Properties of Synthetic Lubricating Oils Cyclic Hydrocarbons with TwentyTwo Carbon Atoms per Molecule E. NEYMAN-PILAT AND S. PILAT Laboratory of Petroleum Technology, LwBw, U. S. S. R.
T
HE chemical structure of hydrocarbons in lubricating the major constituents of lubricating oils. Their properties oil fractions has not yet been established by direct analydeviate too much from those of average lubricating oils. Analytical investigations, too (20, %), confirm the opinion that sis. Investigators a t the National Bureau of Standards who are carrying on a complete analysis of the lubricant fracthe isoparaffins are present in lubricating oils in small quantition of a mid-continent petroleum, have not yet succeeded ties only, if at all. The present work is restricted to aromatic and in isolating any chemically pure components. They have, however, obtained narrow fractions of naphthenic and aronaphthenic hydrocarbons with paraffinic side chains. Seven carbon hydrocarbons were prepared, each containing twentymatic hydrocarbons, highly concentrated and consequently two carbon atoms. Since the molecular weights of the well defined with respect to molecular size and type. Relations between the physical properties and the constiseven are almost the same, an important variable is thus eliminated. tution of lubricating hydrocarbons may be established on an indirect basis by compariAverage lubricating oil son of the properties of fractions have somewhat higher molecular weights synthetic h y d r o c a r b o n s with those of natural lubrithan these hydrocarbons. c a t i n g oils. T h i s w o r k This is due to the fact that in the present work emwas started some years ago Seven new liquid hydrocarbons of the phasis was laid upon the by Hugel, Lerer, Landa, purity of the compounds. Suida, Mikeska, and others naphthenic and aromatic series containing Some of the hydrocarbons who synthesized some hytwenty-two carbon atoms per molecule obtained, for instance, by drocarbons of high molecuwere synthesized for the purpose of comMikeska (28) do not form lar weight and determined paring their physical properties with those homogeneous and chemitheir physical properties. of lubricating oil fractions and of studying cally pure products, as may On the basis of their data be seen from their wide boilit is rather difficult, howthe relation between physical properties ing range (24) as well as ever, t o determine the and chemical structure. from the large discrepancies identity with or even simiConclusions concerning the influence of between the observed and larity of the synthetic hystructure on physical properties are coxncalculated refractions (3%). drocarbons to those which pared with the results of Milreska and those Consequently we decided t o constitute the lubricant investigate lighter hydrofractions of petroleum. A of investigators at the National Bureau of carbons which are certain t o further study of this quesStandards. Some of the data for known b e p u r e . M e t h o d s of tion therefore seems adhydrocarbons of the same molecular weight synthesis (29) were chosen visable. were used for comparison. which would lead to definite The applicability of the ring analysis to products, and make possible Materials an easy separation of the bypure hydrocarbons as well as to lubricating According t o the work products from the desired oil fractions has been widely discussed. A compounds. The position of others, as well as to the naphthenic oil of unknown structure was of different groups and radiinvestigation in which the analyzed indirectly by comparing its physicals is well established, in properties of a paraffinic cal properties with those of synthetic contrast t o those of Mikeska hydrocarbon with thirtywhere this was not always two carbon atoms were hydrocarbons. the case. Table I (Nos. 2 described (14), it seems t o 8) shows the structure of almost certain that liquid the synthetic hydrocarbons. isoparaffins do not belong to 1382
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
November, 1941
TABLRI. Number
Structure
.c
--ClaHss
D
....
CONSTITUTION AND
1383
FORMULASOF 22-CARBON HYDROCARBONS -Mol. WeightTheoretical Found
Formula
Name
Mol. Vol.
CnzHaa
2-Dodecyl-p-cymene
351.7
302.3
296.3
87.43
87.33
12.51
12.67
CaHu
2-Dodecyl-p-menthane
370.7
308.36
304.2
85.45
85.62
14.46
14.38
CznH44
a-Perhydrocarvaaryl-8-diisoamylethano
370.8
308.36
308.0
85.73
85.62
14.38
14.38
CsHr
a-Perhydrocarvacryl-p( l-decahydronaphthalenebethane
336.8
304.32
302.2
86.68
86.75
13.42
13.25
CiiHan
a,@-Dicarvacrylethane
314.2
294.24
284.7
89.71
89.74
10.24
10.26
CinH42
a,8-Diperhydrooarvacrylethane
351.1
306.34
299.2
86.29
86.18
13.81
13.82
CnHa?
I-Dodecyldecahydronaphthalene
348.1
306.34
297.4
86.29
86.18
13.93
13.82
CPSHUCetylcyclohexane ( 8 )
372.1
308.36
...
...
...
...
...
CmHas
Cetylbenzene (7, 0, 1 6 )
351.6
302.30
...
...
...
...
CmHw
Dihexylnaphthalene (9, B3)
317.0
296.26
...
...
...
...
347.8
303
...
...
...
...
(n)
Ca.sHan.s Hydrocarbon from naphthenic acids
Fc?n?$%& --%Found Hydro8alcd. en-.
.
I
.
...
(96')
"The hydrocarbon with 32 carbon atoms (14) was called No. 1.
Determination of Physical Constants Several physical constants were determined to establish the purity of the products, and t o make possible a comparison with light lubricating oils: Densities were determined at 20°, 40°,and 60' C. with a pycnomet er. Refractive indices were obtained with an Abbe refractometer a t 20", 40°, and 60" C. Refractive dispersions were obtained from readings on the compensator drum of the same instrument
at 20" C. The Gladstone and Dale formula was used to calculate specific dispersion. Specific and molecular refractions were calculated by the Lorentz-Lorenz formula, and were compared with the values obtained from Eisenlohr's atomic data. Aniline points were determined in the usual manner with 1-mL portions each of oil and freshly distilled dry aniline. The results were compared with those given by Waterman (31) and by the investigators a t the National Bureau of Standards ($2). Kinematic viscosities were determined in a Vogel-Ossag viscometer. To plot viscosity-temperature curves, Polhohe's W ,
was calculated from viscosities at 20", 50", and 100' C. I t was not possible to compute the viscosity indices (with the exception of oil 5) because of their low viscosities at 210' F. Molecular weights were determined by the cryoscopic method, with naphthalene as solvent. Concentrations of 1 per cent or less were used throughout. Boiling oints of hydrocarbons at 1mm. mercury pressure were determine3 during distillation from Claisen's flasks provided with wide tubes. Some authors give boiling points at very low pressures. For the most part such data are erroneous unless special equipment is used. In the ordinary apparatus the pressure over the surface of the boiling liquid is higher than that recorded by manometers, which are usually placed outside of the distilling apparatus. This difference is due to the drop in pressure between the two oints, and it increases when narrow tubular joints are used. Aythough this fact has been known for a long time (IS, 2 7 ) , some authors give the boiling points a t pressures of 0.001 mm. of mercury (88). To avoid those errors it is advisable to determine boiling oints of high molecular substances in an apparatus like that oPSchicktanz (27); or it is possible to determine boiling range in an ordinary distilling apparatus without reflux condenser or fractionating column (e. g., a Claisen flask) under pressures of at least 1 mm. of mercury. The instrument for measuring the pressure (e. g., the shortened McLeod manometer) must be placed as near as possible t o t,he distilling flask and must not be separated from it by adsorption columns. To protect the pump from vapors and fumes, it is better to use refrigerators with liauid air or carbon dioxide-acetone mixture than adsoration columns. Carbon and hydrogen contents were determined by careful combustion analysis carried out by Liebig's method. Table I describes synthetic hydrocarbons 2 to 8 as well as some other compounds with twenty-two carbon atoms taken from the literature.
TABLE11. DESSITYA S D REFRACTIVE INDEX HyNo. of dro- C Atoms oar- in Naphbon thene No. Rings
--Density20' C. 40' C.
60"
---Ref1 active Tndex-20' C. 40' C. 60' C
...
6.9
6.9 15.6
...
B
c
D a
13.0 6.5
... ...
11.9
-13
$
t
0.3300
0.3200
, 1
0.31 0 0 260
280
303
320
340
-
M
FIGURE 1. SPECIFIC REFRACTIOT OF CZZHYDROCARBOXS COMPARED WITH THEORETICAL CURVES FOR NAPHTHENES AND PAR~FFINS
lower for the hydrocarbon with straight chains (No. 3) than for the corresponding hydrocarbon n ith branched side chains (No. 4). The values a t 20" C. show the following characteristics: Refractive index increases as the number of rings increases. Aromatization is affected by increase in refractive index. The monocyclic aromatic hydrocarbon (No. 2 ) shows a higher index than the dicyclic naphthenes (7 and 8). The branching of the paraffinic side chain decreases refractive index slightly (3 and 4). Condensation of naphthenic (7 and 8) or aromatic rings (6 and C) is followed by a rise in refractive index.
Specific and Rlolecular Refraction
12.1 A
Vol. 33, No. 11
INDUSTRIAL AND ENGINEERING CHEMISTRY
1384
0.8267 0,8597 0.9347 0.8712
. . . . . . . .
........ ........ . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
1.54345 1,4509
........ ........
At 25' C.
Density and Refractive Index From the densities determined at 20°, 40", and 60" C. (Table 11) the temperature coefficient of density was calculated to be from -0.00061 to -0.00069 per O C. Comparidon of the densities a t 20" C. brings out the following: The density increases as the number of rings is increased. Reduction of the aromatic to the corresponding hydroaroinatic rings decreases the density. The monocyclic aromatic hydrocarbons ( 2 and B) have lower densities than the naphthenic hydrocarbons with two rings per molecule (7 and 8) ; this is in accordance with the observation of Mair and Willigham (20). The branching of the paraffinic side chain does not affect the density of mononuclear naphthenes. The number of carbon atoms in naphthene rings, calculated according to Davis and McAllister (6), is also shown in Table 11. With the exception of hydrocarbon 8, the calculated values are almost theoretical and indicate the usefulness of this method for naphthenic or hydrogenated oils. The temperature coefficients of refractive index, dnldt, vary from -0.00035 to -0.00042, and are higher for aromatics than for naphthenes. The 1-alue of d n l d t is distinctly
The specific and molecular refractions were calculated by the Lorentz-Loreiiz formula. For hydrocarbons 3 , 4 , 5 , 7 , and 8 (all naphthenic) they agree well with the values computed from Eisenlohr's atomic data; the discrepancies between the found and calculated refractions at 20" C. do not exceed 0.3 per cent (Table 111). However, for the aromatics (hydrocarbons 2 and 6), the difference is inore than 1 per cent. This confirms the statement of Griffith and Hollings ( 1 1 ) that "aromatic hydrocarbons are known not to give experimental results in agreement with those calculated". For both naphthenic and aromatic hydrocarbons the found values are higher than those calculated, and the former approach the refractions of corresponding paraffins (Figure 1). Vlugter, Waterman, and van Vesten (31) found that a temperature increase of 20" C. increases the specific refraction of mineral oils by 0.0005. I n the present investigation
REFRACTIORS TABLE111. SPECIFICA N D MOLECULAR Hydro- ----Specific Refractioncarbon -Zoo C.No. Found Calcd. 40" C. 0.3351 0.3314 0.3386 0.3297 0.3295 0.3300 0.3291 0.3295 0.3294 0.3205 0.3194 0.3209 0.3333 0.3283 0.3338 0.3240 0.3245 0 3245 0.3239 0.3245 0.3243 D 0.3266 ~
-iMolecular
60' C. Found 0.3361 101.3 0,3305 101.66 0.3299 101.48 0.3213 97.535 0.3344 98.05 0.3249 99.26 0.3246 99.23
RefractionDifference
Calcd. 100.195
101.596 101.596 97.196 96.594
99.396 99.390
fl.1
+0.06
-0.11
C0.35 +1.4S
-0.14 -0.17
..........................
November, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
this increase varies from 0.0003 to 0.0006 per 20” temperature rise, and seems to be higher for aromatics than for naphthenic hydrocarbons.
TABLEv. EFFECT OF HYDROGENATION ON THE HYDROCARBON 7 Incompletely Hydrogenated
Specific Dispersion
SD.disversion
Von Fuchs and Anderson (IO) investigated the specific dispersion [(ng - no)104/d] of synthetic hydrocarbons, principally those of low molecular weight, and found that the values for naphthenes and paraffins are between 95 and 102. They point out that these values are practically independent of molecular weight. All of our naphthenes have specific dispersions in the range 99.8 to 102 (Table IV). This proves their complete saturation. Hydrogenation of those hydrocarbons which were obtained from corresponding aromatics was continued until the specific dispersion decreased to 102 or less. TABLEIV. SPECIFIC DISPERSIONS OF SYNTHETIC HYDROCARBONS Sp. Dispersion,
uc 104 d
Hydrooarbon No.
Found
7.8
1.7
3.0
From Mair et az. ($#I 132
.,. ...
1x5
Abbe No., ng
- no
41
53
53
Von Fuchs and Anderson also give values for aromatics, and point out that dispersion depends on molecular weight and on the type of aromatic ring. On the basis of their data and taking into account the dispersions of other hydrocarbons with higher molecular weight, investigators at the National Bureau of Standards ($2) plotted a diagram in which specific dispersion is shown as a function of molecular weight. Curves were drawn for naphthalene, benzene, and diphenylmethane homologs. Table IV includes the values taken from this diagram for aromatics 2 and 6. The differences between these values and the dispersions found seem to be of the same order of accuracy as the diagram. Table IV also includes the Abbe numbers, which are closely related to specific dispersion. The properties of hydrocarbon 7 can be taken to prove the usefulness of the specific dispersion method for controlling the saturation of oils. This hydrocarbon was obtained by hydrogenating the corresponding aromatic hydrocarbon 6. Before saturation was reached, the properties were determined and are shown together with those of the completely hydrogenated oil in Table V. Further hydrogenation was continued only on the basis of specific dispersion that was too high. Table V shows that the drop in dispersion from 106.3 to 101.7 (only 4.6 units) corresponds to a marked change in other properties, particularly density and refractive index. At the same time the specific refraction shows practically no change. This means that the agreement of calculated and found refractions is not adequate proof of purity. On the other hand, the discrepancy in found and calculated refractions indicates for the most part that the substance contains impurities. To make sure that a hydrocarbon is identical with the substance expected from a synthesis, it is necessary to control not only the specific refraction but also other properties such as dispersion, molecular weight, and carbonhydrogen ratio. The careful determination of hydrogen content (Table V) shows the degree of unsaturation, just as specific dispersion does.
106.3
51.55
Abbe number Density., 29” C. Refractive index 20° C. Sp. refrqotion. fdund Mol weight found Aniline poi& O C. H content Theoretical Found
0.5813 1.4830
0.3241
298.5 85.2
d
13.82 13.70
PROPER‘iYES OF Completely Hydrogenated
101.7 53.78
0.8725 1.4773 0.3240 299,2 90.6 13.52 13.81
Aniline Point T’lugter, Waterman, and van Westen (SI)gave a series of curves showing the relation between aniline point and specific refraction for hydrogenated fractions of different molecular weight (Table VI). Our aniline points were obtained from the theoretical values of refraction and molecular weight. Data from the diagram of Mair, Willingham, and Streiff (22) are also included in Table VI; they represent the relation between aniline points of various types of hydrocarbons and the number of carbon atoms per molecule. Considering the fact that the discussed curves (22, 31) were plotted from only a few points so that their course is not exact, the agreement seems to be quite good. Unfortunately Mair, Willingham, and Streiff gave no curve for aromatic hydrocarbons with two uncondensed rings, and there is no point of comparison on their diagram with our hydrocarbon 6. It may be assumed, however, that the curve for diphenylmethane homologs (CnH~,,-la)will be lower than the curve for naphthalene homologs (CnH2n--lt).
TABLEVI. ANILINE
POINTS OF SYNTHETIC
c
Hydrocarbon
No.
a
This worka
HYDROCARBONS
Aniline Point, C.Vlugter WatermLn, Mair Willingand van ha&, and Westen (81) Streiff ($8)
For hydrocarbon C the value is -3.3.
Application of the ring analysis, according to Waterman
,
et al. (31) to aromatic hydrocarbons 2 and 6 proved a failure
in both cases, even by using the theoretical specific refractions as well as those found. Table VI1 shows $he theoretical composition and the results of analysis for both aromatic hydrocarbons. The discrepancy - between predicted and found aniline points after complete hydrogenation is apparent; this is the cause of the difference in theoretical and calculated composition of the aromatics investigated.
Viscosity Figure 2 gives curves for kinematic viscosity us. temperature. The data for hydrocarbons A, B, C, and D were taken from the literature, and are not always complete; they were recalculated by Walther’s formula (SO). The influence of the chemical composition of hydrocarbons dn viscosity and temperature coefficient of viscosity, as expressed by Polhohe’s W,, is shown in Table VIII. The hydrogenation of aromatics increases viscosity slightly but does not affect the W , value appreciably (Table VIIIa). The
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1386
TABLEVII. RESULTSO F RINGAN.~LYSIS FOR HYDROCARBONS Hydrocarbon No. Theoretical %,of: Paraffin chains Aromatic rings Molecular weight Aniline point found,
C.
9
refraction found &lcd. aniline'point after hydrogenation, Calcd. 7 of paraffinic chains Calcd. of rings Calcd. % of aromatic rings
C.
J
6 refraction theoretical &lcd. aniline(point after hydrogenation Calcd. 9 of p,araffinic chains Calcd of rings Calcd: of aromatic rings Aniline point found after hydrogenation,
9 9
C.
AROMATIC
2
6
75 25 302 43.2
49 51 294 -28.5
0.3351 94.6 66 34 44
0.3333 77.4 25 75 90
0.3314 90.6 67 43 40
0.3283 71.7 10 90 85
100.7
Vol. 33, No.
90.6
42 38
34
30 26
- F \ H c\
B.
/ (CH2)15CH3
22
branching of a paraffinic side chain or splitting it into more side chains with the same number of carbon atoms (Table VIIIb) invariably increases W,, but has a slight, irregular effect on viscosity. The cyclization of paraffinic side chains (Table VIIIc) greatly increases W , (decreases viscosity index); this effect is much more marked in the case of uncondensed than of condensed ring systems. The influence of condensing the rings is still more striking when the hydrocarbons in section d are compared; the viscosity a t 50" C. is slightly and the W , value strongly decreased. This means that hydrocarbons with two condensed rings show a slight decrease in viscosity with temperature, regardless of whether they are aromatics or naphthenes. I n contrast, the hydrocarbons with two uncondensed, strongly alkylated rings of
18
14 IO
6 2 m W
I5
I
2OoC
!z ~
TABLE VIII. Hydrocarbon
--Viscosity, 200
50' C. 8.82
.... n.
9
Centistokes-
c.
No. D B
EFFECTOF CHEMICAL COMPOSITION ON VISCOSITY 100' C.
2.74
Temp. Coefficient, Wp 1.816
Effect of Hydrogenating the Aromatic Rings 5.66 0.6625 *... 7.20 0.7888
....
.. ..
2 3
16.94 10.52
6.32 6.99
2.39 2.57
1.125 1.193
6
77.23 119.14
13.48 18.28
3.25 3.92
6.081 6.020
7
b.
Effect of Branching 6.9'3 2.57 10.02 2.72
5
2 6
16.94 77.23
3 5
19.52 119.14
6.90 18.28
2.57 3.92
1.193 6.020
30
1.193 3.879
....
3:39
0.7888 1.359
20
7.20 6.99
2: 57
0.7885 1.193
5.68 6.32
2:30
0.6628 1.125
70
80
90
I00
j
60 50
40
31.94
7.20 10.40
B C
.... ....
5.66 12.2
3:i3
0.6625 2.243
8 5
31.94 267.5
10.40 31 46
3.39 5.44
1.359 6.207
7 8
118.14 31.94
18.28 10.40
3.92 3.39
6.020 1.359
6
77.23
13.48 12.2
3.25 3.33
6.081 2.243
....
50
'
1,125 6.081
8
C
40
90
70
16.94
d.
I
1
80
80
B 2
-4
l
1
70
90
19.52
Effect of Cyclization 6.32 2.39 13.45 3.25
-
60
'1 00
.4 3
c.
50
U
19.52 42.28
....
*--
1
40
W
3 4
,...
7
I
50
10
Effect of Condensing Rings 2OOC.
30
60
FIQUW2. KINEMATIC VISCOSITIESOF HYDROCARBONS :WITH TWENTY-TWO CARBON ATOMS Above, monocyclic: below, polyoyclio
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1941
aromatic or naphthenic character, show a large decrease in viscosity with temperature. If further investigation confirms the preceding statements and if the work of Mikeska ($3) is taken into consideration, the conclusions on the relation of the chemical structure of hydrocarbons to the trend of viscosity-temperature curves may be summarized as follows:
1387
decrease in ring content per molecule. It is possible that the continuity of the physical properties of these fractions is due to this fact.
1. The slope of the viscosity-temperature curve of monoand dicyclic hydrocarbons (expressed as W, values) does not depend on the charaoter of the rings (aromatic or naphthenic). 2. Hydrocarbons with one ring have low Wp values. 3. Increase in the number of rin s increases W,. 4. Hydrocarbons with two con$!ensed rings have low Wp values. 5. Hydrocarbons with two uncondensed and unalkylated rin s have low W pvalues (e. g., the diphenyl homologs of Mikeseka) 6. Hydrocarbons with two or more uncondensed carvacrylic or perhydrocarvacrylic rings (or, in general, strongly alkylated rings) have high Wp values.
.
It appears that the theoretical composition of hydrocarbons (expressed as percentage of paraffinic chains, naphthenic rings, and aromatic rings) has no relation to the slope of temperature-Wp curves. Table I X shows that naphthenic hydrocarbons 3, 4, and A, which contain about 26 per cent naphthenic rings, have different W , values; also aromatic hydrocarbons 6 and C have similar compositions but quite different W pvalues. On the other hand, oils with different compositions-e. g., No. 2 with 25 per cent aromatic rings and No. 3 with 26 per cent naphthenic rings-have almost identical W , values; the same is true of hydrocarbons 6 and 7. Therefore the percentage of naphthenic or aromatic rings cannot serve as a measure of the slope of viscosity-temperature curves (at least for mono- and dicyclic hydrocarbons). Davis and McAllister (6) showed that a linear relation exists between the viscosity index of an aromatic-free oil and the percentage of carbon atoms in naphthenic rings. Mair, Willingham, and Streiff (99) reported, however, that for water-white oil and for fractions resulting from the hydrogenation of the extracted fractions of mid-continent crude, this relation may be presented in the form of a curve. To facilitate comparison of our hydrocarbons with those of Mair et al., W, values were computed from their results. I n Figure 3 the percentage of rings per molecule is plotted against W, values for our hydrocarbons (numbered circles) and for some of Mikeska's hydrocarbons (triangles). The curve represents the results of Mair, Willingham, and Streiff. Only the point corresponding to hydrocarbon 5, which contains three naphthenic rings, falls close to the curve. The hydrocarbons of Mikeska are closer to but not directly on the curve. However, whereas our hydrocarbons have the same molecular weights and different compositions, the fractions of mid-continent crude differ in both molecular weight and composition. The fractions derived from the extracted portion are characterized by a simultaneous increase in molecular weight and TABLEIX. COMPOSITION OF HYDROCARBONS Hydroosrbon
No.
W p
2 3
1.125 1.193 3.879 6.207 6.081 6.020 1.359
4 5 6 7 8 A
B C D
0.7888 0.6625 2.243 1.816
Paraffin Naphthenio Chains, 7% Rings, % 75 74 26 74 26 28 72 49 47 53 55 45
..
73 75 43 . I
Aromatic Rings, % 25
..
.. .. .. 51
....
25 57
27
..
....
..
..
07
20
40
GO
80
FIQURE3. W , VALUESOF SATURATED
HYDROCARBONS us. PERCIDNTAGE OF NAPHTHENIC RINGS PER MOLECULE
From a practical standpoint, lubricating hydrocarbons may be divided into two fundamental groups. Those with high viscosity indices are the principal components of the so-called raffinates, and those with low viscosity indices, of the extracts. Our hydrocarbons are grouped on this basis in the following table : Hydrooarbona Expected in the Raffinate Aniline No. WP point, C. 2 1.125 43.2 3 1.193 100.7 B 0.6625 A 0.7888 8 1.359 89.0
... ...
Hydrocarbons Expected in the Extract Aniline No. W, point, C. 4 3.879 99.2 7 6.020 90.6 6 6.081 -28.6 5 6.207 86.7
Thus, the monocyclic aromatic hydrocarbons should belong to the raffinate group. If a solvent similar to aniline were used in solvent extraction, the mononuclear aromatics, in ppite of their high viscosity indices, would be drawn into the extract and lost as valuable components of the raffinate. Mair and Willingham pointed out this fact earlier ($0). Three of the four hydrocarbons in the extract series have aniline points higher than 86" C., which means that during the extraction process they would remain in the raffinate portion of the extracted oil, in spite of their low viscosity indices. Their presence in the raffinate would obviously diminish its value as a lubricating oil. It would therefore be advisable to find another method of separation than solubility in selective solvents, in order to extract hydrocarbons according to the scheme of the above table, if these or similar hydrocarbons occur in natural lubricants.
Boiling Points According to Mair and Willingham ($O), who determined the boiling points of lubricating oil fractions from mid-continent crude, boiling point is a function of the number of carbon atoms and does not depend on the number of naphthenic
INDUSTRIAL AND ENGINEERING CHEMISTRY
1388
VOl. 33, No. 11
TABLEX. EFFECTOF CHEMICAL COMPOSITION ox BOILING POINT Series ,No.
--Boiling From literature
Formula
C.
Point--
M m . Hg
A t 1 mm., Hg a
Series NO.
C. b.
a. EFFECTO F HYDROGENATION
230
15
17 2
226
16
168
...
..
..
III
234
12
--Boiling PointAt 1 mm., From literature Hg ' C. M m . U s C.
Formula
EFFECTO F BRAVCKINQ (Cont'd: 12
182
20
181
16
166
..
155-6
10
137
240
16
182
...
..
163-4
224.5
15
166
230
15
172
234
12
182
224.5
15
166
=-Cla"3*
226
15
168
C>-(CHZ)IO-CI>~
240
16
18'2
224.5
16
166
163-4
159-60
182
C I > - ( C H ' ) m - a * CIIa
240
15
iz>
182 ecrCHz-CHz-
IV
...
,.
155-6
CH
\cH
CH3 /\CHa
CH3 /\CH?
b H
e.
/\
CHfiHa
CHI
CHI
CHa
I
n-Docosane
EFFECTO F NUMBERO F RINQS (16)
a - C i ~ H s s ( 16 )
C,H3
..
153-4
bH
/\
CHa
I1
CHa
..
V
.. b.
n-Docosane
218 168
(16)
EFFECTOF BRANCHING
n-Docosane ( 1 6 ) 4-Propylnonadeoane ( 1 7) 7,14-Dimethyleicosane*
224.5 210.6 205
I1
15
10
166 164
12
156
15
172
n-Dooosane (16)
I11
d/ C
n
H
2
,..
163- 1
...
165-6
CH
/\
...
..
CH3 CHa
CHI 163-4
CH3
/
CH
/\
CH3
III
CHs
CH 226
15
168
Cd%I13
IV
...
n-Dooosane (16)
l5Q-60
b H /\
CHI
CH 224.6
15
166
...
..
l5Q-60
/
AH
CH3
150-2
CHI
d/ :CHp--CEzCH CIdbHs
...
153-4
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1941
TABLE X. EFFECT OF CHEMICAL COMPOSITION ON BOILINQ POINT (Conl'd) Series NO.
Formula C.
--Boiling PointFrom At 1 mm., literature Rr C. M m . He C.
EFPEWO F NUMBER O F RINGS(Cont'd)
...
..
170-1
...
..
...
224.5
16
106
226
15
168
...
..
170-1
...
..
158-60
..
166-6
15
168
0.5
221
15
172
0.4
210
CHa
VI1
c" Cd'CHa
'1I'
-ClaHss*
226
230
*
Based on private information from the late J. v. Braun. Z . A second cpmpound wps included, but the. data had been t o o badly torn in the mails to, be legible. The uncertaint of being able t o reach the author an Russia any time soon makes it a d h a b l e to print No. V without the missing data. b This hydrocarbon is the only one included in the tables.
rings in the molecule (at least within about 3 " C.). Therefore, the boiling points of naphthenic hydrocarbons with 22 carbon atoms per molecule should be about 168" a t 1 mm. mercury (22). However, our work as well as comparison with other 22-carbon hydrocarbons has shown that boiling point is influenced appreciably by chemical structure. The change from aromatic to naphthenic rings, the introduction of side chains, and the number and kind of rings and the type of branching all have a strong effect on boiling point. Table X includes hydrocarbons 2 to 8 as well as other
1389
known 22-carbon hydrocarbons. Olefinic hydrocarbons are omitted, as well as some of the polynuclear compounds with very high boilink points (e. g., picene) and compounds with rather doubtful constitution (e. g., those obtained through alkylation in the presence of aluminum chloride). Data from the literature are recorded together with boiling points at 1 mm. of mercury recalculated on the basis of the Beale and Docksey nomograph ( I ) , which has been found sufficiently exact. The boiling points of 22-carbon hydrocarbons vary from 137" C. for dibutyl diphenyl ethane to 221" C. a t 1 mm. of mercury for cyclohexene phenanthrene ethane. The compounds may be divided into four general groups: (1) hydrocarboq with branched chains and strongly alkylated nuclei (carvacrylic or perhydrocarvacrylic) have the lowest boiling points, in the range 150-164° C.; (2) mononuclear hydrocarbons with once-alkylated rings (hexadecylbenzene and hexadecylcyclohexane), n-docosane, and polynuclear naphthenic hydrocarbons with condensed rings have boiling points in the range 185-172" C. a t 1 mm. of mercury; (3) 181-183" c. a t 1 mm. pressure is the boiling range of symmetrical hydrocarbons with two unalkylated rings (e. g., diphenyl- and dicyclohexyldecanes) and polycyclic hydrocarbons with condensed rings containing a small amount of aromatic nuclei (e.. g., the fraction with the formula CnH2n--14 from Mair, Willingham, and Streiff, $2) ; (4)the polycyclic hydrocarbons with condensed rings, containing chiefly aromatic groups, have the highest boiling pointa, in the range 199-221" C. a t 1mm. of mercury (e. g., the fraction C22.5H24.1 from Table X a , fraction CnHW-ls from Mair, Willingham, and Streiff, and hydrocarbons with four rings per molecule from Table Xc). The effect of hydrogenation on boiling point is shown in Table Xu, where four pairs of synthetic hydrocarbons and one pair of petroleum fractions of similar molecular weight are included. I n the case of mono- and dicyclic uncondensed hydrocarbons, hydrogenation lowers the boiling points by 2 " to 4". For a hydrocarbon with four condensed rings the decrease is much more pronounced and amounts to 50". This observation is in contrast to the data of Mikeska (24) who found that hydrogenation raised boiling point in almost every case. We agree with Mair, Willingham, and Streiff (2%) that these results of Mikeska are probably due to error. Branching the paraffinic side chains (Table X b ) decreases the boiling point. Of the three known paraffins (series I), n-docosane has the highest boiling point. The introduction of the first side chain (propyl) causes a small decrease (2"); the introduction of two side chains (methyl), larger decrease in boiling point (10"). Splitting of the side chain into smaller radicals attached to the benzene or cyclohexane rings (series I1 to V) invariably lowers the boiling point. Branching a side chain (series 111) lowers the boiling point by 7" to 9". I n Table Xc the hydrocarbons are arranged t o show the effect of the number of rings per molecule on the boiling point; not only the number but also the kind of nuclei is important. By introducing phenyl and cyclohexyl rings (series I and II), the boiling point is raised, the increase being smaller for one than for two rings. A quite different effect is evident when paraffin chains are replaced by rings alkylated with small radicals (carvacrylic or perhydrocarvacrylic rings), as shown in series 111, IV, and V. The continuous drop in boiling point is obviously due to the fact that replacing the paraffin chains by unalkylated rings (series I and 11) increases the boiling point only slightly, while splitting of the side chain and alkylating the rings with shorter chains lowers it (Table Xb, series I1 and 111). The same phenomenon exists for lowmolecular hydrocarbons with carvacrylic and perhydrocarvacrylic radicals (CI,, CIS, etc.), with the exception of pcymene and carvacrylmethane, the boiling points of which are somewhat higher than those of n-decane and n-undecane.
1390
INDUSTRIAL AND ENGINEERING CHEMISTRY
The introduction of condensed naphthene rings (Table Xc, series VI and VII) into the molecule raises the boiling point by 2-3' per ring. Inadequate data do not permit us to generalize this statement. The polynuclear aromatics increase the boiling points appreciably (series VI11 and IX) . The conclusions on the relation of chemical structure t,o boiling point may be summarized as follows: 1. Reduction of the aromatic to the corresponding hydroaromatic rin s decreases the boiling point (a)for mono- and dicyclic unconjensed compounds by 3-4" C. and ( b ) for polycyclic compounds with condensed rings by 50". 2. Branching of the paraffinic side chain lowers the boiling
point. 3. Splitting the side chain and alkylating the ring: with shorter chains decreases the boiling point. 4. Introduction of unalkylated rings into the molecule increases its boiling point, ( a ) slightly for condensed naphthenic rings (2-3'), (b) somewhat more for uncondensed benzene and cyclohexane rings and (c) distinctly (38.50° C.) for condensed polycyclic aromiitics. 5 . Introduction of strongly alkylated rings (carvacrylic or perhydrocarvacrylic) Iowers the boiling point by about 5' per ring. Many authors have tried to establish a mathematical relation between boiling point and molecular weight for some classes of hydrocarbons or even for mineral oil fractions. The results discussed above show, however, that boiling point depends not only on molecular weight but also on chemical structure. The application of any formulas in which the boiling point is expressed as a function of molecular weight only may lead to erroneous conclusions. For instance, Lucy (19) gave the following equations:
Vo!.
33, No. 11
Since the presence of aromatics was established with certainty, oil may be regarded as a dicyclic naphthenic hydrocarbon with 10 to 15 per cent aromatic content. The comparison of this oil with synthetic hydrocarbons containing the same number of carbon atoms per molecule gives the following results: 1. The density of oil D is similar to that of hydrocarbons 7 and 8 (Table 11). 2. The refractive index is similar to that of 8 (Table 11). 3. The specific refraction i s somewhat higher (owing t o the presence of aromatics) than the specific refraction of hydrocarbons 7 and 8 (Table 111). 4. The molecular volume i s almost identical with that of 8 (Table I), 5 . Oil D contains two naphthenic rings (Table 11). 6. The viscosity at 50' C. is similar to that of hydrocarbon 8 but different from 7 (Table VIII). 7. The W , value of oil D is somewhat higher (owing to the presence of aromatics) than that of hydrocarbon 8 and is quite different from that of hydrocarbon 7 (Table VIII).
We may conclude, therefore, that the chemical structure of oil D is similar to that of hydrocarbon 8, which means that i t contains condensed naphthenic rings. This comparison has been made in order to show the applicability of the data obtained by the synthesis of pure hydrocarbons, The physical properties of hydrocarbons of known constitution may serve as a tool for determining the general character of the chemical structure of narrow fractions of mineral oils.
Literature Cited Beale, F. S. L., and Dooksey, P.,J . Inst. Petroleum Tech., 21, 860 (1936).
where T = boiling point at 1 mm. Hg, ' K. n = number of carbon atoms per molecule M = molecular weight According to these relations the boiling points of hydrocarbons with 22 carbon atoms per molecule would be 160' C. at 1 mm. pressure. This value corresponds approximately to the boiling points of monocyclic naphthenic hydrocarbons with strongly alkylated rings. On the basis of Equation 2 the boiling point of a paraffin with 22 carbon atoms would be 162.2' C. (the true value being 166"); and for an aromatic with four condensed rings i t would be 144.5' (the true value being higher than 221' C., as seen in Table Xc). Because of cyclization, the decrease in molecular weight in the latter case is due to a n appreciable increase in boiling point. Therefore, mathematical functions based on molecular weight and boiling point cannot be applied to pure hydrocarbons and probably not to narrow cuts of mineral oils. It seems plausible that boiling point does depend directly on the "physical" molecular weight-that is, on the molecular weight multiplied by the association factor at boiling temperature, as proposed by Billig (3). The association factor is obviously smaller for paraffins and higher for aromatics, and thus increases the boiling points of the latter.
Comparison of a Naphthenic Oil with Synthetic Hydrocarbons From a heavy fraction of naphthenic acids a naphthenic corresponding to the formula C21.8H40.6 oil was obtained (M), (hydrocarbon D), by reducing the naphthenic esters to alcohols, changing them into the corresponding iodides, and reducing the latter to hydrocarbons. Besides naphthenic hydrocarbons, oil D contains some aromatic material. If no aromatics were present, the formula CnHan-swould indicate that the oil is a mixture of di- and tricydic naphthenes.
Beilstein's Handbuch der organischen Chem., Suppl. Vol. V, p. 298, Berlin, Julius Springer, 1930. Billig, K., Be?., 68, 591 (1935). Boedtker, Chem. Zen.fr., 1929, 11,2558. Cook, J. W., Dansi, A., Hewett, C. L., Iball, J., Mayneord, W. V., and Roe, E., Ibid., 1936, I, 769. Davis, 6. H. B., and McMlister, E. N., IND. Exa. CHSM.,22, 1326 (1930).
Egloff, Gustav, and Grosse, A. V., Universal Oil Products Co., Booklet 217. Evans, E. B., J . Inst. Petroleum Tech., 24, 321 (1938). Ibid., 24, 537 (1938). Fuchs, G. H. von, and Anderson, A. P., IND.ENQ.CEIP~M., 29, 319 (1937).
Griffith, R. H., and Hollings, H., J . Inst. Petroleum Tech., 20, 255 (1934).
Hewett, C. L., Chem. ZentT., 1938, I, 3469. Hiokman, K., and Weyerts, W., J . Am. Chem. Soc., 52, 4714 (1930).
Klos, S.. Neyman-Pilat, E., and Pilat, S., J . Applied Chem. (U. S. S. R.) 13, 1369 (1940). Krafft, F., Ber., 15, 1718 (1882). Ibid., 19, 2987 (1886). Landa, S., eeoh, J., and Sliva, V., CoZlectioit'Czechalovak.Chem. Commun., 5 , 204 (1933). Liepins, A., Chem. Zentr., 1931, I, 2619. Lucy, F. A., IXJ.ENQ.CHEX.,30, 959 (1938). Mair, B. J., and Willingham, C. B., J . Research Natl. BUT. Standards, 17, 923 (1936). Mair, B. J., Willingham, C. B., and Streiff, A. J., Ibid., 21, 565 (1938).
Ibid., 21, 581 (1938). Mikeska, L. A., IND.ENQ.C E ~ M 28, . , 970 (1936). Mikeska, L. A., Smith, C. F., and Lieber, E., J . Org. Chem., 2, 499 (1938).
Muller, J., and Neyman-Pilat, E., J . Inst. Petroleum Tech., 23, 669 (1937).
MBller, J., and Pilat, S., Brennstoffchem., 17, 461 (1936). Sohicktanz. S. T.,J . Research Natl. Bur. Standards., 14, 685 (1935).
Suida, H., and Gemassner, A,, Ber., 72, 1168 (1939). (29) Turkiewicz, N., Ibid., 73, 861 (1940). (30) Ubbelohde, L., "Zur Viskosimetrie", 1935. (31) Vlugter, J. C., Waterman, H. I., and Westen, H. A. van, J . Inst. Petroleum Tech., 21, 661 (1935). (32) Waterman, H. I., and Leendertse, J. J., Ibid., 25, 89 (1939).
