J. Chem. SOC., Faraday Trans. I , 1987, 83 (lo), 3167-3175 Binary Systems of 1,2-Dichloroethane with Benzene, Toluene, p-Xylene, Quinoline and Cyclohexane Part 3.-Dielectric Properties and Refractive Indices at 308.15 K Jagan Nath* and Geeta Singh Chemistry Department, Gorakhpur University, Gorakhpur 273 009, India Measurements of dielectric constants, E, and refractive indices, n, have been made for binary liquid mixtures of 1,2-dichloroethane (CH,ClCH,Cl) with benzene, toluene, p-xylene, quinoline and cyclohexane at 308.15 K. The values of the quantity AE, which refers to the deviations of the dielectric constants of the mixtures from the values arising from volume fraction mixture law, have been calculated. The values of A& have been found to be positive for CH,ClCH,ClLquinoline, and negative for CH,ClCH,Cl- benzene, CH2C1CH,C1-toluene, CH2C1CH,C1-p-xylene and CH,ClCH,Cl- cyclohexane.The positive values of AE for CH,ClCH,Cl-quinoline are attributed to the existence of a specific interaction leading to the formation of a strong complex between CH,ClCH,Cl and quinoline in the liquid state. Values of the equilibrium constant Kf for the formation of a 1 : 1 complex between CH,ClCH,Cl and quinoline have also been calculated using the dielectric constant data, and it has been found that these values of Kf are in accord with the theory of Barrio1 and Weisbecker, which is based upon the electrostatic interactions of the solute with the liquid. The values of the apparent dipole moments papp of CH,ClCH,Cl at various mole fractions in the non-polar solvents benzene, p-xylene and cyclohexane have also been calculated. The values of papp suggest that there exists a specific interaction between CH,ClCH,Cl and the aromatic hydrocarbons.Binary systems of 1,2-dichloroethane (CH,ClCH,Cl) with aromatic hydrocarbons, quinoline and cyclohexane are of considerable interest from the viewpoint of the existence of specific interaction, leading to the formation of adducts between the components of the various systems in the liquid state. The specific interaction of CH,ClCH,Cl with aromatic hydrocarbons and quinoline can be visualised to be due to the presence of two C1 atoms and four H atoms in CH,ClCH,Cl on account of which it can act as a a-acceptor toward, and be involved in the formation of hydrogen bond with aromatics and quinoline.The aromatics in their interaction with CH,ClCH,Cl will act as 7r-donors, whereas quinoline will act as an N-donor. The system CH,ClCH,Cl- cyclohexane, in which only dispersion, dipolar and induction forces are believed to be present between the components, can be used as a reference system. Extensive studies of the interactions between the components of such systems have not been made. Recently,' however, the measurements of excess volumes for binary liquid mixtures of CH,ClCH,Cl with benzene, toluene, p-xylene, quinoline and cyclohexane were carried out at different temperatures and the ultrasonic velocities and adiabatic compressibilities of binary liquid mixtures of CH,ClCH,Cl with benzene, toluene, p-xylene, quinoline and cyclohexane have also been determined., These studies have indicated the existence of specific interaction between the components of the binary systems of CH,ClCH,Cl with aromatic hydrocarbons and quinoline.Dielectric constant measurements of binary liquid mixtures are k n ~ w n ~ - ~ to give reliable information concerning the existence of specific interaction between the components. Furthermore, the values of the apparent 31673168 Dielectric Properties of Binary Systems dipole moments, pUapp, have also provided evidence concerning the existence of specific interactions between the Values of papp can be obtained from dielectric constant measurements of binary liquid mixtures. Hence, in order to gain conclusive evidence about the existence of a specific interaction between CH,ClCH,Cl and aromatic hydrocarbons or quinoline, we have measured dielectric constants for binary mixtures of CH,CICH,Cl with benzene, toluene, p-xylene, quinoline and cyclohexane at 308.15 K.Since the values of the refractive indices of the various pure components and binary mixtures were needed to calculate papp of CH,ClCH,Cl in various solvents, and the molar polarisations of binary mixtures, refractive index measurements have also been carried out at 308.15 K, and the results of present measurements of E and n are interpreted in this paper. Experiment a1 Materials Benzene, toluene, ,p-xylene, quinoline, cyclohexane and 1,2-dichloroethane were purified and their purity was checked as described previously.' Method The measurements of dielectric constants ( E ) were made at 308.15 K and at a frequency of 1.8 MHz, with a dekameter (type DK,,, Wissenschaftlich-Technische, Werkstatten, Germany), using one cell (MFL l/S, no.2078) for mixtures having dielectric constants < 7.0 and another (MFL 2/S, no. 2084) for mixtures having dielectric constants > 7.0, as described previously.6 The cells were thermostatted by the outer jacket using a water thermostat whose temperature was maintained at 308.15&0.01 K. The two cells were first calibrated using purified liquids whose dielectric constants are available from the literature'' and then the measurements of E were made for pure liquids and binary mixtures investigated in the present study. The precision of the measurements of E is of the order of 0.0004 units for dilute solutions of CH,ClCH,Cl in benzene, p-xylene and cyclohexane, and ca.0.001 units for mixtures of CH,ClCH,Cl with quinoline, and for mixtures having higher concentrations of CH,ClCH,Cl in benzene, p-xylene, and cyclo hexane. The measurements of refractive index (n), accurate to within +0.0002, were made using a thermostatted Abbe refractometer at 308.15 K. The values of n were obtained for sodium-D light. Results and Discussion The experimental values of the dielectric constants for the binary systems CH,ClCH,Cl- benzene, CH2C1CH,C1-toluene, CH,CICH,Cl-p-xylene, CH,ClCH,Cl-quinoline and CH,CICH,Cl<yclohexane at 308. I5 K are given in table 1, where x, refers to the mole fraction of CH,ClCH,Cl. The values of E (see table 1) for benzene, toluene, p-xylene, cyclohexane and CH,ClCH,Cl at 308.15 K are 2.2557, 2.3543, 2.2447, 1.9989 and 9.808, respectively, and are in excellent agreement with the corresponding literature values'" of 2.2540, 2.3547, 2.2460, 1.9990 and 9.803, respectively.The present experimental values of n for benzene, toluene, cyclohexane and CH,ClCH,Cl at 308.15 K were found to be 1.491 8, 1.4880, 1.41 80 and 1.4372, respectively, in excellent agreement with the literature values" of 1.49 1 79, 1.487 82, 1.41 8 18 and 1.437 39, respectively. The experimental values of the refractive indices, nl,, for the various mixtures of CH,ClCH,Cl at 308.15 K, as obtained in the present study, were fitted by a least-squares method to the (1) equation n,, = a + hx, + cxf where x, refers to the mole fraction of CH,ClCH,Cl, and a, h and c are constants characteristic of a system at a given temperature.The values of the constants a, h andJ . Nath and G . Singh 3169 Table 1. Dielectric constants for various mixtures of CH,CICH,CI at 308.15 K X l E -x 1 & CH,CICH ,C1-benzene 0.0000 2.2557 0.0060 2.2776 0.0 106 2.2940 0.0323 2.3674 0.0461 2.422 0.050 1 2.428 0.0956 2.603 0. I772 2.934 0.2058 3.079 0.2984 3.546 0.3463 3.815 0.4598 4.58 1 0.5492 5.149 0.6974 6.327 0.7075 6.443 0.7226 6.586 0.8777 8.135 0.91 14 8.537 0.9 190 8.652 0.9539 9.059 0.9889 9.482 1 .0000 9.808 CH ,CICH ,ClLquinoline 0.0000 8.628 0.0930 8.830 0.1 644 8.969 0.4062 9.384 0.4432 9.414 0.4648 9.472 0.5734 9.600 0.6473 9.632 0.8273 9.728 0.8786 9.732 1 .0000 9.808 CH,C1CH,CI-toluene 0.0000 2.3543 0.0768 2.577 0.0823 2.592 0.1539 2.8 19 0.2477 3.157 0.2633 3.238 0.3786 3.802 0.4000 3.936 0.4027 3.948 0.44 12 4.196 0.5460 4.900 0.6 173 5.356 0.8633 7.775 0.9229 8.552 I .0000 9.808 CH,ClCH,Cl -p-xylene 0.0000 2.2447 0.0057 2.2598 0.0089 2.2680 0.01 29 2.2804 0.0362 2.3392 0.0398 2.35 16 0.0448 2.3564 0.0546 2.38 17 0.0922 2.472 0.1049 2.498 0.1234 2.553 0.2225 2.853 0.4344 3.592 0.6745 5.356 0.7629 6.218 0.7875 6.528 0.8334 7.065 0.9693 9.097 0.9802 9.31 1 1 .0000 9.808 CH ,CICH ,Cl-cyclohexane 0 .0000 1.9989 0.0039 2.0 I05 0.0151 2.0321 0.0 199 2.0427 0.0278 2.0641 0.0425 2.0838 0.048 1 2.096 1 0.0842 2.1721 0.1768 2.415 0.3047 2.847 0.3284 2.942 0.3993 3.260 0.4378 3.476 0.51 85 4.043 0.6535 5.214 0.7042 5.641 0.7088 5.713 0.8462 7.328 0.8593 7.561 0.9352 8.682 0.9670 9.166 1 .0000 9.8083170 Dielectric Properties of Binary Systems -1.4 -la2* 0.0 0.2 0.4 O .0.6 0.8 1.0 Table 2. Values of the constants a, b and c of eqn (l), and the standard deviations (T (n) for the various systems of CH,ClCH,Cl at 308.15 K system a b C 44 CH,ClCH,Cl-benzene 1.491 33 -0.060 93 0.006 609 0.000 27 CH,ClCH,Cl-toluene 1.488 15 -0.045 066 - 0.006 08 1 0.000 17 CH ,ClCH ,Cl-p-x ylene 1.487 83 -0.038 218 -0.012 442 0.000 19 CH,ClCH,Clquinoline 1.617 87 -0.105 019 -0.074 836 0.000 79 CH,ClCH,Ckyclohexane 1.418 19 -0.008 403 0.026 714 0.000 37 o.2t O O 0 0 0.01- - 0 . 2 p 6 c along with those of the standard deviations, a(n), for the various systems are given in table 2. The values of the quantity ALE, which refers to the deviation in the experimental values of the dielectric constants of the mixtures from values obtained from the volume- fraction mixture law, have been calculated from the relation ALE = &12-$41&1-$42% (2) where E, and E, are the dielectric constants of the pure components 1 and 2 for which the volume fractions in the mixture are and $4, respectively, and E,, is the dielectric constant of the mixture.The values of ALE for CH,ClCH,Cl-benzene, CH,ClCH,Cl-J . Nath and G. Singh 150 3171 - 0.0 0.2 0.4 0.6 0.8 1.0 X1 Fig. 2. Plot of the apparent molar polarisation, P A , us. mole fraction, xl, of CH,ClCH,Cl for the system CH2C1CH2C1-quinoline at 308.15 K. toluene, CH2C1CH,C1-p-xylene, CH,CICH,Cl-quinoline and CH2C1CH,C1-cyclo- hexane have been plotted us. mole fraction of CH,CICH,CI, x,, in fig.1. It has been shown that the dielectric constants of polar mixtures can be represented as linear functions of the volume fractions of the components.12 Comparison on a volume-fraction basis largely compensates for the 'dipole-dilution' effect, as has been discussed by Franks and Ives.13 Fig. 1 shows that the values of A& are highly positive for CH,ClCH,Cl-quinoline and highly negative for CH,ClCH,Cl-benzene, CH2C1CH2C1--toluene, CH,ClCH,CI- p-xylene and CH,CICH,Cl-cyclohexane. At x, = 0.5, the values of A& for the various systems are in the sequence (AE)quinoline > ('Elbenzene ('&)toluene > (A&), xylene > (A&)cyclohexane. The negative values of A& for CH2C1CH2C1-cyclohexane can be ascribed to the decrease in the degree of alignment of the dipoles with the changing composition of the solu- tion.It has been pointed out that A& is found to be positive in the case of systems in which molecular complexes due to the specific interactions between the components are believed to be f ~ r m e d . ~ The experimental results of measurements of excess volumes, ultrasonic velocities and adiabatic compressibilities for CH,CICH,Cl-benzene, CH,ClCH,Cl-toluene and CH,CICH,Cl-p-xylene have indicated that there exists a specific interaction between CH,ClCH,Cl and aromatic compounds. The negative values of AE for CH2C1CH,C1-benzene, CH,CiCH,Cl-toluene and CH,CICH,Cl-p- xylene can be attributed to the predominance of contributions to A& arising from dipole, induction and dispersion forces over those due to specific interactions.3172 Dielectric Properties of Binary Systems The highly positive values (see fig.1) of AE for CH,ClCH,Cl--quinoline show that CH2ClCH,C1 forms a tight intermolecular complex with quinoline, a consequence which is similar to the case of the system chloroform-pyridine, in which a 1 : 1 intermolecular complex is believed to be formed3 on account of the hydrogen-bond formation between the two components in the liquid state. In order to obtain further evidence concerning the formation of a strong complex between CH,ClCH,Cl and quinoline, we have calculated the values of the total molar polarisations, P, for CH2C1CH,Cl, quinoline and for mixtures of CH,ClCH,Cl with quinoline, by using the Kirkwood-Frohlich equation :14 ( E - n2) ( 2 ~ + n2) V 9 E P = (3) In eqn (3), E , n and V refer to the dielectric constant refractive index and molar volume, respectively.During the present analysis, the density data available in literature" were used to obtain molar volumes for pure liquids, whereas the molar volumes for the various mixtures were obtained from the molar volumes of pure liquids and the measurements on excess The values of the refractive indices, n, of the mixtures used in these calculations were obtained from eqn (1). The total molar polarisations of the mixtures were used to calculate the apparent molar polarisations, PA, of CH,ClCH,Cl at various concentrations in quinoline, in a similar manner to that described by Rastogi and Nath." The values of PA so obtained have been plotted us.mole fraction of CH,ClCH,Cl, x,, in fig. 2, which shows that PA increases sharply with the decreasing mole fraction of CH,ClCH,Cl, thus giving strong evidence that there exists a specific interaction leading to the formation of intermolecular complexes between CH,ClCH,Cl and quinoline in the liquid state. Similarly, the apparent molar polarisations, P,, of quinoline at different compositions of its mixtures with CH,ClCH,Cl were calculated. Considering that a 1 : 1 complex is formed between quinoline (D) and CH,ClCH,CI (A), the value of the molar polarisation, PDA, of the complex formed between D and A was calculated in a manner similar to that described by Earp and G1asstone.l' The value of P,, was found to be 376.36 cm3 mol-l.The values of the equilibrium constant, K,, for the formation of the complex DA, were also calculated from the data on molar polarisations of D, A and DA, and those of the mixtures of D and A, following the procedure described by Earp and Glasstone." The results show that the values of Kf exhibit significant variation with the composition of the mixture. Rivail and Thiebaut3 have also found that in the case of the system pyridine-chloroform, the values of Kf as obtained from the dielectric constant data show a significant variation with the composition of the mixture. It has been pointed out by Rivail and Thiebaut3 that a theory,' based upon electrostatic interactions of the solute with the liquid predicts a linear variation of the logarithm of Kf with the quantity: where E , refers to the infinite-frequency dielectric constant of the mixture.To calculate f i ~ ) from eqn (4), we have taken E, equal to n2. In fig. 3 are plotted the values of logK, us. AE). It is seen that there is a linear variation of logKf with JTE) for CH,ClCH,Cl-quinoline, suggesting that the values of Kf calculated using the simple approach of Earp and Glasstonel' are in accord with the theory17 based upon the electrostatic interactions of the solute with the liquid. Campbell et aL8v9 and Stokes and Marsh7 have shown that the values of the apparent dipole moment, papp, of polar solutes in non-polar solvents furnish useful information concerning the association of the solute molecules with the molecules of the solvent. Hence, in order to know if a specific interaction exists between CH,ClCH,Cl and the aromatic hydrocarbons, we have calculated the values of the apparent dipole moments,J.Nath and G. Singh - 7 -0.60 n .,-I +, 22 5 -0.80 E W --- G - -1.00 Y Kl 3173 - -_____rY___ 0 - I I 1 I I I I 0.15 0.16 0.17 0.18 0.19 0.20 f (€1 Fig. 3. Plot of log [K,/(mole fraction)-'] us. f l c ) for the system CH,ClCH,Cl-quinoline at 308.15 K. papp, of CH,ClCH,Cl at its various mole fractions in the non-polar solvents benzene, p-xylene and cyclohexane using the equation'. l8 where x, is the mole fraction of the polar solute; E is the dielectric constant of the mixture; E , is the dielectric constant of the non-polar solvent (benzene, p-xylene or cyclohexane) and E; is the internal dielectric constant of the polar solute; Vm, Vl and V2 refer to the molar volumes of the solution, the polar solute and the non-polar solvent, respectively ; k is Boltzmann's constant; N is Avogadro's constant; ps,o is the moment of the isolated polar molecule and g is the Kirkwoodl correlation parameter.lg As mentioned by Stokes and M a r ~ h , ~ we have taken (gp,',,)? to be equal to the apparent dipole moment, papp, of the polar solute in tFe non-polar solvent.The values of Vl, V2 and Vm needed for calculations of (gp&)Z from eqn ( 5 ) were obtained from the densities of pure liquids and the values of the excess The value of &; for CH,ClCH,Cl, obtained from the refractive index of CH,ClCH,Cl as described by Stokes and M a r ~ h , ~ was 2.1390. Fig. 4 shows the concentration dependence of pipp for CH,ClCH,Cl in (i) cyclohexane, (ii) benzene and (iii) p-xylene, with a logarithmic scale along the abscissa.Fig. 4 shows that the values of pEpp for CH,ClCH,Cl increase as its concentration in benzene increases. Fig. 4 also shows that the values of pZPp for CH,ClCH,Cl first decrease, reach a minimum and then increase in cyclohexane, whereas the values of pEpp first increase, then decrease to a minimum and increase again in p-xylene. The initial rise in pipp is indicative of the formation of the first, high-dipole-moment species. Fig. 4 therefore indicates that CH,ClCH,Cl monomer is stabilised little by interaction with the non-polarisable solvent (cyclohexane) and stabilised more by association with the solvents benzene and p-xylene. The association of CH,ClCH,Cl with benzene and p-xylene can be considered to be due to the existence of a specific interaction between CH,ClCH,Cl and aromatic compounds, which may be due to the formation of a weak hydrogen bond via interaction of the H atom of CH,ClCH,Cl with the n-electrons of the aromatic ring. However, there is also a possibility that CH,ClCH,Cl is involved in the formation of a weak charge-transfer complex through the interaction of the chlorine atoms of CH,ClCH,Cl with the aromatic n-electrons.Fig. 4 also shows that the initial rise in p:pp for CH,ClCH,Cl in p-xylene solution is delayed more than that in benzene solution, thus showing that the strength of interaction of CH,ClCH,Cl with p-xylene is greater than that with benzene.3174 Dielectric Properties of Binary Systems 4 .O 3.0 N \ CI a N O 2.c 3.1.0 i I I I I -2.0 -1.0 0.0 1 .o log (clmol dm-3) Fig. 4. Plot of ,u&, us. the logarithm of solute concentration for CH,ClCH,Cl in the solvents: (i) cyclohexane, (ii) benzene, (iii) p-xylene. This can be attributed to the fact that the z-electron density of the aromatic ring is increased in p-xylene owing to the presence of two CH, groups. Conclusions In conclusion, we note that the values of ALE show that CH,ClCH,Cl forms a strong intermolecular complex with quinoline in the liquid state. The values of the equilibrium constant Kf for the formation of 1 : 1 complex between CH,ClCH,Cl and quinoline, as estimated from the dielectric constant data, are in accord with the theory of Barrio1 and Weisbecker,l' based upon the electrostatic interactions of the solute with the liquid.The plots of the values of pipp for CH,ClCH,Cl in fig. 4 show that there exist specific interactions of CH,ClCH,Cl with benzene and p-xylene, and that the strength of interaction of CH,ClCH,Cl with p-xylene is greater than that with benzene. This has been attributed to the increased z-electron density of the aromatic ring in p-xylene. The presence of a specific interaction of CH,ClCH,Cl with benzene and p-xylene can be explained as being due to the formation of a weak hydrogen bond between the H atoms of CH,ClCH,Cl and the z-electrons of the aromatic ring. There is, however, a possibility of the existence of a charge-transfer interaction between CH,ClCH,Cl and the aromatic hydrocarbons on account of the interaction of the chlorine atoms in CH,ClCH,Cl with the z-electrons of the aromatic ring.On the other hand, the complexation between CH,ClCH,Cl and quinoline can be attributed to the formation of strong hydrogen bonds between the hydrogen atoms of CH,ClCH,Cl and the lone-pair electrons on the nitrogen atom of quinoline. We are highly thankful to Prof. S. C. Tripathi (Head of the Chemistry Department, Gorakhpur University, Gorakhpur) for providing laboratory facilities. We are also extremely grateful to Prof. R. P. Rastogi (Vice-Chancellor, Banaras Hindu University, Varanasi) for invaluable suggestions. Thanks are also due to the Indian Council of Scientific and Industrial Research, New Dehli, for financial support.J. Nath and G. Singh 3175 References 1 J. Nath and G. Singh, J. Chem. Eng. Data, 1986, 31, 115. 2 J. Nath and G. Singh, J. Chem. Eng. Data, 1986, 31, 327. 3 J. L. Rivail and J. M. Thiebaut, J. Chem. SOC., Faraday Trans. 2, 1974, 70, 430. 4 J. Nath and S. N. Dubey, J. Phys. Chem., 1980, 84, 2166. 5 J. Nath and S. S. Das, Indian J. Pure Appl. Phys., 1981, 19, 343. 6 J. Nath and A. P. Dixit, J. Chem. SOC., Faraday Trans. 2, 1985, 81, 11. 7 R. H. Stokes and K. N. Marsh, J. Chem. Thermodyn., 1976, 8, 709. 8 C. Campbell, G. Brink and L. Glasser, J. Phys. Chem., 1975, 79, 660. 9 C. Campbell, G. Brink and L. Glasser, J. Phys. Chem., 1976, 80, 686. 10 N. A. Lange, Lange’s Handbook of Chemistry (McGraw-Hill, New York, 1973). 1 1 J. Timmermans, Physico-Chemical Constants of Pure Organic Compounds (Elsevier, Amsterdam, 12 T. B. Hoover, J. Phys. Chem., 1969, 73, 57. 13 F. Franks and D. J. G. Ives, Q. Rev., 1966, 20, 1. 14 C. Moreau and G. DouhCret, J. Chem. Thermodyn., 1976, 8,403. 15 R. P. Rastogi and J. Nath, Indian J. Chem., 1967, 5, 249. 16 D. P. Earp and S. Glasstone, J. Chem. SOC., 1935, 1709. 17 J. Barrio1 and A. Weisbecker, C. R. Acad. Sci., Ser. C, 1967, 265, 1372. 18 H. Frohlich, Trans. Faraday SOC., 1948, 44, 238. 19 J. G. Kirkwood, J. Chem. Phys., 1939, 7, 911. 1950). Paper 61241 5 ; Received 15th December, 1986