CONDUCTANCE OF THE TRIALKYLSULFONIUM IODIDES
1037
The Conductance of the Trialkylsulfonium Iodides in Water, Methanol, and Acetonitrile at 10 and 25" by D. Fennel1 Evans and T. L. Broadwater Department of Chemistry, Case Western Reserzle University, Cleveland, Ohio &I06
(ReceiGed September 86, 1967)
Precise conductance measurements were carried out on Me&, Et3SI, and Pr3SI in water, methanol, and acetonitrile at 10 and 25" and on BuaSI in water and methanol at 25". In the nonaqueous solvents, the trialkylsulfonium iodides are found to be considerably more associated than the corresponding tetraalkylammonium iodides, a result consistent with the shorter cation-anion contact distance of the sulfonium iodides. I n aqueous solution, there is no evidence for any greater ionic association of the sulfonium over the ammonium salts, indicating that the large deviations from ideality that have been observed for the R& halides are not the result of coulombic stabilized ion pairs. The change in the limiting conductance viscosity product in water with temperature is interpreted in terms of water structural effects.
Introduction Aqueous solutions of the tetraalkylammonium salts have been carefully investigated by a number of physical These measurements have, in general, been interpreted in terms of the enforcement of water structure around the hydrocarbon side chains of these ions. This type of interaction has readily been detected in those measurements which have limiting ionic properties such as conductance5 and viscosity.g However, there is considerable disagreement concerning the interpretation of the concentration dependence of aqueous solutions of these electrolytes. The large deviations from limiting-law behavior have been discussed in terms of cation-anion pairing,14 cation-cation ~ a i r i n g ,micelle ~ formation,2 and salting-in eff ects.I5 I n order to investigate to what extent coulombic forces are responsible for these large departures from ideality, we have carried out measurements on the trialkylsulfonium iodides (R3SI) in water and in nonaqueous solvents. The trialkylsulfonium salts are pyramidal in configuration'c with the sulfur atom located a t one of the apices. Consequently, the positively charged atom resides at the surface of the molecule rather than being buried in the center of a tetrahedron as in the case of the tetraalkylammonium compounds. Therefore, if coulombic stabilized cation-anion pairing is an important factor in these aqueous solutions, it should be accentuated in the trialkylsulfonium series.
Experimental Section All of the trialkylsulfonium iodides were prepared by treating the alkyl iodide with the corresponding dialkyl sulfide in methanol. The reaction of the trimethyl compound was carried out at room temperature and
was complete within 3 hr. The triethylsulfonium iodide was formed by refluxing the methanol solution overnight. Both salts were recrystallized three times from methanol by the addition of ether, dried for 12 hr in a vacuum oven at room temperature, and stored in a darkened desiccator. The tripropyl and tributyl salts were prepared by heating the reactants in sealed flasks for 1 month at 50". The unreacted starting materials were removed by extraction with ether, and the desired salts were obtained from the resulting residues by recrystallizing six times from acetone-ether mixtures. The salts were dried in a vacuum oven overnight at room temperature and stored in the dark. Upon standing, all of the salts showed some sign of (1) H. 9. Frank and W. Y. Wen, Discussions Faraday Soc., 24, 133 (1957). (2) S. Lindenbaum and G. E. Boyd, J . P h y s . Chem., 68, 911 (1964). (3) W. 3'. Wen and S. Saito, ibid., 68, 2639 (1964). (4) H. G . Hertz and hl. D. Zeidler, Ber. Bunzenges. P h y s i k . Chem., 68, 821 (1964). ( 5 ) R. L. Kay and D. F. Evans, J . P h y s . Chem., 69, 4216 (1965); 70, 2325 (1966). (6) D. F. Evans and R. L. Kay, ibid., 70, 366 (1966). (7) S. Lindenbaum, ibid., 70, 814 (1966). (8) W. Y. Wen, S. Saito, and C. Lee, ibid., 70, 1244 (1966). (9) R. L. Kay, T. Vituccio, C. Zawoyski, and D. F. Evans, ibid., 70, 2336 (1966). (10) B. E. Conway and R. E. Verrall, ibid., 70, 3952, 3961 (1966). (11) G. E. Boyd, J. W. Chase, and F. Vaslow, ibid., 71, 573 (1967). (12) R. H. Wood, H. L. Anderson, J. D. Beck, J. R . France, W. E. de Vry, and L. J. Soltzberg, ibid., 71, 2149 (1967). (13) R. Zana and E. Yeager, ibid., 71, 4241 (1967). (14) R. M. Diamond, ibid., 67, 2513 (1963). (15) J. E. Desnoyers and M. Arel, Can. J . Chem., 45, 359 (1967). (16) S. Lindenbaum, J . P h y s . Chem., 72, 212 (1968).
V o l u m e 76. Number S M a r c h 1968
D. FENNELL EVAWAND T. L. BROADWATER
1038 Table I : Equivalent Conductances 104c
104c
A
illesSI
2.877 9.882 19.951 30,581 42.342 56.526 70.752 85.174
164.65 157.14 150.06 144.45 139.42 134.51 130.37 126.74
A = 0.067 2.418 6.201 10.160 15,718 21.453 27.474 34.646
191.67 186.08 181.73 176.82 172.61 168.80 164.88
'-
5.180 11.388 18.782 26.780 35.489 48.644 63.162
100.31 97.01 94.15 91.72 89.53 86.80 84.31
A
Etas1 CHsCN loo 2.302 158.03 7.931 152.51 15.432 147.73 23.480 143.82 31.605 140.56 40.515 137.53 51.282 134.38 64.027 131.17 25O A = 0.079 2.068 184.09 5.868 179.11 11.990 173.82 18.279 169.65 25.731 165.64 33.218 162.23 41.420 159.01 51.833 155.45 62,297 152.31 MeOH loo 3.419 96.69 10.731 92.55 19.475 89.21 29.122 86.45 38.773 84.21 48.993 82.20 80.17 61.116 73.916 78.30
A
10'C
7.795 14.787 22,920 32.622 43.336 55.997 70.204
A
104c
-
Pr8SI
142.96 138.73 134.93 131.28 127.93 124.60 121.41
Me81
104c
A
Et631
104c
6
PnSI
10" 5.076 11.227 17.411 24.640 31.997 40.302 49.678
87.31 86.52 85.97 85.42 84.94 84.46 83.96
4.289 15.802 20.322 25.782 32.888 41.092 50.250
77.72 76.37 75.98 75.58 75.11 74.63 74.14
4.520 11.139 18.440 26.884 37.028 49.288 61.523
71.64 70.82 70.14 69.49 68.84 68.12 67.50
25 O
A A
= 0.076
1.972 6.723 13.902 21.439 29,263 37.261 46,325 56.457
172.82 167.17 161.65 157 32 153.64 150.45 147.33 144.25 I
=
9.806 20.490 31.082 45.578 53.022 66.643 78.464 89.946
0.088 121.59 120.27 119.26 118.12 117.59 116.74 116.08 115.48
3.593 11.376 20.825 30.701 40.689 50.754 60.938 76.798
87.46 83.38 80.09 77.48 75.34 73.51 71.90 69.76
13.725 22.493 33.373 44.952 55.094 63.984 71,944 82.818
108.14 107.12 106.11 105.17 104.44 103.87 103.40 102.77
A = 0.082 6.884 14.770 23.191 32.112 39.666 49.030 59.072
100.65 99.49 98.54 97.68 97.04 96.31 95.60
BuaSI
HzO, 25" A = 0.074
#
A = 0.086
6.486 13.549 21.803 29.724 36.318 45.402 52.312 57.344
97.23 96.07 95.04 94.20 93.57 92.78 92.23 91.85
MeOH, 25" A = 0.15 4.933 99.91 10.271 96.52 15.849 93.92 20.987 91.94 26.625 90.07 33.764 88.03
25 O
A = 0.102 5.739 11.624 18,563 26.697 33.106 40.434 48.899 57.312
122.32 118.49 115.12 112.01 109.94 107.89 105.80 103.97
A
= 0.104
6.032 12.065 18.734 27.249 33.953 42.937 48.912 60.808
116.57 112.80 109.68 106.53 104.44 102.05 100.66 98.18
A = 4.821 9.598 16.177 23.909 33.353 40.681 49.585 56.196
0.105 106.32 103.22 100.02 97.05 94.18 92.30 90.30 88.97
decomposition as evidenced by the disulfide odor that could be detected upon opening the salt container. The rate of decomposition appeared to increase with increasing cation size and caused such difficulty in the case of the tributyl compound that we obtained only enough material for two conductance runs. The salts were always recrystallized before use, and the actual conductance runs were completed within 3 hr. Conductivity water was prepared by passing distilled water through a 1.2-m column of analytical grade ion-exchange resin, Amberlite MB1. The preparation of conductance grade methanol17 and acetonitritela was the same as previously described. The electrical equipment, conductance cells, and The Journal of Physical Chemistry
general techniques were similar to those reported elsewhere17t19 except for the conductance bridge, which consisted of a Leeds and Northrup ratio box, Catalog No. 1553, and a General Radio decade resistance box, Catalog No. 1432. Briefly, the conductance runs were carried out in Kraus type conductance cellsz0 (17) R. L. Kay, C. Zawoyski, and D. F. Evans, J . Phu8. Chem.,
69, 4208 (1965).
(18) J. F. Coetaee, G. P. Cunningham, D. K. MoGuire, and G. R. Padmanabhan, Anal. Chem., 34, 1139 (1962). (19) C. G. Swain and D. F. Evans, J . Am. Chem. SOC.,88, 383 (1966). (20) H. M. Daggett, Jr., E. Bair, and C. A. Kraus, ibid., 73, 799 (1951).
CONDUCTANCE OF THE TRIALKYLSULFONIUM IODIDES which were stirred constantly with a Teflon-covered magnetic stirring bar. The cells, which had cell constants of 1.3, were calibratedz1 with reagent grade potassium chloride.20 The usual small frequency correction was applied to all resistance measurements. The salts were added to the cell in small Pyrex cups with the aid of a Hawes-Kay cup dropping device.zz The temperature of the baths was set with a calibrated platinum resistance thermometer and controlled to =t0.005".
1039 I
I
I
I
h
+
A =
A0
- LS'(C~)~''+ ECr log Cy (J - BAo)Cr - K A C ~ A(1) ~
-0.8 - O * I
-1.0
0
1
I
I
'iI 1
Results The density increments used to calculate the volume concentration at 25" are given in Table I. They were obtained by density measurements on the most concentrated solution used in the conductance measurement and were assumed to follow the relationship d = do A&, where do is the density of the pure solvent and f i is the concentration in moles per kilogram of solution. The value of A for the trialkylsulfonium iodides was assumed to be independent of temperature, an assumption that has bieen verified6 in the case of the tetralkylammonium salts. The measured equivalent conductances and corresponding concentrations in moles per liter are also given in Table I. The data were analyzed by the FuossOnsager equationz3in the form
I
I>-
0.21
,
,
I
IO
20
30
c
40
50
60
70
104
Figure 1. A plot of eq 2 for trialkylsulfonium iodides in water a t 25'.
Discussion Limiting I o n Conductances. Although enforcement of water structure about the hydrocarbon chains of the tetraalkylammonium salts is perhaps the most cited feature of their behavior in aqueous solution, this type of interaction is not observed with all of the members of the seriese6 In fact, there appears to be a critical size of hydrocarbon chain below which enforcement of water structure is not observed for these cations.2e This effect is most readily detected by studying aqueous solutions of these salts as a function of temperature since water structural effects decrease as the temperature increases. From a number of such studies the following picture has emerged. The smallest member of this series, Me4N+ ion, disorganizes water structure in its immediate vicinity in a manner similar to the larger alkali metal ions. This results, for example, in a negative temperature coefficient for the limiting ionic conductance-viscosity product, X07,5 a positive viscosity B temperature coeffi~ient,~ and a negative dAt/ dt,27 where At is a structural temperature difference between water and Me4N+solutions for the 0.97 nearinfrared band. The tetrapropylammonium ion and
where ionic association was detected, and with y = 1 and KA = 0 otherwise. Since the viscosity of these solutions has not been determined, the value of B, which corrects Eor the effect of the electrolyte on the viscosity of the solution, was set equal to zero. This correction does not alter the value of A0 nor the association constant and only changes the value of J and consequently of it, the ion size parameter. The solvent properties of dielectric constant, viscosity, and density required in the computation have been given in convenient tabular form already.z4 The conductance parameters Ao, it, and K A were obtained by a least-squares fit using a computer programz6 and are shown in Table 11. Also given are u, the standard deviation of the individual points, and Xo+, the limiting conductance of the cation. The following Xo(I-) values were used in this calculation: water, (21) J. E.Lind, Jr., J. J. Zwolenik, and R. M. Fuoss, J . Am. Chem. 25", 76.98; water, lo", 55.39; methanol, 25", 62.78; SOC.,81, 1557(1959). methanol, lo", 50.9; and acetonitrite, 25O, 102.7.5823 (22) J. L. Hawes and R. L. Kay, J. Phys. Chem., 69, 2420 (1965); G. P. Cunningham, B. J. Hales, and R. L. Kay, ibid., 71, 3925 The agreement between the least-squares fit of the (1967). Fuoss-Onsager equation and the data in water at 25" (23) R. M. Fuoss and F. Accascina, "Electrolytic Conductance," can be seen by inspection of the A' plot, Figure 1, Interscience Publishers, Inc., New York, N. Y.,1959. where A' is (24) G. P. Cunningham, D. F. Evans, and R. L. Kay, J. Phye. A'
E
A
- A0 + Sl/C - EClog c
=
JC
(2)
The solid line drawn through the experimental points is the least-squares best value of J .
Chem., 70, 3998 (1966). (25) R.L. Kay, J. Am. Chem. Soc., 82, 2099 (1960). (26) D. F. Evans, G. P. Cunningham, and R. L. Kay, J. Phy8. Chem., 70, 2974 (1966). (27) K.W.Bunzl, ibid., 71, 1358 (1967).
Volume 7.9, h'umber 3 March 1,968
D. FENNELL EVANS AND T. L. BROADWATER
1040
Table 11: Conductance Parameters at 25 and 10' for Water, Methanol, and Acetonitrile KA
d
A0 +
CHsCN MeaSI
25 10
199.23 f 0 . 0 2 171.67 f 0 . 0 4
3.6 3.0
fO.1 hO.1
35.7 f 0 . 6 31.7 f 0.6
0.02 0.04
96.53
EtaSI
25 10
190.25 f O . 0 5 163.57f0.04
3.5 3.3
f0.2 f0.l
19.8 f 0 . 8 17.9 f 0.7
0.05 0.03
87.55
Pr;SI
25 10
178.64 f 0 . 0 3 153.27f0.01
3.4 f O . l 2.76 1 0 . 0 2
18.7 f 0 . 6 13.5 f 0 . 1
0.03 0.01
75.94
Me8SI
25 10
130.32 f 0.03 106.23 f 0.04
MeOH 3.69 f O . 0 9 3 . 6 320.2
23.4 f 0 . 6 23 f 1
0.02 0.03
67.54 55 * 33
EtaSI
25 10
124.62 0.03 101.28 f 0 . 0 4
3.6 3.5
23.9 f 0 . 7 24 h 1
0.02 0.03
61.84 50.38
PraSI
25 10
113.09 1 0 . 0 2 91.97&0.03
3.52 f 0 . 0 8 3.35 f 0 . 0 8
24.2 f 0 . 6 24.2 i 0 . 7
0.01 0.02
50.31 41 07
Bu~SI
25
106.67 f 0 . 0 5
4.3
31
0.02
43.89
Me3SI
25 10
124.49 f O . O 1 88.71f0.02
Hz0 0.88 f 0 . 0 2 0.30 1 0 . 0 5
0.01 0.02
47.51 33.32
Etas1
25 10
111.51fO.01 78.95 f 0.03
0.27 f O .02 0.03 f 0 . O l
0.02 0.03
34 * 53 23.56
PraSI
25 10
102.99 f 0 . 0 0 3 72.92f0.01
0.054 f 0'.001 0.005 f 0 . O O l
0.002 0.02
26.01 17.53
BQSI
25
99.57 f 0 . 0 1
0.002
22.59
*
2.0
higher homologs behave in just the opposite manner, promoting water structure and producing a Xoq product which has a positive temperature coefficient, a negative viscosity B temperature coefficient, and a positive dAt/ dt. Tetraethylammonium ion shows no temperature dependence, indicating that structure breaking and promoting effects apparently cancel each other. A comparison of the behavior of the symmetrical R49+ions to the asymmetrical Me3RN+ ions (where R = CzHsto C14H2B) has shown that the geometry of the molecule is also an important factor.2s For example, the limiting ionic mobility of the trimethyltetradecylammonium ion (17 carbon atoms) is 9% larger than that of the Bu4N+ (16 carbon atoms). The promotion of water structure around all portions of the Bu4N+ ion results in its moving slower than it would in the absence of structural effects, a conclusion that can be verified by comparing the Xoq value for nonaqueous solvents with that for water. For methanol, ethanol, acetonitrile, and nitromethane, Xoq for Bu4N+ ion is 0.2120, which is 23% larger than the corresponding value for water at 25°.5 The asymmetrical trimethyltetradecylammonium ion appears to exhibit more complex behavior in that the three methyl groups attached to the charged nitrogen atom disorganize water structure while the fourth larger alkyl group promotes water structure.24 The polar end of The Journal of Phgeical Chemietry
f0.l f0.l
f0.4
f0.4
f 2
3.6 f 0 . 2
I
the asymmetrical series behaves like a Me4N+ ion, moving more rapidly than it would in the absence of structural effects. Thus the effect of an R4N+ ion on water structure depends upon both the size and shape of the ionic species. A comparison of the behavior of the pyramidal trialkylsulfonium ions with the tetrahedral tetraalkylammoium ions allows these generalizations to be extended. Shown in Figure 2 is a plot of Xoq us. temperature for both series of compounds. Thc quantity Rdo given by
should be greater than 1 for structure-breaking ions and less than 1 for structure-making ions. RzP is 1.02 for Me4N+ and 1.03 for hleaS+. This result is consistent with the RiIe3S+ ion being a smaller species with greater surface charge density than Me&+ ion, and hence somewhat more like a large alkali metal ion in its behavior in aqueous solution. Rzj10for cesium ion is 1.07. Like the Et4N+, the EtsS+ ion shows no temperature dependence, a result in accord with the idea that the structure-promoting and destroying properties of an ethyl group cancel each other. R2b10 is 0.968 for Pr4N+ and 0.988 for Pr3S+, indicating that the hydrocarbon-water interaction dominates in aqueous
CONDUCTANCE OF THE TRIALKYLSULFONIUM IODIDES
0.35
i
I I
---. t
Et3S+
I
020~-.-+
:- I
1 I
I
-
Figure 2. Change of the limiting Walden product for the trialkylsulfonium and tetraalkylammonium ions with temperature.
solution although to a lesser extent in the case of the pyramidal ion. That water :structure effects are responsible for the variation in R26'0 can be verified by considering this same ratio in nonaqueous solvents. The average R251° in methanol for all the R3S+ ions is almost constant, 1.01 f 0.003, in contrast to the 4y0variation observed in aqueous solution. The same type of analysis for acetonitrile solutions is not possible because of the lack of transference data for this solvent at 10". Thus the conclusions drawn concerning the behavior of the R4N+ions in aqueous solution are confirmed by the R3S+ions. Concenlration Dependence. Since the stabilization of ion pairs is mainly coulombic in origin, it is anticipated that a trialkylsulfonium iodide should be more associated than the corresponding tetraalkylammonium iodide. The pyramidal shape of the RaS+, with the charged sulfur atom at one of the apices, should allow the cation-anion centers to approach one another more closely than in the case of R4N+, where the charged nitrogen is shielded by hydrocarbon on all sides.
1041 Such behavior has been verified in nonaqueous solvents.5 In methanol all of the ammonium iodides, methyl through amyl, gave K A = 17 f 1.'' Similarly, as can be seen in Table 11, a constant value of K A is also obtained for the trialkylsulfonium but they are about 35% more associated than the corresponding ammonium salts. I n acetonitrile, K A decreases with increasing cation size, as was the case with the ammonium but the R3SI salts are 50 to 100% more associated than the corresponding R4NI salts. Thus the association behavior of the sulfonium salts is the same as that of the ammonium salts, but the degree of association is greater for the sulfonium iodides as expected, due to the shorter contact distance. In both methanol and acetonitrile the slight change jn the association constants with temperature is not considered significant. It is interesting to note that the d for all the sulfonium salts is 3.5 0.3 in exact agreement with the value 3.5 f 0.2 found for all the tetraalliylammonium halides in nonaqueous solvents. 3o I n aqueous solution, in contrast to the results in nonaqueous solvents, there is no evidence for any greater association of the sulfonium over the ammonium salts. Only one of the salts, Bu3SI, analyzed for any association in water, giving K A = 3.6 f 0.2. A K A of the same magnitude was found for Buds1 in water.6 As can be seen in Table 11, the values of d for the other sulfonium salts are all unrealistically small. Equally small values of d were obtained for the RdN+ iodides in water, Pr4n'I d = 0.03, E t N 8. = 0.5, and Me4NI d = 1.32; B set equal to zero. Comparison of the two cation series shows that they have the same concentration dependence to within 0.1%. This latter fact shows that the peculiar concentration dependence of salts of these large cation series is not the result of coulombic interaction. A similar argument excludes ion-pair stabilization by dispersion forces depending on polarizability.
*
Acknowledgment. We wish to acknowledge the technical assistance of Mr. John Nadas under the sponsorship of the Undergraduate Research Participation Program of the National Science Foundation. This work was supported in part by Contract Yo. 14-010001-1281 with the Office of Saline Water, U. S. Department of the Interior. (28) The somewhat larger value of K A obtained for Bud31 reflects a bad split in the least-squares estimation of the best values of 6 and KA. (29) D. F. Evans, C . Zawoyski, and R. L. Kay, J . Phys. Chen., 6 9 , 3878 (1965). (30) The published value for the R4N + halides is 3.5 plus 0.2 for the
viscosity correction.
Volume 76, Number 9 March 1968
