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, JK.,
W. k!. P A D G E T T , 11, A N D I.
SCIIWAGER
was evacuated. In 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 that 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 that 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 The 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
ACIDITYOF HYDROCARBOXS
2923
in an inert atmosphere. The results are not as accurate as conventional conductivity measuremen ts in aqueous solutions but do have features of interest. Exper imen tz 1 The conductivity apparatus is diagramed in Fig. I. Its use with lithium cyclohexylamide solutions is described by the following procedure.4 The reaction vessel, R, is similar in dimensions to those used in our other exchange studies in this series. The lithiurn cyclohexylamide was prepared from ethylljthium or butyllithium and cyalohexylamine in the usual way Vacuum
ferred to flask D and the solvent was distilled from D to R. The residue of lithium cyclohexylamide in D was removed and titrated. The diluted solution in R %vas drawn up into C for measurement. Several such successive dilutions are possible in a given run. The fluorenyllithium runs were handled in a similar manner. A weighed amount of recrystallized fluorene was treated with excess lithium cyclohexylaniide in cyclohexylamine in R. Fluorene is converted conipletely to lithium salt with lithium cyclohexylan~ide.~ I n the subsequent conductivity measurements, the generally small contribution of the excess lithiuni cyclohexylamide to the conductivity was subtracted. The lithium perchlorate run used a weighed amount of the dried salt in cyclohexylamide with the technique of successive dilutions described above. The results obtained are summarized in Table I in which are tabulated the formal concentrations of salt, the specific conductivity, L,, and the equivalent conductance, &. The viscosity of cyclohexylamine was determined to be 1.16 C.P.S. a t 49.0" by comparison with water with a flow viscometer. Results and Discussion
W Figure 1. Conductivity apparatus for use in an inert atmosphere.
with the normal precautions of degassing, freezing, and thawing of solutions. By proper manipulation of stopcocks and the ube of vacuum and argon pressure, the solution was drawn up into the conductivity cell, C. This cell conhains in its glass stopper a therinocouple well and two electrodes of shiny platinum foil, 3 mils thick, attached to platinum leads which extend from glass support rods. The cells used were calibrated with 0.01 denial KC1 solutions. The entire conductivj t y cell is contained within a vapor jacket Refluxing cyclopentane (b.p. 49.5') was used in this study so that the temperature would correspond to that, a t which many cheiuical studies were made in thie system. After temperature' equilibrium was reached, resistance of the cell was measured using a comniercial bridge, Model 250-IIA, Electronieasurei~ients, Inc.. Portland, Ore. The bridge was checked using standard resistors. After this measurement, the solution waa allowed to flow back into R and was cooled to rooin temperature. A known amount of solution was trans-
I n aqueous solvents and in other solvents of high dielectric constant, conductivity properties of ions are usually considered in terms of ion-atmosphere effects. I n solvents of low polarity or dielectric constant (smenogenic solvents),3the formation of ion pairs is important. The actual concentration of free ions may be so small that ion-atmosphere effects are negligible.6 Much early literature exists on conductivity studies in amines. Some examples are silver nitrate and lithium iodide in ben~ylaniine,~ lithium, sodium, and potassium iodides in diniethylaiiiine,x and tetraalkylamnioniuni iodides in aniline.9 Plots of the equivalent con(1) (a) Taken in part from the dissertations of W. 11. P., 1962, and 1. S., 1964, University of California. This research was supported in part by grants from the Petroleum Research Fund administered by the American Chemical Society, and from the United States Air Force Office of Scientific Research. (b) National Science Foundation Summer Fellow, 1960. (2) For paper SII, see A. Streitwieser, Jr., and W. 11. Padgett, 11, J . P k y s . Ckem., 68, 2919 (1964). 3) R. M . Fuoss, J. Chem. Edzic., 32, 527 (1955). (4) For more complete details, see W 11. Padgett, 11, Dissertation, University of California, 1962. (5) A. Streitwieser, Jr., and J. I. Brauman, J . Am. Chem. Soc.. 8 5 , 2633 (1963). (6) C. A. Kraus, J . Chem. Edzic., 3 5 , 324 (1958). (7) 1'. Walden, Z . p h y s i k . Chem., A148, 45 (1930). (8) E. Swift, Jr., J . A m . Chem. Soc., 60, 2611 (1938). (9) P. Walden in Ostwald and Drucker's "IIandbuch der allgemeinen Chemie," Band I\', Teil 11, Leipaig, 1924; J . N . Pierce, 6.P h y s . Chem., 19, 14 (1915).
Volume 68, .\-umber
IO
October, 1964
A. STREITWIESER, JR.,W. 11. PADGETT, 11, A X D 1. SCHWACER
2924
Table I :
Conductivity Studies in Cyclohexylamine at 49.5" Concn.,
Run
X 10s rnoles/l.
x
IOs X Ao,
mhos/cm.
m h o cm.2,'equiv.
108
Lithium perchlorateb
45.4 22.7 11.4 5.68 2.84 1.42 0.71 0.36 0.18
61.3 11.8 3.13 1.18 0.597 0.311 0.288 0.163 0.203
13.5 5.2 2.7 2.1 2.1 2.2 4.1 4.5 11.3
Lithium fluorenylc
Ab
Bb Cb Dd
40 10 5 2.4 1 0.5 66 30 14
800 24.4 8.2 2.4 1.5 1.0 2000 390 45
200 24 16 10 15 20 300 130 32
Cd
Dd
Ed Fd Gd
225 125 60 80 38 21 9 3.5 . .
103 55 26 ... ,..
Hb Ib
118 58 22 7.5
5
I
I
/ / I
I
I
I
/
\
I
/ / I
I
'1
4-
"/
3a/
Lithium cyclohexylamide
Ad Bd
mum A, occur are almost independent of the nature of the amine, although the magnitude of A, a t these points reflect clearly the polarity and viscosity of the solvcnt. This behavior of &i,with increasing concentration is generally interpreted to reflect the following phenomena.Io At low concentrations, ion-pair formation is important and the free ions responsible for conduction appear to obey the Ostwald dilution law. At higher concentrations, ion triplets become important, causing the minimum in A- and the subsequent increase in A, with increasing concentration. Two effects combine to hold A, to a maximum value-the onset of ion quadruplets and higher aggregates and the increase in viscosity of the solution, which, in lowering the ion mobilities, overcomes the effect of increase in ion-triplet concentration. Plots of '4, vs. log c for lithium perchlorate and fluorenyllithiurri in Fig. 2 show clearly t h a t both salts follow
10.6 7.0 4.4 5.2 4.6 2.8 1.71 1.04
