Excess molar enthalpies of methanol + pyridine, + methylpyridine, and + 2,6-dimethylpyridine

M-1835 J Chem. Thermo&namics 1985, 17, 909914

Excess molar enthalpies methanol + pyridine, + methylpyridine, and + 2,6-dimethylpyridine HIDEKAZU

TOUHARA

and KOICHIRO

Department of Industrial Chemistry. Sakyo-ku, Kyoto 606, Japan (Received I I February

of

NAKANISHI

Kyoto University.

1985)

Excess molar enthalpies have been measured at 298.15 K for methanol + pyridine, + 2-methylpyridine (a-picoline), and + 2,6-dimethylpyridine (2,~lutidine), with an isothermal dilution calorimeter. All the mixtures exhibit negative excess enthalpies, the absolute values of which increase in the sequence: pyridine < a-picoline < 2,dlutidine. The signs and relative magnitudes of thermodynamic excess functions are as follows: Hk < TS,f, < Gfi, < 0. Enthalpies of hydrogen-bond formation between methanol and pyridines are estimated from the present His and the results are discussed in terms of the base strengths of pyridines. specific solvation, and hydrogen bonding.

1. Introduction In previous papers,“, ‘) we have reported thermodynamic excess properties of (methanol + an aliphatic or an alicyclic amine). The influence of specific interactions between a hydroxyl group and an amino group on the behaviour of excess functions has been discussed. As a continuation of our study of (a polar + an associated liquid), pyridine and its derivatives have been adopted as the polar liquid and the excess enthalpies of their mixtures with methanol have been measured. Such a choice will enable us to investigate the specific interactions between hydroxyl and amino groups and the influence of neighbouring methyl groups and n-electrons on these interactions. We shall report here the excess molar enthalpies for methanol + pyridine, + 2-methylpyridine (ol-picoline), and + 2,6-dimethylpyridine (2,6-lutine). Based on the results and literature values, we shall estimate the apparent enthalpies AHhb of hydrogen-bond formation. A brief discussion will also be given of the magnitude and sequence of AHhb in terms of the base strengths of the pyridines and their ionization energies.

2. Experimental The samples used were “Chromatographic” a-picoline 0021-9614/85/100909+06

$02.00/O

“Spectrograde” and 2,64utidine.

methanol and pyridine and Methanol was used without further

0 1985 Academic Press Inc. (London) Limited

910

H. TOUHARA

AND

K. NAKANISHI

purification. All the samples of pyridine bases were dried over anhydrous potassium hydroxide and distilled with a fractionating column having 30 theoretical plates under dry nitrogen at reduced pressure. Middle fractions were sealed in evacuated ampoules because of the high reactivity and hygroscopic nature of these compounds. The purities of purified samples were checked by g.1.c. analysis with a Porapak-R or -S column. No impurities were detected; the densities were 0.78675 g.cme3 for methanol, 0.97806 g.cmm3 for pyridine, 0.93958 g.cmW3 for a-picoline, and 0.91817 g .cm- 3 for 2,6-lutidine at 298.15 K, in good agreement with literature values.‘3*4) The isothermal dilution calorimeter described previously’5’ was used for determining excess enthalpies. In the present application of the calorimeter, a Viton O-ring E-60C, found to be considerably resistant to highly corrosive pyridine solutions, was used as the sealing material whenever it was in contact with sample. In this calorimeter, a known amount of the second component can be added successively to a weighed amount of the first component in the vessel in the absence of any vapour space. Negative excess enthalpies were compensated by Peltier cooling and the electrical energy needed to maintain a set temperature in the mixing vessel was determined. Deviations of temperature from the set value were less than f0.005 K and there was a close compensation between positive and negative deviations.

3. Results and discussion The experimental excess molar enthalpies Hfi, of {xCH,OH + (1 -xWJ%),C,H,-,Nf f or m = 0, 1, and 2 at 298.15 K are given in table 1. The results can be fitted to the equation: Hv(J.mol-‘)

= x(1-x)

f

,4,(2x- l)“-l.

n=1

The coefficients A, (n = 1 to 6), the root-mean-square deviations s, and the rootmean-square fractional deviations sy are listed in table 2. The curves of equation (1) and the experimental points are shown in figure 1, together with the Hvx(l -x) against x plot at low values of x. All the mixtures exhibit negative excess enthalpies, the absolute values of which increase in the sequence: pyridine < a-picoline < 2,6-lutidine. HE(x) is slightly asymmetric about x = 0.5, each curve having a minimum at about x = 0.6, and being similar to that for an alcohol + an aliphatic amine).“) Asymmetries of HE(x) in such mixtures may be explained by the postulate that the Hi is a result of the combination of a positive asymmetrical contribution due to the breaking of hydrogen bonds between alcohol molecules with a negative symmetrical contribution due to the formation of hydrogen bonds between alcohol and amine. If this is correct, the strength of (alcohol-pyridine) hydrogen bonds increases in the sequence: pyridine < cr-picoline < 2,64utidine. The excess molar Gibbs free energy GL and excess molar entropy SE at 298.15 K have been evaluated from the present His and our previous vapour pressures.‘@ The results reveal that in (methanol + a pyridine) the formation of hydrogen bonds

Jf:[xCH,OH+(i

-x){C,H,N

OR 2-CH,C,H,N

911

OR 2,6-(CH3),CSH,N}]

TABLE 1. Excess molar enthalpies of (methanol + a methylpyridine) at 298.15 K HZ

x

J.mol-’

X

f&t J.mol-’

x

ff:

x

J,mol-’

xCH,OH + (1 - x)C,H,N -518.5 0.5909 - 736.5 - 587.8 0.6103 - 735.3 -636.7 0.6280 -731.4 -670.2 0.6437 - 727.4 - 699.2 0.6583 -721.1 -717.4 0.672 1 -714.0 - 729.2 0.6847 - 706.1 - 736.0 0.6965 - 697.4

0.022 1 0.0345 0.0480 0.0651 0.1090 0.1499 0.1912 0.2494

- 28.34 - 50.30 - 74.89 - 106.6 - 187.8 -259.7 -332.3 -426.4

0.3125 0.3660 0.4110 0.4520 0.4876 0.5186 0.5460 0.5698

0.0522 0.083 1 0.1059 0.1494 0.1771 0.2167

- 177.5 - 282.6 - 348.9 -497.8 -574.9 -703.5

xCH,OH+(l-x)2-CH,C,H,N 0.2412 -763.1 0.4111 -1135.4 0.2705 - 842.5 0.4550 - 1195.1 0.3197 - 957.9 0.4828 - 1240.2 0.3282 - 979.2 0.5079 -1276.1 0.3641 - 1049.0 0.5348 - 1291.8 0.3892 - 1092.3 0.5674 - 1301.4

0.0587 0.0906 0.1274 0.1728 0.2091

-292.5 -435.0 -605.7 - 758.0 -891.1

0.2415 0.2991 0.3640 0.4245 0.4813

-

___

Hfi,

J.mol-’

0.7077 0.7175 0.7390 0.7567 0.7753 0.7948 0.8142 0.8346

- 687.6 -678.1 - 647.8 - 626.0 - 599.2 - 567.7 - 532.3 -491.9

0.5955 0.6183 0.6558 0.6795 0.7009 0.7167

- 1300.2 - 1290.5 - 1262.0 -1241.6 - 1208.6 - 1192.3

xCH,OH+(l -x)2,6-(CH,),C,H,N 1030.4 0.5231 - 1654.6 0.7307 - 1566.4 1212.6 0.5618 - 1681.2 0.7730 - 1423.5 1353.8 0.5946 - 1685.5 0.8178 - 1229.2 1503.6 0.6551 - 1670.5 0.8663 -963.7 1616.9 0.7012 - 1614.3 0.8945 - 780.4

x

JCl J.mol-’

0.8558 0.8784 0.9017 0.9269 0.9519 0.9764

-448.3 - 397.5 - 337.6 -268.2 - 184.9 -98.24

0.7539 - 1099.9 0.7937 - 979.0 0.8373 -813.6 0.8778 -640.5 0.9343 - 369.4 0.9794 - 122.1 0.9474

- 393.6

between unlike components might result in an entropy loss compensated by an enthalpy gain and that there is only a small negative deviation of Gfi, from ideality. The signs and relative magnitudes of excess functions are HE < TS: -c Gft,< 0 for the three mixtures studied. Similar results have been reported by Orszagh and Kasprzycka-Guttman (‘) for (methanol + pyridine) at 293 and 303 K. This behaviour of excess functions is similar to that of (methanol + an aliphatic amine)“*2’ and that of (ethanol + pyridine).@’

TABLE

2. Coefficients A,, standard deviation s, and root-mean-square fractional deviations sf of H3(J.mol-‘) for {xCH,OH+(l-x)(CH,),C,H,-,N} at 298.15 K by equation (1) xCH,OH +

A, A2 A3 A4 A5 4. s lo%,

(1 -x)&H,N -2828.8 - 1268.1 -73.0 1023.3 81.4 - 1409.5 2.15 1.19

(1 -x)2-CH,C,H,N -5043.7 - 1994.0 -69.0 1409.1 480.8 - 808.4 5.81 0.74

(1 -x)2,&(CH,)&,H,N -6504.2 -2525.2 - 1582.9 -11.9 1819.2 1700.6 9.56 0.93

912

H. TOUHARA

AND K. NAKANISHI

.

I-

A

\

,

\

B

I\

\ o-

.

.. c

‘--i-

0 X

FIGURE 1. Excess molar enthalpies at B, (1 -x)2-CH,C,H,N; C, (1 -x)2,6-(CH,),C,HsN. coefficients given in table 2.

4

6 I

298.15 K for xCH,OH+: 0, Experimental values; -,

8 I

A,

(i-x)C,H,N; equation (1) with

10 I

E,F-'iV FIGURE 2. Plot of enthalpy of hydrogen-bond formation in (methanol + a pyridine base) against pK, and ionization energy E,. A, pyridine; B, 2-methylpyridine; C, 26dimethylpyridine.

H:[xCH,OH+(l TABLE

3. Estimation

-x){C,H,N

of apparent

enthalpies

AHUP

9.

AH, kJ.mol-’ ~__---~

-1.2 -3.3 -5.1

8.5 1.3 6.5

913

OR 2,6-(CH&C,H,N}]

of hydrogen-bond 298.15 K

kJ.mol-’ Pyridine a-Picoline 2.6-Lutidine Methanol ’ Reference

OR 2-CH&H,N

formation

’

in (methanol

+ a pyridine)

b

AH,,

AH, kJ.mol-’

at

kJ.mol-’ -30.1 -31.0 -32.0

20.4 * Reference

5.

The enthalpy AH,, of hydrogen-bond formation between methanol may be estimated from the present Hf,s and an enthalpy cycle: AH,,, = AHM,, - AH, - AH, + AH,,

and pyridine (2)

provided that only a l-l complex is formed. Here AH,,, AH,, and AH, are the enthalpies of hydrogen-bond formation for methanol-pyridine complexes in an excess of pyridine, of the self-association of methanol, and of the self-association of a pyridine, respectively, and AH, is a correction term due to the difference in dipolar stabilization energy. The HffJx(1 -x) against x plot shown in figure 1 has been used to establish AHMVIP,while AHM and AH, values have been determined from similar plots for mixtures with cyclohexane of methanol@) or a pyridine,“) because, as an inert solvent, cyclohexane has a hexagonal structure similar to that of a pyridine. The enthalpies obtained are given in table 3. The AH,, values listed are apparent ones and any differences from those estimated from, say, spectroscopic results might be equivalent to AH,. The AH,, values from infrared spectral studies range from - 13 to - 18 kJ~mol-l.oo) If we adopt - 16 kJ .mol-’ as a standard value, AH, can be estimated to be about - 13 kJ . mol- ‘, which is by no means small compared with AHhb itself. Finally we examine possible correlations of AH,, with the acidity constant pK, and the ionization energy E,. The difference in AH,, among the pyridines studied is coincident with that of pK, which increases from 5.17 for pyridine, via 5.97 for m-picoline, to 6.75 for 2,6-lutidine with the introduction of methyl groups in the 2or 2,6-positions into the pyridine molecule. (11) On the other hand, E, for pyridine bases has been reported as 8.91 kJ . mol- ‘,(‘*) and one methyl substitution lowers E, by 16 to 18 kJ 1mol- ‘.(l*) Linear correlations of AH,, can be seen in figure 2 with pK, and E,. These results have been predicted by an extended Hiickel theory’13’ or CND0/2 calculation.(14) Examination of the correlation of the hydrogen-bond energy with pK, and E, indicates that OH . . . N hydrogen bonds are not hindered by the steric effect of methyl substitution as has been observed for methylpyridine + benzene or + tetrachloromethane.‘9* “) This work Foundation.

has been supported

by a grant

from

the Kawakami

Memorial

914

H. TOUHARA

AND K. NAKANISHI

REFERENCES 1. Nakanishi, K.; Touhara, H.; Watanabe, N. Bull. Chem. Sot. Jpn 1970, 43. 2671. 2. Nakanishi, K.; Wada, H.; Touhara, H. J. Chem. Thermodynamics 1975, 7, 1125. 3. Timmermans, J. Physico-Chemical Constants of Pure Organic Compounds. Elsevier: New York. 1965. 4. Riddick, J. A.; Bunger, W. B. Organic Solvents. Wiley-Interscience: New York. 1970. 5. Touhara, H.; Ikeda, M.; Nakanishi, K.: Watanabe, N. J. Chem. Thermodynamics 1975. 7, 887. 6. Nakanishi, K.; Ashitani, K.; Touhara, H. J. Chem. Thermodvnamics 1976, 8, 121, 7. Orszagh, A.; Kasprzycka-Guttman, T. Bull. Acad. Polon. SC;., Ser. Sci. Chem. 1972, 20, 349. 8. Findlay, J. T. V.; Copp, J. L. Trans. Faraday Sot. 1%9, 65, 1463. 9. Murakami, T.; Murakami, S.; Fujishiro, R. Bull. Chem. Sot. Jpn 1969,42, 35. 10. Perkampus, H. H.; Kerim, F. M. A. Spectrochim. Acra 1968, 24, 2071. II. Brown, H. C. J. Chem. Sot. 1956, 1248. 12. Watanabe, K. J. Chem. Phys. 1957, 26, 542. 13. Adam, W.; Grimson, A.; Hoffmann, R.; Zuazaga de Oritiz, C. J. Am. Chem. Sot. M&90, 1509. 14. Murthy, A. S. N.; Bhat, S. N.; Rao, C. N. R. J. Chem. Sot. A 1970, 1251. 15. Morcom, K. W.; Travers, D. N. Trans. Faraday Sot. 1967, 62, 2063.