Vol. 66
XOTEB
956
D1SPLACE:MENT REACTIONS A T THE SULFUR ATOM. 1x1. THE REACTION OF CYANIDE WITH THIOSULFATE’ B Y ROBERT EARLDAvrs Department of Chemistry, Purdue University, Lafayette, Indiana Received December 16, 1961
Cyanide ion has a high nucleophilic character toward displacement at saturated carbon atoms. In a previous study on the mechanism of reaction of cyanide with sulfur,2 the term “thiophilic” was suggested to describe the displacement on sulfur atoms. Several single-sulfur transfer reactions also were discussed.
xs + Y +x + SY
(1)
The reaction of thiosulfate (X = S03-2) with cyanide (Y = -CN) was briefly revieweda and new data2were presented. The rate expression (2)
rate = k2(CN-)(S208-a)
was founds and confirmed.2 Ishikawa reported a linear dependence between k2 and the ionic strength, p, a t moderately high salt concentrations. In view of the importance of the reaction of thiosulfate and cyanide to bio~hemistry,~-’~ analytical chemistry,10-12 and the theoretical chemistry of displacement r eactions at the sulfur atom, 1,18--16 new data on the forward and reverse reaction are presented. ka CN-
+ SSOa-2
SCN-
+ SO&-’
(3)
k- 2 Experimental All salts were Merck or Mallinckrodt analytical rea ent grade. Stable salts were dried at 120’ while thiosubate solutions were standardized against potassium iodate and cyanide solutions against silver nitrate. All solutions were prepared at 25.0” using conductivity water. Experiments were performed in deoxygenated solutions under pure nitrogen. Special care was taken to avoid contamination by copper( 11) and saliva, both excellent catalysts. Individual sealed tubes and closed flasks with aliquot-withdrawing ports were used. Aliquots were withdrawn and the reaction was quenched by cooling. The optical densit2 then was Alternameasured at 270 or 260 mp2Jaa t 25.00 f 0.01 tively aliquots were quenched in acidified iodine solution in a
.
(1) Part 11. R. E. Davis, J. Phys. Chem., 62, 1599 (1968). (2) P. D. Bartlett and R. E. Davis, J. A m . Ckem. Soc., 80, 2513 (1958). (3) F. Ishikawa, T.Murooka, and H. Hagisawa, Sed. Reports Tohoku Universdty I , 21, 611 (1932). (4) Thiosulfate has been used as an antidote for cyanide poisoning. (5) B. Mukerji, Indian Med. Gaz., 73, 353 (1937). (6) R. G.Smith, B. Mukerji. and J. H. Seabury, J . Pharmacot., 88, 351 (1940). (7) B. H.Sorbo, Acta Chem. Scand., 7 , 32 (1953). (8) B. H.Sorbo, {bid., 7, 1192 (1953). (9) C. A. McChesney, Nature. 181, 347 (1958). (10) B. H.Sorbo, Biochim. Biophus. Acta, 28,412 (1957). (11) A. Gutman. 2.anal. Chem., 47, 294 (1908). (12) 0.A. Nietzel and M. A. Desess, Anal. Ckem., 87, 1839 (1955). (13) 0.Foss, “Ionic Scission of the Sulfur-Sulfur*Bond,” in “Organic Sulfur Compounds,” Vol. I, N. Kharasoh, ed.. Pergamon Press, New York, N. Y., 1961,pp. 83-96 and references cited. (14) W. A. Pryor, “Mechanisms of Sulfur Reactiohs,” McGrawHill BookrCo., Inc., New York, N. Y., 1962. (16) R. E. Davis, “A Critique of Some Reactions of Elemental Sulfur,” in “Organic Sulfur Compounds,” Vol. 11. N. Kharasch, ed., Pergarnon Press, New York, N. Y., 1962. (16) D. P. Ames and J. E. Willard, J. Ana. Chem. Soc., 76, 3267 (1963).
good hood (caution: HCN) and then titrated with standard thiosulfate. Both methods gave identical results within 2%.
Results and Discussion The rate expression (2) adequately describes the reaction when the cyanide ion concentration has been varied from 2 X to 2.5 X 10-l M and the thiosulfate concentration varied from 2 X to 1.2 X 10-1 M (Table I). The value of the observed second rate constant is a function3of the pH. The data indicate that cyanide ion is many thousands of times more effective than hydrogen cyanide. The second-order rate constant, kz, is a function of the ionic strength. Salts such as potassium chloride and potassium thiocyanide increase the rate. The data are in accordance with the predictions of the Bronsted-Christiansen-Scatchard TABLE I T,OC.
NazSzO:, KCN, M M
Salt
1.1,
M
kl, M - l 8ec.-I
1.20 x 0.002 0.002 1.71 x .005 .005 2.21 x .050 .050 3.96 x 3.90 x .120 .006 .070 .007 3.92 x 4.50 X .251 .12 .01 1.92 x .002 .002 KC1 .002 .002 KCl .02 2.18 X .25 4.30 X .002 ,002 KC1 .002 .002 KSCPJ .01 1.94 X .15 3.79 x .002 .002 HSCN .20 4.34 x .002 .002 KSCN 0 1.23 x 69.84 0 0 0.009 1.81 x 0.003 0.002 .20 4.01 x .OS2 .053 .09 2.94 x .070 .007 .250 .123 .63 6.98 X .8S 7.68 X .253 .125 KaC1 0 3.61 X 90.00 0 0 0.008 5.12 X 0.002 0.002 .021 6.32 x .006 .005 .is 8.90 x .140 ,005 .09 8.60 x ,007 .026 Extrapolated to ,u = 0.
25.00
0
0
0 0.008 .02 .20 .14 .09 .63
10-6” 10-6
10-6 10-6 10-4a 10-4 10-4 10-4
10-4 10-4 10-4
(BCS) equation. A plot of log k2 us. p’’~ is linear with a slope of 1.9 below pl’~ = 0.15. Above p v 8 0.15 the slope decreases. Ishikawa’s data, obtained a t p ‘ / ~S 0.70, show a more linear dependence of log k2 with p. Extrapolation to infinite dilution gives ka = 1.20 f 0.06 X M-l see.-’ at 25”, 1.23 f 0.03 X lo-* at 69.84”, and 3.61 f 0.09 X 10-4 at 90.00’. From these data AH” = 12.4 i 0.4 kcal./mole and AS* = -39 rt 3 cal./mole deg. at 25”. The sign and magnitude of AS” agrees with the positive slope of 1.9 for the reaction of two anions.l7 A mechanism postulated by Foss‘g and consistent (17) Simple electrostatic theory>*predicts t h a t
while the slope of the BCS equation is equal t o 1.018Z~Z~ at 25’. (18) A. A. Frost and R. G. Pearson, “Kinetics and Mechanism,” 2nd ed., John Wiley and Sons, Ino., New York, N. Y., 1961,pp. 136. 150. (19) 0.Foas, Acta Chem. Scand., 1, 307 (1947).
May, 1962
COMMUNICATIONS TO THE EDITOR
with the data is a direct displacement of the cyanide ion on the thio sulfur. Even though this sulfur atom bears negative charge, a negatively charged thiophile can abstract it in a reaction of low activation energy, We are biased by electrostatic and symmetry considerations and suggest a linear CSS system in the activated complex. The position of the equilibrium of reaction 3 lies far to the right. Sorbo2O using the enzyme rhodanase could not detect more than lod6 M cyanide remaining after reaction of 0.1 M cyanide with 0.1 M thiosulfate. We have heated 0.1 M thiocyanate with 0.1 M sulfite a t 90” for ten weeks. No cyanide was detected using the cupric acetate benzidine reagent.2l The limit of detection of cyanide is about 9 X 10-7 M . Thus the equilibrium constant of reaction 3 is a t least 10+1° or greater. Assuming K = the upper limit of Jc-2 is of the order of 10-l6 M-I see.-’ a t 25’. This rate constant corresponds to a first halftime of about one billion years at 0.01 .M concentrations. F 0 s s ~ ~ ~has * 2 suggested that thio-containing thiophiles can be ranked in order of their thiophilicity on the basis of their oxidation potentials for 2XS- -+ XSSX -l-2e-. The necessary potentials are given in Table 11. The values for sulfite and cyanide ion have been calculated from the free (20) B. H. Sorbo, Acto Chem. Scand, 7 , 1132 (1953). (21) F. Feigl, “Spot Tests,” Vol. I, 4th ed., Elsevier Press, Houston,
Texas, 1954, pp. 258-260, 267. (22) 0. Boss, Acta Chem. Scand., i, 307 (1947).
957
energies.28 The usefulness of FOAA’R treatment can be generalized to include other ions such as cyanide. The data of Table I1 are of interest in several respects, (1) An ion of higher EO value will displace an ion of lower Eo. Thus cyanide displaces sulfite. (2) The EO values of sulfite and thiosulfate are nearly identical but subject to differential changes as the pH is varied. Thus thiosulfate will displace bisulfite (as in the decomposition of thiosulfate in acidZ4) and sulfite will displace thiosulfate (reaction of sulfite with sulfur). TABLEI1 OXIDATIONPOTENTIALS IN AQVEOUS SOLUTION X-
EO
Ref.
SCN -0.77 b SzOa-2 - .os c SOa-2 ( - .03) d CN .15 a Value reported by N. Bjerrum and A . Kirschner, “Die Rhodanide des Goldes und das freie Rhodan,” Copenhagen, 1918. * Reference 23, p. 75. Calculated from AFO’s given in ref. 23. AF0s20aTZ may be too large, ref. 23, p. 77. Calculated from AFO’s given in ref. 23, p. 129. (1
+
Acknowledgment.-This work on the theory of displacement reactions at the sulfur atom was supported by a grant from the Walter Reed Army Institute of Research. (23) W. M. Latimer, “The Oxidation Potentials of the Elementa and their Potentials in Aqueous Solutions,” 2nd ed., Prentice-Hall, Inc., New York, N. Y., 1952, pp. 72, 129. (24) R. E. Davis, J . Am. Chem. SOC.,80, 3565 (1958).
COMMIJNICATIONS TO THE EDITOR THE IONIZATION POTENTIALS OF CYCLOPIiOPYL RADICAL AND CYCLOPROPYL CYANIDE1
Sir: Only recently have determinations of the ionization potentials of cycloalkyl radicals been made.2 Pottie, Harrison, and Lossing2 made a direct determination of the ionization potential of cyclopropyl radicals formed in small yields by the thermal decomposition of cyclopropyl nitrite and found I(cyclopropy1) = 8.05 f 0.1 e.v. From a study which is briefly described below, we have determined I(cyclopropy1) = 7.8 f 0.4 e.v. by an indirect method. Cyclopropyl cyanide (Aldrich Chemical Company) was admitted in the vapor phase to the ion source of the time-of-flight mass spectrometer previously describeda and appearance potentials were determined for the various ions using both the extrapolated voltages difference method4 and the (1) This work was supported in part by the U. 6. Atomic Energy Commission under Contrac t No. AT(ll-1)-751 with Kansas State University. (2) R. F. Pottie. A. G. Harrison, and F. P. Lossing, J . Am. Chem. Soc.. 83, 3204 (1961). (3) E. J. Gallegos and R. W. Kiser, ibid.. 83, 773 (1961). (4) J. W. Warren, Nature, 165, 811 (1950).
energy compensation technique.6 For the determination of I(cyclopropyl), we are concerned with the ions of m / e = 41 (70 e.v. relative abundance = 100%) and m/e = 26 (70 e.v. relative abundance = 14%), since their formation involves complementary reactions; Le. CsH6CN
+C8Ha+ + CN + e-
(1)
CaHsCN
+CN+ + CaHS + e-
(2)
and
We found the appearance potential of m/e = 41 to be 12.70 f 0.15 e.v. and the appearance potential of m/e = 26 to be 19.5 f 0.4 e.v. These deviations were derived from the reproducibility of the measurements, and do not necessarily indicate the absolute limits of error. From the above results, the heat for the reaction CN+
+ CsHs +CN + CaHbf
(3)
is -6.8 e.v. However, the ionization potential of the CN radical has been determined to be 14.55 e.v.6--8 Therefore, we calculate the ionization poten( 5 ) R. W. Kiser and E. J. Gallegos. J . Phys. Chem., 66, 947 (1962).
(6) V. H. Dibeler, R. M. Reese, and J. L. Franklin, J . A m . Chem.
affc.,83, 1813 (1961). (7) D. P . Stevenson, J . Chem. Phus., 18,1347 (1950). (8) J. T. Herron and V. H. Dibeler, J . A m . Chrm. Soc., 82, 1555 (1960).
