THERADIOLYSIS OF ETHYL MERCAPTAN
also varies from salt to salt, and there is a smooth correlation between water activity and preferential hydration. On t,he basis of these data, each obtained in a different salt, Bruner and Vinograd7 suggest that the preferential hydration of a macromolecule in a given salt should vary with salt concentration since the activity of water does so. Their interpretation would argue against the possibility that the sedjmentation behavior of ovalbumin may be accounted for by the constancy with salt concentration of preferential
2951
hydration and the frictional ratio. It should be possible to approach the problem more directly by measuring the buoyant density of ovalbumin in cesium chloride and deuterium oxide. The salt concentration and thus the water activity a t the protein band should be different in water and in deuterium oxide, and it should be possible, in this way, to determine whether or not the preferential solvation of the protein is the same a t two different concentrations of the same salt.
The Radiolysis of Ethyl Mercaptan1
by J. J. J. Myron and R. H.Johnsen Department of Chemistry and the Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida (Received March 81,1966)
A study of the radiolysis of liquid ethyl mercaptan has been undertaken. The data strongly suggest that the radiolytic behavior of the mercaptan differs significantly from that of the corresponding alcohol. A comparison of the two radiolyses in the light of the present results is presented.
Introduction I n the past, radiolytic studies of compounds containing sulfhydryl groups have generally been carried out on dilute aqueous solutions of polyfunctional thiols. Except for data on the esr spectra of irradiated methyl and ethyl mercaptan2 taken at 77”K, no studies on the radiolytic behavior of these compounds appear to have been made. Investigation of simple mercaptans in the “pure” state should be of interest as a comparison to that of the corresponding aliphatic alcohols which have been extensively studied by various workers. 3--6 The possible importance of thiol radiolysis studies is evidenced by the number of publications dealing with biological systems in which mercaptans were present cysteas additives. Compounds such as aminello and glutathione11*12 have been widely used as “protectors” of biologically significant systems against radiation damage. Oxidation of the thiol group by radiation-produced radicals from the other compo-
nents of the system is probably an important mode of “protection” or “inhibition” afforded by the sulfur compound. The corresponding disulfide and small amounts of hydrogen sulfide are the usual products of oxidation. ~
~~
(1) Research supported in part by A.E.C. Contract AT-(40-1)-2001 and in part by A.E.C. Contract AT-(40-1)-2690. (2) C. L. Luck and W. Gordy, J. A m . Chem. SOC.,78, 3240 (1956). (3) (a) W. McDonnell and A. S. Newton, ibid., 76, 4651 (1954) ; (b) I. A. Taub and L. M. Dorfman. ibid., 84, 4053 (1962). (4) (a) J. G. Burr, J. Phys. Chem., 61,1477 (1957); (b) G. E. Adams, J. H. Baxendale, and R. D . Sedgwick, ibid., 63,854 (1959). ( 5 ) E. M. Hayon and J. Weiss, J. Chem. SOC.,3962 (1961). (6) R. H. Johnsen, J. Phys. Chem., 65, 2144 (1961). (7) S. L. Whitcher, M. Rotheram, and N. Todd, NucEeonics, 1 1 , 30 (1953). (8) P. Rieez and B. E. Burr, Radiation Res., 16, 661 (1962). (9) C. N. Trumbore, et al., J . Am. Chem. SOC.,86, 3177 (1964). (10) B. Shaprio and L. Eldjar, Radiation Res., 3 , 225 (1955). (11) G . E. Woodward, Biochem. J.,27, 1411 (1933). (12) E. S. G. Barron and V. Flood, J. Gen. Physiol., 33, 229 (1950).
Volume 70, Number 9 September 1966
2952
J. J. J. MYRON ASD R. H. JOHKSEN
Much less work has been published concerning the radiolytic behavior of aqueous solutions containing simple aliphatic mercaptans such as methanethiol or ethanethiol. However, Armstrong and Wilkening13 have recently demonstrated marked yield dependences on pH for both hydrogen and methane production from aqueous methanethiol. Their results indicate that mercaptans may function as solvatedelectron scavengers as well as radical traps by means of the reaction eas-
+ RSH +R + SH-
(1)
The following is a report of some experiments that have been made on the radiolysis of ethyl mercaptan in the gaseous, liquid, and solid phases.
Experimental Section Eastman White Label ethyl mercaptan was used for all irradiations. Gas chromatographic analysis revealed the presence of five impurity peaks which amounted to approximately 0.5 mole % of the substrate. Because of the extremely noxious character of ethanethiol, it was decided not to attempt any rigorous purifying procedures on the substrate. However, some purification was effected by using only the middle third portion of distillate from a distillation into a reservoir on a vacuum line (impurity content 0.2 to 0.3%). Whenever possible, the mercaptan was handled in a Pyrex vacuum system or in a glove box that was vented to an exhaust system. Various other containment procedures employing fume hoods, vacuum desiccators, etc., were also necessarily employed during sampling for product analysis. Liquid samples consisted of approximately 1 ml of liquid contained in 15-nim 0.d. tubes of about 4-ml capacity. All samples were prepared on the Pyrex vacuum system and were thoroughly degassed before the sample cell was sealed prior to irradiation. Blank unirradiated samples were prepared and analyzed at intervals to check on the composition of the substrate and to check preparative and analytical technique. The amount of substrate was determined by volume measurements at 0” in a calibrated graduated tube in the vacuum system. A few gas phase samples were run at room temperature (at pressures of about 270 torr). These were irradiated in a Pyrex cylinder of 400-ml capacity which was fitted with a stopcock and small “freeze down” side arm for sample preparation. An esr spectrum of the radicals formed and trapped at 77°K was obtained using a quartz cell and a Varian esr spectrometer. All irradiations were carried out by X-rays generated by allowing 3-Mev electrons from a Van de Graaff The Journal of Physical Chemistry
accelerator to impinge upon a tungsten target. The temperature during irradiation for both liquid and gaseous samples was 25 f 2 ” . For liquid samples, dosimetry measurements were made during each irradiation employing the Fricke dosimeter. To obtain dose rates for mercaptan samples, the assumption was made that the rate was proportional to the ratio of the electron densities. The dose rate for mercaptan was approximately 1.1 X 1020 ev/’g hr. Total doses ranged from 0.25 to 10 X lozo ev/g of mercaptan. Gas pha.se dosimetry was carried out by measuring the hydrogen yield from propylene radiolysis using the ratio of electron densities and a G(H2)from propylene = 1.1.14
Gases that were noncondensable at 77°K (hydrogen and methane) were measured together by breaking the ampoule (or for a gas sample, opening the stopcock to the cell) and toeplering the gas mixture into a calibrated gas buret. ,4n aliquot of this mixture was then analyzed in a Perkin-Elmer 154-B gas chroniatograph using a 6-ft charcoal column. Chromatograms of liquid products were obtained from a Perkin-Elmer 800 (flame ionization detector) gas chromatograph using a di-n-decyl phthalate on Chroniosorb W column and a silicone 200 oil on Chromosorb W column.
Results Figure 1 shows a plot of G(H2) vs. total dose from 0.25 to 10 X lozo ev/g of liquid mercaptan. Although there is considerable scatter in the points, particularly at the low dose end of the scale, there is no apparent dose dependence of hydrogen yield over this dose range. Consequently, the initial G value for hydrogen production was taken as the average of all determinations, G,(HZ) = 7.1. A plot of the yield of methane us. dose over the same dose range in the liquid phase is given in Figure 2. The scatter is greater than that of the hydrogen plot due to the fact that the methane amounted to only 1-2% of the gas mixture, and accuracy in its measurement, especially at low conversions, is difficult to attain. The average of all measurements gave G(CH4) = 0.10, so that in the liquid phase, G(Hz)/G(CH4)= 70. After subtraction of impurity peaks from “blank” samples were made, gas chromatography showed a total of twelve product peaks at the highest dose employed ( = 1 X loz1ev/g). However, the three largest peaks accounted for 97y0 of the total liquid products that could be detected. These products were diethyl ~~
~
~~
~
J. Chem., 42, 2631 (1964). (14) K. Yang and P. L. Grant, J . P h y s . Chem., 6 5 , 1861 (1961).
(13) D. A. Armstrong and V. G. Wilkening, Can.
THERADIOLYSIS OF ETHYL MERCAPTAN
10
2953
-
8-
-;.o
0
0
0
0
6c
E?4-
21
0
*
2
I
I
1
4
6
I
,
8
10
faded upon warming. The photolytic behavior of the solid is now under investigation. 012
Discussion
2 008 0 0 04
O
'
DOSE
^^
:IO
(loLU e v / p m )
Figure 2. Methane yields from liquid ethyl mercaptan as a function of dose.
disulfide, diethyl sulfide, and probably 1,4-butanedithiol, and these constituted 80, 15, and 2%) respectively, of the liquid products. There was no evidence for the presence of 2,3-butanedithiol. The sulfide and disulfide could be measured from 0.25 to 10 X lozoevjg, and no definite dose dependence was evident. If it is assumed that relative retention times of sulfur compounds on silicone oil are an indication of molecular weight, it appears that six of the remaining nine peaks that constitute the other 3% also have a molecular weight greater than the substrate. As these products are minor ones and authentic samples of possible product compounds were not available, no attempt was made at identification. A solution of thioacetaldehyde in methanol was chromatographed to obtain its retention time and special attention was paid to determine if it was present as a product of the radiolysis. If thioacetaldehyde was present, it was in very small amounts even at the highest doses. Gas yields from irradiation in the gas phase a t room temperature a t a dose of 7 X 1019ev/g were G(H2) = 17 f 2 and G(CH4) = 3.1 rt: 0.4. Therefore, the ratio G(Hz)/G(GH4)is reduced from 70 to 5.5 in going from the liquid to the gaseous state at room temperature. The esr spectrum obtained at 77°K (Figure 3) was a broad asymmetric peak with indications of some possible hyperfine structure. After irradiation, the solid mercaptan exhibited a deep orange color which
Due to the structural similarity of the ethyl mercaptan and the ethvl~ alcohol molecules and the fact that the radiolysis of ethanol has been widely investigated, the discussion of the results will involve, to a large extent, a comparison of the radiolysis of the two compounds. As is usual with aliphatic compounds, hydrogen is a major product [G(HZ) = 7.11. The initial hydrogen yield from the oxygen analog (ethyl alcohol) is Gi(H2)= 5.6g15116The substantially higher yield from the mercaptan might be due to the lower ionization potential of the mercaptan (I.P. = 9.2 ev)17compared to that of the alcohol (I.P. = 10.5 ev).l* This could result in a greater amount of the initial reaction CHaCHzSH --...-+ CH3CH2SH+
+ e-
(2)
as compared to the analogous alcohol reaction. The ionization would presumably be followed by CHaCHzSH+
+ CH3CHZSH
4
CH3CHzSHz+
+ CHsCHZS
+ mCH3CHtSH +eaolCH3CH2SHz++ esol- +nCH3CH2SH+ H H + CHsCHzSH Hz + CHaCH2S e-
(3)
(4) (5)
(6) Reactions similar to (3) to (5) have been postulated in a mechanism for the radiolysis of ethanol.16 --j
(15)G.E.Adams and R. D. Sedgwick, Trans. Faraday SOC.,60,865 (1964). (16) J. J. J. Myron and G. R. Freeman, Can. J . Chem., 43, 381 (1965). (17)I. Omura, K.Higasi, and H. Baba, Bull. Chem. SOC.J a p a n , 29, 504 (1956). (18) K.Watanabe, T.Nakayama, and J. Mottl, J . Quant. Spectry. Radiative Transfer, 2, 369 (1962).
V o l u m e 70, N u m b e r 9
September 1966
J. J. J. MYRONAND R. H. JOHNSEN
2954
However, in the ethanol system there is evidence that the reaction similar to (3) occurs to a significant extent in the liquid but not in the gas phase.Ig This appears to be due to the fact that the hydrogen bonding present in the liquid phase could not aid the formation of the transition complex I in the gas phase.lg CH3-CHZ-0-H
+ 1I
H-- -0-C H2-C H,
I Hydrogen bonding in ethyl mercaptan occurs to a much lesser extent than in ethyl alcohol due to the significantly smaller electronegat,ivity of the sulfur atom as compared to the oxygen atom. Therefore, the formation of the transition complex necessary for reaction 3 is not as likely in liquid mercaptan as it is in liquid ethanol and, therefore, this mechanism for Hz is relatively less important. The various bond strengths (in kilocalories) that have been reported (or estimated) for the ethyl mercaptan and alcohol molecules are listed in Table I.20*21The
Table I Bond strength, kcsl
Type of bond
Ethanol
0-H C-0 C?-C
C-H (methylene) C:-H (methyl)
99" 90" 83b gok 96'
Ethanethiol
8-H C-S
87" 73c
a See ref 20. 'Estimated bond strength, see text. ref 21.
See
bond strengths that were estimated for the ethanol molecule were arrived a t as follows. The C-H value for the methyl group was assumed to be that quoted for ethane by Cottrell,22 who also cited a 6-kcal difference in dissociation energy for the primary and secondary C-H bonds in propane. Hence, the approximate value of 90 kcal for the C-H bond in the methylene position for ethanol is given. The C-C bond strength is that quoted for ethane.22 The corresponding bond strengths for the mercaptan molecule will, in all likelihood, be somewhat greater due to T h e Journal of Physical Chemistry
the sulfur atoms' smaller electronegativity. These figures, while approximate, should give a reasonable estimate of the relative bond strengths for the molecdes. It thus appears that the 0-H bond in the alcohol is the strongest and methylene C-H bond is the weakest among the bonds to hydrogen atoms. This supposition is supported by the esr spectra of ethyl alcoh012~*24 at 77°K and the nature of the major liquid products from the radiolysis.3*l6 Both these types of data indicate that at some stage in the radiolysis CHGHOH is present in appreciable quantities. On the other hand, the S-H bond in the mercaptan appears to be the most labile of all bonds to hydrogen in both molecules. I t is possible that the greater hydrogen yield is due to the relatively great ease of abstraction by hydrogen atoms (produced by reaction 5 and/or other reactions) from the S-H group. The lability of the bond to the sulfhydryl hydrogen atom should result in products that attest to the presence of CH,CH2S radicals in the system. Diethyl disulfide and diethyl sulfide constitute 95% of the condensation products while the 1,2-dithiol was not observed. As the formation of these products is most easily explained by the combination of two ethylthiyl radicals and the combination of an ethylthiyl and an ethyl radical, respectively, the presence of thiyl radicals can be inferred. Further evidence for the presence of thiyl radicals at some stage in the radiolysis can be obtained from data on mercaptan samples that were irradiated and subjected to esr analysis before and after photolysis with selected wavelengths a t 77°K. These experiments show that the species giving rise to the original esr signal is converted by light to another paramagnetic intermediate which has been identified as CH3CH2S.25 This interpretation is consistent with T r ~ b y ' s ~ ~ r ~ ' work on disulfides in which he postulates that the esr spectrum of irradiated disulfides consists of contributions from ionic species (predominantly) and RS (19) J. J. Myron and G. R. Freeman, Can. J . Chem., 43, 1484 (1965). (20) P. Gray, Trans. Faraday Soc., 52, 344 (1956). (21) J. L. Franklin and H. E. Lumpkin, J . iim. Chem. SOC.,74, 1023 (1952). (22) T. L. Cottrell, "The Strengths of Chemical Bonds," Butterw o r t h and Co. Ltd., London, 1958. (23) H. Zeldes and R. Livingstone, J . Chem. Phys., 30, 40 (1959). (24) B. Smaller and M. S. Matheson, ibid., 28, 1169 :1958). (25) S . B. Milliken, K. Morgan, and R. H . Johnsen, to be published. (26) F. K. Truby, J . Chem. P h w , 40, 2768 (1964). (27) F. K. Truby, D. C. Wallace, and J. E. Hess, ibid., 42, 2845 (1965).
THERADIOLYSIS OF ETHYLMERCAPTAN
2955
(thiyl) radicals and alkyl radicals. It appears that the initial (prebleached) spectrum of ethyl mercaptan is largely due to the presence of entities such as CH3CHzSH+ and CH,CH2SH-. Upon photolysis the following reactions are feasible.
+ hv -+ RSH + e+ RSH+ 4RSH*
RSH-
(7)
e-
(8)
RSH*
----f
RS
+H
(9)
These reactions can reasonably account for the observed (postbleached) thiyl radical production. The fact that diethyl sulfide constitutes 15% of the liquid products seems to indicate that there is appreciable rupture of the C-S bond (as would be expected from its low bond strength). However, the FranckRabinowitch cage effect would obscure the true extent of the C-S bond scission. Evidence for the operation of a cage effect can be gotten from the G(HZ)/G(CH,) ratio in the liquid and gas phase. The ratio decreases from 70 to 5.5 on going from the liquid to the gas phase. If it is assumed that the methane yield is due to G C bond scission followed by abstraction from the substrate by the methyl group, the G ratio reflects the greatly decreased cage effect in the gas phase. CH3
+ CHSCH2SH
---f
CHI
+ CzHsS
(10)
Kerr and Trotman-DickensonZ8 have demonstrated that ethanethiol is an efficient methyl radical scavenger due to the efficiency of reaction 10.
The enhanced over-all decomposition of the mercaptan in the gaseous phase 5ts compared with the liquid phase is probably due to two effects. The first is the lack of a cage effect in the gas phase. The second factor is the much larger probability that ion pairs escape immediate recombination and are thus capable of undergoing reactions that result in stable radiolytic products. It is interesting to note that the sulfur analogs of the two major liquid products from ethanol radiolysis (2,3-butanedithiol and thioacetaldehyde) were present, if at all, only in negligible quantities. This appears to minimize reactions such as
+H CH3CHS + Hz
CH3CHzSH +CHsCHSH
(11)
CH3CH2SH
(12)
in the radiolysis. The data obtained to date indicate that the radiolytic decomposition mechanism for ethyl mercaptan is dictated largely by the two bonds to the sulfur atom. Mercaptan radiolysis products, therefore, differ in nature from those of the corresponding alcohol in which the methylene hydrogen atoms largely govern the decomposition.
Acknowledgment. The authors wish to thank Mr. D. Pritchett for technical assistance and Mr. S. Milliken for helpful discussions concerning the work, and for providing the esr spectrum. ~
(28)
~~~
~
J. A. Kerr and A. F. Trotman-Dickenson, J . Chem. SOC.,3322
(1957).
Volume 70,Number 9 8eptember 1966
