Meats of Hydrogenation of Large Molecules Part 3.-Five Simple Unsaturated Triglycerides (Triacylglycerols) BY DONALD w. ROGERS* AND DEBENDRA N. CHOUDHURY Department of Chemistry, The Brooklyn Center, Long Island University, Brooklyn, New York 11201, U.S.A. Received 27th February, 1978 We have determined the enthalpies of hydrogenation of the simple triglycerides of oleic, elaidic, petroselinic, linoleic and linolenic acids. These compounds include the largest molecules ever studied by hydrogen calorimetry and one of them (trilinolenin) produces the largest heat of hydrogena- tion yet observed. Heats of hydrogenation per double bond for the five triglycerides reported here are remarkably consistent with each other and with those of the free fatty acids and methyl esters previously studied.These results suggest a molecular conformation in hexane solution in which the unsaturated acid residues are well separated and the double bonds are thermochemically independent. In Parts 1 and 2 of this series,'. we found that the heats of hydrogenation per double bond of several unsaturated and polyunsaturated fatty acids and their methyl esters are remarkably self consistent. Cis acids and esters have a heat of hydrogena- tion of z - 125 kJ mol-l regardless of whether they are mono-, di- or tri-unsaturated while the trans acid and trans ester included in the study were x4 kJ mol-l more stable. The natural question to ask at this point and the motivation of this paper is, " does this energetic consistency remain when three fatty acids are linked by a glyceride backbone to form one of the biologically important triglycerides ? " EXPERIMENTAL Briefly, the sample is injected in the form of a hexane solution into a stirred slurry of Pd catalyst and hexaiie by means of a mm3 syringe. Sample injections are alternated with injections of a standard (methyl oleate) for which the heat of hydrogenation is assumed to be known (-125.1 kJmol-i).ii The ratio of heat output per mole of the standard to that of the sample leads to the heat of hydrogenation of the latter.Completeness of reaction was tested by transesterifying the reaction product at 80°C in methanol containing sodium rnetho~ide.~ The resulting methyl ester was extracted into benzene and subjected to g.1.c. analysis. In no case was any unsaturated methyl ester detected in the transesterification product.Tests with saturated esters intentionally doped with small amounts of unsaturated esters showed that unsaturated ester present in an amount of 0.4 % of the saturated ester gave a clearly discernible chromatographic peak. Detailed discussion of experimental design and method has been RESULTS AND DISCUSSION Heats of hydrogenation for five simple unsaturated triglycerides (triacylglycerols) are shown in table 1. Each entry in table 1 gives the mean of 16 thermochemical runs on aliquot portions of one solution. The 95 % confidence limits include errors, presumably random, resulting from the method but do not include weighing errors, dilution errors or variations in purity from one sample to the next. Thus, agreement between two sets of experimental runs may be slightly outside the confidence limits 2868D .W . ROGERS AND D . N . CHOUDHURY 2869 of either one because more sources of error are included in the deviation between sets than are reflected in the deviations among members of the same set. Systematic nomenclature for the compounds listed in table 1 is : glycerol cis-9- octadecenoate, glycerol trans-9-octadecenoateY glycerol cis-6-octadecenoate, glycerol cis-9-cis- 1 2-octadecadienoate and glycerol cis-94s- 12-cis- 1 5-octadecatrienoate in the order shown. Determinations were carried out on 5-10 mg of triglyceride and reaction times were x 12 s, permitting repetitive determinations every 2 to 3 min. The high sensitivity of hydrogen microcalorimetry makes it the method of choice for thermo- chemical studies of the unsaturated triglycerides because they are commercially available only in small quantities.Each datum in table 1 is the arithmetic mean of aliquot portions of a solution made up from 500 mg of triglyceride in 5 cmS of hexane. We are presently refining the method so that an entire set of thermochemical runs can be made on 100-200 mg of sample with the intention of determining the heats of hydrogenation of monoglycerides, diglycerides and mixed triglycerides. The latter compounds are more difficult to prepare in high purity than simple triglycerides, consequently they are available in even smaller quantities. TABLE 1 .-HEATS OF HYDROGENATION* OF SOME SIMPLE UNSATURATED TRIGLYCERIDES compound formula wt -AHh/kJ mol-1 95 % C.L./kJ mol-1 triolein triolein trielaidin trielaidin tripetroselinin tripetroselinin trilinolein trilinolein trilinolenin t ri lin olenin 885.4 380.0 381.1 885.4 378.1 375.7 885.4 377.0 379.1 879.4 755.6 758.5 873.3 1132 1133 3.5 3.2 1.9 2.1 0.5 0.8 1.4 2.1 1.3 3.3 * Relative to a value of - 125.1 for methyl oleate.While this work was developing, the principal problem was poor accuracy. We feel that this problem has been largely overcome for large molecules, partly by technical refinements which have reduced the standard deviation and partly by exploiting the rapidity of the method so as to perform many replicate measurements of AH,.,. At comparable levels of the sample standard deviation, s, many replicates give more reliable results than few as shown by the curve of the 90 and 95 % confidence limits as a function of the number of measurements, N, in fig.1. Accordingly, each entry in table 1 represents the arithmetic mean of 16 hydrogenation runs. The confidence limits, C.L., t s C.L. = - JNY where t is Student’s t parameter, are comparable with the best in the thermochemical literature. None of this is to suggest that we have refined hydrogen calorimetry to the point that it gives relative errors as small as those obtained in combustion calorimetry where measurements can be made to kO.01 %. The advantage of hydrogen calori- metry comes about because its absolute error is independent of molecular size, as discussed in Part 2 of this series. Large molecules and small molecules pose quite different challenges to the thermochemist.Because pure samples of large molecules2870 HEATS OF HYDROGENATION OF LARGE MOLECULES are usually expensive, frequently of limited solubility, and because the interesting part of the molecule is a double bond, a functional group, or a strain site, located at one specific point in a molecule which is otherwise inert, sensitivity is at a premium. For example, when we want to look at the isomerization energies of cis and trans isomers of alkenes, the double bond is the only thermochemically interesting part of the molecule. The saturated parts of the molecule, while they influence its energy through steric interference, are, from the point of view of hydrogen thermochemistry, inert. The larger the unreactive part of the molecule, the more sensitive must be any thermochemical method ; a hydrogenation method sensitive enough to determine AHh using 5 mg samples of pentene would require 20 mg of eicosene.I a I2 16 20 N FIG. 1.-Variation of t / z / N with the number of measurements contributing to a sample standard deviation, s. Upper curve, 95 %; lower curve, 90 %. Conversely, the exacting criteria of accuracy demanded of thermochemistry done on small molecules are not realistic criteria for large ones. The triglycerides studied here are not available in >99 % purity. While avoidance of cumulative error encourages one to reduce thermochemical error as much as possible, modifications of the method which increase accuracy much beyond the present level at the expense of sensitivity are not justified at the level of sample purity currently attainable.Taking precision to be a good estimate of acc~racy,~ the results of table 1 are probably reliable to better than 1 %. Since these compounds were described as " 99+ % pure ", the limit of accuracy imposed by the method is about the same as that imposed by sample purity. In answer to the question posed iii the introduction, heats of hydrogenation of the triglycerides of cis unsaturated acids (the naturally occurring form) do indeed show aremarkable regularity, being almost exactly - 12672 kJ mol-l(- 30n kcal mol-l) where n is the number of double bonds in the compound. Each heat of hydrogenation is slightly larger in absolute value than would be expected by taking three times the heat of hydrogenation of the corresponding fatty acid residue.2 For Irregularities are small and close to the limits of experimental error.D.W. ROGERS AND D. N. CHOUDHURY 287 1 example, in the case of triolein, 380 > 3(125.1), indicating a slightly greater structural relaxation on going from the cis unsaturated triglyceride to the saturated triglyceride than we observe on going from the unsaturated free acid to its saturated product. These observations support the model of fatty acid residues which suffer little constraint imposed upon them by the glycerol backbone in triglycerides. Crowding effects, which might be expected to appear when nine double bonds occupy the same molecule, are not observed. There is neither increased strain in the reactant molecule brought about by introducing presumably planar elements into its structure, nor is there a stabilizing effect brought about by cooperative interaction between two or three double bonds in an acid residue, as contrasted to one.On the contrary, the double bonds in an acid residue are energetically entirely independent of one another, which is consistent with the picture of alternant (as contrasted to conjugated) double bonds existing in acid residues which are well separated. In an inert solvent like hexane, the acid residues probabJy radiate away from a central glycerol axis at mutual angles of 120". The evidence is also consistent with a strained, unsaturated trigly- ceride going to an equally strained saturated compound, but we think this is unlikely. The triglyceride containing all trans acid residues, trielaidin, has a heat of hydro- genation which is smaller in magnitude than the cis triglycerides but the effect is not as pronounced as one would expect from the usual stabilization of a trans carbon chain relative to the cis configuration.Trans isomers are usually 4-5 kJ mol-1 more stable than cis isomers 1p 2* 6 s unless very severe steric interference are involved as they are not in the present case. This leads to an expected lowering of 12-15 kJ mol-1 of the magnitude of the heat of hydrogenation of the trans triglyceride relative to the cis. The actual lowering is only 3.6 kJ mol-l. Evidently steric interactions which make the cis free acid and ester less stable than the trans form are replaced by inter-chain crowding in the triglyceride which removes most of the stabilizing effect of the trans configuration.None of these effects is large, however, and we feel that they should be regarded as second order perturbations on a fundamentally open structure which suffers little in the way of steric constraint. or ring strain CONCLUSION This paper represents a significant step in our attempt to apply hydrogen calori- metry to large molecules. The largest molecules examined by this method prior to the present work have a formula weight of less than 30OY1s lo* l1 while all data in table 1 are for molecules about 3 times as large. The largest molar heat of hydrogen- ation previously observed is -584.5 (1, 7 octadiyne)12 while the value for trilinolenin reported here is - 1132 kJ mol-l. We hope that the innate advantages of hydrogen calorimetry over combustion calorimetry for large hydrocarbons and related un- saturated molecules will lead to better understanding of their thermochemistry and of the forces involved in determing their structure.The authors acknowledge the support of the US. National Institutes of Health during this work. D. W. Rogers and N. A. Siddiqui, J. Phys. Chem., 1975,79, 574. D. W. Rogers, 0. P. A. Hoyte and R. K. C. Ho, J.C.S. Farahy I, 1978, 74,46. D. W. Rogers and P. A. Papadimetriou, Mikrochemica Actu, 1974, 937. Chromatography/Lipids, Bulletin 721B-EsterificationY Supelco Inc., Bellefonte, Pa., 16823, (1975). J. L. Jensen, in Progpzss in Physical Organic Chemistry, ed. R. W. Taft (John WiIey and Sons, New York, 1976), vol. 12, p. 189. 1-912872 HEATS OF HYDROGENATION OF LARGE MOLECULES G. B. Kistiakowsky, J. R. Ruhoff, H. A. Smith and W. E. Vaughan, J. Amer. Chem. Soc., 1936, 58, 137. R. B. Turner, D. E. Nettleton, Jr. and M. Perelman, J. Amer. Chem. SOC., 1958, 80, 1430. D. W. Rogers, H. von Voithenberg and N. L. Allinger, J. Org. Chem., 1978, 43, 380. lo R. B. Turner, W. R. Meador and R. E. Winkler, J. Amer. Chem. SOC., 1957,79,4122. D. W. Rogers and S. Skanupong, J. Phys. Chem., 1974,78,2569. lZ T. L. Flitcroft, H. A. Skinner and M. C. Whiting, Trans. Furuday Soc., 1957, 53, 784. ' J. L. Franklin, Ind. and Eng. Chem., 1949, 41, 1070. (PAPER 8/357)