32
FRANK V. DUNDERMAN AND S. H. BAUER
The temperature of the column was then raised to 130°C.; after 125 hr. 15 ml. of ester had left the column. Ester (boiling point = 92°C. at 16 mm.). ....................... Alcohol. .......................................................
aD = OID
-31.4' $0.04"
SUMMARY
Diastereoisomers of the type of dl-sec-butyl 1,2-methoxypropanoate and dlsec-butyl 1,2-acetoxypropanoate were partially separated by selective adsorption on activated charcoal.
T H E RAMAN AND ULTRAVIOLET-ABSORPTION SPECTRA OF THE
DIMETHYL ETHER-%ORON TRIFLUORIDE COMPLEX^ FRANK V. DUNDERMAN
AND
S. H. BAUER
Baker Laboratory of Chemistry, Cornell University,Ithaca, N e w Y o r k Received August 21, 1946
As an extension of our determination of the structure of dimethyl ether-boron trifluoride (l),we undertook a study of the Raman and ultraviolet-absorption spectra of the liquid, in order to determine the effect of coordination on the vibrational frequencies of the parent molecules and on the ease of excitation of the electrons associated with the C-0 bond in ether. The liquid2 was distilled several times in a vacuum train which was free from grease contaminations. The resulting product was clear and colorless, and remained so even after several hundred hours' exposure to the light of a mercury arc which had been filtered through Pyrex glass? THE RAMAN SPECTRUM-DATA
The Raman tube was constructed with a jacket for circulating a light-filtering solution and for temperature control. It was mounted vertically, at the common focus of two elliptical reflectors having a pair of mercury arcs (G. E. Uviares) at each of the two foci. When these are operated at 25 volts and 3 amperes, the X 4358 line appears in great intensity, whereas neighboring lines are quite weak. The spectrograph used was a large Hilger constant-deviation instrument with a dispersion of about 40 A. per millimeter a t X 4600. 1 Prepared for the 1945 Meeting-in-Print of the Division of Physical and Inorganic Chemistry of the American Chemical Society. 2 The material used was a sample kindly loaned by Professor Laubengayer ( 6 ) , to whom we wish t o express our sincere appreciation. 3 We believe that the often-observed discoloration of dimethyl ether-boron trifluoride on standing arises from a slow reaction with traces of grease and is not due t o a photochemical reaction, as has sometimes been stated.
SPECTRA O F DIMETHYL ETHER-BORON
TRIFLUORIDE
33
The compound was observed to fluoresce somewhat in the ultraviolet; hence, it was not surprising that the ratio of line intensity to that of background was low in the region of X 4358. By regulating the exposure time and underdeveloping, it was possible to record (on Eastman 103-a plates) displaced lines on the linear portion of the sensitometric curve, whereas the background was held down to the toe of the curve, thus leading to maximum contrast. The positions of the Raman lines were determined by linear interpolation between the lines of an iron arc, both on microphotometer recordings and on tenfold-enlarged prints. On eliminating those frequencies which could arise from the scattering of the various mercury lines in the region X 4046 to X 4358,twelve reliable displacements were observed. These are listed in table 1; the limits of error given were estimated from the self-consistency of repeated measurements. DISCUSSION
In order to appreciate the tremendous effect the association has on the energy levels of the parent molecules, reference should be made to figure 1, wherein TABLE 1 cm.-'
328 f 14 496 f 7 666 f 16 804 f 4 918 f 15 1014 f 5
cWL-1
(weak and diffuse) (moderately strong, broad) (weak and diffuse) (moderately strong) (stronger than 804) (weak and diffuse)
I
1085 f 5 1216 f 2 (1267) f 2 1309 f 10 1454 f 3 3035 2977
*
(weak and diffuse) (strong, sharp) (weak and diffuse) (weak and diffuse) (moderately strong, broad)
} (doublet, moderately strong)
the observed frequencies of boron trifluoride4 and dimethyl ether6 are compared with those of the associated complex. The initial symmetry of boron trifluoride (D3h)has been reduced t o a local symmetry in the vicinity of the boron atom which is Caw,a t most; the local symmetry for the oxygen atom is C,. Changes in molecular dimensions as deduced from electron-diffraction measurements are summarized in table 2. Since the vibrational analysis for dimethyl ether is still in doubt, it is not possible to learn much from the present Raman data alone. Clearly, many more lines than we have observed are to be expected; these must be weak and apparently could not be distinguished from the background. Roughly, one may estimate from the most probable electron-diffraction values that the symmetrical B-F stretching should decrease from 888 cm.-l to about 610 em.-'; perhaps the line a t 666 cm.-1 is that one. Other possible correlations are indicated in figure 1. The one definite conclusion which follows is that in liquid (CH3)20:BF3very little of the undissociated parent molecules can be present, since the strongest As summarized by D. M. Gage and E. F. Barker: J. Phys. Chem. 7,455 (1939). As summarized by G. Herzberg: Infrared and Raman Spectra of Polyatomic Molecules, p. 353, D. Van Nostrand Company, New York (1945).
34
FRANK V. DUNDERMAN AND S. H. BAUER
a
SPECTRA O F DIMETHYL ETHER-BORON
35
TRIFLUORIDE
Raman lines of these substances do not appear in the spectrum of the complex. The complex CH3CN:BFa would have been more suitable for a study similar to the above, since the symmetry of the methyl cyanide would remain unchanged and that of BF3 would be reduced to C S ~ .Unfortunately, this complex is a solid at room temperature, and is but slightly soluble in suitable liquids. Finally, we might note that previous recordings of the Raman spectra of mixtures wherein the formation of molecular complexes was to be expected resulted in spectra which showed appreciable displacements of the line positions, but the similarity of the patterns to that of the parent substance was evident (2, 4, 5 ) . Only in the case of diethyl ether with aluminum trichloride (3), wherein the formation of a donor-acceptor bond of the type proposed for dimethyl ether-boron trifluoride appears plausible, did the resulting spectrum differ appreciably from that of the ether. TABLE 2 Ch.an,qes in molecitlar dimensions as deduced jrom electron-difraction measurements (CHs)zO*
B-F . . . . . . . . . . . . . . . . . . . . . . . . . . . C-0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-O., . . . . . . . . . . . . . . . . . . . . . . . . . L FBF. . . . . . . . . . . . . . . . . . . . . . . . . LCOC . . . . . . . . . . . . . . . . . . . . . . . . .
* Reference
1 . 4 2 f 0.02 if.
~
BFst
(CHdz0:BFat:
1.30 f 0.02 if.
1.43 f 0.03 if. 1.45 f 0.03 1.50 f 0.06 A.
120" 111" f 4"
4.
110" 110" (assumed)
9.
t Reference 7 .
1Reference 1. THE ULTRAVIOLET-ABSORPTION SPECTRUM-DATA
For purposes of comparison the absorption of liquid diethyl ether and of dimethyl ether-boron trifluoride mas measured over the range 5000-2100 8. Boron trifluoride does not absorb in this region. The source was a small hydrogen discharge tube running a t 3 mm. pressure and excited by about 12 kilovolts, The spectrograph mas a large oBauschand Lomb single-prism quartz instrument, with a dispersion of about 7 A. per millimeter in the region X 2200. Sensitized 1-0plates were used; densities mere converted to intensities for each wave length through the use of screens whose relative transmissions were determined by means of the inverse-square law. Corrections for the differences in reflection at normal incidence for full and empty cells were applied, and precautions taken to eliminate fluctuations in sensitivity in going across a plate. Since fused-quartz cells were not available, cemented cells with an optical path of 1 em. were used; these were filled as indicated in figure 2. In spite of strenuous precautions to avoid contamination by the wax, apparently enough reaction took place in the vapor phase so that after about 2 hr. the absorption spectrum of the liquid became noticeably intensified and shifted toward the red. The extinction coefficients plotted in figure 3 mere obtained during the first hour
36
FRANK V. DUNDERMAN AND S. H. RAUER
t o distillation unit
sealed off when filled
a l cap f i [led- by
a c.
la x
cooling
f i l l e d by
sy phoning
lp(:
FIG.2. Filling procedure for the cemented quartz cell ' DIETHYL
ETHER
0 D I M E T H Y L E T H E R : BORON T R I F L U O R I D E
a
5000
3000
2500
2100
A.
FIG. 3. The ultraviolet-absorption coefficients of liquid diethyl ether and dimethyl ether-boron trifluoride a t room temperature. Path length = 1 cm.
SPECTRA OF DIMETHYL ETHER-BORON
37
TRIFLUORIDE
after filling, when the extent of contamination could be considered negligible. Nevertheless, the E, (log 1/T) values for (CH3)20:BF3 are probably large. DISCUSSION
Two facts have t o be accounted for. Firstly, in the complex a faint absorption begins a t a wave length appreciably longer than in the ether; secondly, the intensity of absorption by the complex is much lower in the region X 2300, where thc ether begins t o absorb strongly. The explanation, in words, may be that the non-bonding electron pair of oxygen whose excitation is responsible for the absorption region beginning a t X 2300 in aliphatic ethers (10) is involved in the formation of the donor-acceptor electron-pair bond. Hence the threshold of the corresponding absorption is shifted to higher energy values. On the other hand, the electron pairs participating in the C-0 bonds as a consequence of association are held less tightly and their excitation may be responsible for the long wave toe in the extinction curve. The intensity of this absorption is low, owing to obvious Franck-Condon restrictions. In the language of molecular orbitals, the association process may be described as follows: Assuming local symmetries Czu and CBvaround the oxygen and carbon atoms, respectively, in dimethyl ether, one may write for the electron configuration in that molecule when in the ground state (8): (2saJ2 Approximate location..
. . . . . .O
[2t31;+2
CH3+ CH3
[2/hJ2 C-&C
[za#
TI^'^
(2zhJ2,
C-SC
CH3+ CHI
0
'AI (1)
The ultraviolet-absorption region beginning with X 2300 arose from the excitation of one of the non-bonding (2zb1)o electrons to an essentially atomic ( 3 . ~ orbital. )~ After coordination with boron trifluoride, the resulting electronic milieu of the oxygen atom resembles that of a nitrogen atom in (CH3)2NQ,where Q is a highly electronegative group. This is based on the assumption that the oxygen valence angles in the associated compound are pyramidal, a point of view which is in best agreement with the diffraction data. Roughly, the same number of electrons may be allocated to the vicinities of the oxygen and nitrogen atoms; they differ only in that the oxygen with its higher nuclear charge will have a somewhat more compact electron atmosphere. Of course, one cannot write an electron configuration for (CH&NQ with Q not specified. However, with Q sufficiently electronegative, the electrons with the lowest ionization potentials would very likely follow the order (assuming the symmetry C,, and that Q is on the XZ plane):
.. . Approximate location.. . . . . . .
[y-J"]2
C-N-C
-
[guyZ
[yu'l2
CH2
CH
[zuq2,
IS-Q
(2)
N
As in ammonia, the excitation of one of the non-bonding [ z d ] N electrons to an essentially atomic (38a')N orbital would result in an absorption region beginning
38
FRANK V. DUNDERMAN AND S. H. BAUER
at about X 2200. Thus the association reaction involves an electronic distortion from configuration 1 to an arrangement similar t o 2 (figure 4). In the case of the complex the electronic locations are less readily specified.
... [yu”I2 Mostly a t . . . . . . . . .C-0-C To some extent. . . . (0-B)
[yu”I2
CH2
[yu’l2 CH
[3Ca‘I2 0-B
[ZU’I2,
0
‘A1
(3)
7 -
(C-0-C)
The following may be noted: (a) Owing to the reduction of symmetry, the degenerate [n] orbitals of the methyl groups have been split into symmetric and
(C)
FIG.4. Coordinates for the molecular orbital description of the association process.
antisymmetric orbitals, relative to the xx plane. ( b ) The essentially non-bonding atomic electrons of oxygen (2xb# become the molecular electrons needed in the B-0 bond. Their excitation now requires higher energies. (e) The [ Z U J electrons which participated in the C-0-C bonds remain molecular, but become essentially non-bonding. Their excitation should require less energy, and bonds become explains the long toe in the absorption curve. The C-0-C weaker, as is borne out by the electron-diffraction data (lengthening of the C-0 separation from 1.42 8. to 1.45 8.)and by the chemical facts that (CH&O:BF3 is a good methylating agent, and that BF3 is commonly used as a splitting agent for aromatic ethers. Finally, since excitation of one of the [xu’] electrons would bonds, resulting in an re for the excited further greatly weaken the C-0-C state which is much larger than for the normal, the intensity of absorption should be lorn at the long-wave-length limit, as is observed.
SODIUM STEARATE-CETANE-WATER
SYSTEMS
39
SUMMARY
The Raman and ultraviolet-absorption spectra of liquid dimethyl ether-boron trifluoride have been recorded. The complex shows a set of vibrational frequencies which differs radically from those of the parent molecules. Very little of the unassociated substances are present in the liquid phase. The complex shows a faint absorption at appreciably lower frequencies than do the ethers. However, the intensity of absorption of the complex is much less in the region below X 2300, where ethers begin to absorb strongly. An explanation is presented in terms of a molecular orbital description of the association process. REFERENCES (1) BAUER,S. H . , FINLAY, G. R., (2)
(3) (4) (5) (6) (7) (8) (9) (10)
AND LAUBENGAYER, A. W.: J. Am. Chem. SOC.66, 889 (1943); 67, 339 (1945). BRIEGLEB, G., AND LAUPPE,W . : Z . physik. Chem. B36, 42 (1937); stannic chloride dissolved in alcohol and in ether. BRIEGLEB, G . , AND LAUPPE,W.: Z . physik. Chem. B36, 56 (1937). BRIEGLEB, G . , AND LAUPPE,W . : Z.physik. Chem. B37, 260 (1937); simple alcohols and ethers with halogen acids. GERDING,H . , AND SMIT,E . : Z. physik. Chem. B61, 200 (1942); aluminum chloride (A12Cla.2HzO). LAUBENGAYER, A. W., AND FINLAY,G. R . : J. Am. Chem. SOC.66, 887 (1943). LEVY,H . A . , AND BROCKWAY, L. 0.: J. Am. Chem. SOC.69,2085 (1937). MULLIKEN, R. S.:J . Chem. Phys. 1, 492 (1933); 3, 506 (1935). PAULINQ,L., AND BROCKWAY, L. 0.: J. Am. Chem. SOC.67, 2684 (1935). THOMPSON, H. W., AND LINNETT,J. W . : Proc. Roy. SOC.(London) A166, 108 (1936).
THE BEHAVIOR OF SODIUM STEARATE WITH CETANE AND WATER ROBERT D . VOLD
AND
JOSEPH M. PHILIPSON
Department of Chemistry, The University of Southern California, Los Angeles 7 , California Received September 26, 1946
This paper presents data on the transition temperatures detected in mixtures of sodium stearate, cetane, and water, and describes the appearance and properties of these systems. Some attempt is made to represent the data in terws of a phase diagram, and to relate the changes in observed behavior to the changes in the nature of the solvent. These systems exhibit a great variety of colloidal phenomena, existing as clear solutions of “solubilized” oil, as oil-in-water emulsions, as soft liquid crystalline phases, and as gels which vary from transparent jellies t o hard wax-like solids. They are also of industrial importance in such products as cosmetic preparations, lubricating greases, etc.
