HYDROGEN CHLORIDE I N ANHYDROUS BENZONITR~LE
889
Hydrogen Chloride in Anhydrous Benzonitrile: Electrical Conductance, Time Effect, and Ion-Solvent Interactions
by George J. Jam, Iqbal Ahrnad,Ia and H. V. VenkatasettyIb Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York
(Received November 14, 1965)
The conductance of hydrogen chloride in benzonitrile for the concentration range 0.823 X to 0.13 mole/l. a t 25' has been investigated. The conductance-time effect is found to be less marked than in the acetonitrile solutions. The conductance data for the aged solutions were examined by the various extrapolation techniques for weak electrolytes. The values for Ao, K,>nd a thus obtained are, respectively, 1.5 f 0.2 ohm-' cm.-', 2.5 f 0.5 X lov4,and 1.6 A. The sharp decrease in A with concentration in the dilute solution range can be attributed to ion pair formation. Two solid substrates of composition corresponding to C & , C N .HCl and C&,ChT * 2HC1 can be isolated from the saturated solutions. The structural data and physical properties of these substrates are considered in the light of possible chemical interactions in the system.
Introduction Benzonitrile is the simplest aryl derivative of hydrogen cyanide. The relatively high dielectric constant (25.2 at 25') is understood in large part as due to the polar nature of the nitrile group which gives the force field a highly directional character (dipole moment 4.05). Like acetonitrile (D = 35.99 a t 25") the values of its viscosity and Trouton constant indicate that association in the liquid state is much less than in other organic solvents, making it a less complex solvent for the study of ion-solvent interactions. The properties of HCI as an electrolyte in acetonitrile have recently have been reported from this laboratory.2 The present communication reports an investigation of the electrical conductance of this solute in anhydrous benzonitrile, particularly with reference to the values for aged solutions. Attention was also directed to the problem of the time effect and the solid substrates, first noted for CH8CN-HCl systems. Experimental Benzonitrile. Benzonitrile (Eastman Organic Chemicals) was magnetically stirred with CaHz (20 g./l.)3 for 24 hr., decanted, and then refluxed for 18 hr. with freshly added CaHz (20 g.). It was next fractionally distilled using a vacuum-jacketed strip-silvered distil-
lation column (100 X 25 cm.) packed with 0.125-in Hastelloy stainless steel helices (Podbielniak Inc.) and a solenoid-controlled partial take-off condenser. Separate fractions were collected using a magnetically controlled policeman distributor. Unless otherwise indicated, a t all times the exits to the atmospheric pressure on the flasks and conductance cells were through hlg(ClO& guard tubes. The middle cut of this distillate (b.p. 191.0') was again refluxed (18 hr.) with fresh CaHz (20 g . ) and refractionated. The specific conductivity of the solvent thus purified was 5-9 X ohm-' cm.-' (25'). The water content ('Karl Fischer titration) was found to be less than 0.009 wt. %. Hydrogen Chloride. The hydrogen chloride gas, available commercially (Matheson Co.), was passed successively through a sulfuric acid bubbler and columns of Si02 gel and Mg(C10& prior to use. Preparation of Solutions. For the conductancetime effect study, the solutions were made directly in the conductivity cell (cylindrical electrode type).ZG ( 1 ) (a) Postdoctorate Fellow, 1962-1963; (b) Postdoctorate Fellow,
1961-1962. (2) (a) G.J. Janz and S. S. Danyluk, J. Am. Chem. Soc., 81, 3846 (1959); (b) ibid.. 81, 3850 (1959); ( c ) ibid.. 81, 3854 (1959). (3) J. F . Coetsee, et al., Anal. Chem.. 34, 1139 (1962). (4) G. J. Jans and E. J . Rock, ibid., 22, 626 (1950).
Volume 68,Number 4
A p r i l , 1964
G. JANZ,I. AHMAD,AND H. VENKATASETTY
890
Excess hydrogen chloride gas from the solution and the cell flask was removed by a dry nitrogen sweep. For regular concentration-conductance runs, a concentrated stock solution was made in a similar fashion and the dilutions in an experiment were by weight-pipet technique. All concentrations were checked after each conductance measurement by a volumetric analysis of a small amount of the solution. The accuracy of this, down to 0.005 M HC1, was d = l % ; at lower concentrations because of the poor end point, it was not
~
b
3 Y
"el-
2 *O-
s
f
01
la/-
le
Table I : Conductance-Time Effect in HC1-Benzonitrile Solution (25")
Concn., mole/l.
0.0575 0,0693 0.0810 0.2660 0.130 0.130
-
Aging period, hr.
75 49 49 50 40 400
2.87 3.73 6.06 25.11 18,90 21.66
Sp. conductance increase;
%
69 55 16 77 11 27.5
Equivalent Conductance. In Table 11, the specific conductivity and equivalent conductance data for various aged solutions are summarized. At lower The Journal of Physical Chemistry
I
"
'
Figure 2. Phoreogmm of the ''aged" HC1-benzonitrile solutions.
-Sp. conductance,-ohm-' om.-' X 108 Initial Final
1.70 2.40 5.20 14.20 17.00 17.00
"
concentrations, the conductlvlty of the solvent was significant, and therefore due account of this was taken In calculating the A-values for these concentrations. Figure 2 gives the phoreogram of this system. The limits of accuracy shown In this table have been calculated by taking into account the errors involved in the chemical analysfs ( f1-3%), resistance measurement, and temperature variation of the bath ( + O . l % on resistance), as well as errors due to the conductancetime effect. Solid Substrates. When a saturated solution of HCl in benzonltrlle was chilled to -8" and allowed to
HYDROGEN CHLORIDE IN ANHYDROUS BENZONITRILE
Table I1 : Equivalent Conductance of the “Aged” HC1 Solutions in Benzonitde (25”)” 8 p . con-
Concn., mole/l.
v/C
ductivity, ohm -1 om.-’ x 106
0,0686 0.0545 0.0342 0.0285 0.0188 0.0119 0,0091 0.0057 0.0026 0.00124 0.000823
0 . 2820 0.2335 0.1849 0.1688 0.1371 0.1091 0.0!354 0.0:755 0,0510 0.0352 0,0286
11.080 8,854 6,044 5.064 3.805 2.870 2.550 1.932 1.406 0.774 0.156
Sp. conductivity corrected for solvent
x
106
996 760 950 970 3 711 2 776 2 456 1 838 1 312 0 680 0 062
10 8 5 4
a Specific conductivity of benzonitrile ern.-’.
=
Equivalent conductance, ohm -1 om. -1
0 0 0 0 0 0 0 0 0 0 0
160 f 370 161 i 3’30 174+ 3Y0 175f3% 1 9 7 h 370 233f337, 270 f 3Y0 323 f 47, 510f 67, 547 f 10% 753 =k 10%
0.094 X 10-6 ohm-’
stand for a long time, a white crystalline substrate was obtained. This was insoluble in benzene, carbon tetrachloride, and diethyl ether, but soluble in methyl alcohol and acetone. ‘The substrate was fairly stable at room temperature; in a sealed capillary, it sintered Analytical results conat 57” and melted at 62-65’. firmed it to correspond stoichiometrically to a 1: 1 addition compound CeH6CN.HC1 (%€I C1: theoretical, 26.13; experimental, 25.3, 25.16, 24.26). The infrared spectrum of the crystalline product was obtained with a Perkin-Elmer Model 21 spectrometer having NaCl optics and the KBr disk technique. It showed a complete absence of C=X stretching frequency (2240 cm,-l). A strong abslorption band at 1640-1660 cm.-’ was also apparent; the latter corresponds to the characteristic C=N stretching mode of imino-type compounds.6 When HCl gas was bubbled for prolonged periods into beneonitrile cooled in a freezing mixture (- 10”) another crystalline compound was obtained. This was highly unstable and readily decomposed into 1 : l substrate and HCl. On analysis it was found to correspond stoichionnetrically to a 1:2 addition compound C6H6CN*PIHCl: (% HCI: theoretical, 41.45; experimental, 38.00, 39.70, 39.32). 13ecause of its high instability, attempts to gain the infrared spectrum were not successful.
Discussion The only other work on the conductance of HCI in benzonitrile reported in the literature is by Zil’berman, et d 6 Unfortuna,tely, the results are reported as a
891
small scale phoreogram which precludes precise values. It can be estimated that the value of A is less than 0.1 ohm-’ em.-’ (0.04-0.12 M HC1). The solutions were apparently aged for 24 hr.; this may a c c o h t for the over-all lower values as compared with those obtained in this study (Table 11, Fig. 2 ) . The phoreogram in Fig. 2, for solutions which were aged for 21 days, conclusively shows that HC1 behaves as a weak electrolyte in benzonitrile. By a free-hand extrapolation of a large-scale phoreogram, the value of 1.5 ohm-’ cm.-l is obtained for A,‘. Attempts to use this value for subsequent Shed1ovsky’-type extrapolation were not fruitful, since the function S
s ( z ) = l + z +2 - + 2-83 2 2
does not coverge. With somewhat higher values, i.e., 16, this function is convergent, and the extrapolation for A0 and K , gives 1.44 ohm-] cm.-I and 2.33 X respectively (Fig. 3(i)). Less accurate relationships for the weak electrolytes were also examined and are listed here. (i) From the equation given by Robinson and Stokes8 A
= A,” - (&’a
1
+ @)fl + xu
+
where (aho’ 8 ) = Onsager function (A,’ taken as 1.5), x = (2.9127 X lo8 dC/D1’zz50and u = distance of closest approach (taken as 4 A. in this case), the A,” values were gained and plotted against C. The extrapolated A,,” a t C = 0 is A0 (Fig. 3(ii)). (ii) The Ostwald dilutiong law: A, and K are obtained, respectively, from the intercept and slope of the graph of l / A us. CAL(Fig. 3(iii)). (iii) Davies’ methodlo: the dissociation constant K is obtained from the graph of d C l (where C, is the ionic fraction of the solute) us. log C,2/Ic,(where C, is the fraction of the undissociated molecules) (Fig. 3(iv)), (iv) K can be obtained” from a graph of C, us. (log K - log f*2), where the activity coefficients are ( 5 ) L. J. Bellamy, “Infrared Spectra of Complex Molecules,” J. Wiley and Sons, Inc., New York, N. Y., 1956. (6) E. N. Zil’berman, et al., J . Gen. Chem. USSR, 31, 1905 (1961). (7) T. Shedlovsky, J . Franklin Inst., 225, 739 (1938). (8) R . €1. Stokes and R . A. Robinson, “Electrolyte Solutions,” Academic Press, Inc., New York, N . Y., 1955, p. 144. (9) C. A. Kraus and W. C. Bray, J . Am. Chem. Soc., 35, 1315 (1913). (10) C. W. Davies, ”The Conductivity of Solutions,” John Wiley and Sons, Inc.. New York, N. Y., 1933. (11) H. S.Harned and B. B. Owen, “Physical Chemistry of Electrolyte Solutions,” Reinhold Publishing Corp., New York, N . Y., 1958, pp. 171, 312.
V o l u m e 68, Number Q
A p r i l , 1064
G. JANZ, I. AHMAD, AND H. VENKATASETTY
892
Table I11 : Summary of 4 and K Calcuiated by Various Extrapolation Methods bo
K
1.5 1.4 1.3-1.7
...
Method
Free-hand extrap. Robinson and Stokes Ostwald Davies Taking into account activity coeff. in Davies Shedlovsky -
0 0
I 4 CAS,,,f$ld
t
-0 . 1 0
I
4
-log K
c xrd
e
4
CA x l d
( iv)
CaHs(Cl)C=NH
0
K)
bq
IO
SO
x IOZ
Figure 3. Extrapolation curves according to various relationships: (i) Shedlovsky; (ii) Robinson and Stokes; (iii) Ostwald’s dilution law; (iv) Davies; ( v ) taking into account the activity coefficient in Davies.
estimated by the Debye-Huckel limiting law (Fig. 3(v)). The values of noand K thus obtained are summarized in Table 111. Taking into consideration the lack of refinement in the latter analyses, the values of 1.5 f 0.2 ohm-’ cm.-l and 2.5 f 0.5 X for A0 and K , respective15 are recommended. The initial rapid decrease in A with concentration suggests strong association. The slope of log h us. log C (inset Fig. 2) is -0.5 in this concentration range (characteristic of ion pair formation12). The critical distance calculated from Bjerrum’s13theory is 11.2 A., ie., ionic association is not improbable for simple solutes in benzonitrile. The distance of the closest approach can be calculated if the value of K is known from the relati~nshipl~ T h e Journal of Physical Chemistry
I
... 1.44
=
6.12 - 3 log D
x x x
10-4 10-4 10-4
2 . 3 3 x 10-4
+ log Q ( b )
where D is the dielectric constant and &(b) is a numerical function of b, values of which are avai1able.l’ From Q ( b ) thus gained, a can be readily calculated. Taking K = 2.5 X 10-4, the a is found to be 1.6 k. in this manner. Recognizing that the internuclear distance in the HC1 molecule is 1.28 A., the distance of the closest approach thus found seems quite reasonable. From the infrared spectral evidence it follows that the 1-1 benzonitrile-HCl substrate may be formulated as an imino chloride, C8HS(C1)C=”. Further interaction with HC1 can be foreseen, e.g.
It-
00
.
a
(iii)
‘E 6
6
.
... 3.1 2.5 2.5
+ HCl e [CeH6(Cl)C=NHz]+Cl-
Thus an imino dihydrochloride is seen the likely structure of the 1 : 2 compound. Such compounds are notably unstable and evolve HC1 readily. The time effect for conductance also warrants comments. This effect is not as marked as in the HC1acetonitrile solutions. After 50 hr., the specific conductance increased at the most by 77% as compared with as much as 400% in acetonitrile. This is one of the major reasons of the larger scatter in the results of the equivalent conductance us. concentration in acetonitrile as compared with benzonitrile. Some insight in the nature of the time effect can be gained from a consideration of the various possible solute-solvent interactions in these solutions. The ionization process of hydrogen chloride in CN, by analogy with the scheme advanced for acetonitrile,zs may be expressed as CeH6CN
+ HC1
CeHbCN. HC1
CeHbCN HC1 CeH&XH+Cl-
(2) (3)
(12) C. A. Kraus, Trans. Electrochem. SOC.,66, 182 (1934). (13) N. Bjerrum, I
