Conversion of Naphthenic Acids to Naphthene Hydrocarbons CHEMICAL CONSTITUTION
.
GILBERT E. GOHEEN, Sun Oil Company, Norwood, Penna.
A fraction of naphthenic acids isolated from the lubricating oil portion of a Gulf Coast crude petroleum has been investigated in detail. The fraction had an average molecular weight of 317 and an average molecular formula of C20.7HS5.202.1. The corresponding naphthene hydrocarbons were prepared from these acids by way of the ethyl esters, alcohols, and iodides. These naphthene hydrocarbons were found to have higher viscosity indices than those of a typical naphthene-base oil. Evidence was obtained to indicate that the acids were monobasic compounds containing on the average about 2.6 rings per molecule. No more than about 5 per cent by weight of aromatic material was present.
I
NVESTIGATION of fractions of naphthenic acids obtained from the lubricating oil portion of a Gulf Coast petroleum has demonstrated that they are monobasic acids containing from 14 to 29 carbon atoms per molecule (11). Although the acids were saturated, they had a hydrogen deficiency below the fatty acid series of 4 to 10 atoms per molecule. Average type formulas ranged from C,Ht,0 2 to CnHz- 1002. The possible presence of aromatic acids was recognized. I n order to obtain further information with regard to the properties and composition of high-molecular-weight naphthenic acids from this source and of naphthene hydrocarbons derivable from them, the present investigationwas undertaken. A sample of naphthenic acids of high molecular weight was subjected to extensive purification and to fractional distillation in a molecular still a t a pressure of less than 0.005 mm. of mercury. A fraction of the purified acids was then converted to the ethyl esters which, in turn, were fractionated a t the low pressure. The esters were reduced to naphthenic alcohols which were also distilled in the molecular still. Finally the alcohols were transformed into the iodides and the latter were reduced to the corresponding naphthene hydrocarbons. The naphthenes were hydrogenated and the properties of the aromatic free material were determined. The naphthenes were also refined with concentrated sulfuric acid. Physical properties of all of the naphthenic compounds were determined.
fied with sulfuric acid, dissolved in ether, and washed until the aqueous layer was free of mineral acid. The acids were dried and were freed of ether by evaporation under diminished pressure. They had an acid number of 174. I n order to obtain a more homogeneous fraction and to produce further purification, the acids were distilled a t a pressure of less than 0.005 mm. of mercury in a molecular still as previously described (11). Of the six fractions obtained, only fractions 2 and 3 were used in the following conversions. The molecular weights of the naphthenic acids utilized for the present investigation ranged from about 309 to 338. They were practically free of acids of molecular weight lower than 292 (oontaining 19 carbon atoms per molecule) and higher than 348 (containing 23 carbon atoms per molecule). The average value was 317 as calculated from an acid number of 177. The average number of carbon atoms per molecule was 20.7.
Esterification of Pure Naphthenic Acids with Ethyl Alcohol The extracted and fractionated naphthenic acids were esterified by a modification of the method described by Corson, Adams, and Scott (6). It consisted in passing ethyl alcohol in the form of its vapor through the esterification mixture while the latter was maintained a t 115-120' C. This not only allowed the reaction to take place a t a higher temperature than is possible by ordinary refluxing in alcoholic solution, but also removed the water as it was formed and thus drove the reaction to completion. The esterification was continued until 930 ml. of distillate had been collected. This required about 2 hours of passage of alcohol vapor. The reaction mixture was dissolved in ether and the ethereal solution was thoroughly washed with 6 per cent sodium carbonate solution. The addition of a little ethyl alcohol was necessary to prevent emulsification during the washing process. The ethereal solution was dried over anhydrous sodium carbonate, and the ether was removed under diminished pressure. The yield of ethyl naphthenates was 91 per cent of the theoretical amount. I n order to produce further purification and fractionation, the esters were distilled in the molecular still (Table I).
Purification of Naphthenic Acids The naphthenic acids used for this investigation originated in Gulf Coast crude oil and were similar to the acids studied in the previous investigation (11). These acids were freed from most of the hydrocarbon oil contamination by neutralizing with soda, distilling out the oil, liberating the acids, and redistilling; fractions from the front and tail ends were discarded. Further purification was effected by neutralizing this material with strong alkali and subjecting the soaps to a pressure extraction with a liquid propanebutane mixture until no more extract was obtained. The extracted soaps were acidi503
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Fractions 1 and 6 were discarded, and fractions 2, 3, 4, and 5 were combined to give a uniform sample of lemon yellow color. This sample weighed about 1344 grams and consisted of 72.4 per cent of the distilled product. The discarded fractions had average molecular weights of about 335 and 412, respectively. The fractions which were utilized ranged from 339 to 370 in average molecular weight. The molecular weights were calculated from the saponification numbers. First attempts to determine the saponification numbers of the ethyl naphthenates by refluxing the samples in alkaline ethyl alcohol solution gave inconsistent results which were always low. Satisfactory agreements were finally obtained by 1.5 to 2 hours of refluxing of the samples in a measured volume of a solution containing 14 grams of potassium hydroxide in 1liter of butyl alcohol. A blank was run with each set of samples. The physical properties of the final combined sample are given in the tables of data.
Reduction of Ethyl Naphthenates to Naphthenic Alcohols The ethyl naphthenates were reduced in small portions to naphthenic alcohols by means of sodium in anhydrous ethyl alcohol. This method was originally used by Bouveault and Blanc (3) and has since been modified by various investigators (8, 22, 33) for application to diverse esters. From a search of the literature it seemed improbable that this type of reduction would attack a naphthene ring. For our purpose the following modification was adopted: I n a 3-liter round-bottomed three-necked flask, equipped with a large reflux condenser and a 500-ml. separatory funnel, were placed 48.3 grams (2.1 moles) of sodium metal freshly cut into pieces approximately the size of large peas. The open neck of the flask was then stoppered with a rubber stopper. By means of the separatory funnel there was added in a continuous stream a solution of 120.7 grams (0.35 mole) of ethyl naphthenates (molecular weight 345) in 100 ml. of completely anhydrous ethyl alcohol. This was followed a t once by the quick addition of 375 ml. of anhydrous ethyl alcohol. The alcohol had been made completely anhydrous by the method of Lund and Bjerrum (29).
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column. This distillation was continued until the temperature of the reaction mixture was about 112" C. The residue was allowed to cool, and sufficient water was added to dissolve all of the solid. The oily layer which separated was dissolved in ether, and the ethereal solution was washed with two portions of 10 per cent sodium carbonate. Some alcohol was added to inhibit emulsification. After the ethereal solution had been carefully separated and dried over anhydrous sodium sulfate, the solvent was removed under diminished pressure. Eleven reductions were carried out in this manner, and the average percentage yield of crude product was 91 per cent of the theoretical amount. Since the saponification number as well as the acid number of the product was zero, no ester remained. The hydroxyl number of these crude naphthenic alcohols was about 134, Tvhich corresponded to a molecular weight of 418, assuming the product to be pure monohydric alcohols. However, the theoretical molecular weight calculated from the molecular weight of esters used was only 303. Thus t o produce further purification, the crude naphthenic alcohols were distilled in the molecular still. Owing to the viscous nature of the alcohols, some difficulty was encountered in obtaining fractions free from contamination by charging material. Severtheless, after four distillations, two main fractions were obtained. Fraction I consisted of light yellow material with a molecular weight of 303, which corresponded with that required by theory. The analysis for carbon and hydrogen indicated that this material %'as the desired naphthenic alcohol. It weighed 391 grams or was 41.2 per cent of the weight of substance first charged into the molecular still. Fraction I1 comprised the residue or undistillable material. It was black and very viscous. The physical properties of fraction I which was used for the remainder of this investigation are given in the tables.
Conversion of Naphthenic Alcohols to Naphthenic Iodides
The naphthenic alcohols were converted to iodides by a method similar to that described by Hartman, Byers, and Dickey (12) for the preparation of n-hexadecyl iodide. It was found best to carry out the reaction in a ventilated hood. In a 500-ml. round-bottomed three-necked flask were DISTILLATION OF ETHYL NAPHTHENATESplaced 80 grams (0.26 mole) of naphthenic alcohols and 4.1 TABLE I. MOLECULAR grams (0.13 mole) of red phosphorus. The flask was placed Av. Weight of % of SaponiRefractive Charge, Fraction Temp., Fraction, Charge in fication Mol. in a paraffin bath, the temperature of which was about 70" C. N0.b Weightc Index, ny Grama No. a C." Grams Fraction The flask was then fitted with a mercury-sealed mechani633.6 A-1 105 56.0 8.8 168 334 1.4810 cal stirrer, a reflux condenser, and a rubber stopper. To the A-2 117 209.7 33.1 165 339 1.4825 A-3 121 147.3 23.3 160 349 1.4832 warm mixture of alcohol and phosphorus were added 50.3 8-4 127 89.9 14.2 157 358 1.4841 A-5 132 68.3 10.8 154 363 1.4843 grams (0.4 mole) of resublimed iodine over a period of about A-6 157 48.3 7.6 ... ... .... an hour with continuous mechanical stirring. About half 166 .... B-1 10.9 337 1262 0 114 137.3 of the iodine was introduced during the first 5 minutes, and 165 1.4827 325.0 B-2 25.8 340 116 1.4836 21.6 348 161 119 272.3 B-3 the remainder was then added more slowly. The temperature 1 7 . 0 152 1.4839 2 1 4 . 5 8-4 369 127 of the paraffin bath was gradually raised until it had reached 152 1,4842 13.2 370 134 166.0 B-5 136 .... 412 120.3 B-6 9.5 147 120". It was then maintained a t that point for 5 hours with a Pressure approximately 0.005 mm. Hg. continued mechanical stirring. b The saponification was produced by refluxing the ethyl naphthenates with a measured volume of a solution of 14 grams of IiOH dissolved in 1 The reaction mixture was dissolved in about 200 ml. of liter of butyl alcohol. Calculated from the saponification number. ether and was transferred to a separatory funnel. A small amount of black colored liquid which settled to the bottom of the funnel was drained away. The ethereal solution was decanted and filtered t o remove phosphorus. It was then The reduction was not especially vigorous, but some foamwashed in turn with three portions of water, three portions of ing occurred. A preheated hot plate was placed below the re5 per cent sodium hydroxide, and finally with water. After action flask a t once, and refluxing was alloved t o continue being dried over calcium chloride and filtered, the ether was until all sodium had gone into solution. At this time the reremoved under diminished pressure. The yield of naphthenic action mixture was allowed to cool somewhat, the flask was iodides containing 31.25 per cent iodine (Parr bomb method) removed from the condenser, and 75 ml. of water were slowly was 88 per cent of the theoretical amount. The properties added with agitation of the flask. The solution was transof the final combined product of five runs are given in the ferred to a 1-liter round-bottomed flask, and the ethyl alcohol tables. was removed by distillation through a small fractionating
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Preparation of Naphthene Hydrocarbons from the Naphthenic Iodides
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-TOPPING 1 The naphthenic iodides were reduced to h h l - 8 a r L / N G fRACNO ALKALI TREATM€NT e Y the corresponding naphthenes by a proce- & of CRUDE V A C U U M 01s T I L L A T I O N RcsiauE dure similar to that utilized by Levene (18) r A C l O l f l C A TION, DEH Y D R A TION, VACUUM '71.5 T I L L A TION for the conversion of n-hexadecyl iodide to nE & NAPH THENATE hexadecane. The iodides were dissolved in NAPYTYENIC a VACUUM DISTILL ATION RESIDUE glacial acetic acid and then treated with ~ A C l D I F I C A T l O N , DEH YDRATlON, VACUUM DIJTIL L A T I O N gaseous hydrogen chloride and zinc dust. It was found best to add the zinc dust in small portions to the warm solution over a period of 2 hours with mechanical stirring. Stirring and heating were continued for a total of 24 hours, during which time the NAPHTHENIC & s reaction mixture was saturated with hydroACID NO RANGE:IJS-lSO gen chloride gas a t the end of each 4 hours. The reaction mixture was cooled and was -i '"T E R I f l C A TION, MOL E CUL A R D I S T I L L A T I O N poured into an equal volume of water con, tained in a 2-liter beaker. Some ether was used to wash out the flask and was added to the mixture. The latter was then filtered on a Buchner filter to remove the zinc residue. This residue was mashed with water and finally with ether. The two layers in the filtrate were separated and the aqueous layer was extracted with two portions of ether. The combined ethereal solution was thoroughly washed with four 100-ml. portions of 20 per cent sodium hydroxide solution. A little ethyl alcohol was added to inhibit the tendency for emulsification. After being washed with water until free of alkali, the organic solution was dried over anhydrous sodium sulfate. The solvent was removed under diminished pressure. The average yield of naphthenes from four runs was 88 Der cent of the theoretical amount. The physical 3000 pounds per square inch (190-211 kg. per sq. cm.) presproperties of the combined uniform product are given in the sure. Raney nickel catalyst m-as used. The solvent was tables. The material contained no iodine, but it had a small methylcyclohexane, and the hydrogenation was continued hydroxyl number ( 5 ) indicating the presence of a trace of for 25 hours in one run and for 50 hours in another. The alcohol. The acid number was negligible (0.9). physical properties of the products of the two runs are given The crude naphthenes were stable towards alkaline potasin the tables. sium permanganate solution, which indicated that no olefinic unsaturation mas present. The decoloration of a bromineProperties of Naphthenic Compounds carbon tetrachloride solution was only slight. The naphthenes for the most part had a boiling range of 300-340" C. (760 A flow sheet depicting the transformation of the naphthenic mm.) with some decomposition. acids to hydrocarbons is given in Figure 1. -4fairly narrow cut of naphthenic acids was used as starting material for the Refining of Kaphthenes with Sulfuric Acid conversion, and then the esters and finally the alcohols were distilled in a molecular still. KO attempts were made to isoA 50-gram portion of the crude naphthenes mas refined by late individual acids. The possibility of doing that becomes shaking with three 25-ml. portions of concentrated sulfuric much smaller as the molecular weight of the naphthenic acids acid with frequent cooling in ice water. Each treatment increases. lasted about 30 minutes. Very little coloration appeared in The average properties of the naphthenic acids and of the third extraction. their derivatives are given in Tables 11, 111, IV, and V. The After the final acid layer had been drained an-ay, the naphobserved molecular weights were calculated from the various thenes were diluted with ether. The ethereal solution was chemical and physical properties determined. The theoretical washed with water, with two portions of 20 per cent sodium values are those calculated on the basis of 317 as the value for hydroxide solution, and finally v+th water until free of alkali. the naphthenic acids used. The molecular weight of 317 was It was dried over anhydrous calcium chloride, and the ether was calculated from the acid number, assuming one carboxyl removed under diminished pressure. The hydroxyl number group to be present in the molecule. Previous work as well of the product was zero. The physical data indicated that, as the present data demonstrate this to be the true situation. although a small loss in oxygen and sulfur content was effected, For if the acids were actually dibasic, the observed molecular only a trace of the aromatic content was removed. weight of the naphthenes finally obtained would be twice the theoretical value. Actually they were found to correspond. Hydrogenation of Naphthene Hydrocarbons In order to obtain agreement in results for the saponification A portion of the crude naphthenes was hydrogenated in numbers of the ethyl naphthenates, it was necessary t o reflux an Adkins hydrogenation apparatus a t 230-250 " C. and 2700them in alkaline butyl alcohol solution.
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limits of experimental error, until the naphthenes are hydroTABLE11. PHYSICAL CONSTANTS OF NAPHTHENES, NAPHTHENIC genated. In that case it practically disappears and thus ACIDS,AND DERIVATIVES gives evidence that the hydrogenated naphthenes are purely Saponi- Hy- Aniline Mol. Weight naphthenic and paraffinic in structure. Acid fication droxyl Point TheoCompound No. No.0 N0.b OC.' Obsvd. retical The analytical data indicate that the naphthenic acids had Naphthenic acids 177 177 0 ,. 317C ... an average molecular composition of 20.7 carbon and 35.2 Ethyl naphthenates 0 163 0 .. 344; 346 hydrogen atoms. A small amount of sulfur (0.11 per cent) Naphthenic alcohols 0 0 185 .. 303 303 Naphthenic iodides 0 ... 6 .. 406f 413 was found. The amount of oxygen (determined by difference) Na- hthenes
0 5 77.8 2888 287 8rude Refined with concd. Hd30r 0 ... 0 78.5 2880 287 Hydrogenated 25 hr. . .. 0 86.0 287Q 288 Hvdronenated 50 hr. 0 .. . . . 8 4 . 2 286Q 288 5 See footnote b Table I. b Determined b; the method of Smith and Bryant ( 6 7 ) ; see also Kaufmann and Funke ( 1 6 ) . c Calculated from acid number. d Calculated from saponification number. e Calculated from hydroxyl number. f Calculated from iodine analysis. P Calculated from viscosity data by the method of Fenske, XcCluer, and Cannon ( 7 ) ; see also Keith and Roess (16).
. ..
.
The correspondence between the theoretical and observed molecular weight values is satisfactory in all cases except that of the iodides. The low value of 406 would indicate that slightly more than 1 atom of iodine was present. This, as well as deviation in the case of some other constants of the naphthenic iodides, is probably due to the fact that they were not distilled because of the possibility of decomposition. The kinematic viscosities of all products were determined a t the three standard temperatures by means of the modified Ostwald viscometers (2). The results are given in Table 111, and the viscosity-temperature curves are shown in Figure 2. The presence of the carboxyl group produces the greatest viscosity value; next come the hydroxyl group, the iodine atom, and the ethyl ester group in turn; and finally the hydrogen atom produces the smallest value for viscosity. For the sake of comparison, the viscosity index was calculated for the intermediate compounds as well as for the naphthenes. The values for viscosity gravity constant and for gravity index are given for the hydrocarbons. The difference between the theoretical and observed molecular refractivity (molecular refractive exaltation = 1.25 in the case of the acids) indicates the presence of a small amount of unsaturation. This value shows little change within the
TEMP€RATURE IN DEGR€CS E
FIGURE2. KINEMATIC VISCOSITYTEMPERATURE RELATIONS FOR NAPHTHENIC COMPOUNDS
TABLE111. VISCOSITY DATAOF NAPHTKENES, NAPHTHENIC ACIDS,AND DERIVATIVES Compound Naphthenic acids Ethyl naphthenates Naphthenic alcohols Naphthenic iodides Na hthenes
-Kinematic, 10O'F. (37.8' C.) 2166.10 23.26 472.77 49.05
Centistokes130° F. 210' F. (54.4' C.) (98.9' C.) 448.22 12.12 122.62 21.12
32.82 3.82 11.50 5.09
-5aybolt 100' F. (37.8' C.)
Universal, 9ec.a130' F. 210' F. (54.4' C.) (98.9' C.)
9846 111 2149 226
2038 66 558 102
153 39 64 43
13.89 7.76 2.76 73 $&d with concd. HI SO^ 13.41 7.61 2.72 71 Hydrogenated 25 hr. 13.05 7.39 2.69 70 Hydrogenated 60 hr. 11.22 6.62 2.53 63 a Calculated from, kinematic viscosity by the 1934 A: S. T. 31. method ( 1 ) . b From Saybolt viscosity a t 100' F., using the equation of Hill and Coats (14).
51 50 50 47
36 36 36 36
Viscosity Index (1s) 109 64 - 307 - 28
-
25 30
32
2.3
ViscosityGravity Constant6
.... .... ....
Gravity Index (80)
..
....
.. ..
0.8629 0.8604 0.8644 0.8541
52 55 63 64
.
I
SPECIFIC DISPERSION, AKD REFRACTIVITY DATAON NAPHTHENES, NAPHTHENIC ACIDS,AND DERIVATIVES TABLEIV. DENSITY, ComDound
Densitya, di0 0.9860 0.9543 0.9464 1.2076
Refractive Index, n q 1.4966 1.4835 1.4966 1.525s
1.4849 0.8920 1.4837 0.8896 1.4794 0.8839 1.4781 0.8815 a Density determinations were made using a pycnometer of about b Lorens-Lorents specific refractivity, n3 2 ;i'
5 +
Sp. Refractivityb, rso 0.2966 0.2995 0.3090 0.2541 0.3212 0.3216 0.3210 0.3212 24-ml. capacity.
-Mol. RefractivityObsvd. Theoretical 94.01 92.76 103.34 102.14 93.63 92.72 102.55 103.81
Mol. Refractive Exaltation 1.25 1.20 0.91 -1.26
Specific Refractivity Dispersion, Intercept, n z x 10' n-d/2 (31) 1.0035 1.0064 1.0234 0.9220
1.02 1.0388 92.19 91.17 1.0390 92.26 91.23 1.03 0.19 92.14 91.95 1.0375 0.04 1.0374 92.19 92.15 The values were corrected for the buoyancy of the air.
...
97 101
...
107 105 99 98
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~ NAPHTHENES, NAPHTHENIC ACIDS,AND DERIVATIVES TABLEV. ANALYTICALD A T ON Compound Naphthenic acids Ethyl naphthenates Naphthenio alcohols Naphthenic iodides Naphthenes Crude Refined with ooncd. HzQ04 Hydrogenated 25 hr. Hydrogenated 50 hr. cz
%C
%*oH
% Ea
78.20 78.96 81.88
11.17 11.47 12.38
0 11
...
86.51 86.54 86.47 86.46
.
.
I
13.08 13.10 13.34 13.41
.. ,. ..
% Oby Difference 10.52 9.46 5.65
0.10
0.01 0:01
% 0b.L
S or H No.b
t . .
0.31 0.35 0.18 0.12
10.10 9.28 5,27
...
yo I
d v . &.Iol. Formula
... ...
Av. Type Formula CnHzn - a.zOt.1 CnHzn ~JOZ., CnH2n 4 . 2 0 1 . 1 (CnHzn k.2)Il.Q
CnHm -
, . I
31.25 0.0
...
CnHzn CnHm CnHzn
... ...
--
4.200.1
~i00.i 1.4 a.2
Determined by the method of Waters (3%). b Acid, saponifioation, or hydroxyl No,
was slightly greater by about 0.42 per cent than could be accounted for in the carboxyl groups. The average type forThe . esterification and the mula was therefore CnHZ.- ~ 0 2 . ~ reduction of the esters to alcohols proceeded according to theory. Since a carbon and hydrogen analysis was not run on the iodides, the average formula given is the theoretical one. The values for the crude naphthene hydrocarbons agree with the theoretical of C20.7H37.2, corresponding therefore to CnHzn- 4.p. This would be approximately the theoretical type formula for a tricyclic naphthene hydrocarbon. However, no conclusion as to structure could be made until any aromatic content possibly present was removed, since a small amount of aromatic hydrocarbons with their small hydrogen to carbon ratio could produce a large part of the difference in hydrogen to carbon ratio between a bicyclic and a tricyclic naphthene. The analytical data as well as the values for the molecular refractive exaltation, aniline point, and specific dispersion indicate that the refining of the naphthenes with concentrated sulfuric acid produced practically no effect upon the indicated aromatic content. However, these same physical properties reveal that the hydrogenation process produced a hydrocarbon mixture which was purely naphthenic and paraffinic in constitution.
Constitution of Acids Deduced from Physical Properties of Naphthenes and Hydrogenated Naphthenes Since a large amount of work has been published recently
After hydrogenation, the above mentioned physical properties demonstrate that the hydrogenated material was free of aromatics and was purely naphthenic and paraffinic in constitution. Thus the aniline point rose to 84.2' C., the specific dispersion dropped to 98, and the molecular refractive exaltation dropped to approximately zero within the limits of experimental error. The specific refraction was 0.3212 and the hydrogen-carbon ratio was 1.846. Thus the hydrogenated naphthenes had an average type formula of C,H2, - 5.2. Therefore, since bicyclic naphthenic hydrocarbons have the type formula CnHzn 2, and tricyclic have CnHz,- 4, it a p pears that the material of the present investigation contained some molecules with more than 2 rings. On the basis of the above data, the average number of rings per molecule is 2.6. This is believed to be accurate within a t least *0.3 and probably within +0.2. Although the rise in aniline point upon hydrogenation would indicate an aromatic content of about 5 per cent, as calculated by the method of Vlugter, Waterman, and van Westen (68), this is not confirmed by the increase in hydrogen content, since about 0.3 per cent of sulfur and oxygen (determined by difference) was removed. Thus the actual aromatic content is probably less than the above figure. Since the variation from the paraffinic type formula of CnHzn + 2 gives definite information with respect to the number of saturated carbon rings present in hydrocarbons free of olefinic or aromatic unsaturation and not to the ratio of cyclic to open-chain carbon atoms, assumption would need to be made in order to estimate a t the present time the percentage of naphthenic and paraffinic carbons in the material. Thus bicyclic saturated molecules having a formula of C12Hn (CnH2,- 2) may be of the following common types (the position of the alkyl group has no significance) :
-
concerning methods of estimating the structure of lubricating oils from physical data, it is possible to gain some insight as to the structure of the naphthenic acids by determining the structure of the naphthenes which were synthesized from them. The methods of Vlugter, Waterman, and /)Ci" p~---CnHa C -J L ~ -I/ -CH3 C3H7 van Westen (68) and of Grosse (IO) were applied. \/\/ J V The crude naphthenes had I IT IV V VI an aniline point of 77.8" C., a specifii refraction of Many other examples of hydrocarbon ring systems are de0.3212, and a molecular weight of 287. If no aromatics were picted by Grosse (IO). present, the aniline point should have been about 84.8" C. Data present in the literature seem to indicate that bi(since aromatics have high values of specific refraction, the cyclic aromatic rings present in petroleum are, in general, true aniline point would be slightly less than this). Thus the linked through two common carbon atoms as in type I1 rather aniline point as well as the values for the molecular refractive than through a single carbon-carbon bond as in type I (61). exaltation indicate that a small amount of aromatic material Vlugter, Waterman, and van Westen (18) assume that the was present. The specific dispersion, oils with which they worked contained 6-carbon condensed naphthenic rings as in type 11. However, that 5-carbon rings may be present in petroleum is evidenced by the fact that several substituted cyclopentanes have been isolated from of all paraffins and naphthenes is practically a constant value fractions of American petroleum (26). (9, 29, 30, S1)-about 99. On the other hand, the specific I n the field of naphthenic acids the work of previous investidispersion of aromatic substances is much higher. Therefore gators (4,24,66,SS, 34) reveals that 5-carbon naphthene rings the value of 107 obtained for the crude naphthenes also deare generally present. Also according to recent publications, notes a small aromatic content.
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von Braun (4,6) believed that the bicyclic naphthenic acids probably consist of two separate rings. However, the question of the type of rings present in highmolecular-weight naphthenic acids has not yet been completely answered. Kurtz and Lipkin (17) showed that if the molecular weight and the number of rings per molecule are accurately known, it is possible to predict the density for either the 5-carbon or 6-carbon ring types in naphthene hydrocarbons. Also they found that there is an appreciable difference between these densities. Apparently this method is practically independent of whether the rings are of the separate or condensed type. The relation between the density, molecular weight, and number of rings per molecule can therefore be used to obtain an indication of the number of carbon atoms per ring. An application of this method to the data obtained in the present investigation for the naphthenes hydrogenated for 50 hours gives an approximation of 5.2 carbon atoms per ring. Even though the number of rings per molecule must be known accurately for this method to be of definite value, it is felt that the figure obtained is of use in a confirmatory sense. It should be noted, however, that the value of 2.6 for the number of rings in the molecule is independent of the size or type of the rings present. The naphthenicity index (IO) for the hydrogenated naphthenes is 25.1. The change in viscosity of the naphthenes upon hydrogenation deserves some consideration. The hydrogenation caused a small decrease in kinematic viscosity and an increase in kinematic viscosity index. Mikeska (93) observed that naphthenes with one and two rings have viscosities slightly higher than those of the corresponding aromatics. However, Mair, Willingham, and Streiff ( d l ) hydrogenated extract portions of the lubricant fraction from a mid-continent petroleum and found unexpectedly that the viscosities were lower in many cases. They surmise that “the hydrogenation of one or more aromatic rings, when those rings are attached to two or more naphthenic rings (probably in the condensed form), results in a decrease in viscosity, and the decrease in viscosity becomes greater, the greater the number of aromatic rings hydrogenated”. They found that the viscosity index increased somewhat. Therefore, although the amount of aromatics in the present case was so small that little change was produced in viscosity and viscosity index by hydrogenation, the trend corresponds to the results of the latter investigators. This trend would indicate that the aromatic rings are condensed directly to naphthenic rings.
Summary Saphthenic acids isolated from the lubricating oil portion of Gulf Coast crude petroleum have been purified and fractionated in a molecular still. A fraction having an average molecular weight of 317. an average molecular formula of C20.7H35.202.1, and an average type formula of CnH2, - 6.202.1 has been investigated in detail. The object was to gain further information regarding the properties and composition of high-molecular-TTeight naphthenic acids and of naphthene hydrocarbons derivable from them. The acids \\ere converted to ethyl esters, the esters were reduced t o naphthenic alcohols, the alcohols were transformed into the iodides, and the latter were finally reduced to the corresponding naphthene hydrocarbons. The hydrocarbons were refined with concentrated sulfuric acid. They 1Tere also hydrogenated until only naphthenic and paraffinic material was present. Physical properties of the various naphthenic compounds were determined. The naphthene hydrocarbons were found to have higher viscosity indices than those of a typical naphthene-base oil. After being refined with concentrated sulfuric acid, they had a
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kinematic viscosity index of 30, a viscosity gravity constant of 0.8604, a gravity index of 55, and a kinematic viscosity of 13.41 centistokes a t 100” F. and 2.72 centistokes a t 210” F. (71 and 36 seconds Saybolt, respectively). From a consideration of the physical properties of the naphthenes before and after hydrogenation, it mas revealed that the deficiency of hydrogen in the naphthenic acids from which they were derived mas due only in small part t o the presence of aromatic or other foreign material. Evidence was obtained to indicate that the acids consisted mainly of polycyclic naphthenic acids having an average of about 2.6 rings per molecule. Thus this evidence supports a theory that naphthenic acids containing more than two rings in the molecule do exist. The investigation provides further proof that acids of this type are monobasic and that they contain mainly &carbon rings.
Acknowledgment The author desires to express his appreciation to Johannes H. Bruun, under whose supervision this investigation was carried out, for advice and assistance. The assistance of W. B. M. Faulconer in providing the organic combustion data is gratefully acknowledged.
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