PINIC ACID DIESTERS Preparation, Properties, and Lubricant Applications C. M. MURPHY, J. G. O'REAR, AND W. A. ZISMAN Naval Research Laboratory, Washington 25, D . C.
*
b
A
resistance to oxidation manifested by the pinonates was caused by the keto group, it was concluded t h a t the pinates were the more promising materials from which to develop stable lubricants. This does not imply that the pinonates may not be very useful for plasticizers or other applications. Diesters containing cyclic groups have not found extensive use in low temperature lubricant applications because of either their large temperature coefficients of viscosity or their high freezing points (or pour points). Experience with such esters is based largely upon the phthalate esters and others prepared from alcohols containing phenyl or cyclohexyl groups. Since the cyclobutyl ring is much smaller than a six-membered ring, its presence in the oil molecule should not cause such adverse effects on the combination of properties required of a low temperature oil (4, 26). The authors were confident t h a t the dimethyl substituents on the cyclobutyl ring, which increase the asymmetry of the molecule, would act to prevent the close alignment of neighboring moleculcs and so produce very low freezing points. T h e presence in such compounds of four geometrical and optical isomers, which will be discussed subsequently, should cause a n additional lowerCHs ing of the freezing points. It has been demonstrated t h a t I branching near the center of the molecule is more effective in C lowering the freezing point and has a less adverse effect upon viscosity index than does branching near the end of t h e molecule (26). Diesters of pinic acid made with normal alcohols and especially the Ca t o Clo alcohols would have this type of configuraOxidation I1 / \ tion, and therefore would be expected to have high viscosity CH3-C-CH HC-CH, -OH C indexes, low freezing points, and lower viscosities a t -40" F. than \ / the phthalates. Because of the similarity of these dialkyl pinates CHz t o other aliphatic dibasic acid esters used as low temperature H2C, plasticizers, i t was conjectured t h a t such pinates should have \ I / interesting possibilities for this application. \ I C/ I n the next phase of the program, the Southern Regional ReOxidation H search Laboratory was asked t o prepare small quantities of the n-amyl, n-hexyl, n-octyl, and 2-butoxyethyl diesters of pinic acid a-Pinene for further study by this laboratory. One ester, CHs CHa CHI CHs dihexyl pinate, was t o be prepared in larger volume \ / \ / 0 for lubricant studies. Soon afterwards, these and 0 C' 0 0 % /I / \ Oxidation I1 / \ other pinate diesters of exceptional purity were preEO-C-CHHC- H Ho--C-cH HC--CH2-8-oH pared t o obtain more data on the relation of struc\ / ture to properties and for comparison with the CHz materials prepared in pilot plant quantities. Later Norpinic acid Pinic acid the S R R L undertook the development of oxidation methods suitable for the low cost commercial production of pinic acid. Although the earlier oxidation product pinonic acid appeared to be cheaper to produce than pinic acid, it was not expected to be sufficiently oxidation stable for the intended uses. However, it was SYNTHESIS AND PURIFICATION O F PINATE DIESTERS agreed that esters of this acid should also be prepared and studied. An examination of the structure of pinic acid reveals that stereoA plan was evolved for cooperative research in which the Southern isomers may result from both geometrical and optical isomerism Regional Research Laboratory was t o make the 2-ethylhexyl (Figure 1). If the cis configuration is assumed t o be the ddester of pinonic acid and the corresponding diester of pinic acid enantiomorph, a total of four enantiomorphs are possible: ddfor a preliminary study by this laboratory. cis-; dl-cis-; dl-trans-; and Id-trans-pinic acids (as shown beEarly experiments demonstrated t h a t both compounds had low). Moreover the two cis-enantiomorphs may form a racemic pour points below -56" F. and viscosity indexes of 60 and 86, dimer and the two trans-enantiomorphs may likewise form a respectively. Accelerated thermal oxidation tests using air at racemic dimer. 125' and 150' C. revealed t h a t not only was the pinate diester Until recently, the formation of cis-norpinic acid as the end prod $he more stable, but that it was also more effectively stabilized uct of the oxidation of pinic acid had been taken by some investib y a good antioxidant like phenothiazine (26). Since thelesser
LIPHATIC diesters are in increasing demand for the extreme low temperature lubrication of turbojet engines (21), instruments ( 7 , 2 2 ) ,machine guns ( 6 ) ,and aviation and ordnance greases (18-20). Certain of these diesters are also useful as low temperature plasticizers. A variety of end products compete for t h e dibasic acids, the branched-chain alcohols, and even the diesters themselves. Since 1942 the growing demand for diesters has made it desirable to increase the domestic supply of cheap dibasic acids suitable for the preparation of lubricants and low temperature plasticizers, Products obtainable from native raw materials are especially desirable. During a joint discussion on applications of terpene chemistry with representatives of the Southern Regional Research Laboratory (SRRL) of the U. S. Department of Agriculture, the authors pointed out the possibilities of using certain dibasic acids derivable from or-pinene, a major constituent of turpentine. The principal acids formed in the stepwise oxidation of a-pinene are shown below with structural formulas written t o emphasine the analogy with an aliphatic dibasic acid:
\ /CH3
/ha 1 --
4
--
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INDUSTRIAL AND ENGINEERING CHEMISTRY
120
gators to indicate t h a t pinic acid had a cis configuration. Now it is generally agreed ( 1 2 , 13, 6%) t h a t both synthetic and natural pinic acids, as well as hydroxypinic acid, have trans configurations and t h a t the change to the cis form occurs only at the last stage, when trans-hydroxypinic acid is oxidized to cis-norpinic acid.
HzCOOH
GO
CH2COOH
dd-cis-Pinic acid
GO
dl-trans-Pinic acid
co HzCOOH
11-cis-Pinic acid
Id- trans-Pinic acid
c
J-
Racemic dimer Racemic dimer *Asymmetric carbon atom Figure 1
The preparation of pure pinic acid free from racemic modifications has been described by Grandperrin ( 1 2 ) . The acid obtained by the alkaline permanganate oxidation of &-pinene was particularly resistant to cyclization and was therefore believed to be d-trans-pinic acid. Its properties are shown in Table I. Small scale synthesis of pink acid by the stepwisealkaline oxidation of pinene to pinonic acid and of pinonic acid to pinic acid has been reported by several investigators (8, 1.2, IS). The acid from SRRL was obtained by the successive oxidation of d-a-pinene t o pinonic acid with alkaline permanganate solution and of pinonic acid to pinic acid with alkaline hypochlorite solution (11). The crude acid assayed 80 to 85yGpinic acid, the major impurity being pinonic acid.
TABLE I.
PROPERTIES O F P l K I C
Experimentally Detd. Values Property 73-76a Melting point, C. 185-187/1.0 mm. Hg Boiling point, C. 163-165/0.2 mm. Hg Optical rotation, - 1 . 2 (Chloroform, [el$? c = 4.10)b
Weutraliaation equivalent
- 1 . 6 (water, c = 4.29) 6 . 6 (acetone, c = 7.60)
ACID
Literature Values (1.2)
74a
185-186/2.0 mm. H g
- 1 . 4 (chloroform, c = 9.7)"
- 1 . 9 (chloroform,
c = 9.7)C - 4 (water, c = 4 . 9 ) " 5 acetone,^ = 6 . 2 ) C
93.10 (theory) 9 3 . 2 2 (found)
Calculated Elemental assay 68.09 58.05 % Carbon 7.84 7.58 % Hydrogen 5 After being distilled twice a n d worked on a clay plate. b e = grams solute/100 ml. solution. C After recrystallizing the redistilled acid from a mixture of petroleum ether and diethyl ether.
The crude acid (8.0 moles) was converted to the dibutyl ester and further purified by the procedure described: Preliminary distillation gave a 79.0% yield of material; boiling point 180-188" C./lO.O mm. Hg. Redistillation through a 20plate column yielded a product: boiling point 182-184' C./ 10.0 mm. Hg; n % O 1.4480; dZ,O0.9710. On the assumption that t h e impurity was butyl pinonate, the purity of the dibutyl pinate was estimated t o exceed 99.0%. The redistilled dibutyl pinate (5.00 moles) was saponified over a 24-hour period with an excess of alcoholic potassium hydroxide. After removal of the alcohols t h e reaction mixture was acidified with an excess of hydrochloric acid and dried in vacuo. Ether extraction separated the pinic acid from the potassium chloride. Two distillations of the acid
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from t h e ether extract gave the product in 74.8% yield (based on dibutyl pinate). The acid solidified completely into white crgstals after standing a few hours. On the basis of properties shown in Table I, this acid is believed to be identical with the trans-pinic acid reported by other investigators (8,12,13, $ 7 ) . Each alcohol used in the preparation of the various esters waq distilled through a 20-plate spiral wire column. Their properties agreed with the best literature values. All of the esters of thip report were prepared by a general procedure which involved refluxing a mixture of the appropriate alcohol with pinic acid in tho presence of toluene and a small amount of p-toluenesulfonic acid monohydrate. T a t e r formed during the reaction was removed azeotropically and collected in a calibrated water trap. Refluxing was continued until the formation of water ceased. Crude esters were recovered from the residual mixture by distillation through a 16-inch Vigreux column a t reduced pressure. Essential data on the preparation of the various pinate esters are given in Table 11. I n all instances the distilled esters (water-white in color) were stripped through a helix-packed column against a counterflow of carbon dioxide gas (5 mm. of mercury pressure, 100' C.) ($4)and then percolated through Florisil and alumina to obtain the specially purified esters with neutralization numbers of less than 0.05. Physical and chemical constants of interest in the identification and characterization of the various pinate esters are given in Tables I11 and IV. The surface tensions were measured by the ring method using the Harkins and Jordan (16)corrections. The results are correct within &0.3%. Attempts to obtain the saponification numbers of the esters using A.S.T.M. Method D 9448T were unsatisfactory as the compounds were incompletely saponified (approximately 657,) in 3 hours. Refluxing for 8 hours resulted in about 80 t o 85% saponification. Doubling the excess of alcoholic potassium hydroxide and refluxing for 48 hours gave complete saponification; however, the end points were obscured by the presence of silicates from the glass vessel. The saponification numbers obtained agreed within zklyo with the theoretical. Since the compounds synthesized were new, they were further identified by comparing the calculated and observed molecular refractions and parachors. Excellent agreement between the calculated and the observed refractions and parachors was obtained, as shown in Table IV. It has been observed by many investigators that branching lowers both the molecular refraction and the parachor. The extent of this deviation depends on the class of compound and the position of the branching. The small lowering observed on both the molecular refraction and the parachor is consistent for the entire series of pinate esters and is evidently due to the gem dimethyl branching on the cyclobutane nucleus. I n going from dibutyl pinate t o didecyl pinate it is noteworthy t h a t the observed increase of either the molecule refraction or the parachor agrees very closeiy with increases calculated from either structural or bond constants. PROPERTIES O F PINIC ACID DIESTERS O F INTEREST IN LUBRICATION
FREEZING POINT.Some of the properties of the pinates of special interest in the development of lubricants are given in Table V. Since -65' F. is the lowest temperature required in current military specifications for lubricants, every pinate not having a freezing or a melting point was subjected to storage tests of 72 hours' duration at 5" F. intervals down to -75' F. Only the bis(2-butoxyethyl) and the didecyl pinates could be made to crystallize. The dimethyl substituted cyclobutane ring in the pinic acid molecule hinders the close alignment of neighboring molecules, and thus liquids with low freezing or pour points were obtained. There v a s no important difference in the properties listed in Table V of the purified pinate diesters prepared by this laboratory and the SRRL except that the bis(2-butoxy-
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1953 TABLE11. DATA ON Diester Dibutyl pinate Diamyl pinate Dihexyl pinate Diheptyl pinate Dioctyl pinate Didecyl pinate Bis (2-butoxyethyl) pinate
Grams 55.6 52.1 149.9 130.3 235 0 186.2 151.1 93. I 247.4 149.0 300 8 149.0 271.8 186.2
THE
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0nA.S.T.M. Chart D 341 of log-log viscosity versus log temperature. The graph of bis(2-butoxyethyl) pinate curved away from the temperature axis a t M1* subzero temperatures. Found Both bis(2-butoxyethyl) and diheptyl pinate 10.8 have 21 atoms in the principal chain of the mole26.5 cule and they also have essentially the same molecular cross section. They differ only in that two 36 0 methylene groups in the heptyl ester have been replaced by ether oxygen atoms. From a compari18.3 son of dialkyl ethers and alkanes (26) the 2-bu28.8 toxyethyl diester should be the less viscous a t high temperatures and have the greater A.S.T.M. slope. 28.8 However, the viscosity of the former a t subzero 37.8 temperatures is greater than would be predicted from its A.S.T.M. slope (100" to 210" F.) as the graph curves upward. The upward curvature of the graph is possibly a manifestation of association. VOLATILITY.I n many applications, relubrication intervals are governed by the volatility of the lubricant, hence the oil should be as nonvolatile as possible, consistent with the other characteristics required. The weight losses of the pinates a t 210' F. by A.S.T.M. Method D 972-48T are given in Table V. Only the dibutyl and diamyl pinates had weight losses exceeding 1%-4.5 and I.8%, respectively. SPONTANEOUS IGNITION TEMPERATURE. The spontaneous ignition temperature of each pinate diester was determined in the apparatus described by Sortman, Beatty, and Heron (29). Drop sizes of approximately 10 mg. with an air flow of 125 cc. per minute were used, as in previous work ( 4 , SO). The spontaneous ignition temperatures of the pinates (last column of Table V) were all above 700" F., increasing with increasing molecular weight of the alkyl esters. It is interesting to note that the spontaneous ignition temperature of bis(2-butoxyethyl) pinate is more than 20' F. below its analog, diheptyl pinate. This is in conformity with the conclusion that ethers have lower spontaneous ignition temperatures than paraffins of equal chain length (IO). As the spontaneous ignition temperature of a compound varies with the fuel t o air ratio (IO,d9), it is likely that the values reported here are not the minimum that could be obtained. Properties of the dialkyl pinates are also compared with those of the 2-ethylhexyl diesters of aliphatic acids in Table V. These two series of esters have an equal number of atoms in the princi-
PREPARATION OF PINATE DIESTERS
Reflux Reactants Yield, Time", Moles % ' Hours 0 . 7 5 1:Butanol 93.0 3 0 . 2 8 Ptnic acid 1 . 7 l-.Pentanol 90.0 4 0 . 7 Ptnic acid 2 . 3 1-Hexanol 85.0 5 1 . 0 Pinic acid 95.8 3 1 3 1-Heptanol 0 . 5 Pinic acid 1 . 9 1-Octanol 85.5 4 0 . 8 Pinic acid 1 . 9 1-Decanol 89.4 5 0 8 Pinic acid 2 . 3 2-Butoxyethanol 92 5 4 1 . 0 Pinic acid
Time required for removal of water.
Theory 10.1 25.2 36.2 18.3 28 8 28.8 36.0
ethyl) pinate of the SRRL could not be made to freeze a t -50°F.; however, it failed to pour after 72 hours a t -75" F. From these data it is evident that the pinates prepared from the normal alcohols C* through Ca have sufficiently low freezing points (or pour points) to care for current requirements for low temperature lubricants. VISCOSITY.There is a progressive increase in the viscosity of the normal pinate diesters with increasing chain length, increasing from 7.1 centistokes at 100" F. for the butyl to 20.9 centistokes for the decyl diester. The cyclobutane group adds t o the over-all length of the pinate molecule in its normal or "stretched out" configuration approximately the equivalent of two aliphatic carbon atoms. As in earlier work (4, 26) the ether oxygen atom in each ester group has been counted as one of the atoms in the principal chain. Thus, the total number of atoms in the principal chain of the molecule of the pinates of Table V varies from approximately 15 for the dibutyl ester to 27 for the didecyl ester. These pinate diesters are in the same chain length and viscosity range a~ the aliphatic diesters made from sebacic, azelaic, and adipic acids-all of which are now in wide use in lubricant applications. VISCOSITYINDEX. The temperature coefficient of viscosity decreases with increasing chain length if the molecular chain i s f l e x i b l e ( 2 8 ) . Thus the A.S.T.M. (Dean and Davis) viscosity index increases from 97 for the dibutyl ester to 158 TABLE111. ANALYTICAL DATAON PINATE DIESTERS for the didecyl ester and the Mole& Boiling A.S.T. M. viscosity-temperaular PointMm. Specific Rotation, Carbon, % Hydrogen, % Empirical Weight ture slope decreases from 0.77 Compound Formula (Theory) O C. of Hg [a]%0 Calcd. Found Calcd. Found to 0.67. A s t h e m o l e c u l a r Dibutyl pinate CliHsoO4 298.41 126 0.4 -0.28 68.56 10.13 68.42 10.36 cross section remains constant Diamyl pinate Ci~Ha404 326.46 150 0.4 -1.03 69.90 69.92 10.50 10.14 CziHaaOi 354.51 164 0.4 -0.58 71.14 71.50 10 81 10.71 while the length increases in CnrHaOi 382.57 178 0.4 -0.19 72.20 72.28 11.07 11.01 192 0.4 -0.39 73.12 73.07 going t o the higher homologs, Dioctyl pinate C26H4604 410.62 11.29 11.07 Dideoyl pinate CasHsrO4 466.72 219 0.4 -0.13 74 63 74.69 11.66 11.48 the viscosity index should inBis(2-butoxyethyl) CziHaaOa 386.51 178 0.4 -1 22 65.25 65.28 9 91 9.85 pinate c r e a s e , s i n c e t h e r a t i o of breadth t o length decreases. Because b i s ( 2-e t h y 1h e x y 1 ) TABLE IV. MOLECULAR REFRACTIONS AND P A R A C I ~ OFOTHE R ~ PINATE ESTERS pinate contains two ethyl side Surface Moleaular Refraction Parachor chains which increase the effecIndex of Calculated0 Refraction, Tension, Calculateda tive cross section of the mole20' C StrucStruccule, it is considerably more Compound n so G./Mi.' Dynes/&. turd Bond Found tural Bond Found 1.4478 0.9702 30.49 82.37 82.54 82.30 736.6 735.4 722.7 viscous than dihexyl pinate and 1.4498 0.9578 30.40 91.56 816.6 815.4 8 0 0 . 4 91.66 91.83 has a smaller viscosity index. Dihexyl pinate 1.4512 0.9468 30.73 100.95 101.13 100.86 896.5 895.4 881.4 1.4525 0.9380 30.96 110.23 110.42 110.13 976.5 975.4 962.3 Like the aliphatic diesters ( 1 , Dioctyl pinate Diheptyl pinate 1.4540 0.9313 31.10 119.52 119.72 119.42 1056.4 1055.4 1041.3 Djdec 1 pinate 1.4562 0.9198 138.09 138.31 137.98 1216.4 1215.4 1203.1 31.58 the pinate diesters made Bts(2-gutoxyethyl) 1.4514 0.9981 104.47 104.70 104.44 936 4 935.6 926.7 32.20 f r o m p r i m a r y a l c o h o l s expinate hibitedessentially linear graphs a Calculated from Vogel's structural constants and from Vogel's bond constants (81-54). of viscosity versus temperature
~~~$$"~&
?;!SF,
~i~t,Y:$~,na&ttee
122
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 1
TABLE v.
COMPARISOS O F PROPERTIES O F S O M E P I S A T E ESTERSWITH THE DIESTERS O F 2-ETHYLHEXANOL AND -1LIPHATIC -kCIDS s o . of A.S.T.M. A.S.T.M. Slope Freezing Evaporation Spontaneous in Viscosity, Cs. a t O F. \-iscosity -650 io Empirical Atoms Principal or Pour Lossa, Ignition Formula Chain 210 100 0 -40 -65 Index 210' F. Point,, 'I?. Wt. % Temp., O F . 15 Dibutyl pinate 632 2945 < -75 97 0.78 4 . FJ 720 Diamyl pinate 17 972 5620 < -75 109 0.76 ... 1.8 Bis (2-ethylhexyl) glutarate 19 715 4200 6 0.80 < -75 106 ... 731 19 1136 Dihexyl pinate 124 6480 0.74 < -75 737 0.3 Bis (2-ethylhexyl) adipate 20 807 0.77 < -73 121 5000b 0.2 743 Bis (2-ethylhexyl) pimelate 21 878 0.74 < -75 137 5300 b 753 0.2 21 1437 Diheptyl pinate 0.71 8540 < -75 138 0.2 757 Bis (2-butoxyethyl) pinate 21 3000 0.74c -5Od 116
