1104
R. M. DEANEBLY AND L. T. CARLETON
PHYSICAL CONSTANTS OF THE NORMAL PARAFFIN HYDROCARBONS R. M. DEANESLY AND L. T. CARLETON Shell Development Company, Emeryville, California Received September $S, 1940
INTRODUCTION As the last step in the establishment of accurate physical constants for hydrocarbons it is necessary to build up exact relationships between the various members of a homologous series, so that the accuracy of the data for any one compound can be examined not merely in the light of the experimental accuracy of the work on that compound, but by consideration of ita fit in the homologous series to which it belongs. The most widely used relationship of this sort is that of molecular refraction according to the Lorenz-Lorentz formula. Another, for which no simple mathematical equation has been set up, is to the effect that the increment of molecular volume in a homologous series of hydrocarbons is nearly constant, showing a slight smooth trend towards a higher value as the molecular weight increases. In this study the densities and refractive indices for compounds of the normal paraffin series have been correlated so as to form a smooth series, both in respect of increment of molecular volume and in respect of constancy of increment of molecular refraction. In this way it is believed that the most accurate values have been found for the physical constants of these compounds. I n order to carry out the smoothing with satisfactory accuracy and reliability, it became evident that it was necessary to establish a rpliable reference point a t some fairly high molecular weight. n-Hexadecane was chosen as the most suitable, for several reasons: (1) It is the highest member of the series to be liquid at 2OOC. (2) It is well known, although the data on ita physical properties were formerly rather conflicting and unsatisfactory. (3) It was found to be easy to purify. Accordingly, the physical constants of this compound have been redetermined with the greatest care, and after detailed critical study of the results and comparison of them with those of other workers, the data obtained were accepted as more nearly correct than any previous data and were used as a basic point in smoothing the whole series. The second part of this report describes the experimental work to obtain these constants and those of n-dodecane. Normal dodecane was the subject of experimental study in these laboratories to give reference values at an intermediate molecular weight, since, at the time this work was done, the published data on this compound were discrepant and appeared weak. The purification and subsequent measure menta were carried out with the utmost care. We have since noted the
PHYSICAL CONSTANTS OF HYDROCARBONS
1105
most recent and exact measurements of the K'ational Bureau of Standards (8), with which our data are in excellent agreement. Mean values from these two independent preparations may therefore be accepted with confidence.
PARTI I. STEPS USED I N OBTAINING THE SMOOTHED DATA
The first step in correlating the data mas the selection of best values from a study of all data in the chemical literature up to the present, supplemented by those from our own preparations, mentioned above. Selection of the best value of a constant was made for each compound individually, with the utmost care. No mathematical process of selection was considered justified, since the weighting assigned any item must depend on various factors difficult to evaluate numerically: agreement with other items, authority of the investigator, purity of the sample, and accuracy of measurements. Both the thoroughness of our original compilation and our selections are confirmed in nearly all cases within very close limits by the recent publication of Egloff (3). 11. SMOOTHING O F DENSITY DATA
The selected 'density data from which the smoothing is developed are given in column 3 of table 1; Egloff's selected data are given for comparison in column 4. From the selected density data and 1940 atomic weights are derived the molecular volumes given in column 5 and the differences in molecular volume in column 6. These differences in molecular volume are plotted against number of carbon atoms in figure 1. Our selection of the correct smooth curve through these differences is taken to be subject to three conditions: (1) The molecular volumes of pentane, hexane, heptane, dodecane, and hexadecane are accepted as the most nearly correct and are assigned approximately equal weight; all other data are considered far less basic. (2) The intermediate portion of the curve must be both smooth and a good fit to the remaining data as a whole. Of these other data, those for the lower members are probably more reliable, and entitled to more careful consideration, than those for the higher.' (3) A gradual approach to a constant increment of molecular volume, A ( M / d ) , with increasing molecular weight is postulated. Hence higher 1 I t is also worth noting that in purifying compounds of this series the contaminants are almost in every case of higher densityif theyare not of lower boiling point, so that experimental data are liable to err in the direction of higher density more frequently than the reverse. A good fit of lower density than the selected best data is inherently more probable than one of higher density.
1106
R. M. DEANESLY AND L. T. CARLETON
derivatives of M / d with respect to number of carbon atoms per molecule must continuously diminish as the molecular weight increases. Under these conditions the smooth curve of differences of molecular volume (column 7 , table 1) was developed by trial and successive approximations. This curve fulfills condition 3, and at the same time yields what TABLE 1 Density of normal paraffins (81
(1)
dy
NO. OF CARBON ATOMS
5
(SELECTED
BEST DATA PBEBENT AUTEORS)
72.146 0.62631
(0
(4)
d r
A(Mld)
(SELECTED 3EBT DATA. EQLO~)
FROM 2OLUMN
6
0.62638 115.192
115.188 0.62633 15.486 15.495
6
86.172 0.65942
0.65942 130.678
130.683 0.65940 15.879 15.869
7
100.198 0.68368
0.68375 146.557
146.552 0.68370 16.016 16.033
8
114.224 0.7026
0.70283 162.573
162.585 0.70255 16.073 16.142
9
128.250 0.7179
0.71790 178.646
178.727 0.71757 16.146 16.214
10
142.276 0.7304
0.72985 194.792
194.941 0.72984 16.170 16.261
11
156.302 0.7409
211.202 0.74006
0.74045 210.962 16.524 16.291
12
170.328 0.74874
0.7493
227.493 0.74872
227.486 15.886 16.310
13
181.354 0.7575
0.7568
213.803 0.75616
243.372 16.016 16.322
14
198.380 0.7648
260.125 0.76263
0.76360 259.388 16.751 16.330
15
212.406 0.7692
0.7688
276.455 0.76832
276.139 16.655 16.336
16
226.432 0.77335
292.791 0.77336
0.77499 292.794 16.119 16.341
17
240.458 0.7784
0.7781
309.132 0.77785
308.913 16.345
18
254.484
(0.7821)
- 325.477
0.78188
appear to us to be the best possible agreements with the selected data in keeping with conditions 1 and 2. From this smooth difference curve* and the values for n-pentane, n-hex2 The authors agree that there is no theoretical reason for assuming the graph of molar volume of a normal hydrocarbon versus number of carbon atoms to be a smooth curve. It is certain, however, that the experimental data a t Ca, Cs,G, GI,,and
PHYSICAL CONSTANTS OF HYDROCARBONS
1107
me, n-heptane, n-dodecane, and n-hexadecane were derived the smoothed molecular volume and density values of columns 8 and 9. 111. SYOOTHINQ OF REFRACTIVE INDEX
Refractive index data for CS,C, (212, and CU hydrocarbons are conspicuously better established than for other members of the series. The selections for these, as well as for other members, are presented in column 3 of table 2. Values of molecular refraction have then been calculated from the smoothed molecular volumes (column 1) and these find selections of
i
B
..
Numbar ot Corbon Atoms
FIG.1 . Smoothing of molecular volume of normal paraffins. A t 20°C.iM/d for n-hexane = 130.683
the refractive index. Assuming equal increments of molecular refraction per added carbon atom, the molecular refraction is expressible &s
+
Mr = A Bx in which x is the number of carbon atom per molecule and A and B are constants. When equal weights are assigned to each of the values of Mr for (26, Cr,C12, and (216, these yield, by the method of least squares, M r = 2.0752 4.640412 rt 0.0010
+
GI@,the basic reference points discussed above, indicate that any deviations from smoothness are small. For simplicity, and in the absence of further evidence, we have assumed a smooth curve, and it is evident from the above discussion and from figure 1 that the possible errors thus introduced are inappreciable.
1108
R. M. DEANESLY AND L. T. CARLETON
This expression is adopted, and smoothed refractive indices (column 7) are calculated from the values of Mr thence obtained and the smoothed molecular volumes from table 1. The increment of molecular refraction is thus determined as 4.6404, with an uncertainty of several units in the last place of decimals, and cannot at present be defined more precisely. Final values of d and n are recorded in table 3. It is noteworthy that Hulst (e), without many very accurate recent data at his disposal, derived the value 4.640 for the series of normal paraffins. For a series of nitriles he derived the value 4.636. TABLE 2 P normal
Refractive index (1) NO. OF CARBON ATOMS
(2)
(3)
Mld
.20'
SMOOTHED;
PROM COLU M N 8, TABLE 1)
(6)
(4 )
D [SELECTED )EST DATA, PRESENT AUTEORS)
.20' D (SELECTED
BEST DATA,
M . ? ' IMOOTHED)
EQLOFF)
8 9 10 11 12 13 14 15 16 17 18
115.188 130.683 146.552 162.585 178.727 194.941 211.202 227.493 243.803 260.125 276.455 292.791 309.132 325.477
1.35782 1.37505 1.38770 1.3977 1.4056 1.4120 1.4173 1.42156
1.35768 1.37506 1.38774 1.39764 1.40562 1.41205 1.41902 1.42182
1 ,43449
1,4352 (1.4380) (1.4399)
(7)
(6)
-+
.20'
n a 1 __ 12% 2 FROM COLMNS 2 AND
___
~
5 6 7
wafins
25.2773 29.9177 34.5581 39.1985 43.8389 48.4793 53.1197 57.7601 62.4005 67.0409 71.6814 76.3218 80.9622 85.6026
D IMOOTHED; FROM COLU M N 6)
0.219444 0.228933 0.235808 0.241095 0.245284 0.248687 0.251511 0.253898 0.255946 0.257726 0.259288 0.260670 0.261902 0.263007
The values of column 7 are in agreement with the equation and the smooth values of d of column 9, table 1.
TZ
=
1.35772 1.37503 1.38770 1.39752 1.40535 1.41174 1.41707 1.42158 1.42547 1,42886 1.43184 1.43448 1,43684 1,43896
0.350363 0.347186 0.344898 0.343172 0.341824 0.340741 0.339853 0.339111 0.338482 0.337942 0.337474 0.337063 0.336700 0.336377
0.52167d
+ 1.03104
IV. RELATION BETWEEN REFRACTIVE INDEX AND DENSITY
Smooth values of density and refractive index having been obtained in the manner described, another relationship, which is not predictable, becomes apparent. This is that above C6the smoothed values of n and d are in exact linear relation according to the equation n = 0.52167d
+ 1.03104
Values of n calculated from smoothed densities by means of this equation agree with the smoothed values of table 2 derived from molecular refraction to 0.00004 or less for all compounds within the range Ce to CIS, inclusive. This observation k in some measure independent support of the
1109
PHYSICAL CONSTANTS OF HYDROCARBONS
correctness of the smoothed molecular volumes obtained by the method described. TABLE 3 Summary of the physical constants of normal parafins
(1)
(7)
NO. OF
IOLECULA
CABBON
WEIQHT
ATOMS
(1940)
-
BOILlNQ
da d&
at MO'C.
POINT AT 780 M Y .
11
12 13 14 15 16 17 18
-
72.146 86.172 100.198 114.224 128.250 142.276 156.302 170.328 184.354 198.380 212.406 226.432 240.458 254.484
0.62633 0.65940 0.68370 0.70255 0.71757 0.72984 0.74006 0.74872 0.75616 0.76263 0.76832 0.77336 0.77785 0.78188
0.000975 0.000891 0.000840 0.000803 0.000774 0.000761 0.000733 0.000719 0.000708 0.000699 0.000692 0.000686 0.000681 0.000677
F R E E B l N Q POINT AT 780 XM.
-1-
'C.
5 6 7 8 9 10
(8)
1 .35772 1.37503 1.38770 1.39752 1 .4O635 1.41174 1.41707 1.42158 1.42547 1.42886 1.43184 1.43448 1 ,43684 1.43896
36.1 0.000542 68.7 0.0005O8 98.4 0.000483 125.6 0.000463 150.7 0.000447 174.0 0.000435 195.8 0.000425 216.2 0.000417 235.5 0.000410 253.6 0.000405 270.6 0.000400 286.5 0.000396 301.4 0.000393 315.3
OC.
-129.7 -95.3 -90.6 -56.8 -53.7 -29.7 -25.6 -9.60 -6.0 +5.5 10.0 18.145f0.003 22.0 28.0
I n view of the accuracy of the data surveyed and the principles of smoothing employed, i t is believed that all thevalues quoted in this table are not likely a t any time to be revised by more than the values shown below: f0.0022 (z = number of carbon atoms) for M AO.000010for dnldt A0.00005 for d A0.5 for the freezing point fO.00010 for n f O . l for the boiling point &0.000005 for ddldt of C I ~ and below; f0.3 for the boiling point above CI2 The above data fit the following relationships: I. n = 0.52167d 1.03104 11. MI.:' = 2.0752 4.640412 (z= number of carbon atoms) Note: The selected experimental boiling points through CI2fit the series of smooth values tabulated above to closer than 0.1"C. The extrapolation from there onward is made to agree with the value 286.4OC. a t 760.6 mm. directly observed on our best sample of n-hexadecane. There is also fair agreement a t all points in this range with the experimental data of Krafft (7). Freezing points are best experimental data; they form a fairly smooth series, but criteria are too uncertain for more acciirate smoothing (see figure 4).
+ +
V. CHANGES IN DENSITY AND REFR4CmVE INDEX WITH TEMPERATURE
In order to be able to apply the smoothed values for d and n at 20°C. to observations at other temperatures, a plot was made of the best values
1110
R. M. DEANESLY A N D L . T . CARLETON
of ddldt and dnldt which could be derived from the literature. Within the accuracy of this smoothing, dnldt and ddldt are constant for a range of approximately 20' above and below 2OOC. The sources used were as follows: Timmermans et al. (10, l l ) , Shepard et al. (9)) Wibaut et aL3 (12), Brooks et aZV4(1, 2), Egloff (preferred values) (3)) and the work of the present authors. Data of Eykman (4)have been considered, but are omitted from the graph a.s they are too few and scattered to assist in establishing the mean line. Each value was plotted against the corresponding value of d or n, respectively (figure 2). For the coefficient of density there are a large number of well-established values, and accordingly a good smooth curve could be established by inspection. Between CISand the value of d corresponding to infinite molecular weight (approximately 0.86)) a number of values from Egloff's compilation form a guide to the general trend of the curve. The values taken from this smooth curve are given in the summary (table 3, column 4). In the plot of dnldt versus n, the values are fewer and often less reliable, and the location of the smooth curve was accordingly derived in the following manner: Since n has been shown to be a linear function of d a t 2OoC., it should likewise be a linear function a t any adjacent temperature, 25'C., for example. That is, n=Ad+B in which A and B are functions of temperature alone. Differentiating, d - - n = Add - + d - + dA ;It dt dt dt
dB
+ 1.03104, dn -at -- 0.52167 dd -+ (d)m dt
Since, a t 20°C., n = 0.52167d
After careful consideration, the values of dnldt = 0.000542 and 0.000400, at C g and Cis, respectively, are chosen as the best that can be established, taking into consideration each and all of the data of figure 2. Substituting 3 Evidence from the data of Wibaut et al.,taken as a whole,is such that there is a slight probability in favor of a slight trend in the direction of a larger dn/dt with decreasing A. The scatter of the data is, however, greater than any such trend. Accordingly, mean values from Wibaut's data have been selected by inspection, since no rigorous mathematical system of averaging is justified. Data from the two papers have been averaged.
1111
PHYSICAL COXSTANTS O F HYDROCARBOXS
8
x
I
c
0
50 n
0
(
A
5c
-
&
-0
40.
1.375
I.
1
1.475
FIG.2 . Temperature coefficients of density and refractive index at 20°C. for normal paraffins
1112
R. M. DEANESLY AND L. T. CARLETON
these values and corresponding values of d and ddldt in the equation above, we obtain for Ca
dn - -- 0.000542 = 0.52167 X 0.000891
+ 0.65940
dn - _ - 0.000400 = 0.52167 X 0.000686 dt
+ 0.77336
dt
-
These two equations may be solved for the two unknowns (dA/dt)t-20 and (M/dt)r-m. When values from this solution are substituted, the equation becomes dn dd = 0.52167 - 0.0003077(d)rd dt
+ 0.0002801
This last equation and the previously derived curve of dd/dt versus d establish dn/dt for C6 and all compounds above. Within this range, it appears to give the best possible representation of the individual values. The resulting smooth curve of dn/dt versus a is shown in the lower portion of figure Z 5 An additional check on the validity of this smoothing is given by calculation of the temperature coefficient of specific refraction, dr/dt, from various correvponding values of dd/dt and dn/dt from the two curves. A plot of drldt, so obtained, against the number of carbon atoms shows a succession of values from 0.000021 a t C6 to 0.000030 a t CIS,forming, within the accuracy of the calculation, a smooth curve which agrees well with the best individual values. The smooth curves thus obtained for ddldt and dnldt versus d and n, respectively, have been used to construct the curves of figure 3, where these functions are plotted for greater convenience against the number of carbon atoms. A comparison of all good individual values of ddldt and dnldt for branched-chain paraffins shows such small and uniform scatter about the two smooth curves of figure 2 in which dd/dt and dn/dt are plotted against d and n, respectively, that these curves may be accepted as giving best values for all paraffis. This is of course not true of the graphs in figure 3, where the basis for the plot is the number of carbon atom.B 6 It should be noted that the equation, like the linear relation between ~t and d which forms ita basis, appears not t o be applicable t o n-pentane. For this reason the curve is not extended below Ca. e However, if high precision is not required, the graphs in figure 3 may be accepted aa giving approximate values, not only for all paraffins, but for all other saturated hydrocarbons a8 well.
PHYSICAL CONSTANTB OF HYDROCARBONS
1113
VLI. OTHER PHYSICAL CONSTANTB
I n order to complete the tabulation of the properties of the normal paraffins for convenient reference, the data for boiling points and freezing points have been listed in table 3, columns 7 and 8, respectively. Those for boiling points have been taken from a smoothed curve (not shown) which exactly fits, within &0.loC., well-established experimental data from more than one source for each member from C4 to Clz, inclusive. As may be seen, it establishes with fair precision best values for the higher mem-
Numbrr of Carbon Atoms
FIG.3. Temperature coefficients of density and refractive index a t 20°C.for normal paraffins. From these two curves dr/dt varies from O.ooOo21 at CSto 0.000030 at
(218.
bers. The values quoted for freezing points are best values from the literature (from Egloff, with the exception of C6 and C,) except in the case of Cia, for which our latest value is accepted, and in the case of CU, for which our new value is in exact agreement with that of the Bureau of Standards. They are not smoothed, but the diyergences from a smooth alternating series are small, as may be seen in figure 4. Extrapolation of the difference curves on this graph predicts within 0.5"C. the literature value of the freezing point of Cda. The one distinct deviation of the experimental values from the smooth difference curve is in the region Cg to CS,where the experimental values of several observers are in good
1114
R. M. DESNESLY AND L. T. CARLETON
agreement. This is curious, and it is not clear whether one of the experimental values is in error or whether, as is quite likely, some factor in the crystalline state of one or the other of these compounds is causing an abnormality of the freezing point.
Number of Carbon Atoms
FIG.4. Freezing points of normal paraffins
PARTI1 I. PREPARATION O F PURE n - D O D E C A N E
Lauryl alcohol (from the Eastman Kodak Company) was refractionated and a pure heart cut was taken. This was converted to the iodide, following the directions given in Organic Syntheses (Vol. 15, page 29). The crude iodide, after filtration and .washingto remove acid, wasreducedto n-dodecane by means of zinc, according to the directions given in Organic Syntheses (Vol. 15, page 27). The dodecane (2060 cc.) removed by ether extraction and distillation was then fractionated at atmospheric pressure. Cuts of 100 cc. were taken, and their refractive indices were measured. Those of constant nto' = 1.42156 were combined for further purification (1350 cc.).
1115
PHYSICAL COXSTAKTS OF HYDROCARBONS
This material was subjected to repeated recrystallization without solvent, the freezing point being observed with a Beckmann thermometer after each step in the process. After five recrystallizations, the freezing point had risen 0.05"C. At this point the remaining n-dodecane, amounting to approximately 100 cc., was washed with 98 per cent sulfuric acid containing 3 per cent silver sulfate. This step caused a rise in the freezing point of 0.02"C. Two further recrystallizations without solvent were without effect on the freezing point (Le., they affected the freezing point by less than 0.OOZ"C.). The freezing point of this pure sample was observed to be -9.604"C., using a resistance thermometer calibrated by the Bureau of Standards. As a check on the purity of the product, it was recrystallized twice from acetone, but this also Jvas without effect on the freezing TABLE 4 Physical constants of n-dodecane Freezing point.
..............................................
Density.. , . , . , . , . . , , , . , , , . , , , , , , , , , . . d::'
-9.604 f 0.003"C.
= 0.74876 & 0.00001
dd = dt
0.000718
Refractive index, 20°C. : nc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.41936 f 0.00006
nD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.42152 f 0.00006
nF.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.42673 f 0.00006
dn
- = 0.00G404 dt
Hence, n2-1
Specific refraction
rEo = -
Specific dispersion
&o" -=
nz+2
nF - n c ~
d
1
x -d
dr dt
- = 0.000039
= 0.33905
X 10' = 98.4
point. The sample was therefore judged pure and its physical properties were determined. These properties are as shown in table 4. From the consistency of agreement of the measurements obtained (especially on the freezing point), as well as the extreme carefulness of the techniques, this preparation and that recently reported by Mair and Streiff (8) are outstanding. The measurements on the latter sample, the impurity in which is estimated to be not more than 0.06 mole per cent, are summarized below : MELTING POINT
BOILING POINT
'C.
'C.
-9.6OfO.02
216.26f0.03
di!'
0.74512f0.00005
1
7l
A T 25%.
= 1.41736fO0.00O10
nc = 1.41951f0.00010 nF = 1.42469 f 0,00010
1116
R . M. DEANESLY AND L. T. CARLETON
11. SELECTED VALUES OF THE CONSTANTS
The present authors and Mair and Streiff agree to O.OO°C. on the freezing point of n-dodecane; the latter and Shepard and coworkers agree to 0.03"C. on its boiling point. In selecting best values of d:' and n:" for the smoothings of the first section, however, there is more uncertainty, especially since the measurements of the Bureau of Standards must be converted from 25°C. A preliminary smoothing of the temperature coefficient of density for the entire series of normal paraffins yields a vaiiie of 0.000722 a t CI2. This is in good agreement with the value of 0.000718 obtained on our sample, and the figure 0.000720 is accordingly used in correcting the Bureau of Standards density to the figure quoted in table 5. The selected density is an average of this corrected figure with our directly measured value. The conversion of the refractive index is more uncertain. Our sample yields a coefficient of 0.000404 for the D line, the consistency of which with the values for the F and C lines, 0.000414 and 0.000401, respectively, TABLE 5 Selection of constants o j n-dodecane BOURCm
1
,I
Present authors.. . . . . . . . . . . . . . Mair and Streiff. . . . . . . . . . . . . . . Selected values. . . . . . . . . . . . . . .
.I
oc.
'C.
-9.604
1:::;
216.26 216.25
0.74876 0.74872 0.74874 I
1 ,42152 1 ,42161 1.42156
is evident. At the same time, this figure is distinctly low with reference to values for the series of normal paraffins. as a whole, and especially with reference to the carefully chosen value for n-hexadecane; a graphical smoothing of these other values gives 0.000425 a t Clz. In table 5, conversion to 2OOC. of the Bureau of Standards n:' (which reproduces almost exactly an earlier determination by Mair) has been made by the value 0.00042. The arbitrariness of this or any other assignment is, of course, clear, but the wide scatter of the data for dn/dt shown in figure 2 precludes more exact choice. Finally, then, the selected refractive index is the average of this calculated n:" figure with the measured nzo' of the present work. 111. PREPhRATION O F PURE n-HEXADECANE
Two completely independent purifications of this compound have been made a t these laboratories, starting from different batches of commercial cetane reference fuel for Diesel engine testing. The data are in close agreement and are supported by those recently published by Wibaut et al. (12). For reasons stated later, our second preparation is believed slightly superior
1117
PHYSICAL CONSTANTS O F HYDROCARBOKS
to the earlier preparation and to that of Wibaut and coworkers, and the data from it have therefore been used in the foregoing correlation of the series of normal paraffins. This choice has been made only after very critical appraisal of the work, as the authors were reluctant to appear biased in favor of their own experimental work to the exclusion of that of others.
A . First preparation A preliminary purification of a 100-cc. sample of freezing point 16.215.8OC. by recrystallization four times, each time from five volumes of acetone yielded a sample having the following constants: f.p., l7.7tioC., d$", 0.7738; ntoo, 1.4343. A fresh start was then made, using waste cetane of freezing point 15.15"C. from the former preparation and twenty volumes of acetone for each crystallization. A single recrystallization yielded 37 per cent of crystals of the following properties: f.p., 17.65"C.; dz!', 0.7740; n io', 1.4343. After two more recrystallizations the constants were: f.p., 1i.95"C.; d:', 0.7737; ntoo, 1.4344. The first sample of purified cetane of freezing point 17.78OC. was added at this point, and after two more recrystallizations the constants were: f.p., 18.15"C.; d;'?, 0.7738; nZOa , , 1.4344. Another recrystallization from acetone and another from methyl ethyl ketone had no further effect on the freezing point. The final properties when tested again were: f.p., 18.15"C.; d;!', 0.7737; nio', 1.4344. Cold concentrated sulfuric acid had no effect on the final sample. The residual cetane recovered from the mother liquor from the last step had a freezing point of 18.05"C. The density measurements above were not considered of the highest precision, and a new pycnometer, calibrated against certified weights, mas used on the final sample. The results were as follows: TEMPERATURE
MEAN
'C.
20.0
0.7735(7) 0.7735(6)
25.0
0.7704(3) 0.7704 (2)
30.0
0.7667 (2) 0.7666 (5) 0.7667 (2)
di? = 0.7735(7)
$ = 0.000685
In order to determine accurately the temperature coefficient of the refractive index of cetane, a sample having a freezing point of 17.95"C. (the best sample remaining) was placed in the cell of the Pulfrich refractometer and the temperature was varied gradually. The mean straight line of eighteen
1118
R. M. DEASESLY A S D L. T. CARLETON
observations at ten different temperatures between 20" and 30°C. gave dnldt = 0.00040. Applying this to the best observation on the pure sample (at 25.0"C.; room temperature, 25°C.) yields nto' = 1.4344. To verify these values, the properties of the impurities removed were studied. Starting with a fresh sample of the crude material, a single recrystallization was made from twenty volumes of acetone, and both crystals and residuum were separated from solvent. The results are tabulated below : SAMPW
1
VOLUME
~
POINT
cc.
Original material. . . . . . . . . . . . . . Crystals ....................... Residuum from solvent, . . , . . , .
65 51 12
17.85
0.7745 0.7737 0.7792
1.4346 1.4346 1.4361
This procedure was repeated with the following results : MMPLl
Crystals above. . . . . . . . . . . . . . . . Crystals (product) . . . . . . . . . . . . . Residuum from solvent. . . . . . . .
45 29 14
II
_~ 1;:5 18.10 17.35
0.7737 0.7737 0.7742
1.4346 1.4345 1.4347
1119
PHYSICAL COXSTASTS OF HYDROCARBONS
freezing curve was too small to be mea*ured. A tenth crystallization was without effect, changing the freezing point to 18.147"C., i.e., nithin experimental error. This tenth sample of material aq the final product and yielded, after drying with sodium, the follon ing nieawrements for the refractive index and density: COXSTANTS
7':
t = 2O.W"C.
1
1.43215: 1.43212 1.43436: 1.43434 1.43077: 1.43375 0,77336; 0,77335
. . . . . . . . . . . . . .l
~
nD . . . . . . . . . . nk . . . . . . . . . . . . . . . .
d6 . . . . . . . . . .
t
~
-
25.03"C.
1.43026;1.43021 1.43250; 1.43245 1.43787; 1.43781 0,76991; 0,76992
The final freezing-point measurements n ere carried out with a cryoscopic molecular weight apparatuq, using a Beckmann thermometer calibrated against a resistance thermometer certified by the Bureau of Standards. The thernionieter foi the pycnometer bath and the refractometer circulating liquid i\a+ calibiated against the same resistance thermometer. The calibrations of the pycnometer (approximately 8 cc.) n-ith water a t 20" and 25°C. nere conqistent and justify the assertion that the absolute accuracy of the density measurements is f 0.00002. The measurements with the Pulfrich refractometer n ere reproducible to f 0.00003, but after the accuracl- of observing the zero point of the instrument is taken into account, an accuracy of only f 0.00006 is probable. In selecting the most nearly coriect value of the refractive index, it must be taken into consideration lion accurately the temperature of the refractometer circulating liquid iq a measure of the temperature of that small portion of the liquid in the cell through which the light passes dn -
SOURCE
dl
n a v e length
l
6563
_
C
,
I. Present work, preparation I I I
5891
5Si6
_
d
_
D
~ 0.00040
I
11. Present work, preparation 11. . . . . . . . . . . . . .. I 0.00037S
0,000373 O 000382
111. Wibaut et al., tables I1 and I11 1 0 000412 I V Smooth-curve value for series of paraffins (ref. 5, P. 13)
-
0 000408
I
0 000412
0 00043 I
I
i 0 00040
~
.
1120
R. M. DEhNEGLY AND L. T. CARLETON
It is clear that if the room temperature is higher than the refractometer temperature, the effective sample will be At warmer than the circulating liquid, and vice versa. Whether At is large enough to be significant is debatable. The comparisons given in the table a t the foot of page 1119 are pertinent. The value of dn/dt will not be affected by the minor difference in purity between our samples and Wibaut's. There is, accordingly, evidence that our value for dn/dt is slightly in error, and it is concluded that our value for the refractive index a t 25.03'C. for preparation I1 is most nearly correct, being nearest room temperature a t the time, and that the value of dn/dt = 0.00040 should be applied to it to give the correct refractive index a t 20'C. This is the basis for the values given in table 6. TABLE 6 Physical constants of n-hexadecane Freezing point. ............................................... Density. . . . . . . . . . . . . . . . . . . di!'
=
0.77335 i 0.00002
2
18.145 =k 0.003"C.
= O.OOO684 It 0.000002
Refractive index, 20°C.: % ,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
nD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . nm.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hence, Specific refraction Specific dispersion
=
1.43225zk 0.00006 1.43449 i 0 . OOOOB 1.43985 rt 0 .OOO06
:G2$ -
X
dn
F
= 0.33707(4)
=
0.00040 zk 0.00001
dT
;ii = O.OooO29
nF nc ~ ' = 3 ~ ~ ~x 104 = 98.3 i 0.2
IV. COMPARISON OF THE DATA WITH OTHERS
Apart from these two preparations of pure n-hexadecane and that of Wibaut et al. (12), all other published data show much higher densities. The probable reason for this iq clear from our measurements of the density of the impurities, probably mostly cetyl alcohol. The following arguments further go to show that the association of the lolyest density with the highest freezing point is in this case strong evidence of the highest purity. For what contaminants are possible and what would be their effect on the physical constants? (1) The sample was inert to cold concentrated sulfuric acid and had zero bromine number; therefore no unsaturates were present. ( 2 ) Lower molecular weight paraffinq would lower not only the density but also the freezing point. In any case, fractional distillation would eliminate them. (3) Higher molecular weight paraffins would raise the density, so that, if they are present in this sample, the true density is even lower. In any
Q 2
0
$I
: h
d
h
n
cv
ku:
6'
53uj 0
0
3
0
0 h h
h
h
h
T
P
a
8& h
h
P
*
8 II
3s h
9
tX
0 h
0
0
0
0
i
1121
0
0
0
1122
R. M . DEANESLI' AND L. T. CARLETON
case, fractionation did eliminate a few per cent of a higher boiling (unidentified) impurity. (4) Isomeric branched-chain paraffins would have, by analogy with the rest of the paraffin series, lower freezing points. The chance of paraffinic impurity of any sort, which, because of crystal isomorphism, formed a complete range of mixed crystals of intermediate higher freezing point and was therefore not easily eliminated by recrystallization, is out of the question here, since such a compound would have to combine higher freezing point with lower density to fit the observed trend on purification. (5) Any conceivable non-paraffinic impurity inert to bromine and sulfuric acid would have higher density like cetyl alcohol and be easily eliminated,-as, in fact, the impurities were,-by selective solvent recrystallization. The data from our first preparation are rejected on the grounds that purification was less thorough-no fractional distillation, and work only from a small original sample-and also because the measurement of the physical constants wa.s carried out with a little less precision. The data of Wibaut et al. are rejected on the ground of distinctly lower freezing point associated with higher density. For these reasons we used the corrected data of our second preparation (table 7), rather than any average of our own work with that of others in the smoothing of the series described in the first section of this report.
SUMMARY From the extremely careful preparation and measurement of the physical constants of n-dodecane and n-hesadecane, and from a complete survey , of data in the chemical literature on these and other normal paraffins, values of the physical constants for members of the series from Cg through Cls have been selected. From these ?elections series correlations are obtained, yielding highly accurate smooth values for all members in the range considered. Constants treated in this manner include freezing point, boiling point, density ($), refractive index (n:"), and the temperature coefficients of the last two constants. The authors are indebted to Mesare. C. J. Gaiser and K.C. Castner for assistance in carrying out the synthesis and purification of n-dodecane and of n-cetane. REFERESCES (1) BROOKS, D. B. : J. Research S a t l . Bur. Standards 21,847 (1938). (2) BROOKS,D. B., HOWARD, F. L., A X D CRAFTON, H. C., J R . : J. Research xatl. Bur. Standards 24, 33 (1940). (3) EGLOFF,C. : Physicul Constants of Hydrocarbons, Vol. I. Reinhold Publishing Corporation, S e w \-ark (1939).
FUNCTION O F C.4RBOKATE IK SYNTHESIS O F GLTCISE
1123
(1)E r ~ z r a ~J., F. : Recherches Refractometriques. (5) (6)
(7) (8) (9)
(10) (11)
(12)
de Erven Loosjes, Haarlem (1919). GROSSE,A. V., A N D EGLOFF,G.: Physicul Comtants o,f Parafin Hydrocarbons. Bulletin 219, Universal Oil Products Company, Chicago (1938). HULST,L. J. K.V A N D E R : Rec. trav. chim. 64, 518 (1935). KRAFFT,F.: Ber. 16, 1687 (1882). MAIR,B. J., A N D STREIFF,A. .J. : ,J. Research T a t l . Bur. Standards 24, 395 (1940j . SHEPARD, A. F., HEKSE, A. L . , ASD MIDGLET, T., JR.: J. .hi, Chem. Soc. 63, 1918 (1931). T I M M E R M A N S , J., A N D l , i A R T I S , F.: J. chim. phys. 26, 111 (1928). TIN\IERMASS, J., A N D HESS.AST-ROL.ASD, hfarz: J. chim. phys. 32, 501 (1935). \vIIIB.%UT, J. P., HOOG, H., LASGEDIJK, s. L., O V E R H O F F , J . , A S D S M I T T E S B E R G , J.: Rec. trav. chim. 68, 329 (1839).
QL-ASTITATIVE ISVESTIGA4TIONS O F AXIhTO ,1CIDS ASD PEPTIDES. VI THE FUNCTIOK OF CARBONATE IN
SYNTHESIS OF GLYCINEFROM HYDROXIDE, AND A M M O N I ~CARBOKATE' M THE
CHLORO.4CETIC ACID, i h M O S I U h l
LIAY S DV:",
A W BUTLER,
AND
EDWARD H F R I E D E S
Department of Chemastry, Cnauerszty of Calafornza, Los Angeles, Culzfornza Recezued February 16, 1041
The first synthesis of glycine from a-halogen acids and ammonia,namely, that reported in 1858 by Perkin and Duppa (12),-was folloned by a number of investigations (4), of nhich the one by Robertson (11) is the most comprehensive. In the early studies glycine was isolated as the copper salt in order to separate it from the mixture of the amino acid and the ammonium salts of hydrochloric (or hydrobromic) acid, iminodiacetic acid (so-called diacid), and trimethyleneaminetricarboxylic acid (so-called triacid) (equations 1 to 3, table 1). Studies by Robertson (11) on the rates of formation of chloride ion and primary amine under varying conditions led to the discovery that the production of secondary products is markedly depressed, and the yield of glycine is increased nearly to the theoretical quantity, by the use of high 1 Presented before the annual meeting of the American Society of Biological Chemists, held in Toronto, Canada, April, 1939 (see reference 5). The authors were aided in this work by a grant from the Univemty of California. For the fifth communication in this series see Dunn and Porush: J Biol Chem. 137, 2G1 (1939)
