2358
NOTES
reaction of CH3 plus CH-CH-CHt radicals. The methyl radicals are formed in reaction 8 and are the CH3
+ CHFCH-CH~
+Cas-1
(17)
precursors of ethane, while allyl radicals results from H-atom abstraction from propene, reaction 5.2 The relatively large yield of isobutane provides evidence that scavenging of H atoms by propene to form isopropyl radicals followed by their association with methyl radicals is an important reaction. The preferential addition of H atoms to the terminal carbon is demonstrated by the relative yield of isobutane to nbutane, this ratio being 10 : 1.
Conductance Studies of Ammonium and Phosphonitrilium Salts in Acetonitrile at 25' by Ismail Y. Ahmed and C. D. Schmulbach2 Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania (Received November d8, 1966)
The nature of solute species in acetonitrile is of increasing interest because of expanded use of acetonitrile as a reaction solvent.* Conductance measurements were made on a series of typical 1 : 1 electrolyte ammonium salts as well as phosphonitrilium salts of type a where X is C1-4 and SbCle-.5 The principal
well with previously reported values of 0.7-1.5 X 1O-l and 1-2 X 1O-l ohm-' c111-l.~ Values of 2-5 X ohm-' cm-' have recently been reported."J PurGcation of Salts. Compound I, [(C6H6)4P2N3H41C1, was prepared by a method previously described.l 1 It was recrystallized from acetonitrile and dried for 2 hr under vacuum at 110", mp 244-246 " . Compound 11, [(CsH&P2NaHd]SbC16, was provided by Dr. C. Derderian. This compound was recrystallized from carbon tetrachloride and petroleum ether and dried at 25" in vacuo for several hours. The pale yellow crystals melted sharply at 143-144". A suspension of 6.30 g (0.038 mole) of tetraethylammonium chloride (Eastman White Label) in 200 ml of anhydrous dichloroethane was cooled in a Dry Iceacetone bath. An excess of boron trichloride (Matheson) was added from a syringe. The solution was allowed to warm to room temperature to give a homogeneous solution. White needlelike crystals of tetraethylammonium tetrachloroborate precipitated upon cooling and were filtered from the chilled solution. The crystals were washed several times with chilled dichloroethane and dried under vacuum overnight at room temperature. The solid melted with decomposition to a brown liquid at 165-170" in a sealed ampoule. AnaE. Calcd for CsHZoNBC14:C, 33.95; H, 7.07; N, 4.49; C1, 50.20. Found: C, 33.73; H, 7.09; N, 4.89; C1, 49.80. Elemental analyses were performed by Galbraith Laboratories, Knoxville, Tenn. Tetraethylammonium chloride and tetraethylammonium perchlorate were Eastman White Label and were used without further purification.
Results The results of conductance measurements are summarized in Table I. The limiting molar conductance was calculated by means of the Onsager equation. A ~~~~~~
a
objectives were to establish the extent of association of ions (ion-pair formation) in acetonitrile and to elucidate the nature of the solute phosphonitrilium salts.
Experimental Section Purification of Acetonitrile and Conductance Measurements. Acetonitrile (Fisher Certified reagent) was purified by the method described by Coetzee.6 The distillation apparatus together with the conductance cells and bridge are described e1sewhere.l The specific conductance of the purified solvent was in the range of 0.8-1.7 X lo-' ohm-' cm-' at 25". This compares The Journal of Physical Chemistry
~
(1) Supported in part by the National Science Foundation. (2) To whom all correspondence should be addressed at the Department of Chemistry, Southern Illinois University, Carbondale, Ill. (3) See R. A. Walton, Quart. Rev. (London), 19, 126 (1965). (4) I. I. Bezman and J. H. Smalley, Chem. Ind. (London), 839 (1960). (5) C. Derderian, Doctoral Thesis, The Pennsylvania State University, 1966. (6) J. F. Coetzee, G. P. Cunningham, D. C. McGuire, and G. R. Padmanaphan, A d . Chem., 34, 1139 (1962). (7) 1. Ahmed, Master's Thesis, The Pennsylvania State University, 1965. (8) I. M.Kolthoff, S. Bruckenstein, and M. K. Chantooni, J . Am. Chem. SOC.,83, 3927 (1961). (9) I. M.Kolthoff, M. K , Chantooni, and J. W. Wallis, ibid., 85, 426 (1963). (10) J. F. Coetzee and G. P. Cunningham, ibid., 87, 2529 (1965). (11) C. D. Schmulbach and C. Derderian, J . Inorg. Nucl. Chem., 25, 1395 (1963).
2359
NOTES
Table I : Conductance of Typical 1 :1 Electrolytes in Acetonitrile Y E t t N C l c x 104, mole/I.
1 .035 2.030 2.988 4.800 5.657 7.280 10.22 12.18 15.08 19.80 21.74
A
170.0 166.0 163.9 163.5 161.0 160.7 158.3 157.4 149.7 148.0 147.3
-EttNCIOrc x 104, mole/l.
1.687 3.290 4.830 7.688 10.29 13.84 18.87 23.06 29.65 34.75 38.44
[(CIHS)~PZN:H~]C~104, A mole/l.
-EttNBCI--
7
A
185.5 173.8 170.8 168.7 169.1 155.8 152.7 150.8 148.6 147.5 143.7
mole/l.
1.629 3.330 4.658 6.070 11.13 13.45 18.21 22.26 25.68 29.68 33.38
least-squares treatment of the data was applied. For compound I, the Fuoss method1* for associated electrolytes was used to obtain the value for A,,, 148.9 ohm-' cm2 mole-', and the dissociation constant for the ion pair, KD,3.2 X mole 1.-l. The other four electrolytes do not show any association in the range of concentration studied. Acetonitrile is, therefore, a leveling solvent for ionization of four of the five compounds studied. Additional evidence for ion-pair formation for compound I was obtained from molecular weight measurements. The apparent molecular weight of compound I is 305 f 8 mole-' in the concentration range 5.5 X to 1.2 X M.13 The error is expressed as a standard deviation. The formula weight for compound I is 452. The existence of ion pairs in equilibrium with univalent ions would account for this observation. For comparison, the apparent molecular weight of compound I is 444 when measured by vapor pressure osmometry in chloroform, a solvent of low dielectric constant (e30 4.8). The solute concentration was 1.02 X 10-3 and 1.77 X lo-* M . In this case the solute species is predominantly ion pairs or molecules. Furthermore, compound I is a nonconductor in chloroform. The limiting molar conductances for the remaining salts are Et4NC1 (176.6 f 0.5)) Et4NC104 (188.9 0.9)) Et4NBC14 (180.2 f 0.4), and (CsHa)4PzN3H4SbCla (162.5 f 0.3 ohm-' om2 mole-'). In a recent study, Coetzee and Cunningham adopted the reference electrolyte tetraisoamylammonium tetraisoamylboride to evaluate ionic conductances in acetonitrile.'O Transference studies of the reference electrolyte in nitromethane gave equal values for the transference number of the cation and anion and provide experimental support for their claim that the ionic conductance may be equally divided between the reference cation and anion in other solvents such as acetonitrile.
*
A
2.170 2.172 2.654 2.760 3.480 4.550 4.956 5.820 7.440 9.740 12.94
176.2 172.9 170.0 172.6 171.5 170.9 167.9 164.9 163.5 162.5 162.0
[ (CsHs)rPzNtH41SbCI~
7
cx
c x lo',
104.6 104.8 100.5 97.8 91.7 87.3 82.7 80.4 76.3 68.8 58.1
cx
104, mole/l.
A
0.496 1.364 2.601 4,033 5.561 6.885 8.033 9.037 9.923 10.89 11.89
160.1 151.4 147.9 145.4 140.3 138.9 137.2 137.7 134.5 134.2 132.4
A value of 85.05 ohm-' cm2 mole-' was given for the limiting ionic conductance of tetraethylammonium ion in acetonitrile."J We have accepted this as the most reliable value for Xo(Et4N+) and have computed limiting ionic conductances by the application of Kohrausch's law of independent migration of ions to our conductance data. The results are summarized in Table 11.
Table 11: Single Ion Conductances, XO, in Acetonitrile at 25"
Cation
XO,
XQ,
ohm-1 cmr mole-'
ohm -1 cm' mole-1
EtrN + (CsHshP2NnH4'
85.05" 57.3
Anion
c1Cl04BCl4SbCls-
91.6 103.8 95.2 105.2
' Reported in ref 10. This value referred to b(tetraisosmy1ammonium ion) = 57.24.
The limiting ionic conductances of the c104- and BCI4- anions are larger than the value for the chloride ion. Such a relationship is expected because the smaller, more polarizable chloride ion would interact more strongly with the polar solvent than the larger ions. The small ionic conductance of the phosphonitrilium cation, ( C ~ H S ) ~ P ~ N ~ suggests H ~ + , substantial solutesolvent interaction. ~~
~
(12) R. M. Fuoss and F. Accascina, "Electrolytic Conductance," Interscience Publishers, Inc., New York, N. Y., 1959, pp 225-230; R. M. Fuoss, J . Am. Chem. Soc., 57, 488 (1935). (13) Molecular weights were determined on a Mechrolab vapor pressure osmometer, Model 301A. The data in chloroform were obtained by Dr. F. G . Sherif.
Volume 71, Number 7 June 1967
NOTES
2360
Acknowledgment. The generous support in the form of a fellowship to Ismail Y. Ahmed from the National Science Foundation administered by the American Friends of the Middle East is gratefully acknowledged.
Results and Discussion The vapor was found to contain only the species PbCI2, PbBr2, and PbClBr (identified as positively charged ions) ; hence the discrepancy in vapor pressure measurements is explicable not by the formation of PbCla. PbBrz but by the equilibrium PbC12
The Formation of Lead(I1) Chloride Bromide
+ PbBr2 = 2PbClBr
Table I gives a summary of the mass spectral data (listed in order of measurement) using 20-ev electron energy and photomultiplier amplification. The formation constant of PbClBr is given in terms of the partial pressures by
(PbClBr) in the Vapor Phase by H. Bloom and J. W. Hastie Chemistry Department, The University of T a s m n i a , Hobart, Australia (Received October $8,1966)
PPbClBr2
Kp =
PPbClgPbBrn
From the measurement, by transpiration, of partial vapor pressures of PbCl2 and PbBr2 above molten mixtures of the two salts at 770°, Greiner and Jellinek' found that the apparent activities of PbBr2 were slightly lower than Raoult's law values over the whole composition range, but that those of PbCl2 were higher than the Raoult's law values from 0 to 0.7 mole fraction of PbCI2. These observations were contrary to the Gibbs-Duhem relation and also to the results of Salstrom and Hildebrand,2 whose activity values of PbBr2 from measurement of the emf's of formation cells were considerably lower than those reported by Greiner and Jellinek. To account for the discrepancies, Greiner and Jellinek suggested a vapor phase equilibrium PbC12
Its heat of formation (per 2 moles) is
AH =
Experimental Section Using a quadrupole mass spectrometer, details of which will be published separately, the mass spectra and ionization characteristics of vapors above an equimolar PbC12 PbBr2 mixture were investigated between 421 and 495'. The melt was contained in a silica crucible within a silver Knudsen cell and heated in a furnace in the chamber of the mass spectrometer. A shutter was used to eliminate background ions. Temperature was measured by a Pt vs. Pt 13% Rh thermocouple and a Leeds and Northrup millivolt potentiometer.
+
+
The Journal of Physical Chemistry
- AH(v,PbCld
where AH[v,PbClBr), etc., are the heats of vaporization of PbCIBr, etc., and are obtainable from the slopes of the graphs of log P vs. 1/T for each species or, in the present investigation, from the graphs of log I+T us. 1/T. Table I : Ion Current us. Temperature Data Temp,
+ PbBr2 = PbCI2.PbBr2
This equilibrium would cause the apparent vapor pressures of both components and therefore the derived activities to be higher than the real values, but only in the case of PbC12 would apparent activities exceed the Raoult's law values. The present investigation relates to a mass spectrometric study to identify the vapor phase species.
-
~ A H ( v , P ~ c ~ B ~AH(v,PbBrr) )
a
OC
PbCl+
495 486 480 450 454 461 475 466 441 421
200 133 143 37 59 90.5 129 72.2 34.3 19.3
I +T for the vapor speoieaa PbBr + PbClBr +
720 460 450 150 210 310 500 300
.-. ... *
55.6 42.5 39.6 14 19 29.4 39.8 26.3 13.1 5.1
PbBrr+
175 118 131 37 55 83.4 121 71.2 37.2 26.9
Ion current in arbitrary units; T in "K.
Ion currents for PbCl2f could not be determined accurately owing to interference by an intense PbBr+ ion close in mass number to PbCl2; hence AH(,,PbClr) was assumed to be equal to AH(v,PbBrt) and equal to the heats of vaporization for the two pure salts, which have been found by direct measurement3 to be the same within experimental error (32 f 2 kcal mole-'). These assumptions are reasonable in view of the lack (1) B. Greiner and K. Jellinek, 2.Physik. Chem. (Leipaig), 165, 97 (1933). (2) E. J. Salstrom and J. H. Hildebrand, J. Am. Chem. Soc., 5 2 , 4641 (1930). . . (3) H. Bloom and J. W. Hastie, unpublished results.
