NOTES
Nov. 20, 1953
5735
308 mp as erroneously printed in the first paper of this series). Analyses.-Aliquot portions of a stock solution of fluoroplatinic acid were titrated with 0.0219 N sodium hydroxide. The course of the reaction was followed with a Model G Beckman pH meter. The titation curve was typical of the reaction between a strong base and a strong acid. Aliquot portions of the same stock solution were analyzed for platinum content by reduction of the acid with hydrazine. The results confirmed the existence of the dibasic fluoroplatinic acid.
We also tried to obtain evidence for a metastable reaction intermediate by looking for a mass-law effect.6 The pertinent data in 75.00 wt. % ethanolwater a t 50' are shown in Table 11. The Table lists the rate constants for solvolysis in the presence of varying concentrations of salt for sodium bromide, a common ion salt,6 and for sodium perchlorate and sodium p-toluenesulfonate, two unreactive "neutral" salts. The rate constants are Acknowledgment.-Part of this investigation was generally based on at least two rate runs, and their performed under the terms of a contract with the probable errors are about 1.6%. If, to a first approximation, the neutral salt effects are treated AEC, Washington, D. C. as colligative functions of the ionic strength, the DEPARTMENT OF CHEMISTRY rate constants at a given salt concentration should THEGEORGEWASHINGTON UNIVERSITY WASHINGTON, D. C. be equal for the two neutral salts, and possible decreases below this value due to sodium bromide measure the magnitude of the mass-law Some Solvent and Salt Effects in the Solvolysis of should effect. As is shown in the table, the rate constants +Butyl Bromide' a t a given ionic strength are indeed approximately BY W. C. COBURN,Jr., ERNEST GRUNWALD AND HENRY equal for the neutral salts, but are greater rather P. MARSHALL than less in the presence of sodium bromide. JULY 10, 1953 RECEIVED Thus there is no evidence for a mass-law effect. The solvolysis of s-alkyl halides proceeds by a TABLE I1 mechanism which cannot be classified as either EFFECTOF ADDEDSALTSON THE SOLVOLYSIS RATE OF ssN1 or &.X2v3 On the one hand, there is stereo- BUTYLBROMIDEIN AQUEOUSETHANOL,75.00 WEIGHT% chemical evidence of partial racemization in the ETHANOL," 50.00 f 0.02" solvolysis of 2-bromooctane4 and probably of s108 k Salt M (sec.-l)b Salt M S (b-: butyl p-toluenesulfonate,6 suggesting the existence None .. 1.06 NaC104 0.18 1.09 of a metastable solvolysis intermediate. On the 0.20 1.20 .45 1.0 NaBr other hand, there is kinetic evidence for isopropyl .50 1.17 .80 1.09 bromide which indicates the nucleophilic participa.77 1.19 NaOTs .21 1.08 tion of solvent in systems such as ethanol-water.2 .43 1.04 We now wish to report some rate studies on the solvolysis of s-butyl bromide, a representative a 79.22 vol. %. * Probable errors in k : 0.45 M NaC104, member of this interesting group of reagents. 4%; all other values, 1.6%. First-order rate constants have been measured in a Within the limits of present knowledge, this number of solvents, including acetic acid, ethanol conclusion cannot be reversed if the salt effects and ethanol-water mixtures, and are equal, within are treated as specific rather than colligative. a few per cent., to the corresponding rate constants In aqueous solution salt effects on non-electrolytes for isopropyl bromide. The results are shown in often parallel either the salt polarizability or the Table I. Without repeating the analysis of rate electrostriction of the solvent due to the ele~trolyte.~ constants which was made previously for isopropyl The relevant values of the apparent molar refracbromide,2 we may therefore conclude that there is tions, R, and of ( - V,) are summarized in Table nucleophilic participation of solvent in the solvolysis 111. The values are apparently not very sensitive of s-butyl bromide in systems such as ethanol- to change in solvent from water to 75.00% ethanol, water. as is illustrated for sodium p-toluenesulfonate. TABLE I It is seen that, on the basis of the molar refractions, COMPARISON OF SOLVOLYSIS RATE CONSTANTS FOR Isothe neutral salt effects ought to be nearly equal for PROPYL AND S-BUTYL BROMIDE sodium bromide and sodium perchlorate but ought
r:
108 k (sec.-l) for s-Butyl
Solvent
Temp., OC.
Isopropyl bromide
bromide
Abs. EtOH" 50.0 0.112 0.098 80 Vol. % ' EtOH" 50.0 1.18 1.06 60 Vol. % EtOHb 70.0 74.1 80.0 Glacial HOAc 100.0 0.89 0.96 Data for isopropyl bromide in abs. and SO% EtOH were taken from ref. 2. * M. L. Dhar, E. D. Hughes and C. K. Ingold, J . Chem. SOC.,2058 (1948).
TABLE 111
cv." -
v,)a R 12.78d, 12.8' HOH -10.5b HOH - 7.5b 13.32d 43.4" HOH - IO0 75.00% Ethanol -7' 43.2' = partial molar volume of salt in very dilute solution. Vs = molar volume of (metastable) pure liquid salt. Taken from W. F. McDevit and F. A. Long, THISJOURNAL, 74, 1773 (1952). "Own measurements at 25.44". For VB use molar volume of crystalline solid salt. (See Taken B. Lunden, Z . physik. Chenz., 192, 345 (1943).) from H. Fajans and G. Joos, 2. Physik, 23, 1 (1924). Salt
Solvent
NaBr NaC104 NaOTs
vf
(1) Supported in part by the Research Corporation and by the Office of Ordnance Research, U. S. Army. (2) S. Winstein, E. Grunwald and H. W. Jones, THISJOURNAL, 78, 2700 (1951). (3) C. K. Ingold, "Structure and Mechanism in Organic Chemistry," (6) (a) L. C. Bateman, M. 0. Church, E. D. Hughes, C. K. Ingold Cornell University Press, Ithaca, N. Y.,1953, Chapter VII. (4) E. D. Hughes, C. K.Ingold end 5. Masterman, J . Clem. SOC., and N. A. Taher, ibid., 979 (1940); (b) 0. T. Benfey, E. D. Hughcaand l l g 6 (1987). C. K.Ingold, ibid., 2488 (1962). (8) J. Kenran, H.Phillip8 and V, P,Plttman, add,, 1078 (1988), (7) F,A. Long and W,F, MoDevlt, Ckrm, Row., 81, 119 (19IDh
NOTES
5736
Vol. 75
to differsharply for sodium p-toluenesulfonate. On ments, initial concentrations of the alkyl halide were 0.2 0.1 M , and of the sodium acetate were 0.02 and 0.01 M . the basis of the electrostriction of the solvent, all and Over these ranges of concentration the kinetics was first three neutral salt effects ought to be nearly the order and there was no evidence of a second-order term due same. Neither of these hypotheses is consistent to direct reaction of alkvl halide with acetate ion even though the sodium acetatk concentration varied in a typical with the observed effects. from 0.02 to 0.001 M. The present failure to observe a mass-law effect runMeasurements of R for Salt~.’~-Refractive indices were does not rule out the presence of a metastable re- measured with a Bausch and Lomb immersion refractomeaction intermediate in the solvolysis of s-butyl ter; densities were measured with a 50-ml. pycnometer. bromide since the effect is a sufficient but not a The R values are accurate to about *2%. necessary phenomenon. Our results are consistent (15) N. Bauer and K. Fajans, “Physical Methods of Organic Chemwith analogous observations on the solvolysis of istry,” A. Weissberger, editor, Interscience Publishers, Inc., New t-butyl bromidea.8 and chloridegwhere the mecha- York, N. Y. 1949, Chapter 20. nism is s N 1 . Our results are not consistent CHEMISTRY DEPARTMENT with reports of small rate depressions (4-12%) FLORIDA STATEUNIVERSITY due to added halide salts in the solvolysis of iso- TALLAHASSEE, FLORIDA propyl bromide,l o ethyl p-toluenesulfonate” and benzyl chloride12in partly aqueous media. While Polarographic Study of Various Diphenyl Disulfides these rate depressions suggest that there is a massBY EUGENEL. COLICHMAN’ AND DANIELL. LOVE^ law effect, it must be admitted that, within present knowledge, they can also be accommodated as RECEIVED MAY22, 1953 neutral salt effects. The biologically important cystine-cysteine sysExperimental Part tem has been investigated by both potentiometric Materials.-s-Butyl bromide and isopropyl bromide, and polarographic methods. Kolthoff and Linfrom Eastman Kodak Co., were dried over potassium carbonate and fractionated through a 30-plate all-glass column gane3 have summarized and discussed these inbefore use. Middle fractions with the following properties vestigations. It is seen that the disulfide linkage were used: s-butyl bromide, b.p. 90.6’ (759 mm.), n z 1 . * ~ is usually reduced irreversibly to mercaptans at the 1.4353; isopropyl bromide, b.p. 58.9’ (754 mm.). dropping-mercury electrode. Additional polaroSodium bromide was Baker and Adamson C.P. material graphic results on disulfides have been reviewed by and was dried at 110’ before use. Sodium perchlorate was The present investigation concerns prepared halide-free by careful neutralization of Mallinc- Waw~onek.~ krodt 60% perchloric acid with J. T. Baker C.P. sodium hy- the polarography of various diphenyl disulfides.
.
droxide sticks to the phenolphthalein end-point Concentration of the solution gave crystalline material which was washed with a little alcohol and dried to constant weight over magnesium perchlorate in vacuo. The dry salt contained less than 0.6% water by Karl Fischer titration for water. Sodium p-toluenesulfonate was prepared by careful neutralization of toluenesulfonic acid monohydrate with C.P. sodium hydroxide. It was purified several times by dissolving in water and reprecipitating with ethanol. P h y i cal properties of the vacuum-dried salt: d” 1.46; V.” 122.3 (water, 25.44’); 126.4 (75.00 wt. %ethanol, 25.44’). The salt contained less than 0.1% water. The solvents redistilled water, absolute ethanol (
