RALPHK. BIRDWHISTELL
textbook forum
University of West Florida Pensacola, FL32504
Erroneous Explanations for the Limited Water Solubility of Organic liquids Donald B. Alger California State University, Chico, Chico, CA 95929 Most chemical processes of industrial and biological importance occur in solution. The exclusion of nonpolar hydroohobic molecules from water is a n imoortant ohenomenon in industrial and environmental situations, and in the stabilization of bioloeical structures. The understandlngof solubilities is the gasis of separation, purification, and analysis of many chemicals. Recent articles discuss some of these principles of solubilities and hydrophobicity i n relterms. Entropy chance is clearly a atively . so~histicated . major consideration for this process;l, 21. Discussions of solubilities in oreanic courses usuallv are first offered with respect to a k a & or alcohols. ~ e i e r a l izations often are offered. such a s "like dissolves like". or qualltatlve dcscript~onsof hydrophobic efkts arc discussed. These offer useful ecneralizatlons. allow for some predictions, and some textsgo no further. Most full-year organic texts attempt to offer explanations a t the molecular or thermodynamic level including. explanations such a s those listed below. ~
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M o r e hydrogen honds in water must he broken in order to aceammadate the hydrocarhon tails, and the energy required
far this process is not recovered from interactions of the water with the dissolved molecules. Water is highly polar, and it has little aflinity for the distinctly nonpolar hydmcarhon molecules. h the extent that any of the hydrocarhon does dissolve, it will be at the expense of favorableintermolecularinteractions for both water and the hvdrocarhon., vet no strone solvent-solute attrac" tions will replace them. This will not he an energetically favorable process and the solubility is, indeed, low. ~
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Of 13texts examined, seven contained statements of this sort. When students are asked to predict from such explanations, whether the solution process would be exothermic or endothermic, they nearly always predict a n endothermic process.
One text, which discusses solubility in terms of AG = AH - T&S, actually indicates that the nonmiscibility of water with most o r c a ~ clisuids is the result of a positive enthalpy of mixkg. However, the actual enthalpy of solution for most organic liquids, a t least of eight carbons or less is negative (3).This is true even for compounds such a s alkanes, which have no polar part. Therefore, the limiting factor must be a n unfavorable entropy, which is due to the increased ordering of the solvent, and not a n unfavorable enthalpy a s most texts either imply or directly state. Only one text was found that discussed the limited solubility in terms of entropy (4) and explained it a s follows.
As the hydrocarhon chain of an alcohol gets larger, the coinpound hegins to lwk mare like an alkane. To make room for the hydrocarhon chain, hydrogen honds in water form a tighter ieelike structure around the chain. The entropy cost of the extra organization is not compensated for by the enthalpy gain through the van der Waals interaction of the hydrocarbon chains with the ice-likestructure plus the hydrogen bonding to the alcohol. Thus, solubility decreases as the hydrocarhon chain gets larger. I t is, of course, important for students to consider entropy a s a n explanation for phenomena, particularly with respect to solvent effects. I t seems that emphasizing this with the relatively simple phenomenon of solubility provides a n excellent opportunity for introduction or reemphasis of the important concept of entropy. Literature Cited I M ~ r p h jK . P:Pnvdtv.P L .(itll.S J ~ C I N P 19111.247.SSP 2 ller?frld. J Sn#orr 1891 257, IR .I ?.drll, A S ~ L l d m r of . O r m n r C o m p o ~ n d ?3rd . cd. D \bn So%rsnd Sew Ywk. 1941,V0l 2. 4. Seeitwieser, A ; Heathmeh C. H.; Kosower, E. M.Infrduefion to Organic Chsmlaty: 4th ed.;MacMiUan; New York, 1992:p 210.
Volume 71
Number 4 April 1994
281
