shown on each line are thiophenohsulfur. 1. The decrease in concentration of thiophenol was followed by aliquot sampling of the reaction mixture and determination by NMR of the rela tive strength of the mercaptan proton signal. The concentration/time



8.

LANGER

115

Reactions of Mercaptans

AND HYNE

data obtained were examined to determine the order of reaction with respect to thiophenol. Afirstorder analysis was shown to be unsatisfac tory. The second order plots shown in Figure 1 are characterized by an initial curved section followed by a linear relationship between reciprocal concentration and time. The various plots represent different relative concentrations of thiol:sulfur. A range of relative concentrations from 1:18 thiol to sulfur to 1:3 thiol to sulfur were examined. By virtue of the excess sulfur in each run, the concentration of sulfur is assumed to re main relatively constant, at least within the limits of accuracy of the rate constants obtained. The linearity of the reciprocal concentration-time plots supports the pseudo-second order rate expression. _i£^l_

f

c

-

i

[^sH]«

d)

The k values obtained from such a second order analysis are shown as a function of relative sulfur concentration in Table I. The observed dependence of k on sulfur concentration indicates that the rate ex pression shown in Equation 1 is inadequate and that a concentration term in sulfur is required. Assuming an unknown order of reaction b with respect to sulfur, an expanded rate expression is obtained. ob8

0b8

Thus

k

0b8

and

log k

ob8

(3)

= k [S]

b

0V

= log k + b log [S]

(4)

ov

Hence, a plot of log k vs. log [S] should yield a straight line of slope b, the order of reaction with respect to sulfur. Such a plot is shown in Fig ure 2 with lines having slopes corresponding to b values of 2, 3, and 4 shown for comparison. The value of b = 3 accommodates best the experi0b8

Table I.

Molar Ratio SH:S 18.8 16 12 8 6

Thiophenol—Sulfur Reaction

Initial |>S#] mole I'

Initial [S] mole I'

2.540 2.850 3.465 4.411 5.110

47.70 45.64 41.59 35.30 30.67

1

1

0

k X 10 I mole' sec' 3

ob8

1

1

1.84 1.35 0.938 0.585 0.375

Second order (with respect to -SH) rate constants at 130°C as a function of relative sulfur concentration. α


116

SULFUR

RESEARCH

TRENDS

mental data. Thus, the overall rate expression suggested by the rate data for the reaction of thiophenol with liquid sulfur is rf[6

ArS^Ar + H S {x predominantly 2) 8

2

(6)

was investigated by NMR under the same conditions and procedures used in the kinetic studies. In the kinetic studies the primary interest was the rate of disappearance of the thiophenol-SH signal, but in addition to observing the diminution of the - S H signal, the appearance and disap pearance of other NMR signals also characterized the NMR study. By analyzing the NMR spectra of the reaction mixture at various times, it was found that while the absorption of the sulfhydryl peak decreased, new peaks downfield of - S H appeared and then disappeared during the reaction, indicating that intermediates were formed. An illustration of this behavior is presented in Figure 6 where NMR spectra, taken from the reaction mixture of the reaction between benzenethiol and liquid elemental sulfur as a function of time, are shown. Figure 7 is an expanded


122

SULFUR

/

RESEARCH

φ - S H+S

TRENDS

ΙΗ8.8


4.00

'-2

3.00

/

ORDER

RATE

ANALYSIS

Φ

ο

ε ι—ι Ο Ζ Ο ο ^

2.00 Ιο,

/

1.00

0.50

INDUCTION

2

PERIOD INTERMEDIATE CONCENTRATION

0.50

Φ

ο 0.30 ι—ι Ο Ζ ο ο 1 - 1

H-S —Η x

α

0.10

Ρ LJ_

Q

0

Figure 8.

\' L 10

ιI 20

ιI 30 TIME

ι1— 40 (min.)

ι 50 1

—

ο 'ι 60

Φ-Sx-H J *— 70

Relationship between induction period for thiophenol/sulfur reaction and concentration variation of intermediates


8.

LANGER

AND HYNE

123

Reactions of Mercaptans

section of the less detailed reaction spectrum shown in Figure 6. The NMR signals resulting from HS^H species in the spectrum shown in Figure 6 were assigned according to Hyne et al (2). The formation of these sulfanes was not unexpected since it has been shown that HSJH species are formed by the reaction of H S, a product of the reaction, and molten sulfur (6). In addition to the HSJrl NMR signals, however, other transient peaks appeared between the absorption of HS#H and - S H . Since the polysulfanes, HS^H, are products of the reaction of HSH with liquid elemental sulfur, the formation of sulfanes of the type ArS^H might be expected also from the reaction of ArSH with liquid elemental sulfur. The independent synthesis of various unsymmetrical sulfanes of the ArSJH type (4) permitted the assignment of these new NMR signals and clearly indicated that not only HS^H, but also unsymmetrical polysulfanes ArS^H are generated in situ when an aromatic thiol reacts with molten sulfur. The behavior of the intensities of the signals assigned to HS^H and [ Z - C H - S H : S - S - R ] e

4


Z-CeUiS- + H S S R

(H)

In the above reaction the S - H bond becomes polarized giving a partial positive charge on the sulfur ( δ ) with an electron being transferred partially to the attacking radical (δ.)· Substituents in the thiol molecule which favor the polar structures preferred by the attacking radical will speed the reaction. Values of ρ ranging from —0.4 to —1.5 have been observed for radical abstraction of hydrogen in reactions of this kind (II, 12,13,14). The ρ — —1.2 value observed here suggests that there is a moderate degree of charge separation in the rate determining transi tion state involving hydrogen abstraction from the thiol by an ArS** or HS*- radical. It is appropriate now to consider the different behavior of both the p-amino and p-nitrothiophenols. The accumulation of evidence suggests that these two thiols react by a different mechanism. It is known that amines have a marked effect on the opening of S and react with sulfur rapidly to form ions (15,16). Davis (17) has proposed a scheme for the initial steps in the reaction of sulfur and amines. Probably, the p-aminothiophenol reacts with sulfur by an ionic rather than a free radical mecha nism. The question remains, however, as to why p-nitrothiophenol reacts in a similar manner to the p-amino compound. The reduction of nitro groups to amino functions by hydrogen sulfide and polysulfides is well known (18); hydrogen sulfide is a reaction product in the thiol/liquid sulfur reaction. Examination of the N M R spectra recorded during the reaction of p-nitrothiophenol and sulfur shows that small amounts of products having an amino group are formed during the reaction. Traces of such amines would suffice to catalyze the ionic reaction mechanism for p-nitrothiophenol. The concentration-time behavior of the HSJH and ArS*H interme diates presented in Figures 8 and 9 permits further comments on the details of the sequence of steps following the radical initiation. It is not possible to elaborate fully the sequence of steps summarized ( Equation 8 ), but a few limiting criteria can be established. The fact that the con centrations of the HSJH and ArS*H intermediates maximize during the induction period suggests that they are the immediate and relatively stable initial products of reaction of the propagating radical species. It is +

8

B


SULFUR RESEARCH TRENDS

128

suggested that the initial complexed catenasulfur biradical (henceforth •S» - ) reacts initially with thiol to yield a sequence of steps shown in Equations 12 to 15.


ArSH +

^± ArS- + HS„-

(12)

HS*- + ArSH ^± ArS- + H S , H

(13)

ArS- + n S 8 ^ ± A r S * -

(14)

ArS„- + ArSH —ArS- + ArS*H

(15)

The hydrogen polysulfides or sulfanes have been shown (3) to be unstable under the reaction conditions used and to undergo decomposition by disproportionating via other sulfanes to hydrogen sulfide and sulfur (Equation 6). The aryl sulfanes (ArS^H) are generally less stable than the HS*H species and are expected to decompose in a similar manner yielding the diaryldisulfane and hydrogen sulfide (Equation 17). H S * H - » H 2 S + nS 8

(16)

A r f t p H - ^ A r A + nSe

(17)

Conclusion The reaction of aromatic thiols with liquid sulfur in the temperature range between the melting point of sulfur and the equilibrium polymerization temperature is free radical in nature and similar to the reaction of hydrogen sulfide with sulfur under the same conditions. Intermediates of the H S * H and ArSJH type are formed in the reaction but decompose to form mainly H 2 S, Ar 2 S 2 > and sulfur. Kinetic evidence suggests that the initial sulfur radical species formed is complexed with two cyclooctasulfur molecules. While this idea agrees with previous suggestions based on non-kinetic evidence it is not conclusive. p-Amino and p-nitrothiophenol react by a mechanism that is probably ionic initiated by the catalytic effect of the amino group present or generated in situ by H 2 S reduction of the nitro group. Literature Cited (1) Wiewiorowski, T. K., Touro, F. J., J. Phys. Chem. (1966) 70, 3528. (2) Hyne, J. B., Muller, E., Wiewiorowski, T. K., J. Phys. Chem. (1966) 70, 3733. (3) Muller, E., Hyne, J. B., J. Amer. Chem. Soc. (1969) 91, 1907. (4) Langer, H. J., Hyne, J. B., Can. J. Chem., to be published. In Sulfur Research Trends; Miller, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

8.

LANGER AND HYNE

(5) (6) (7) (8)


(9) (10) (11) (12) (13) (14) (15) (16) (17) (18)

Reactions of Mercaptons

129

Jaffé, H. H., Chem. Rev. (1953) 53, 191. Wiewiorowski, T. K., Touro, F. J., J. Phys. Chem. (1966) 70, 234. Tobolsky, Α. V., Eisenberg, Α., J. Amer. Chem. Soc. (1959) 81, 780. Wiewiorowski, T. K., Parthasarathy, Α., Slaten, B. L., J. Phys. Chem. (1968) 72, 1890. Schenk, P. W., Thümmler, U., Z. Electrochem. (1959) 63, 1002. Miller, J. D., "A Theoretical Investigation of the Nature of the Bonding in Sulfur Containing Molecules," Thesis, Tulane University, 1970. Russell, G. Α., J. Amer. Chem. Soc. (1956) 78, 1047. Ingold, K. U., J. Phys. Chem. (1960) 64, 1636. Schaafsma, Y., Bickel, A. F., Kooyman, E. C., Rec. Trav. Chim. (1957) 76, 180. Walling, C., Miller, B., J. Amer. Chem. Soc. (1957) 79, 4181. Jennen, Α., Hens, M., Comp. Rend. (1956) 242, 786. Mayer, R., Gewald, K., Angew. Chem. (1967) 79, 298. Davis, R. E., Nakshbendi, H. F., J. Amer. Chem. Soc. (1962) 84, 2085. Schröter, R., in Houben-Weyl-Müller: "Methoden der Organischen Chemie," 4th ed., Vol. XI/1, Thieme-Verlag, Stuttgart, 1957.

RECEIVED March 5, 1971.


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