Points for N-C7H16, 2,2-(CH3)2C4H8, and 2,2,4-(CH3)3C5H9 Fall Off the Line to Which Most Other Violet Solutions Conform
Total Page：16
File Type：pdf, Size：1020Kb
EFFECT OF METHYL GROUPS UPON THE SOLVENT POWER OF ALIPHATIC LIQUIDS BY JOEL H. HILDEBRAND AND JoHN DYMOND UNWVERSITY OF CALIFORNIA, BERKELEY Read before the Academy October 11, 1966 In 1950, the senior author published' a short paper on "An Irregularity in the Solvent Powers of Paraffins" in which he called attention to the fact that their solu- bility parameters, 5, calculated from energy of vaporization, AEVaP, are too small to give correct values for heats of mixing by the term in the simple solubility equation for regular solutions,2 v24,2(51-52)2. v2 denotes molal volume of liquid solute, 1, is volume fraction of solvent; the 8's are (AEv`P/v)l/2. They are better solvents for iodine and poorer for fluorocarbons than their 8-values indicate. In the meantime, evidence has accumulated that this anomaly of paraffins occurs with many of their mixtures with nonpolar substances. Hiraoka and Hildebrand,' in 1963, illustrated this graphically by a plot of log x2 versus (52 - 51)2. X2 is the solubility of iodine expressed as mole fraction. The points for n-C7H16, 2,2-(CH3)2C4H8, and 2,2,4-(CH3)3C5H9 fall off the line to which most other violet solutions conform. Shinoda and Hildebrand4 in 1965 determined values for additional aliphatic solvents and showed that the pure cyclic compounds, cyclopentane, cis- and transdecalins, fall on the line, as does cyclohexane, but that methyl- and dimethylcyclohexane do not, nor does n-hexadecane. We showed also that the entropy in all these cases is virtually regular, and that the divergence must therefore be attributed to failure of the term(52 - 51) 2 to give the correct enthalpy in the case of aliphatic liquids whose molecules contain methyl groups. The work herein described was undertaken in order to test this inference more fully. Neopentane boils far below the standard temperature, 250, so we turned to triptane, 2,2,3-trimethyl butane, which has 15 methyl hydrogen atoms to one -CH. We were fortunate enough to get a generous supply of it from General Motors through the kindness of Dr. J. M. Campbell. We determined the solubility and entropy of solution of iodine in this liquid, in normal pentane, hexane, octane, dodecane, and ethyl cyclohexane. These liquids were of the highest purity obtain- able from commercial sources. All were fractionally distilled shortly before use. The triptane was washed with sodium bisulphite solution, dried over Drierite, and fractionated after addition of sodium. The observed values of the solubility of iodine in mole per cent, 100x2, are given in Table 1. Table 2 gives 100x2 at 25CC and values of 51 for the solvents. We give these to three figures for the sake of close comparisons, a procedure justified by the single reliable source' of the values for heat of vaporization at 250C. The solutions with which we are here concerned all show virtually regular entropy of solution of iodine; therefore we plot our data on coordinates that reveal any in- adequacy of the right-hand member of the simple solubility equation:2 RT In (a2/X2) = V2.142(52-81)2, (1) 1001 Downloaded by guest on October 2, 2021 1002 CHEMISTRY: HILDEBRAND AND DYMOND PROC. N. A. S. TABLE 1 SOLUBILITY OF IODINE IN MOLE PER CENT, 10OX2 n-C5H1,2 T0C 0.70 17.20 25.00 ... ... 10OX2 0.223 0.428 0.595 ... n-C&H14 T0C 2.60 16.00 25.03 31.90 100X2 0.256 0.443 0.637 0.824 ... n-C,2H26 TOC 1.50 25.00 40.00 ... ... 10OX2 0.455 1.130 1.936 ... ... C-CeH11 - C2H5 TOC 1.40 15.90 25.00 40.00 ... 100X2 0.415 0.737 1.036 1.788 ... 2,2,3-(CH3)aCJH7 TOC 1.61 8.60 16.72 17.90 25.01 10OX2 0.238 0.326 0.448 0.468 0.621 T0C 30.03 35.00 40.71 45.21 ... 100X2 0.746 0.893 1.102 1.314 .. where a2 represents the activity of solid iodine, 62 the solubility parameter of iodine, and 81 that of the solvent. We use as before: a2 = 0.256, v2 = 58.5 cc, 82 = 14.1. Figure 1 shows our new data and nearly all earlier iodine solubilities plotted in this form. The points for 14 solutions fall on line A, but many others diverge markedly, showing quantitatively the discrepancies existing between the measured solubilities, expressed as RT In X2, and heat of solution of liquid iodine calculated from solubility parameters. We invite attention to the following observations: one methyl or ethyl group substituted on cyclohexane causes a divergence of -300 cal, two methyl groups increase it to 400 cal, line B. The higher normal paraffins, line C, diverge to the extent of --420 cal, increasing to 700 cal with n-pentane, which has the largest ratio of methyl to methylene hydrogen. In the four branched chain molecules, lines D and E, the -CH2 groups are so far shielded by -CH3 that the divergence in all cases is nearly the same, -800 cal. Ratios of methyl hydrogen to methylene hydrogen atoms are: triptane, 15; dimethyl butane, 6; pentane, 1; heptane, 0.6. AEV A / AE v AE~v / The term (62 61)2 = + - 2) VI V2 V1 V2 This is twice the difference between an arithmetic and a geometric mean of the co- hesive energy densities of the liquid components. Since the partial molal entropy of solutions of iodine is regular in these cases, its partial molal energy, RT In (a'/X2), is smaller than the figure given by v2 lb12(52 - S1)2. The interaction energy between iodine and solvent is larger than the geometric mean value followed by the 14 solu- tions whose points fall on line A. This finding further emphasizes the need for a theory of intermolecular potentials far more sophisticated than the primitive London theory and 6-12 central, radial potentials.6 The three points for iodine in perfluoro-solvents, at the top of line A, are somewhat off line A, (corresponding to attraction energy smaller than geometric means, as noted TABLE 2 SOI,1BILITY OF IODINE AT 250C, 100X2, ENTROPY OF SOLUTION, CAL 1)EG1, AND SOLUBILITY PARAMETER OF SOLVENT, 5l Solvent lOOX2 S2 - S,: 51 n-Pentanle 0.595 23.3 7.02 n-Hexane 0.637 23.0 7.27 n-Octane 0.791 7.55 n-Dodecane 1.130 21.9 7.84 2,3-Dimethylbutane 0.567 6.99 2,2,3-Trimethylbutane 0.621 22.9 6.96 Ethyl-cyclohexane 1.036 22.0 7.97 Downloaded by guest on October 2, 2021 VOL. 54, 1965 CHEMISTRY: HILDEBRAND AND DYMOND 1003 4 E 0 4 -3 A D 0~~ ~ ~ ~ ~ ~ ~~~~~~~0 0~~~~~~~~~ 0 I2 3 4 V2 2i (82- , kcol FIG. 1.-Relation between energy of solution of iodine derived from measured solubility, xi, and that calculated from solubility parameters. Line A, beginning at lower left: C02, CHCl6, TiCh, cis-CloHlg, trans-CioH,,, CCL, c-C6H12, c-C5H1o, SiC14, CC16CF,, CCl2FCClF2, C-C4ClF7, c-C4Cl2F6, (CXF)sN, C.F,,CFs, C7F11. Line B, left to right: c-C6H12 (on line A), c-C6Hn1C2Hs (below), c-C6H,1CH,, c-C6H1o(CH,)2. Line C, left to right: normal paraffins, C16H34, C12H26, C8H18, C7H16, C6H14, CQH12. Line D, left to right: 2,3-(CH3)2C4H,, 2,2- (CH,)2C4H8. Line E, left to right: 2,2,3-(CH3)2C4H7, 2,2,4-(CH3)3C5Hq. Line F, top to bottom: C6H6, CH6CHa, p-C6H4(CH3)2, m-C6H4(CH3)2, 1,3,5-C6H3(CHs)3. Group G, from top: 1,2-C2H4C12, CH2Ck, 1,1-C6H4Cl2. in 1950.1 The partial molal volume of iodine in these fluorine compounds is far ill excess of 58.5 cc.2 The nonviolet solutions in aromatic solvents, line F, and the group of polar sol- vents, G, have been discussed in the earlier papers. We invite attention to a note by our associate, Dr. Eva M. Voigt, entitled "Visible Absorption of Iodine in Saturated Hydrocarbons," soon to be submitted for publication, which gives spectroscopic evidence of the effects of methyl-groups. We gratefully acknowledge the support of this research by the Atomic Energy Commission, and the triptane received from Dr. J. M. Campbell of the General Motors Corporation. ' Hildebrand, J. H., J. Chem. Phys., 18, 1337 (1950). 'See Hildebrand, J. H., and R. L. Scott, Regular Solutions (Englewood Cliffs, N. J.: Prentice- Hall, Inc., 1962). 3 Hiraoka, H., and J. H. Hildebrand, J. Phys. Chem., 67, 916 (1963). 4 Shinoda, K., and J. H. Hildebrand, J. Phys. Chem., 69, 605 (1965). 6 Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Com- pounds, American Petroleum Institute Research Project 44 (Carnegie Press, 1953). 6Cf. ref. 2, chaps. 6 and 11; also Hildebrand, J. H., J. Chim. Phys., 61, 53 (1964). Downloaded by guest on October 2, 2021.