J. Phys. Chem. 1980, 84, 1381-1386
1381
Specific Interactions of Pyridinium Ions and Pyridine Molecules in Nltrobenzene. Substituent Effects M. C. Haulalt-Plrson” and M. De Pauw’ Depirtment of Chemktty, Unlverstty of Leuven, Celestijneniaan 200F, 3030 Heverlee, BeMium (Received April 11, 1979; Revised Manuscript Received October 29, 1979) Pubihcatbn costs asslsted by Kathoibke Unlversitdt te Leuven
From conductance measurements of a series of pyridinium picrates in nitrobenzene solutions the equilibrium conatants of different hydrogen bonding reactions were determined Kf for the molecular acid-base association HPic + Py + PyHPic, K,for the ionic association PyH+ + Pic- PyHPic, and K1+for the homoconjugation PyH+ + Py == PyHPy+. In addition to these, the formation of 1:2 complexes between the pyridinium ion and the pyridine molecule is observed in the cases of 3,4-dimethylpyridine and 4-(dimethylamino)pyridine. This may be explained in terms of r-r interactions between the second pyridine molecule and that already bound to the cation by a hydrogen bond. Plots of log Kf,log K,, and log K1+vs. aqueous pK,’s of the pyridines yield straight lines with slope of 1.167, -0.218, and 0.061, respectively. The results show that the hydrogen bond fornnation between pyridine and picric acid in nitrobenzene increases with increasing basicity of the pyridine molecule. The value of the first slope indicates that proton transfer is important. The ionic association between the pyridinium cation and the picrate anion increases with increasing acidity of the cation. The increase of the homoconjugation constant Kl+ with pK,’s of the pyridines indicates a greater effect of substituents on the pyridine compared to their effect on the pyridinium cation. This is explained by considering the potential energy curve of the proton for the homoconjugate cation. The increase of the depth of the potential well resulting fronn the hydrogen bond formation is more important when the pyridine is more basic.
Introduction The acid-base interactions between picric acid and pyridine in nitrobenzene have been studied by Witschonke In such and Kraus2 and by Kolthoff and his co-~orkers.~ a system different types of intermolecular hydrogen bonding reactions can simultaneously occur: (a) formation of a “normal” hydrogen bond between picric acid and pyridine PicH + Py PicH-Py (1) [PicH,Py] K, = [PicHI P Y l (b) ion pair formaibion resulting from the proton jump PicH.-Py
Pic--+HPy [Pic-,HPy+] Kt = s[PicH,Py]
(2)
(c) dissociation of this ion pair into “free” ions Kd
Pic--+HPy ‘K:Pic- + PyH+ [Pic-,HPy+] K8/ = [Pic-][PyH+]fk2
(3)
(d) formation of a hydrogen bond between the cation and its conjugate base (homoconjugation) PyH”
K1+ + Py e (PyHPy)+
(4)
The present paper reports the results of our investigations of a series of pyridinium picrates in nitrobenzene. When conductance data were used, it was possible to estimate 0022-3654/80/2084-1381$01 .OO/O
the values of the equilibrium constants for the various competing reactions mentioned above. Our purpose is to study the effect of the substituents of the pyridine molecule (or of the pyridinium cation) on the formation of different types of hydrogen bonds. From this point of view the formation of the homoconjugate cation will be of particular interest since the same substituents are found on both the proton donor cation and the proton acceptor conjugate base. Experimental Section Materials. The pyridinium picrates were prepared by interaction of the pyridine with a solution of picric acid in ethanol. The resulting precipitates were recrystallized from ethanol-acetone mixtures and dried. Nitrobenzene (Fluka puriss) was distilled from activated alumina under reduced pressure. The residual conductivity does not exceed 3 X 10” ohm-’ cm-’. The pyridines (Aldrich) were distilled under reduced pressure just prior to use. 4Aminopyridine was recrystallized twice from water. 4(Dimethy1amino)pyridine was recrystallized from ether. Measurements. Electrical conductances were measured by a Wayne Kerr universal bridge B221 ac conductimeter operating at a frequency of 1592 Hz. Philips P.W. 95/2/01 cells were used. The cells were calibrated by determining the conductance of KC1 solutions in water at 25 OC in the concentration range 0.001-0.01 N and by using the equation for equivalent conductance due to Lind, Zwolenik, and Fuoss.4 The cell constants were 0.7335 f 0.0005 and 0.7064 0.0007 cm-’. The viscosity of the solvent was measured with the automatic MGM Lauda viscosimeter. The density was determined with a hydrostatic balance. The dielectric constants were measured by means of a WTW dekameter operating at 2 MHz. All measurements were carried out at 25 f 0.01 “C. Results and Discussion Molecular Acid-Base Association Constants (Kf)and Ionic Association Constants (K,).Limiting conductances, 0 1980 American Chemical Society
1382
Hatrlait-Pirson and De Pauw
The Journal of Physical Chemistry, Vol. 84, No. 11, 1980
TABLE I: Limiting Equivalent Conductances A , (ohm-' cma equiv-') and Association Constants K , (dm3mol-') s f Pyridinium Picrates in Nitrobenzene at Various Concentrations ([L], mol dm-3)of the Corresponding Pyridine -__I
IO-' [L] 0 2.16 3.10 5.00 6.10 7.84 10.00
0 0.993 2.063 3.268 8.025 11.990 14.100 0 1.467 2.943 4.070 6.100 8.200 9.960 15.036 19.575 0 0.904 1.015 2.000 3.987 5.698 9.896 14.750 20.220
D
rl, cp
A0
K,
Pyridinium Picrate 34.82 1.8510 (22.55)b 34.78 1.8495 34.18 7800 34.75 1.8485 33.95 6491 34.70 1.8460 33.47 4378 34.68 1.8440 33.34 3941 34.62 1.8410 33.19 3155 1.8370 33.10 2454 34.58 4-Methylpyridinium Picrate 34.82 1.8510 (29.45)b 34.80 1.8485 33.64 6602 34.77 1.8460 33.22 4474 34.73 1.8427 33.20 3323 34.56 1.8237 33.00 1599 1.8257 32.91 1130 34.40 34.32 1.8217 32.85 956 3,5-Dimethylpyridinium Picrate 34.82 1.8510 (29.20)b 34.77 1.8460 31.42 4605 34.72 1.8410 30.95 2801 34.69 1.8380 30.95 2164 34.60 1.8340 30.64 1519 34.53 1.8300 30.67 1183 34.46 1.8260 30.65 985 34.28 1.8170 30.60 677 34.11 1.8105 30.50 525 2,6-Dimethylpyridinium Picrate 34.82 1.8510 (31.80)b 34.80 1.847 33.37 6176 34.80 1.8465 33.30 6098 34.78 1.8430 32.93 5397 34.71 1.8370 32.55 4644 34.65 1.8330 32.35 3995 34.48 1.826 32.09 3102 34.28 1.820 32.15 2523 34.05 1.816 32.03 2120
Ra
lO-'[L]
2.17 2.61 3.89 4.34 5.45 7.04
0 2.07 2.07 4.08 5.98 10.00 10.15
1.78 2.63 3.56 7.33 10.84 12.92
0 1.05 2.00 4.00 6.01 7.93 9.89
2.69 4.27 5.55 7.98 10.32 12.50 18.53 24.35
0 1.033 1.560 2.280 3.950 6.100 8.010 9.017 9.926 20.170
1.11 1.12 1.27 1.49 1.74 2.28 2.87 3.50
D
q,cP A0 K, 3-Methylpyridinium Picrate 34.82 1.8510 (26.33)b 34.78 1.8452 32.85 5528 32.85 5489 34.78 1.8452 34.72 1.8407 32.75 3354 1.8367 32-50 2366 34.68 1539 32.05 34.58 1.8287 32.15 1553 34.58 1.8285 4-Ethylpyridinium Picrate 34.82 1.8510 (28.50)b 34.80 1.8477 32.80 6661 32.61 4772 34.77 1.8457 34.70 1.8412 32.30 2912 34.63 1.8367 32.20 2126 32.15 1708 34.56 1.8327 34.48 1.8287 32.10 1352 3,4-Dimethylpyridinium Picrate 34.82 1.8510 (30.80)b 34.79 1.850 31.40 4579 34.77 1.849 30.96 3354 34.74 1.848 30.09 2462 29.95 1557 34.69 1.846 34.60 1.843 28.96 992 34.54 1.841 29.01 762 28.89 677 34.50 1.839 34.46 1.838 28.49 600 34.10 1.826 28.00 270
2,4,6-TrimethylpyridiniumPicrate 34.82 1.8510 32.50 5433 34.78 1.8430 31.80 3946 34.68 1.8320 31.00 2477 34.58 1.826 30.73 1870 34.45 1.820 30.51 1420 4-(Dimethy1amino)pyridinium Picrate 0 34.82 1.8510 33.70 1802 1-00 34.79 1.855 31.14 698 2.00 34.76 1.8575 30.54 414 3.03 34.74 1.860 30.36 306 5.99 34.69 1.865 29.90 169 8.00 34.66 1.866 29.73 119
0 1.985 6.028 9.967 15.010
I _ -
Ra 2.50 2.51 4.14 5.90 9.16 9.08
1.80 2.52 4.16 5.74 7.20 9.17
2.17 2.97 4.07 6.46 10.24 13.40 15.17 17.17 39.76 1.00 1.38 2.23 2.98 3.98 1.00 2.59 4.38 5.95 11.48 15.34
4-Aminopyridinium Picrate 0 1.00 1.8510 33.00 2171 34.82 34.82 1.8520 31.62 1041 2.09 0.99 4.30 34.82 1.8560 30.49 505 3.00 6.62 1.8595 29.86 328 5.257 34.82 34.82 1.8630 28.93 232 7.976 9.34 10.02 34.82 1.8680 28.66 187 11.62 R Kaa/Ka is corrected for the change in the dielectric constant D of the solvent. The values of Ka0 ( K , for [ L ] = 0) obtained as described in text appear in Table 11. Values in parentheses are Aa' obtained by means of eq 8.
Ao, and ionic association constants, K,, of salts in solution can be easily obtained through the different formulations of the conductance equations. When the association is appreciable (K, > 100) reliable values of hoand K, may be obtained by use of the Fuoss linear relationship6 F(z)/A = l / A o + ACfi2Ka/F(~)Ao2 (5)
F(z) is a function tabulated for the useful range of the variable z , depending itself on the experimental conductance A and of a preliminary value of ho: (-33/2(~/2))] F(z) = y3 COS' [(YJ z = (S/Ao3/2)(CA)1/z
S is the Onsager coefficient which depends upon &, the viscosity 7, the dielectric constant D,and the absolute temperature T of the medium. C is the salt concentration. f k is the mean activity coefficient calculated from the Debye-Huckel theory. Consequently a plot of F(z)/A against CAfi2/F(z) will determine A. and K, from the intercept and slope. This method was successfully used to determine the limiting conductances and the association constants of partially
substituted alkylammomium and imidazolium picrates in nitrobenzene. In the present work we have measured the conductance6 of solutions of the following picrates in nitrobenzene: (1) pyridinium picrate; (2) 3-methylpyridinium picrate; (3) 4-methylpyridinium picrate; (4) 4-ethylpyridinium picrate; (59 3,5-dimethylpyridinium picrate; (6) 3,4-dimethylpyridinium picrate; (7) 2,6-dimethylpyridinium picrate; (8) 2,4,6-trimethylpyridiniumpicrate; (9) 4-aminopyridinium picrate; (10) 4-(dimethy1amino)pyridiniumpicrate. On plotting values of F(z)/A against those of ACfk2/F(z),we obtain linear relations but the intercepts of the F(z)/A axis yield limiting conductances A. which are abnormally low. We find, for example, a value of 22.55 for pyridinium picrate, 26.33 for 3-methylpyridinium picrate, 30.80 for 3,4-dimethylpyridinium picrate, 33.70 for 4-(dirnethylamino)pyridinium picrate (see A. values in parentheses in Table I for complete results). As we consider the limiting conductances of other substituted ammonium picrates in nitroben~ene~~' (tributylammonium picrate, 28.86; dibutylammonium picrate, 30.4; n-butylammonium picrate, 32.97; ammonium picrate, 34.4; trimethylammonium picrate, 34.8; N-methylimidazolium picrate, 34.3), it clearly
Conductance of Pyridinium Picrates in Nitrobenzene
The Journal of Physical Chemistty, Vol. 84, No. 11, 1980 1383
TABLE 11: Limiting Equivalent Conductances, Ionic Association Constants, Molecular Acid-Base Association Constants, and pKa's of Pyridines for Substituted Fyridine-Picric Acid Systems in Nitrobenzene K,+ PKad Kfb Kaa substituent A, 57.8 f 3 5.17 16836 34.80 5.7 x 104 1. H 80.3 f 5 6.68 13740 33.50 2.1 x los 2. 3-CH3 5.98 83.0 f 2 11710 4.9 x 105 34.00 3. 4-CH, 80.3 f 2.6 6.02 11970 4.4 x 105 4. 4-C,H, 33.20 5. 6. 7. 8. 9. 10.
32.00 32.70 33.80 8 2.50 33.00 83.70
3,6-(CH,), 3,4-(CH,), 2,6-(CH,), 2,4,6-(CH,), 4-", 4-N(CH3),
11860 9920 6830 5430 2170 1800
1.3 X lo6 2.6 X 10' 1.7 X l o 6
Ka = [PyHPicl/([PyH+][Pi~-]f*~).K f = [PyHPic]/([Py][HPic]). in aqueous solutions.
appears that theee A. values lie in a narrow range and diminish regularly as the hydrogen atoms of the ammonium group are substituted by alkyl groups. In the cases of pyridinium picrates, the abnormally low values of A,, are found for the, picrates of the weakest pyridines. These values increase regularly with the basicity of the pyridines to reach, in the cases1 of strong pyridines, A. values similar to those obtained for the substituted ammonium picrates. These results support the fact that the picrates of weak aromatic bases undergo in nitrobenzene both molecular acid-base and ionic dissociation as previously postulated by K r a w 2 1/4
+ HPic
(6)
PyH+ iPic-
(7)
PyHPic ePy
-+
PyHPic K, =
[PyHPic] [PyH+][Pic-]fA2
~~
By combining eq 6 and 7 with the relation of FUOSS, Witschonke and Kruus2obtained the following equation: F(z)/A = l / A o + lL/Ao(Ka/Kf)1/2+ ACf*2K,/F(~)A02 (8)
with l / A o + l/Ao(K,/Kf)1/2= l/A
