ACIDITY OF HYDROCARBONS
Acidity of Hydrocarbons. XII.
291 9
Aggregation of Lithium Cyclohexylamide
in Cyclohexylamine by Isopiestic Measurementla
by A. Streitwieser, Jr., and W. M. Padgett, IIlb Department of Chemistry, University of California, Berkeley, California
(Recehed April 16, 1064)
Isopiestic inolecular weight measurements of lithium cyclohexylamide in cyclohcxylaniine show a degree of aggregation in semiquantitative agreement with theories derived previously from kinetic measurements. A modified technique for such isopiestic measurements is described.
From previous sludies of the kinetics of hydrogendeuterium exchange of toluene-a-d by lithium cyclohexylamide in cyclohexylamine, it was proposed that monomeric lithium cyclohexylaniide is in mobile equilibrium with dimers, trimers, etcS2 The kinetic results were interpreted with the aid of two simple models for the component equilibria and equilibrium constants were derived. These treatments lead to predictcd values of the average degree of agglomeration of lithium cyclohexylaniide which are capable, in principle, of independent confirmation. For the determination of mean molecular weights with exclusion of air or moisture a t a given temperature (the kinetics was run a t 25 and Z O O ) , the method of isopiestic measurement or isothermal distillation appeared to be most accurate and convenient. The original procedure of Signer3 has borne several modific a t i o n ~but , ~ the general method has been to follow the course of equilibration volumetrically by draining the equilibration chambers periodically into graduated receivers. I n the experiments described here, the distillation is followed instead by spectrophotometric means. The apparatus designed for this purpose consists of a glass vessel in the shape of an H which carries ground-glass stoppers a t the top of each arm, and which can be connected to a vacuum line through a stopcock on the crosspiece. The bottom of one arm is a spectrometer cell; both arms contain small glass-enclosed magnetic stirrers. The glass stoppers allow entry of the reagents m a hypoderniic syringe or pipet and provide a means of cleaning between runs. I n the general procedure, a known volume, of known forinal concentration, of the
solution to be tested was placed in the “sample” arm, and a known volume, of known concentration, of a solution of a suitable chromophore was placed in the arm containing the spectrometer cell (“reference” arm). After degassing and evacuating, the entire apparatus was immersed completely in a 50’ thermostat and the solutions were stirred intermittently. Periodically, the absorption of the chromophore solution was measured and progress to equilibrium was determined. These experimental data are sufficient to give the mean molecular weight of the test substrate. For the present experiinents, no single compound was found which served as a suitable chromophore in the concentration region of interest to us; hence, a mixture was used of a major solute, hexaethylbenzene, serving as a nonvolatile and nonabsorbing substrate, plus a minor solute, I ,2,5,6-dibenzanthracene, whose absorption a t 395 nib could be measured readily. The procedure was tested with triphenylinethane in benzene (Table I). We could determine an experimental molecular weight, but for our purposes we preferred to determine the rate of approach to the theoretical equilibrium. I n eq. 1, which is convenient for this purpose, C and V are the starting concentra(1) (a) This research was supported in part by a grant from the United St,ates Air Force Office of Scientific Research. (b) National Science Foundation Summer Fellow, 1960. (2) (a) A. Streitwieser, Jr., D. E. \‘an Sickle, and 1%’. C Larlgworthy, J . Am. Chem. Soc., 84, 244 (1962): (b) A. Streitwieser, J r . , I t . A Caldwell, M . It. Craiiger, and 1’. %I. Laughton, J . P h y s . Cham., 6 8 , 2916 (1964).
(3) It. Signer, Ann., 478, 246 (1930). ( 4 ) E. P. Clark, Anal. Chem., 13, 820 (1941); 1,. M . White and K . T. Morris, i b i d . , 24, 1063 (1952): C . E. Childs, i b i d . , 2 6 , 1963 (1954).
Volume 68.S u m b e r IO
October, i B G 4
A. STREITWIESER, JR.,A N D W.
2920
tions and volumes, respectively. A0 and A , are the optical densities a t the start and a t equilibrium, and the primes refer to the substrate under test. The equation is a straightforward derivation assuming that A is proportional to the reference concentration and that the concentrations of substrate and reference are equal a t equilibrium. This run shows that close approach to equilibrium can be accomplished but requires several days.
R.PADGETT, I1
tion of the color. Although these limitations restricted the quantitative nature of the results obtained, repeated runs did show that agglomeration of lithium cyclohexylamide is important. Because of such leaks, we present in this paper only a selected set of t h x e runs which were successful over prolonged periods.G Two such experiments, runs 83 and 12,5, are summarized in Fig. 1 and 2 . In these runs, the formal concentration of lithium cyclohexylaniide was close to the concentration of the reference solution. The pronounced distillation of solvent into the reference side shows that the lithium cyclohexylamide is actually more dilute than indicated by the formal concentra-
Table I : Test of Isothermal Ilistillation with Triphenylmethane a t 50" Run 82: 3.0 ml. of 0.073 hf C6Et6 and 8.0 ml. of 0.053M PhaCH (in benzene) Time, hr.
Aa
0 43.5 114.5 158.5 217.5 230 Calcd. a t equil."
0 481 449 ,405 391 386
A3950
R U N 83
0
0.2 5 0 1
0
0
i
. .b
,385
Run 11-4: 3.0 ml. of 0.038kfC6Et6 and 3.0 ml. of 0.025 hf PhrCH (in cyclohexylamine) Time, h r .
A0
0 19.8 53,5 137 159.8 183 5 Calcd. st eyuiLb
0.393 388 382 365 360 358 326
a Absorbance of 1,2,5,6-dibenxanthracene at 395 mp contained in the CsEts side. Cell leaked. Equation 1.
Figure 1. Run 83: 8.0ml. of 0.072 F L ~ N H C G Hus. , , 3.0ml. of 0.070 M CcEts.
mob A395
This technique should be useful for comparatively volatile solvents but is inconveniently long with less volatile solvents ; unfortunately, cyclohexylamine, the solvent of our present interest, falls in the latter class. Run 11-4 in Table I , triphenylmethane in cyclohexylaniine, shows only partial approach to equilibrium a t 50" even after 183 hr. At 200 hr., leakage into the cell from the thermostat ended the run. Such leaks occurred frequently in the runs with lithium cyclohexylamide; hence, in these runs, a small crystal of 4,5-nicthylenephenanthrencwas included in the test solution. l'hc intense red color of t,he corresponding carbanion5 served as a suitable indicator; the developnient of even niinute leaks causcd iinniediate extincThe ,Jowmal of Phldsical C h e m i s t r y
0 tt
I
O
0
I
0 0
0 0 0 0
0
0
I
Figure 2. Run 125: 4.0rnl. of 0.056 F LiSHC6H11us. 3.0 ml. of 0.056 &%f C6% (5) A. Streitwieser, J r . , and J. I. Brauman. J . Am. Cham. Soc., 8 5 , 2033 (1963). (6) For further details, see W. sity of California, 1962.
M.P a d g e t t ,
11, Dissertation, Univer-
Acmiw
OF
HYDROCAHBONS
2921
tion; that is, aggregation does occur. The average degree of aggregation, CY, may be derived by eq. 2 .
C’
V’
Runs 83 and 125 have not reached equilibrium; from the last A values observed we derive a = >1.6 from both runs. For the two models of the aggregation equilibria and the equilibrium constants obtained from the kinetic measurements,2b we may derive the corresponding theoretical values of a as a function of the formal concentration of lithiurn cyelohexylainide. The results are shown in Figs 3. The final concentrations of lithium cyclohexylamide in runs 83 and 125 (0.08and 0.07, respectively) are close to the concentration region where the a-curves for the two models overlap; both theories predict a = 2.6-2.9 in this concentration range. The approach to equilibrium was froin the same side in both of the above runs. Several other experiments showed the same qualitative trends, but leaks frequently developed after only a few days, especially a t the glass stoppers. Consequently, a modified apparatus was developed which had only a single entry tube and which could be sealed off completely to leave only the two arms in an inverted U. Run 11-19 (Fig. 4) was accomplished with this apparatus. After equilibrium was allowed to progress from one side for 400 hr., distillation of additional solvent was forced (dotted line in Fig. 4). The results show that thereafter equilibrium was approached from the other side and that the equilibrium position is bracketed by this procedure.’ The A values at 400 and 500 hr. yield CY = 1.9-2.9. The corresponding lithium cyclohexylarnide concentrations are 0.07 and 0.09, respectively, for which the theoretical models gave = 2.6-3.0. Because of the concentration region chosen, the present experiments do riot distinguish between the two theoretical models. Run 11-19, the most complete and successful of the runs made, appears to lead to an equilibrium a similar to the theoretical models, but probably somewhat lower. These results clearly demonstrate the qualitative correctness of the aggregation picture of lithium cyclohexylamide in cyclohexylamine but point up the simplifying approximations made in rendering an infinite array of equilibrium constants mathematically tractable.
Experimental Hexaethylbenzene (Eastman), m.p. 127-129’, was dried under vacuum and stored over dried silica gel.
I
5-
I
1
1
I
I l l ’ [
--- MODEL -MODEL
I
I
I
I I I I I I
l
l
/ I , K=60
2 , K=500
/
/
1-
Figure 3. The form of CY as a function of the formal concentration of lithium cyrlohexylamide from the theoretical models 1 and 2.
RUN II-19
0 0 0 0 0 0 0
I I I I
I
0
O
b 0
HOURS
400
Figure 4. Run 11-19: 5.0 ml. of 0.060 F LiNHCGH,, us. 10 ml. of 0.050 M CaEta in the sealed apparatus.
1,2,5,6-Dibenzanthracene(Rutgerswerlte-Aktiengesellschaft) was rccrystallized from acetic acid and chromatographed as a benzene solution on Woelin alumina. The white flakes produced had n1.p. 266.0-266.2O (Kofler hot stage) and were stored over silica gel. The reference solutions were prepared by dissolving weighed amounts in benzene. Benzene was removed on a vacuum line, and a known amount of cyclohexylamine was distilled from iiiolecular sieves onto the Volume 68, Number 10
October, 1964
A.
2922
evacuated solid residues. When solution was complete, an aliquot was transferred with a syringe or volurnetrik pipet to the spectrometer cell side of the previously degassed apparatus. Lithium cyclohexylamide was prepared as described previously2a from cyclohexylamine and butyllithiurn in a cylindrical flask from which a capillary tube carrying a stopcock emerged from the bottom and terminated in a syringe needle. Using argon pressure, the solution was forced through the needle and a serum cap into the other chamber of the isopiestic apparatus, which contained graduation markings for determination of volume. During these transfers, any exposures of contents to the atmosphere were avoided by use of an argon purge.. When the apparatus was properly stoppered, both solutions were thoroughly degassed, then frozen, and the apparatus
Acidity of Hydrocarbons. XIII.
STHEITWIESER, J K . ,
W. k!. P A D G E T T , 11, A N D I.
SCIIWAGER
was evacuated. I n the modified apparatus, the single opening was sealed off a t this point. After thawing, the entire apparatus was placed in an underwater magnetic stirring system that provided intermittent stirring, and immersed completely in the thermostat. I'eriodically , the apparatus was placed in a Beckman DU spectrophotometer, and the absorption a t 395 mp was measured under standardized conditions. Dibenzanthracene in cyclohexylamine has bands a t 374, 384.3, and 394.8 niH. Beer's law is obeyed and the molar absorptivity of the last band is 1190. I n principle, equilibrium is reached when the absorbance no longer changes. I n practice, impractically long times are required, and we were forced to be satisfied with an approach to equilibrium Three of the several runs made are summarized in Fig. 1 , 2 , and 4.
Some Conductivity
Studies of Lithium Cyclohexylamide, Fluorenyllithium, and Lithium Perchlorate in Cyclohexylamine'"
by A. Streitwieser, Jr., W. M. Padgett, 1I,lb and I. Schwager Department of Chemistry, University of California, Berkeley 4, California (Received A p r i l 16, 1964)
An apparatus is described for determining conductivities in an inert atmosphere. Measurements with lithium perchlorate, lithium fluorenyl, and lithium eyclohexylamide in cyclot o 10-l2 niole/l. hexylamine at 49.5" give ion-pair dissociation constants in the range Thrse results indicate t h a t the bonds to lithium in each of these salts are about equally ionic; the results confirm the conclusions reached in previous studies of kinetics of exchange with lithium cyclohexylaniide in cyclohexylamine t h a t free ions are not significantly involved in the concentration region used.
As part of our general study of acid-base exchange reactions and equilibria betwcen hydrocarbons and lithium cyclohexylamide in cyclohexylamine,2we have carried out a limited study of the ionogenic3 properties of this system. We report conductivity ineasurernents T h e Journal of Physical Chemistry
of lithium cyclohexylamide and of an ionic organometallic compound, fluorenyllitliium. For comparison, we have studied also a typical salt, lithium perchlorate. An apparatus was developed for preparing the solutions and carrying out the conductivity measurements
