A. STREITWIESER, JR.,R. A. CALDWELL, R I . R. GRASGER, Ah'D P. XI.
2916
cont,rary to what happens with other classical techniqucs. Furthermore, since the slope of e.m.f. us. T curves varies suddenly (and more by increasing tempcra-
Acidity of Hydrocarbons. XI.
LAUGHTON
tures) when a phase transition occurs, an eventual small and gradual variation with the temperature of the electron transport number of the intermediate electrolyte does not affect the applicability of the method.
Activation Parameters for Exchange
of Toluene-& with Lithium Cyclohexylamide in Cyclohexylamine1
by A. Streitwieser, Jr.,2aR. A. Caldwell,2bM. R. Granger, and P. M. LaughtonZc Department of Chemistrg, University of California, Berkeley, California
(Received A p r i l 16, 1064)
The drpendence a t two temperatures of the pseudo-first-order rate constants for exchange of toluene-a-d with cyclohexylamine on the concentration of the lithium cyclohexylainide catalyst leads to the thermodynamic parameters for the aggregation equilibrium of lithium ryclohcxylainidc and the activation parameters for the bimolecular exchange reaction. The cquilibriuni is unusual in that aggregation is accompanied by an increase in entropy; the exchange rcactioii itself has an unusually negative entropy of activation. These results arc interpreted in terms of specific solvation effects.
In prcvious ~ 0 r 1on < ~the exchange reaction of toluenea-d n ith lithium cyclohexylainide, \$-e demonstrated that the kinetics was consistent with reactive mononierir lithium cyclohexylainide being in equilibrium with incrt dimers, trimers, and higher aggregates. The present study is concerned with the temperature dependence of the rraction and the corresponding activation parameters. Because of the equilibria involving the active catalyst, isolated rate measuremerits at different temperatures do not suffice. These equilibria required a study of the kirictic order in catalyst over a wide rangc of concentration; the complete study of the cffect of temperature on rate requires the temperature depcndence of these equilibria and, consequently, a knowledge of the liinetir order of catalyst over a concentration range a t more than one temperature. Experimental Preparation of toluene-a-d was reportrd p r e v i ~ u s l y . ~ The kinetics was followed using exchange procedure The Journal of Physacal ChPrnistrti
B3 with some modifications. Butyllithium from Foote lLIineral Co. was used to prepare the catalyst solutions. The infrared analyses for deuterium content were performed on a Perkin-Elmer 221 or 421 speetrophotometer with 10% solutions in carbon tetrachloride in 1nim. cells or as the pure liquid sample, aftjer isolation by gas-liquid chromatography, in a 0.1-mm. microcell. I n later runs, and particularly the runs a t low base concentration, the formal Concentration of lithium cyclohexylamide was determined by rcaction with excess dry bromobenzene, which was introduced at the A. Streitwieser, J r . , and H. F. Koch, J . Am,. Chem. Soc.. This work was supported by grants from the Petroleum Research I h i d and by the Vnited States A.ir Force Office of Scientific Kesearch of the Air Research and Development Command. (2) (a) Alfred P. Sloan Fellow, 1958-1962; (b) National Science Foundation Cooperative Graduate Fellow, 1962-1964; (c) ACS-PRF Farulty Awardee, 1962--1963; on leave from Carleton Vniversity, Ottawa, Canada. (3) A. St.reitwieser, Jr., D. E. Van Sickle, and W.C. Langworthy, J . A m . Chem. Soc., 84, 244 (1962). (1) Paper);:
8 6 , 404 (1964).
2917
ACIDITY OF fIYDROC4RHONS
end of a run by typical vacuum line techniques, and by \.'oll-iard titration of the liberated bromide ion. The rebults are consistent with a 1 : 1 stoichiometry of lithiuni cyclohexy1:tmide and reacted bromobenzene. Estiniatcd accuracy of the titration is about 2yo. In addition, in these runs, cyclohexylamine was dried by a final distillation on the vacuum line from lithium cyclohcxylaniide. Sevcral rims were carried out with a mixture of toluenc-a-d and tolucnc-a-t. The tritium rates were follon ed by counting aliquots in a scintillation counter (Kuclear-Chicago llodcl 720) or with a radio-assay gas chromatography unit described previously.' The k ~ ) ~ isotope l c ~ effects recorded in Table I will be discussed in a future paper.
Table I : Rates of Exchange of Toluene-& Cyclohesylarnide at 25.00 + 0.01"
with Lithium
[l'oluene-a-
dl, rnole/l.
mole 4.
0.42' ,481 .47 .48 .48 ,49 43' .48 49k 33 48 4xm .47 50" 43' 48' 53/,'
0.0049 0057 ( .O1l)i ( . 017)i ,017 ,021 ,024 ,038 060 ,061 075 097 . 187 027 0051 ,0032 .33
C,Q
1 ODkexptlrh
8ec
1
1 20 1 28 (0 52) (1 45) 2 19 2 3 2 4 2 5 3 0 3 3 4 8
6 8 3 3"
4 3" 3 3" 15 8''
10"k 8ec
1 1 (0 (1 1 2 2 2 2
-1
07' 12h 46) 28) 92 1 1' 2 6 (3 2)' 3 2 3 4 3 8 7 2" 3 8"" 2 go.* 13 7"'
[LINH10Sk~,~ C e H ~ i l , ~1. 1KIOle-l see. - 1 niole/l
0 0018 0020
6.0
0033 0035 0037 0043 0050 0050 0053 0057 0067
5.9
5.5
5.8 5,7 5 .0 5.2 6.4
6 0 6.0 5.7
Forrnal concentration of lithium cyclohexylamide. * Calculated froin slope of log (z- 2,)tis. time. Correc:i,ed rate ronstant from eq. 8 of ref. 3 . Concent,ration of monomeric lithium r~clohex?.larriide froin rnodel 2. e Second-order rate const.ant These runs were carried out with a mixture of from rnodel 2. toluene-& arid toluene-&, and both k n and k~ were obtained. ' /hium cyclohexylamide in our solutions rather than our customary procedure of titrating total base. The results showed that about 0.003 M base was present not as lithium cyclohexylaniide and was presuniably hydroxide resulting from residual traces of water. Better results were obtained inore recently by distillation of cyclohexylamine from lithium cyclohexylainidc rather than from molecular sieves. These results are suinniarized in Table I. The results of recent runs at low base concentration at 50' are shown in Fig. 1. These results lead to I(, = 60 1. mole-' rather than the value of 85 I. mole-' adopted previously. For the activation parameters, a series of runs was made at 25' covering a 40-fold range of c from 0.005 to 0.2 M . Using the same model, the results are also fitted best by I
