Heats of Hydrogenation of Large Molecules Part 2.-Six Unsaturated and Polyunsaturated Fatty Acids BY DONALD W. ROGERS,* OTHO P. A. HOYTE AND RICKEY KAM C. Ho Department of Chemistry, The Brooklyn Center, Long Island University, Brooklyn, New York, 11201 U.S.A. Receiced 7th March, 1977 We have determined the heats of hydrogenation of the C16 and CI8 unsaturated and poly- unsaturated fatty acids, palmitoleic, oleic, elaidic, linoleic, linelaidic and Iinolenic aCids. This paper reports and interprets the results in the light of previous experimental measurements and theoretical predictions. Heats of formation follow from Hess’ Law addition to the reliable values of the heats of formation of the hydrogenation products, hexadecanoic (palmitic) and octadecanoic (stearic) acids. Our results are generally in serious disagreement with the scattered experimental data and predictions gleaned from the literature and represent, we think, a significant improvement over them.Prior to 1930, despite chemical 1. and X-ray evidence to the contrary, oleic acid was widely regarded as the trans-isomer of 9-octadecenoic acid and elaidic acid was regarded as the cis-form. In that year, Keffler investigated the problem from a thermochemical point of view 4* and argued that the configurations are reversed. His conclusion was based on the relative heats of combustion of the isomers, and as abundant subsequent evidence shows, was correct. We feel, however, that the experimental problems of purifying .and preserving samples of fatty acids were so great as to make his conclusion fortuitous and his numerical data suspect.The same is true of the much earlier thermochemical results of Stohmann who also arrived at the correct relative stability of the isomers. More recently, the only thermochemical studies on unsaturated fatty acids have been those of Suito and Aida 7* who estimated the heats of hydrogenation of oleic acid and its methyl ester by a kinetic method and Omil’chenko, who predicted the heats of formation of several saturated and unsaturated fatty acids 9 9 lo “ from molecular structure ”. In contrast to the unsaturated fatty acids, the common saturated fatty acids have been studied very thoroughly. Combustion studies by Adriaanse, Dekker and Coops 11* l 2 and Swain, Silbert and Miller l3 are in good agreement and would seem to be among the most reliable thermochemical data available on large molecules. These data are supplemented by the fusion data of Lebedeva.14 The existence of reliable heats of formation of saturated fatty acids makes it desirable to know the heats of hydrogenation of the corresponding unsaturated and polyunsaturated acids.Heats of hydrogenation, which may be determined with good accuracy on very small samples, lead directly to the heats of formation of the unsaturates by Hess’ Law addition, thus circumventing the meticulous experimental procedures necessary to determine accurate heats of combustion for molecules of this size. 46D. W. ROGERS, 0. P . A. HOYTE AND R. K . C. HO 47 EXPERIMENTAL APPARATUS The apparatus used was as previously described l5 except that the calorimeter was made hydrogen-tight with a rubber gasket instead of silicone sealent.REAGENTS All fatty acids were obtained from Analabs (North Haven, Connecticut 06473) and certified by the manufacturer to be 99+ % pure as determined by g.l.c., t.1.c. or both. Sample treatment was as previously described l5 and the sources and purities of the other reagents have been given.16 PROCEDURE Experimental data were gathered as previously described.16* l7 80 mm3 of a -28 % solution ( - 70 pmoles) of the unsaturated fatty acid in n-hexane were injected into a calorimeter containing hydrogen at 2atm pressure and a stirred slurry of 5 % Pd on charcoal. For acids containing one double bond, these injections were alternated with identical injections of an approximately equimolar solution of 1-hexene in n-hexane.For acids containing two or three double bonds, the concentration of hexene was doubled or tripled so that the standard 1-hexene solution was approximately thermochemically equivalent to the unknown acid. Hydrogenation brought about a temperature change of ~ 0 . 2 K. Assuming the heat of hydrogenation of 1-hexene to be known (-126.4kJmol-' = - 30.22 kcal mol-I) from the combustion studies of Bartolo and Rossini,18 comparison of the temperature rise per mg of the 1-hexene standard to that of the unknown acid leads directly to the heat of hydrogenation of the acid. The specific g.1.c. procedure for fatty acid methyl esters l5 for establishing purity of the reaction product (evidence for completeness of reaction) was modified for free fatty acids by switching to a 6' 10 % SP-216-PS stationary-phase column. The supporting medium was Supelcoport and the prepared stainless steel Q in.column was obtained from Supelco (Bellefonte, Pensylvania 16823). Our reason for switching to the more polar stationary phase rather than converting the free acids to their methyl esters prior to g.1.c. separation was that we wished to subject the reaction product to a minimum of chemical treatment which might obscure detection of trace amounts of unsaturation. Diagnostic tests for unsaturation were designed as before and indicated complete hydrogenation in each case, as elaborated in the discussion section. The carrier gas was helium at -50 psi and the column temperature was 180 to 200°C.RESULTS AND DISCUSSION RELIABILITY At present, we feel that the overall error is within & 1 % which is typical of what earlier workers report for smaller molecules. Clearly, this does not compare with the relative error reported in good contemporary combustion calorimetry. Coinbustion calorimetry suffers one serious drawback, however. As combustion methods are applied to larger molecules, the absolute error of any combustion calorimetric method becomes larger. Restricting our attention to hydrocarbons for the moment, a four carbon chain might have an enthalpy of combustion of a little less than 3000 kJ mol-1 but a 16 carbon chain would have a heat of combustion four times that value. Given a constant relative error in kJ g-l, the absolute error of the second combustion is four times that of the first.This problem is especially serious because the structural information one wishes to deduce from thermochemical data usually depends on small differences in enthalpies of formation of the molecules under comparison, two cases in point being determination of cis-trans isomerization enthalpies and strain enthalpies.48 HEATS OF HYDROGENATION Despite the larger relative error of hydrogen calorimetry, it is clear that if reason- ably accurate hydrogenation data can be obtained, the enthalpy difference between a molecule having a heat of hydrogenation of 125 kJ mol-l and an isomer having a heat of hydrogenation of 120 kJ mol-I can be more confidently compared than can heats of combustion of 12 000 and 11 995 kJ mol-I.In summary assessment of the method, its principal advantages are speed, simplicity and the requirement of only a few mm3 of sample per injection, thus permitting sample purification by g.1.c. and related micromethods. Its principal drawback is a relative error of about rfi 1 % which can be improved upon by making the reaction time faster and instrument response time less so as to shorten extrapola- tions on the temperature-time curve. We believe both of these improvements are possible. DIAGNOSTIC TESTS Immediately upon concluding a series of hydrogenations, tlie calorimeter was dismantled, the catalyst filtered off and the calorimeter fluid containing all reaction products was set aside for analysis. A sample of stearic or palmitic acid was intentionally contaminated with a few percent of the unsaturated fatty acid under study and subjected to g.1.c.analysis. In each case, we obtained good separation of peaks for the saturated and unsaturated components of the solution. Next, the reaction products were subjected to analysis under identical conditions. Contaminated stearic or palmitic acid was injected before the calorimcter fluid of interest rather than in the reverse order so as to establish the optimum conditions for separation. Upon injection of calorimeter fluid, we observed, in addition to the hexane peak, a strong peak at the retention time of saturated acid due to the presence of the reaction product with a distinct minor peak at the retention time of the unsaturated acid. Thus, the preliminary indication was that hydrogenation was incomplete, a result difficult to reconcile with the excellent agreement of the heats of hydrogenation between the acids (see below) and tlie esters previously rep0rted.l If hydrogenation had been incomplete for the acids and complete for the esters, the acid values should be low.In fact, they are equal to or slightly higher than the heats of hydrogenation of the esters. Further g.1.c. investigation of solutions of saturates and unsaturates in hexane showed that injection of a pure unsaturated acid resulted in a chromatogram with an unexpected saturate peak and that injection of a pure saturated acid resulted in a chromatogram with an unexpected unsaturated acid peak. Repeated injection of either pure acid in hexane solution caused the anomalous peak to decrease in height.Our conclusion, supported by the column manufacturer's descriptive bulletin,20 was that both acids were being adsorbed at the metal injection inlet and being released little at a time on subsequent injections, causing the anomalous peaks. Accordingly, we injected repetitive samples of Calorimeter fluid after an intentioilally contaminated sample had been run and observed the behaviour of the peak due to unsaturated acid. The anomalous peak decreased in size until, after five or six injections, it disappeared altogether. The last discernible peak had an area which was of the order of a few tenths of a percent of the peak due to the saturated acid. Further injections of calorimeter fluid brought about no change in the chromatogram and no trace of the anomalous peak.We believe that hydrogenation was complete. In a few cases, we started a series of hydrogenation runs like those from which we drew our quantitative data but we withdrew aiiquots from the calorimeter between runs for g.1.c. analysis. The results were negative for unsaturated acid.49 D . W. ROGERS, 0. P . A . HOYTE AND R . K . C. HO CATALYST ALTERATION As we found to be true of methyl esters, the first few injections of unsaturated fatty acids produced heats of hydrogenation which were more negative than expected (see discussion). We take this to be due to exothermic adsorption of the saturated reaction product on the catalyst or, more probably, on the charcoal support. (5 % palladium on charcoal was used to make the catalyst slurry).As was true of the esters, this exothermic interference faded out after four or five injections and results calculated from the sixth injection on were constant at a plausible AHh of - - 124 kJ mol-1 for the cis-monounsaturated acids and -4 kJ lower for the trans-forms. Accordingly, the results shown in table 1 were calculated from runs after the sixth in any series. TABLE HEATS OF HYDROGENATION OF SIX UNSATURATED FATTY ACIDS AHh per double bond fatty acid AHh/kJ mol-1 a /kJ mol-1 oleic acid - 123.65 1.6 - 123.65 1.6 oleic acid -125.150.8 -125.lrfi0.8 elaidic acid - 120.252.0 - 120.2+2.0 linoleic acid -254.4+ 1.5 - 127.250.8 linolenic acid - 380.2+ 1.9 - 126.75 0.6 palmitoleic acid -125.1_+1.0 -125.1kl.O Iinoelaidic acid -248.8+0.5 - 124.4rfi0.3 a 1 kcal = 4.184 kJ In contrast to the methyl esters, however, there was a limit beyond which the catalyst charge was no longer useful.After sixteen to twenty pairs of alternate injections of 1-hexene and acid, kinetic lag and decreased potentiometer deflection were noted for the standard. Response for acid injections remained normal. This could be due merely to our having exceeded the solubility limits of the product stearic (or palmitic) acids which are not very soluble in hydrocarbons.21 We speculate that the polar stearic acid molecules produced at the catalyst surface tend to agglomerate about the catalyst particles as the solubility limit of stearic acid in hexane is approached. Agglomeration may prevent approach of 1 -hexene on the next injection. The 1-hexene standard, being nonpolar, is not able to compete on an equal basis with acid molecules so that its hydrogenation is retarded, the recorder curve is altered and extrapolation produces low results.Subsequent injections of acid, however, add polar molecules to the system which compete successfully for the catalyst surface with the molecules already there. The curve shape and recorder responses are therefore unaltered for the acid. This behaviour is clearly indicated by altered recorder response and appears well after a useful series of calorimetric runs has been made. A broad " window '' exists between initial adsorption effects and deterioration of the standard curve, thus collection of data as shown in table 1 was possible. Each entry in table 1 represents the arithmetic mean of six experimental data and the error limits are the standard deviations of all six results from their mean.Translation from the nonsystematic nomenclature which is almost universal in fatty acid chemistry is as follows : palmitoleic, olcic, elaidic, linoleic, linoelaidic and linolenic acids are cis-9-hexadecenoic, cis-9-octadecenoic, irans-9-oct adecenoic, cis-9-cis- 1 2-octadecad- ienoic, brans-9-trans- 12-octadecadienoic and cis-94s- 12-cis- 1 5-octadecatrienoic acids respectively.50 HEATS OF HYDROGENATION COMPARISON WITH PREVIOUS WORK Suito and Aida have obtained the value of - 156 kJ mol-1 for hydrogenation of oleic acid over raney nickel by a kinetic m e t h ~ d . ~ A subsequent study yielded - 149 kJ mol-l for both oleic acid and methyl oleate.8 Both of these values seem too negative by comparison with our data.We have encountered exothermic adsorption interference in preliminary studies which give an apparent heat of hydrogenation which is too negative by 20 to 24 kJ mol-1 [see ref. (1 5) for a discussion of adsorption effects]. Table 2 shows the heats of formation and fusion taken from Domalski's critical review 2 2 and table 3 shows the heats of formation of the unsaturated acids by Hess' law addition to the values for the heat of formation of the liquid palmitic or stearic acid listed by Domalski. These in turn were taken from the combustion data of TABLE 2.--"EATS OF COMBUSTION AND FUSION OF STEARIC AND PALMITIC ACIDSa AHc/kJ mol-1 AHf/kJ mol-1 at 298 K A Hfus at 298 K fatty acid (crystal) /kJ mol-1 (liquid) stearic acid - 11280 58.6 - 889.1 palmitic acid - 9978 53.1 - 838.5 a From the review of Domalski.32 Adriaanse, Dekker and Coops 11* l2 and fusion data of Lebedeva.14 Also tabulated in table 3 are predictions by Omil'chenko based on a bond-energy scheme and heats of formation calculated from the heats of combustion of unsaturated fatty acids by Keffler.The results we obtained for the fatty acid methyl esters l5 were based on a value of - 123.7 W mol-1 for the heat of hydrogenation of the 1-hexene standard. This value is in error by 2.76 kJ mol-I. When comparing the present data with those for the esters, each ester datum should be multiplied by the factor (126.4/123.7)4.184. TABLE 3.-HEAT OF FORMATION OF UNSATURATED FATTY ACIDS fatty acid AHt a /kJ mol-1 AHrb /kJ mol-1 oleic acid -764.8 C -710.2 -748.5 (I) -783.2 (s) elaidic acid - 769.0 -910.8 (s) linoleic acid - 634.7 - 603.1 linoelaidic acid - 640.2 linolenic acid - 508.8 - 485.9 palmitoleic acid - 713.4 - 656.7 a Data of Omil'chenko, ref.(10) ; b from data of Keffler, ref. (5) ; C average of two sets of experi- ments. One area in which we expect to find data comparable with those in table 1 is that of cis-trans isomerism studies. The appropriate data involve transition from a reactant to a product which is in the same or nearly the same physical state. Kistiakowsky, et aZ.23 observed the vapour to vapour transition of cis to trans 2-butene to be -4.0 kJ mol-l, Turner et ~ 1 . ~ ~ obtained -3.6 kJ niol-l for the isomerization of 1,1,8,8-tetramethy1-4-octene from the pure liquid to dilute aceticD. W.ROGERS, 0. P. A . HOYTE AND R. K. C. HO 51 acid solution and Franklin estimated the average cis-trans isomerization energy from the vapour to the vapour state as -5.23 kJ mol-l. In previous work on the methyl esters of unsaturated fatty acids, we found the isomerization enthalpies of methyl oleate to methyl elaidate to be -3.6 kJ mol-1 on one set of trials and -4.6 kJ mol-1 on another. Comparison of the value for elaidic acid in table 1 with the two values for oleic acid yields an isomerization energy of -3.4 kJ mol-1 from one experimental value for oleic acid and -4.9 kJ rno1-l for the other. These values are in excellent agreement with our values for the esters and other worker’s values for similar isomerizations of smaller molecules.In contrast, we have the value of - 128 kJ mol-I isomerization enthalpy for the oleic acid to elaidic acid conversion calculated from Keffler’s combustion data on the two acids5 This remarkable value was used by Keffler to support his (correct) assignment of cis-trans isomerism for oleic and elaidic acids. With the advantage of hindsight and the studies of Kistiakowsky and Turner, however, this value has become totally unreasonable and we must now use it as grounds to reject his numerical data for the thermochemistry of the unsaturated fatty acids. Subtracting the value of the heat of hydrogenation of linoleic acid from that of linolenic acid, we obtain - 125.8 kJ mol-1 for the partial hydrogenation of linolenic to linoleic acid.Proceeding by the same method, we find the following step-wise hydrogenation sequence : - 125.8 - 130.7 - 125.1 linolenic I_+ linoleic -129.; oleic We feel that, in view of the cumulative error encountered in determining partial heats of reaction by difference, that these data are quite self-consistent. In contrast, the predictions of Omil’chenko, taken in combination with Adriaanse, Dekker and Coops’ value for the heat of formation of stearic acid yield stearic. - 117.2 - 107. I - 179.1 linolenic --+ linoleic + oleic - stearic. These values are neither self-consistent nor in agreement with our experimental results. We wish to acknowledge support of this work by the National Institutes of Health. R. Robinson and G. M. Robinson, J. Chem. SOC., 1925,127,175 ; 1926,2204.T. P. Hilditch, J. Chem. 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