The Heats of Combustion, Formation, and Isomerization of Isomeric

HEATSOF COMBUSTION, FORMATION, AND ISOMERIZATION OF MONOGLYCERIDES

2887

The Heats of Combustion, Formation, and Isomerization of Isomeric Monoglycerides1T2

by Leonard S. Silbert,8B. F. Daubert, and Leo S. Mason Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania

(Received February 1.9,1966)

Heats of combustion for the isomeric monoglyceride series ranging from monocaprin to monostearin and for one pair of unsubstituted benzoylmonoglycerols were determined and used for calculating the heat of formation, heat of isomerization from the enthalpy difference of the isomers, and energy increment pkr CH2 for their solid states. The heat of isomerization ranged from -1.99 to -3.84 kcal. mole-' for the aliphatic monoglycerides and -1.08 kcal. mole-l for the aromatic series in contrast to a reported value of -9.52 kcal. mole-'. The kinetic equilibrium constant reported for the isomerization of Z-monoglyceride to 1-monoglyceride was compared to the equilibrium constant computed from their isomerization energies. An entropy difference for the monopalmitin pair, AS' = -5.6 cal. deg.-' mole-l, was derived and interpreted. The energy increment per CH2 ranged from 154.65 to 155.87 kcal. mole-', and the possible reasons for the range are briefly discussed.

Introduction Pure aliphatic and aromatic 2-monoglycerides have been prepared and chara~terized,~-~ and their rearrangement to 1-monoglycerides have been studied under acid, alkaline, and thermal conditions.@~ '0 The rearrangement was recently demonstrated to be a reversible reaction, 2-monoglyceride @ 1-monoglyceride, in which the 1-isomer is the predominant component in the equilibrium.lo Clarke and Stegeman" determined the heats of combustion for a single pair of monoglycerides, the monopalmitins, whose enthalpy difference provided the heat of isomerization evolved in the rearrangement. Their value of -9.52 kcal. mole-' afforded thermodynamic evidence of the greater stability of l-monoglycerides, but they also acknowledged this isomerization energy to be unusually large when compared to differences of 0.5-3.5 kcal. mole-' normally obtained for isomeric pairs.12 To confirm the reliability to their 1-monopalmitin heat of combustion value, they also reported preliminary heats of combustion on l-monomyristin whose energy seemed to compare favorably with that calculated from their 1-monopalmitin value by including R , o ~ s i n i ' sheat ~ ~ of combustion per CH2 group.

Because the anomalously large isomerization energy was reported on only a single pair of isomeric monoglycerides, this investigation was undertaken on both (1) This paper is based on the Ph.D. thesis of L. S. Silbert, June 1963. (2) The authors extend their appreciation to Swift and Co. for a research grant that made this work possible. (3) Eastern Regional Research Laboratory, Eastern Utilization Research and Development Division, Agricultural Research Service, U.S.D.A., Philadelphia, Pa. 19118. (4) M. Bergmann and N. M. Carter, 2. physiol. Chem., 191, 211 (1930). (5) B.F.Stimmel and C. G. King, J. Am. Chem. Soc., 56,1724 (1934). (6) B. F. Daubert, H. H. Fricke, and H. E. Longnecker, ibid., 65, 1718 (1943). (7) J. B.Martin, ibid., 75, 5482 (1953). (8) 0. E. van Lohuiaen and P. E. Verkade, Rec. trav. che-m., 78, 460 (1959). (9) B. F. Daubert and C. G. King, J. Am. Chem. SOC.,60, 3003 (1938). (10) 0. E. van Lohuiaen and P. E. Verkade, Rec. trav. chem., 78, 133 (1959). (11) T. H. Clarke and G. Stegeman, J. Am. Chem. SOC.,62, 1815 (1940). (12) (a) F.D. Rossini, J. Res. Natl. Bur. Std., 46, 111 (1951),butenes (2.05kcal. mole-'); (b) M. S. Kharasch, ibid., 2, 339 (1929),methyl butyrates (1.4kcal. mole-'); (c) F. D. Rossini, ibid., 35, 141 (1945), phenylpropanes (0.67kcal. mole-'); (d) G. S. Parks and G. E. Moore, J . Chem. Phys., 1, 1066 (1939),propanols (3.35 kcal. mole-'). (13) F.D. Rossini, I d . Eng. Chem., 29, 1425 (1937).

Volume 69, Number 9 September 1966

2888

series ranging from 10 to 18 carbon atoms per evenmembered fattty acid chain in order to re-evaluate the energy difference between isomers. Reliable procedures for preparing a series of isomeric monoglycerides in high chemical and physical purity and the availability of a modern, high precision calorimeter permitted accurate heat of combustion measurements for these compounds. Two monobenzoylglycerols were also included in this investigation to provide similar thermochemical evidence of relative stabilities for an aromatic monoglycerol pair whose physical properties are in the reverse relationship observed for the aliphatic series. The aromatic 2-monoglycerols exhibit higher melting points and lower solubilities than their 1-isomers, but like aliphatic 2-monoglycerides, they similarly rearrange to their 1-isomers in acidic or basic solution. This reversal in physical properties incurs uncertainty on establishing the relative stability of the aromatic isomers in the absence of thermodynamic information.

L. S. SILBERT,B. F. DAUBERT, AND L. S. MASON

Experimental Materials. Pure fatty acids used in the syntheses

2-monocaprin from an ethyl ether-petroleum ether mixture. 2-Monobenzoylglycero1crystallized from ethyl ether-petroleum ether-methanol mixture (17 :5 :1) at -5" after seeding and the hygroscopic product stored over a drying agent. Criteria of Purity. The purity of each monoglyceride was checked by capillary melting point, periodic acid oxidationlZ1acid number, saponification equivalent, and hydroxyl value.22 Periodic acid oxidation of vicinal hydroxyl groups is a distinctive test to differentiate 1-monoglycerides from the 2-isomers. 1-Monoglyceride values were within 0.5% of the theoretical values. All of the aliphatic 2-monoglycerides gave slight positive values ranging from 0.15 to 0.95%, but these are considered to be These results combined with acid number, saponification number, and hydroxyl value indicate a minimum chemical purity of 99.5%. Two physical techniques were attempted. 24 Samples of 1- and 2-monostearin analyzed by infrared spectra gave spectra that were practically identical. Recent infrared studies25report differences between the isomers, but it is apparent from this work that the technique is

were derived from commercial fatty acid sources by fractional distillation and saponification of their methyl esters followed by repeated acetone crystallizations of the purified acids, excepting capric acid which crystallized from petroleum ether. The purest grade of benzoic acid was used for the monobenzoylglycerol preparations. I-Monoglycerides. Established methods were followed for preparing l-monogly~erides~~-~~ which were crystallized from the following solvents: 1monostearin and 1-monopalmitin from methanolethyl ether cosolvent, 1-monomyristin and l-monolaurin from ethyl ether-petroleum ether (b.p. 35-50") cosolvent, and capric acid from petroleum ether. l-Monoben~oylglycerollgwas repeatedly crystallized from a 3: 1 ethyl ether-petroleum ether cosolvent that was progressively cooled to - 20" and seeded with a crystal of the compound at 5" to prevent oiling. The hygroscopic nature of the crystallized product necessitated storage over a drying agent. 2-Monoglycerides. The 2-acyl-l13-benzylideneglycerol intermediates were prepared by a slight, though critically important, modification of the Stimmel and King p r o c e d ~ r e . ~ ! ~ After reduction of the benzylideneglycerol intermediates, the 2-monolgycerides were isolated from solution and crystallized from the following solvents : %monostearin and 2-monopalmitin from methanol, 2-monomyristin from ethyl ether-petroleum ether mixture containing some methanol, and 2-monolaurin and

(14) E. Fischer, M. Bergmann, and E. Birwind, Ber., 53, 1589 (1920). (15) E. Fischer and E. Pfihler, ibid., 53, 1606 (1920). (16) E. Fischer, ibid., 53, 1621 (1920). (17) H. P. Averill, J. N. Roche, and C. G. King, J. Am. Chem. SOC., 51, 866 (1929). (18) T. Malkin and M. R. E. Shurbagy, J. Chem. SOC.,1628 (1936). (19) M.p. 41.2-41.8O; van Lohuizen and Verkades report 40-41'. (20) The Stimmel and King procedure6 using pyridine as solvent in the acylation step led to a product invariably contaminated by a palladium catalyst poison that either inhibited reduction of the 2acyl-l,3-benzylideneglycerol compounds or induced isomerization to the 1-isomer on reducing the acylals under forcing conditions. Acylation of 1,3-benzylideneglycerol in a neutral solvent like chloroform and restriction to a slight excess in the quantity of pyridine used as acid acceptor eliminated those difficulties formerly encountered in the reduction step of the intermediates and in the isolation step of the desired 2-monoglycericks . (21) (a) W. D. Pohle, V. C. Mehlenbacher, and J. H. Cook, Oil & Soap, 22, 115 (1945) : (b) E. Handschumaker and L. Lenteris, ibid., 24, 143 (1947). (22) C. L. Ogg, W. L. Porter, and C. 0. Willits, Ind. Eng. Chem., Anal. Ed., 17, 394 (1945). (23) It was observed both in this study and elsewherelo that the longer the aliphatic 2-monoglycerides remained in the acetic acid oxidizing solution, the more extensive was transposition t o the 1isomers. The method is more reliable for the more stable aromatic 2-monoglycerides which shift too slowly to be observed in the time interval of the analysis. (24) Since completion of this work, additional physical techniques have been developed for the analysis of monogylcerides such as thin layer chromatography [A. F. Hofmann, J. Lipid Res., 3, 391 (1962)], countercurrent distribution [E. S. Perry and G. Y . Brokaw, J. Am. Oil Chemists' Soc., 32, 191 (1955) 1, and vapor phase chromatography [A. G. McInnes, N. H. Tattrie, and M. Kates, ibid., 37, 7 (1960)l. (25) (a) D. Chapman, J . Chem. Soc., 55 (1956); (b) H. Susi, S. G. Morris, and W. E. Scott, J. Am. Chem. SOC.,38, 199 (1961).

The Journal of Physical Chemistry


unsuitable for determining traces of one isomer in the other. The solubility method of analysis, proposed by HerriottZ6as a physical criterion to test chemical purity, was tried on the 1-monoglycerides. The technique will not be described here owing to its lack of success, the reader being referred to the thesis' for further details. Those solubility results obtained on the three monoglycerides submitted to the test are as follows with the solvents, temperatures, and solubility in g. of compound/100 g. of solvent (reported to two significant figures in lieu of the four figures experimentally obtained) : 1-monostearin (methanol, 30", 2.5) ; 1monopalmitin (methanol, 30°, 8.0; methanol, 25", 7.8; methyl ethyl ketone, 25", 4.4; benzene, 25", 0.46) ; 1-monolaurin (benzene, 25", 2.6). The most stable physical state for these monoglycerides was required for combustion. 1-Monoglycerides are known to exhibit four polymorphs, the PL modification being the most stable form while the 2monoglycerides appear to be monomorphic. Further elaboration of this topic is made redundant by the recent review.*' The melting points obtained for the p L form of the 1-monoglycerides agreed excellently with the highest reported melting values.18s28 Melting points for the :!-isomers also agreed with their reported value^.^^^^ To supplement chemical analysis in support of the purity of monoglycerides, the following methods were used to replace the previous physical procedures. Some samples of 1-monostearin and 1-monopalmitin were solvent crystallized two or three times, followed by combustion of samples from each crystallized lot. Separate preparations of 2-monopalmitin were made from separately prepared palmitoylbenzylideneglycerol intermediates, each of which was also prepared from individually purified palmitic acid and benzylideneglycerol fractions. 1-Monocaprin, 1-monolaurin, and 1-monomyristin were obtained from their shifted 2-monoglyceride isomers, and the combustions were compared to those of the 1-monoglycerides synthesized through the isopropylidene intermediates. The remaining compounds were single preparations. Apparatus and Method. Calorimeter. The calorimeter employed in this investigation was patterned after that of DickinsonZ9and has been de~cribed.~oModifications made in the instrument have been discussed by Pl'athan3I and R u 1 0 n . ~ ~ The calorimeter chamber was thermostaked a t 25.000 f 0.003". The mass of the calorimeter can and water weighed 2200.00 f 0.05 g. using a large capacity balance with a sensitivity of f0.01 g. The "Emerson double-valved bomb" and all connecting parts within the reaction chamber were

2889

stainless steel, the bomb having a capacity of 545 ml. A measured volume of water (1.5 ml.) was added to the bomb prior to each combustion determination. Ignition wire used in the standardization and monoglyceride combustions was approximately 7.5 cm. in length per determination. The bomb was filled to an oxygen pressure of 450 p.s.i.g. after two flushings a t this pressure. The temperature-measuring equipment, consisting of a Xationttl Bureau of Standards platinum resistance thermometer, Mueller temperature bridge, galvanometer, and auxilliary mirror to lengthen the galvanometerto-scale distance, was supplied by Leeds and Northrup Co. Electrical firing energy determinations taken in the absence of sample gave an average value of 6.00 f 0.26 cal. Preparation of Samples. Since all of the compounds submitted to study were solids, the procedures for standardization and samples were identical. Each compound was dried a t 30" and 1 mm. before conibustion. ( a ) Benzoic Acid Standardization. The procedure for the standardization of the bomb was previously e ~ p l a i n e d . ~The ~ , ~benzoic ~ acid used was Sational Bureau of Standards Sample 39g, with a certified heat of combustion of 26.4338 f 0.0026 absolute joules g.-l (weight in vacuo) which calculated to 6317.83 f 0.62 cal. g.-l (weight in vacuo) by using the conversion factor of 1 cal. equals 0.0041840 absolute kjoule and using a density of 1.320 g. cm.+ a t 25" for benzoic acid. The bomb calorimeter was standardized with 1.5 g. of benzoic acid per calibration to give an approximate temperature rise of 3.3". (b) Monoglycerides. Sample weights were chosen to give a temperature rise comparable to the standardizations. A sample was transferred to a pill maker and compressed to a solid disk between two hard-surfaced cardboard disks. Any loose particles on the surfaces and edges were blown away by means of a gentle air jet before weighing the sample in a nickel crucible. The monobenzoylglycerols were hygroscopic compounds, but special precautions were not taken during the weighings as no significant weight changes were

(26) R. M. Herriott, Federation Proc., 7, 479 (1948). (27) D. Chapman, Chem. Rev.,62, 433 (1962). (28) L. J. Filer, Jr., S. S. Sidhu, B. F. Daubert, and H. E. Longenecker, J. Am. Chem. SOC.,66, 1333 (1944). (29) H. C. Dickinson, Bull. Bur. Std., 11, 243 (1915). (30) (a) T. H. Clarke, Thesis, University of Pittsburgh, 1939; (b) T. H. Clarke and G. Stegeman, J. Am. Chem. Soc., 61, 1726 (1939). (31) C. C. Nathan, Thesis, University of Pittsburgh, 1948. (32) R. H. Rulon, Thesis, University of Pittsburgh, 1951.

V o l u m e 69, N u m b e r 9

September 1965

2890

evident on reweighing several samples within a 15-min. period. Combustion Product Corrections. Unburned ignition wire and loss of weight of the nickel crucible were determined, and the appropriate combustion corrections were made for each run. Traces of nitric acid produced in the combustions were not determined on every sample. An average of three nitric acid determinations per tank of oxygen consumed was sufficient for use in the combustion calculations since the contribution of the nitric acid to the combustion values was only 0.003 to 0.00670 of the total energy. Densities of Monoglycerides. Density determinations for the solid aromatic monoglycerides were obtained by a pycnometer procedure.3 3 The insolubility of aromatic monoglycerides in isooctane permitted density measurements by displacement of their saturated solutions. Solubility in isooctane was obtained by evaporation of the saturated solutions. SpeciJic Heats. Computation of the Washburn corrections,34 requires a knowledge of the specific heat of each compound. For this purpose the method of mixtures of an elementary design was employed using for the equipment a silvered flask, copper U-tube slug (approximately 10-ml. capacity) as sample container with a close-fitting cap easily sealed with beeswax, and Beckmann thermometer, and stirrer (cf. thesis for details'). The method, on checking against calorimetric benzoic acid as a test compound, agreed within 2% of the accepted average value for 0-25'.

Results Units of Measurement and Auxiliary Data. All data reported are based on the 1961 atomic weights35 (carbon, 12.01115; hydrogen, 1.00797; oxygen, 15.9994), the 1951 fundamental constant^,^^ and the definitions: 0°C. = 273.15"K.; 1 cal. = 4.1840 absolute joules; R = 1.98717 cal. deg.-l mole-'. The laboratory standard weights had been calibrated a t the National Bureau of Standards. For use in reducing weights in air to in vacuo and in correcting to standard states, densities and specific heats of the monoglycerides are required. The densities of the solid aliphatic monoglycerides were taken from Merker's37 studies. Density determinations for the aromatic monoglycerides, obtained a t 30.00 f 0.05" and accurate to .tO.l%, are 1.302 g. ml.-' for l-monobenzoylglycerol and 1.286 g. ml.-l for 2-monobenzoylglycerol. The derived solubilities in g./100 g. of isooctane are 0.034 for 1-monobenzoylglycerol and 0.007 for 2-monobenzoylglycerol. Results of the specific heat measurements are listed in Table I together with values calculated from Kopp's law for comparison. The Journal of Physical Chemistry


Table I: Specific Heats of Monoglycerides cpr cal. deg.-l --g.

Stearin Palmitin Myristin Laurin Caprin Benzoylglyceride

'-Mono-

2-Mono-

glyceride

glyceride

Calcd."

0.407 0.410 0.411 0.390 0.398 0.291

0.407 0,404 0,400 0.380

0.421 0.416 0.412 0.408 0.403 0.314

0.288

From Kopp's law.

To compute the values of the standard heat of formation of monoglycerides, the following values were used for the standard heats of formation of carbon dioxide and water, in kcal. mole-': C02(g), -94.0517; H20(1), -68.3149. These values are from N.B.S. Circular 5003*& with corrections for changes in molecular weights of COZ and H20.38b Determination of Temperature Rise. All calculations were identical for the standardization and monoglyceride heat runs. The method of D i c k i n ~ o n ~ ~ was used for each temperature rise determination to correct for the total resistance change for heat transfer between the calorimeter and jacket. Reduction to Standard States. The complete procedure for the "Washburn corrections" and reduction to standard conditions is given by W a ~ h b u r n . ~-Go ~ and -AE,M-l are the evolved heat of the bomb process in kcal. mole-' and cal. g.-', respectively, and - A E R O is the standard change in internal energy in kcal. mole-'. Calorimetric Results. An energy equivalent series of 11 calibrations gave an average value of 2859.54 0.43 cal. deg.-' which was used in the calculations for the 12 monoglycerides studied. To ensure against any changes in energy equivalent during the combustion studies, standardizations made once every 3 or 4 days agreed excellently with the determined value.

*

(33) N. Bauer, "Physical Methods of Organic Chemistry," Vol. I, Part 1, A. Weissberger, Ed., Interscience Publishers, Ino., New York, N. Y.,1949,Chapter VI, pp. 253-296. (34) E. W. Washburn, J . Res. Natl. Bur. Std., 10, 525 (1933). (35) A. E. Cameron and E. Wichers, J . Am. Chem. Soc., 84, 4176 (1962). (36) Natl. Bur. Std. (U.8.) Tech. News Bull., 47, 175 (1963). (37) (a) D. R. Merker and B. F. Daubert, J . Am. Chem. Soc., 80, 516 (1958); (b) D. R.Merker, Thesis, University of Pittsburgh, 1951. (38) (a) F.D. Rossini, D. D. Wagner, W. H. Evans, S. Levine, and I. Jaffe, Nationd Bureau of Standards Circular 500,U. S. Government Printing Office, Washington, D. C., 1952; (b) N. K. Smith, D. W. Scott, and J. P. McCullough, J . Phys. Chem., 68, 934 (1964). (39) H.C.Dickinson, Bull. Bur. Std., 11, 189 (1915).


The heat of combustion data obtained for 2-monopalmitin are typical of all the experimental data. These data are shown in Table I1 where AR is the corrected resistance rise in ohms; AT, the corrected temperature rise in "C.;m, the in vacuo mass in grams; Ow, the Washburn corrections; A& the total heat correction for the burned fuse wire, nickel crucible loss, nitric acid formation, and ignition energy; EaPP, the energy equivalent of the calorimeter system; and M , the molecular weight. Table 11: The Standard State Heat of Combustion of 2-Monopalmitin at 25" A&

AT,

cor. rise,

cor. rise, OC.

Sample ohm A-1 2 3 4 B-1 2 3 4

0.312411 0.284771 0.270039 0.388641 0.398507 0.359066 0.391025 0.270703

3.08862 2.81507 2.66963 3 34193 3.94036 3.55060 3.86730 3.66522

- AEo/ M, EsppATv

AEw,

081.

081.

8,832.04 19.25 8,049.79 17.25 7,633.90 16.33 10,986.16 24.54 11,267.60 25.62 10,153.09 22.80 11,058.71 25.38 10,480.86 23.48

cal.

AEZ,

oal. 16.28 13.63 18.90 17.18 15.22 15.47 19.07 20.43

7%

B.

1.05118 0.96789 0.90795 1.30777 1.34146 1.20885 1.31606 1,24712 Mean

g. -1

8404.85 8407.45 8405.01 8406.31 8407.26 8405.03 8407.69 8406.49 8406.26

Std. dev.: s = 1.17; 5 =. 1.33

Results for the two series of monoglycerides are summarized in Table 111. The standard deviations ranged from f0.014% for 2-monopalmitin to *0.045% for 2-monobenzoylglycero1 and averaged f 0.027% for the combined series. Derived Results. The standard heat of combustion, AH,", and the heat of formation, AHr0(298.150K.), Table III: Summary of Combustion Experiments

Compound

Formula

No. of samples measd.

A E o a / M & 3,

cal.

g.-1

2-Monoglycerides cu&o4 8 2-Monostearin CloHaO4 8 %Monopalmitin CI,HuOd 8 2-Monomyristin Cl&H&4 7 2-Monolaurin CiaHzsO4 6 2-Monocaprin 2-Monobenzoylglycer01 C10H1204 7

8617.85 f 1.52 8406.26 f 1.33 8158.17 f 1.41 7863.28 f 1.42 7497.54 f 1.69 5942.66 f 1.31

1-Monoglycerides cu&o4 8 1-Monostearin Ci&&( 8 1-Monopalmitin C1,HarOi 10 1-Monomyristin ClsHsoO4 7 1-Monolaurin ClaH2aO4 8 1-Monocaprin Cl&zO4 7 1-Monobenzoylglycerol

8607.11 f 1.16 8397.18 f 1.56 8150.57 f 1.15 7856.16 f 1.41 7484.79 f 1.21 5937.07 f 1.34

2891

in kcal. mole-l for the solid state are given in Table IV with the results of AE,and AER. In addition to these values, the differences in the heats of formation between 2- and l-monoglycerides representing the heat of isomerization and the energy increment per CH2 group, AH(cH*), within each of the two isomeric series are assembled in Tables V and VI, respectively. Treatment of Datu. The procedure of Rossini and Deming" was followed in the assignment of uncertainties. An estimate of the standard deviation for single observations of a set of n measurements is denoted by s, and the "over-all" standard deviation assigned to the average mean obtained from the combination of all of the standard deviations involved with each of the sets of observations in accordance with the law of propagation of precision indicies is denoted by S. The accuracy interval S assigned to the arithmetic mean of a set of combustion determinations is determined from the uncertainty interval assigned to the thermometric value for the calorimetric standard benzoic acid, the standard deviation of the arithmetic mean of the set of calibrations, the estimate of the standard deviation of the arithmetic mean of a set of monoglyceride combustion determinations, and the uncertainty interval assigned to the Washburn corrections and firing energy, the latter two combined in an estimate of 1.0 ca1./10,000 cal. For a rejection of a few divergent measurements observed in this investigation, Chauvenet's criterion was conveniently applied.41 Interpretation and Significance of Data. The heats of isomerization obtained in this investigation are in the order of magnitude normally expected for isomeric pairs of compounds with values ranging from - 1.99 to -3.84 kcal. mole-I for the aliphatic isomers and -1.08 kcal. mole-' for the aromatic isomers. These results contradict Clarke and Stegeman's isomerization value of -9.52 kcal. mole-'. Their heats of combustion for 1-monopalmitin and 2-monopalmitin are 0.16 and 0.40y0 higher, respectively, and their isomerization energies are approximately three times our values which are internally consistent, The decreasing progression in the heats of isomerization for the monostearin-monolaurin sequence and the relatively large increase for the monocaprin pair are of interest. The data may be reviewed by the following considerations: (1) the progression is spurious and could be represented by an average value since (40) P.D.Rossini and W. E. Deming, J. Wash. Acad. Sci., 29, 416 (1939). (41) A. G. Worthing and J. Geffner, "Treatment of Experimental Data," John Wfiey and Sons, Inc., New York, N. Y., 1943,pp. 170, 319.

Volume 69,Number 9 September 1966

2892


Table IV : Summary of Derived Values for 25'

- AE,,

-' A E R O ,

koal. mole-1

- AH,',

kcal. mole-'

- A H f o (298.15'K.), koal. mole-1

ZMonostearin %Monopalmitin ZMonomyristin ZMonolaurin ZMonocaprin ZMonobenzoylglycerol

3090.07 f 0.55 2778.37 f 0.44 2467.50 f 0.43 2157.72 f 0.39 1847.02 f 0.42 1165.98 f 0.26

%Monoglycerides 3088.85 f 0.55 2777.28 f 0.44 2466.49 f 0.43 2156.76 f 0.39 1846.20 f 0.46 1165.10 f 0.26

3093.89 f 0.55 2781.72 f 0.44 2470.34 f 0.43 2160.02 f 0.39 1848.87 f 0.46 1165.69 f 0.26

315.81 f 0.63 303.25 f 0.52 289.89 f 0.49 275.48 f 0.45 261.90 f 0.50 184.72 i 0.29

l-Monostearin l-Monopalmitin l-Monomyristin l-Monolaurin l-Monocaprin l-MonobenzoyIglyceroI

3086.22 f 0.42 2775.37 f 0.51 2465.21 f 0.35 2155.76 f 0.39 1843.88 f 0.30 1164.88 f 0.26

l-Monoglycerides 3085.01 f 0.42 2774.23 f 0.51 2464.07 f 0.35 2154.77 f 0.39 1843.04 f 0.30 1164.02 f 0.26

3090 05 f 0.42 2778.67 f 0.51 2467.92 f 0.35 2158.03 f 0.39 1845.71 f 0.30 1164.61 f 0.26

319.75 f 0.52 306.30 f 0.58 292.31 f 0.43 277.47 f 0.45 265.06 f 0.35 185.80 f 0.29

Table V : Heats of Isomerization for Monoglycerides"

- [AHoo(2-MG) - AHo"(l-MG)I, koal. mole-1

Monostearin Monopalmitin Monomyristin Monolaurin Monocaprin Monobenzoylglycerol a

koal. mole-1

3.84 f 0.69 3.05 f 0.67 2.42 It 0.55 1.99 f 0.55 3.16 f 0.55 1.08 f 0.37

Enthalpy difference between 2- and l-monoglycerides.

Table VI: Energy Increment per CHt for Aliphatic Monoglycerides -AH(CH~),~ kcal. mole-12-Monoglycerides l-Monoglycerides

Stearin-palmitin Palmitin-myristin Myristin-laurin Laurin-caprin

155.78 f 0.35 155.40 f 0.30 154.82 f 0.29 155.28 f 0.30

155.39 f 0.33 155.08 f 0.31 154.65 z t 0.26 155.87 f 0.25

A.H(CH~)= one-half the enthalpy difference between successive even-carbon homologs.

a heat of isomerization is obtained by the difference between two large numbers that would result in a relatively large error; (2) the progressive decrease in the monostearin-monolaurin sequence is valid, but the monocaprin value is in error; or (3) the observed sequences from monostearins to monocaprins are real and acceptable. With reference to (l), were the average value of -2.89 kcal. mole-' most probable, the extreme values of monostearin and monolaurin The J O U T Wof~ Physical chemistry

~

would not overlap this average within their experimental precisions. However, the difference of nearly 1.9 kcal. mole-' between monostearin and monolaurin exceeds the sum of their experimental precisions by 1.5 S in partial support of the observed progression even though differences between adjacent homologous pairs are exceeded by their combined deviations. With reference to (a), if each of the observed isomerization energies is discrete, the increase exhibited by the monocaprins requires explanation, though only speculation can be made a t this time. The higher isomerization energy for the monocaprins would not likely be attributed to a higher energy polymorphic form as a contaminant in the l-isomer for this would lead to a diminished energy difference between the isomers. A higher energy polymorph for 2-monocaprin, should it exist, could account for the difference. It has been recently reported that 2-monoglycerides show the existence of another polymorphic but this claim, based on solubility studies, must be held in reservation pending confirmation by techniques other than solubility, particularly in view of the complications encountered in our solubility test. In consideration of (3), t,he isomerization energies reported in this paper refer to isomer differences determined for the solid states so that the observed sequence may be a reflection of differences in their sublimation energies. With differences in sublimation energies between successive homologs amounting to about 1.5 kcal. mole-' (vide infra), i t appears unlikely that this large a difference would exist between closely structured long-chain isomers. Unfortunately, the data currently available on sublimation energies (42)

S.c. Smith, Dissertation Abstr., 23, 2326 (1963).


of fatty acid derivatives do not permit more definite conclusions. Since the heats of isomerization show a progression, the energy increment per CH2 for this data must similarly progress. The CH2 energy increments of Table VI differ by 1.4 to 2.6 kcal. mole-' from the CH2 increment reported by Clarke and Stegeman, who claim agreement with Rossini's value13 for gaseous hydrocarbons. The agreement between their CH2 energy increment obtained for the solid state and the gaseous state CH2 energy increment is entirely fortuitous; it is well known that the CH2 energy increments for these different states of aggregates differ by an amount dependent upon the heat of sublimation. The most recent values for the CH2 energy increment in AH,(g) are 157.44 kcal. mole-' for the normal paraffins above C4H10 and 157.46 kcal. mole-' for alcohols above C4H90H.43 Sublimation energy CH2 increments of 1.5 to 1.8 kcal. mole-' are reported for several classes of The increments per CH2 in AH,(s) are, therefore, expected to be in the range of 155.5-156 kcal. mole-' in support of the increments derived for the monoglycerides in Table VI. Consequently, Clarke and Stegeman's CH2 energy increment must be in error by a t least 1.5 kcal. The heat of isomerization (1.08 kcal. mole-') for the aromatic monoglycerides is of the same order of magnitude as reported for the propylbenzenes (0.67 kcal. mole-') 12' but is approximately one-third of the aliphatic monoglyceride values. Although the isomerization energy is small, nevertheless, 2-monobenzoylglycerol is energetically comparable to the aliphatic 2-monoglycerides by having a heat of combustion larger than its 1-isomer. This is supporting evidence of its lower thermochemical stability which is contrary to the relative stability that would generally be deduced from the reversed relationship observed from its melting point and solubility characteristics. Thermochemistry of the Equilibrium. In the relation AF" = AHo- TAS" the free energy change equals the enthalpy change when AS" is zero and is related to the equilibrium constant of the reversible reaction, 2monoglyceride 1-monoglyceride, by the equation AF" = -RT In (l-MG/2-MG). (1-MG/2-R/IG is the per cent ratio of 1-monoglyceride to 2-monoglyceride.) The per cent of each isomer a t equilibrium, calculated from the isomerization energy of - 1.08 kcal. mole-', is 14% Zmonobenzoylglycerol and 86% l-monobenzoylglycerol. This result is in excellent agreement with the position of the equilibrium for the aromatic monoglycerides determined kinetically by van Lohuizen and VerkadelO to be 12% 2- and 88% 1-monoglyceride. (Conversely, by use of l-MG/B-MG = 88:12, AFo

2893

is computed to be -1.18 kcal. mole-' in comparison with our experimental value of - 1.08 kcal. mole-'.) The agreement between AF" and AH" confirms a negligible, if any, entropy difference of AS" = 0.3 A 1.0 cal. deg.-' mole-' between these isomers. The equilibrium position for the monopalmitin pair has been determined by Martinqsto be 10-8% 2monopalmitin and 90-92% 1-monopalmitin, a result substantially confirmed (15-12% 2-isomer and 8588% 1-isomer) by Brokaw, Perry, and Lyman.46 Assuming a zero entropy difference and, as a first approximation, equating AF" to the transition energy of -3.05 kcal. mole-' for the monopalmitins, the equilibrium is calculated to be 0.5% 2- and 99.5% 1monopalmitin, a result that disagrees significantly with the reported equilibrium values. The assumption that AS" for the aliphatic monoglycerides is zero or negligible appears to be unjustified. Calculating for AS" by using the Martin range of values for 1MG/2-MG = 9O:lO and 92:8, AF" = -1.30 and - 1.44 kcal. mole-', respectively, and combining these results with AH" = -3.05 f 0.67 kcal. mole-l, AS" is calculated to -5.6 f 2.2 cal. deg.-l mole-l. (Similarly, Brokaw's values lead to AS" = -6.7 f 2.2 cal. deg. mole-'.) The negative entropy difference for the aliphatic monoglycerides" may be tentatively interpreted on the basis of available, though limited, physical data on the solid compounds. Nuclear magnetic resonance of monoglyceridesPB shows that the ,& forms give rise to large second moments for l-monostearin (19.9 f 1.7 gauss2), l-monomyristin (19.7 1.3 gauss2), and 2-monopalmitin 0.9 gauss2). The smaller second moment (18.2 for 2-monopalmitin shows that for this isomer the hydrocarbon chain has relatively more freedom of motion about the chain axis.

-'

*

*

(43) J. H.S. Green, Chem. Ind. (London), 1215 (1960),andreferences contained therein. (44) (a) A. R. Ubbelohde, Trans. Faraday Soc., 34, 282 (1938); (b) K. L. Wolf and H. Weghofer, 2.physik. Chem., 339, 194 (1938); (c) M. Davies and V. E. Mdpass, J . Chem. SOC.,1048 (1961); (d) H.A. Swain, Jr., L. S. Silbert, and J. G. Miller, J. Am. Chem. SOC., 86, 2562 (1964),and references contained therein. (45) J. B. Martin, ibid., 75, 5483 (1953). (46) G. Y. Brokaw, E. S. Perry, and W. C. Lyman, J . Am. Oil ChemiStS' SOC., 32, 194 (1955). (47) A referee validly objected to the ASocalculation for not having allowed for activities of the components in the measured solution systems for obtaining the equilibrium constant and similarly correcting the heats of formation by heats of sublimation prior to computing the heats of isomerization. Lacking such data, it may be conjectured that the over-all effects are small as the quantitative differences between the isomers are expected to be nil. It is statistically significant that the absolute value of ASois about 2.6 times the standard deviation. (48) D.Chapman, R. E. Richards, and R. W. York, J. Chem. SOC., 436 (1960).

Volune 69, Number 9 September 1966

2894

Infrared spectroscopicexamination of the OH stretching band (3200-3600-cm.-' region) and of the carbonyl stretching band (1700-cm.-l region) has been reported In 1for monoglyceride isomers and polymorph~.~5~~*9 monoglycerides, the OH and C=O stretching bands are observed to move to lower frequencies and higher frequencies, respectively, in the order liquid -t aL -t sub-aL -t PL' -t PL, which is also the order of increasing stability of the polymorphic forms. Chapmanzsa interpreted this to be the order of increasing hydrogen bonding strength with preferential bonding occurring between OH groups rather than between OH and c----O groups. The 2-monoglycerides similarly prefer stronger hydrogen bonding between OH groups than between OH and C=O groups on passing from the liquid to the / 3 ~form, its only known crystal form. A comparison of bands between the PL forms of 1- and 2-monoglycerides indicates the former to have the stronger interaction between OH groups and the weaker OH to C=O group interaction. The comparison lends evidence of stronger interactions and lesser mobility of the chains for the l-isomer. The l-monoglycerides have melting points about 10" higher than the 2-isomers. Malkin60 suggests that the hydrogen bonding of 2-monoglycerides is of the head to head type and that l-monoglycerides, because of their higher melting points, may involve lateral hydrogen bonding in addition to the normal head to head. By having lower melting points, the 2-monoglycerides

The J o u d of Physitd Chemistry

L. S. SILBERT, B. F, DAUBERT, AND L. S. MASON

may be expected a priori to have larger long spacings and, inferentially, larger molar volumes ; however, 28 smaller long spacings were observed We calculated molar volumes from their solid densitiesn$sl from which the 2-monoglycerides are larger by 0.5 to 2.9 ml. mole-' for the monocaprin-monostearin sequence.62 These physical properties, separately and in toto, are evidences of larger restraints in crystalline l-monoglyceridea that would also lower their entropy relative to the crystalline 2-isomers. No supporting evidence is presently available to account for the nearly zero ASofor the aromatic monoglycerides whose crystal structures would not be expected to correspond to those of aliphatic structures. (49) Bands for 1-monostearin and I-monopalmitin are as follows: liquid state (3453 cm.-l; 1706 cm.-l; UL form (3360 om.-'; 1720 cm.-l); sub-q, form (3342 cm.-1; 1730 om.-'); BL' and @L forms are similar by having an OH split, e.g., BL' form E3342 (main component) and 3243 cm.-l; 1736 cm.-l] and p~ form [3243 (main component) and 3307 cm.-1]. Analogous bands for Zmonomyristin are 3527 and 1715 cm.-l for the liquid state and 3347 and 1733 om.-' for the ,TI, form. (Sa) T.Malkin, Progr. C h . Fats Lipide, 2,34 (1954). (51) Solid densities for monoglycerides were reported for measurements a t 2l0.ma Merker's measurements a t 30°s7b in g J d . are listed aa follows for convenience: Zmonostearin, 1.044; I-monostearin, 1.053; l-monopalmitin, 1.060; l-monomyriatin, 1.068; l-monolaurin, 1.077; l-monocaprin, 1.088. (52) Although solid densities of the isomers differ only by 0.0020.009 g./ml. for the monocaprin-monostearin series and these dMerences may be within the experimental precision of their measurements, it is striking that the %isomers are uniformly leea dense (with the exception of l-monostearin a t 21 O) than their corresponding l-isomers.

The Heats of Combustion, Formation, and Isomerization of Isomeric

Recommend Documents