FOTES
3343
the one to be described in a future publication.la The esr spectra were taken in saturated solutions of the complex in dilute acid (1 drop of HN03/150 cc of H20). Observation of 170hyperfine structure in p-peroxobis(pentaamminecoba1t) (5+) ions is much more difficult than for the dibridged (4+) ion discussed herein, as a result of the much smaller solubility and broader epr lines of the former. However, attempts to carry out such measurements are in progress. (13) M.Mori, ‘J. A. Weil, and
M.Ishiguro, submitted for publica-
tion.
The Reactions of Hydrogen Atoms with Simple Thiols by Robert R. Kuntz Department of Chemistry, University of Missouri, Columbia, Missouri 65101 (Received January SO, 1967)
The photolysis of simple thiols in the gas phase results in the production of H atoms and thiyl radicals.’ Several investigators2t3have studied reaction of these H atoms with the parent compound by the competition technique involving reactions 1-3. A steady-state
+ hv +RS + H H + RSH +H2 + RS
RSH
H
+ C2H4
+C2Hs
(1) (2)
(3)
derivation involving H atom reactions leads to eq 1,283where RH: and RH2’ are the rates of hydrogen pro-
duction in the absence and presence of scavenger ethylene. Rate constant ratio values of 1.62and 1.73 have been obtained for H2S and methyl mercaptan, respectively. The present study uses a similar technique to measure this rate constant ratio for ethyl, n-propyl, and n-butyl mercaptans in order to determine the effect of substituents on the rate of H-atom removal from the thiol group.
Experimental Section Hydrogen sulfide (>99.5%), ethane (>99.0%), ethylene (>99.5%), and methyl mercaptan were obtained from Matheson and used without purification except for bulb-to-bulb distillation. Gas chromatographic analysis of the ethane showed 0.1% ethylene
present. Ethyl, n-propyl, and n-butyl mercaptans (Eastman) were used without further purification. Care was taken to exclude mercury vapor from the reaction during transfer and analysis by use of a series of liquid air traps. Pressure measurements were made with a mechanical diaphragm gauge. The reaction cell, low-pressure mercury light source providing 2537-A light, and analysis system have been described p r e v i ~ u s l y . ~Sample decomposition was usually restricted to less than 1% to avoid complications from secondary products. All photolyses were performed a t 24”.
Results and Discussion Table I summarizes the results of this study. For HzS, the ratio k2/k3 is seen to be pressure independent over the range of 8-35 cm total pressure and 5-10 cm H2S pressure with an average value of 2.1 f 0.1. Darwent and Roberts2 reported a value of 1.6 and also noted the pressure independence. The apparent rate constant ratios for the mercaptans all show a strong dependence on total pressure with the value remaining constant within experimental error over the low-pressure range, then decreasing rapidly after a few centimeters of pressure (depending on the particular mercaptan used). In order to investigate this effect, several runs were made in which the pure mercaptan or a constant mercaptan-ethylene mixture was photolyzed over a pressure range effected by the addition of ethane to the system. Ethane is relatively inert to H atom attack in the presence of the more reactive H atom donor mercaptans. These results are shown in Table 11. A comparison of the relative rates of hydrogen evolution in the photolysis of methyl mercaptan with and without added ethylene is shown in Figure 1. It is seen that the apparent decrease in the rate constant ratio is due to the pressure-induced decrease in hydrogen production. Pressure effects have not been noticed in the previous studies of methyl mercaptan photolysis using the full mercury arc and pressure up to 30 ~ m Several . ~ reasonable possibilities for this effect seem to be excluded from the data presented. These are as follows. (i) There are H atom scavenging impurities in the ethane used for pressurization. To explain the entire decrease in hydrogen yield exhibited in Figure 1, one can calculate, using reasonable rate constants, that 2.4% (1) J. G.Calvert and J. N. Pitts, Jr., “Photochemistry,” John Wiley and Sons, Inc., New York, N. Y.,1966,pp 490, 491. (2) B. deB. Darwent and R. Roberts, Proc. Roy. SOC.(London), A216, 344 (1953). (3) T.Inabe and B. deB. Darwent, J . Phys. Chem., 64,1431 (1960). (4) R. R. Kuntz, ibid., 69, 2291 (1965).
Volume 71, Number 10 September 1967
NOTES
3344
Table I PREIH, om
PCnHu
PC%Ho
PREH,
om
om
om
HzS 5.05 5.05 5.05 5.05 5.05 5.05 10.1 10.1 10.1 10.1 10.1
0 3.55 5.68 9.30 14.90 22.00 0 8.67 12.35 17.47 24.6
wCDHISH 1.41 1.04 0.93 0.77 0.60 0.46 2.84 1.91 1.80 1.52 1.40
... 1.98 2.18 2-26 2.18 2.11
...
1.77 2.11 1.98 2.36 Av = 2 . 1 -I 0.1
CHaSH 5.05 5.05 5.05 5.05 5.05 5.05 5.05
0 2.70 4.83 7.30 11.0 16.2 24.1
22.6 18.7 14.0 12.2 9.37 6.77 5.15
... 2.47 1.58 1.75 1.58 1.38 1.41
C~HISH 4.97 4.97 4.97 4.97 4.97 4.97
0 3.90 6.60 8.65 11.9 17.9
~O’RH~, molea/seo
21.1 15.0 12.2 11.0 9.0 6.4
... 1.94 1.83 1.91 1.76 1.57
olefin (C2H4) or 0.02% oxygen would be required. Actual analysis of the ethane by H-flame chromatography using a Porapak Q column shows only 0.1% CzH4. Even though each sample was thoroughly degassed before use, this small amoufit of oxygen might be present. The lack of an effect of added ethane on the H2 yields in H2S photolysis, however, and the dependence of ks/k3 on total pressure only, independent of pressurizer, seems to indicate that impurities are not responsible for the observed effect. (ii) “Hot” H atoms or vibrationally excited C2Hs from reaction 3 cannot cause the observed decrease in H2 production since ethylene is not present in one of the CH3SH systems presented in Figure 1 and the decrease still occurs. In the absence of any reasonable alternative, “hot” H atoms may become thermalized, but should still produce hydrogen by reaction 2.
0 2.34 3.98 5.7 7.9 10.2 13.3 0 3.7 5.25 6.75 9.7 14.4 0 1.35 2.06 2.70 4.05 6.17
3.98 3.98 3.98 3.98 3.98 3.98 3.98 5.1 5.1 5.1 5.1 5.1 5.1 2.06 2.06 2.06 2.06 2.06 2.06
16.5 13.6 11.5 9.8 8.3 6.1 5.35 18.7 14.2 13.1 1’1.0 9.0 7.0 8.90 7.32 6.84 6.28 5.20 3.96
2.75 2.30 2.09 1.98 1.51 1.60 2.30 2.42 1.92 1.79 1.71 3.03 3.32 3.14 2.75 2.42
n-CdHsSH 0 0.69 0.86 1.10 1.46 2.09 0 1.13 2.34
0.78 0.78 0.78 0.78 0.78 0.78 2.8 2.8 2.8
...
4.68 3.76 3.64 3.36 3.14 2.80 1.68 1.51 1.26
3.54 3.85 3.58 3.76 3.92
...
3.62 2.50
(iii) Third-body combinations are usually unimportant in this pressure region. If one assumes the mechanism given is the only one of importance and that thiyl radicals disappear only by re~ombination,~ then the [HI and [RS] may be approximated from eq I1 and I11 using the measured value of kZ6 and kt = lOI3
- -
RH^ = k2 [HI [RSH I RRSSR RH^ kt[RSj2
(11) (111) cc/mole sec. To account for the observed decrease in H2 production H
+ RS + M-RSh
+ 11
(4)
k4 must be ca. 10-26 ccZ/molecule2 sec. This is six orders of magnitude larger than the generally observed values of k4 10-32 cc2/molecule2 sec.7 Thus, third-body effects would seem to be unimportant. N
NOTES
3345
(iv) Quenching of excited RSH molecules does not seem plausible since the mercaptans show a predissociation-type absorption spectrum which is consistent with 0.9).’ the high quantum yield of Hi production (4 Collisional frequencies a t these pressures are such that the lifetime of the dissociating species must be >10-lo see, assuming unit efficiency in deactivation. Again, this seems unreasonable.
I
-
I
0
0
5 18 -
e
3 v
Table 11: Pressure Effect on kz/kg RSH/ CzHd
PTra
om
1O9RH2, moles/sec
k2/ka
0.20 0.109 0.111 0.112 0.109 0.109 0.097
2.03 2.11 2.17 2.03 2.03 1.59
H2S 0.59 0.59 0.59 0.59 0.59 0.59
-
0.79 2.3 3.4 4.8 9.7 20.1 30.4
...
CHiSH
0.70 0.70 0.70 0.70 0.70 0.70 0.70
1.06 2.5 13.0 17.2 25.2 39.2 0.99 2.4 4.6 8.9 20.3 31.2 42.2 54.8
0.84 0.84
1.10 2.4 10.2
0.75 0.75 0.75 0.75 0.75
...
4.37 2.63 2.64 2.56 2.21 1.94 4.26 2.46 2.34 2.40 2.12 1.82 1.73 1.81
2.01 2.04 1.89 1.36 1.07
... 1.93 1.74 1.84 1.41 1,07 0.98 1.05
CzH6SH
a
P T = PH25
3.40 2.20 2.09
... 2.18 1.91
+ PC~H* + Pc*H#in centimeters of mercury.
The pressure effect cannot, then, be explained with the available data. The rate constant ratio for the low-pressure region is consistent with the previous measurements3 for methyl mercaptan. These pressureindependent ratios for the other mercaptans are collected in Table 111.
I
I
6t
0
1
I
12
0
24
36 Pressure, cm.
48
54
Figure 1. Hydrogen yields in the photolysis of methanethiol: 0, 1 em CH3SH; 0, RSH/C2Ha = 0.75, 1 cm CHISH; 0 , RSH/CZHa = 0.70, 1 em CH3SH. Scale adjusted to give equal amounts of hydrogen at low pressures.
Table I11 : Summary of kz/k3 Values for the Mercaptans in the Pressure-Independent Region Mercaptan
HzS CHaSH C2HbSH TL-C~H~SH n-CaHgSH
kz/kaa
Lit. values
2.1 i0.1 1 . 7 zt 0 . 2 (9 values) 1 . 9 zt 0 . 1 (5 values) 3 . 2 f 0 . 1 (3 values) 3 . 7 =t0 . 1 (6values)
l.gZ 1 .7I (50’)
a Reported errors are average fluctuations from the mean. The methanethiol fluctuations are larger owing to interference of the methane and hydrogen peaks in the chromatographic analysis of the product.
Acknowledgment. This investigation was supported by U. S. Public Health Research Grant RH-00321, Division of Radiological Health. ( 5 ) R. P.
Steer, B. L. Kalra, and A. R. Knight, J. Phys. Chem., 71,
783 (1967).
(6) Based on ka = 1.6 X 1011 cc/mole sec: K. Yang, J . Am. Chem. SOC.,84, 3795 (1962). (7) 0.K. Rice, “Proceedings of IXth International Astronautical Congress,” Amsterdam, 1958, p 9.
Volume 71, Number 10 September 1967
