860
A. R . OLSOK, L. D. FRASHIER .4ND F. J. SPIETH
the enol form of the ester. In the mercuration reaction we do not yet know enough about the activated complex to picture the effect of anions, but it should be possible by optical or electrochemical methods to detect whether mercuric ion alone interacts with perchlorate. I believe that no marked interaction would be found in this system. On the other hand, the retarding effect of chloride ion and acetate ion fits in well with their complex formation with mercuric ion, as pointed out by Westheimer.
THE RACEMIZATIOK OF THE DIMETHYL ESTER OF 2-BROMOSUCCINIC ACID BY LITHIUM BROMIDE IN ACETONE' A. R . OLSON, L. D . FRASHIER,? A X D FRANCES J. SPIETH
Department of Chemistry, University of California, Berkeley, California Received August 10, 1960
The addition of anhydrous lithium bromide to a solution of the optically active dimethyl ester of bromosuccinic acid in dry acetone causes racemization of the ester. If the logarithm of the rotation is plotted against the time in the usual way, as shown in figure 1, a straight line is obtained, showing that the reaction is pseudo-unimolecular. The specific rate of racemization, however, depends upon the concentrations of the lithium bromide and of the ester. The addition of other substances, such as lithium perchlorate and mater, has pronounced effects upon the rate. PREPARATION OF MATERIALS
The preparation of 1-bromosuccinic acid from l-aspartic acid has been described previously (2). The dimethyl ester was prepared by refluxing for 2 hr. a solution of the acid in methyl alcohol to which 2 ml. of concentrated sulfuric acid had been added. Water was then added to the solution, and the ester mas extracted with ether. After removal of the ether by distillation, the pale yellow liquid was dried over calcium sulfate and distilled under high vacuum to produce a colorless liquid. Microanalysis for carbon and hydrogen gave 32.02 i 0.14 per cent and 4.20 f 0.13 per cent, respectively. The theoretical values are 32.02 per cent and 4.03 per cent. In acetone [a]:% = -69.5'. The acetone was treated with potassium permanganate and potassium hydroxide and then distilled. Subsequently it was kept in contact with calcium sulfate in an evacuated system for 2 hr., with shaking at frequent intervals, 1 Presented before the Symposium on Anomalies in Reaction Kinetics which was held under the auspices of the Division of Physical and Inorganic Chemistry and the Minneapolis Section of the American Chemical Society at the University of Minnesota, June 19-21,1950. Allied Chemical and Dye Corporation Fellow in Chemistry at the University of California, 1948-49. Present address: Georgia Institute of Technology, Atlanta, Georgia.
RACEMIZATION OF DIMETHYL
I-BROMOSUCCINATE
861
and then distilled a t room temperature. The final drying was accomplished by distilling the acetone so that the vapor passed over phosphorus pentoxide which was suspended on a packing made of short pieces of glass tubing. Timmermans and Gillo (3) report that acetone dried in this manner contains not more than 0.001 per cent water. c. P. lithium bromide was recrystallized three times from water and dried under high vacuum mm. of mercury) for 2 days a t 100°C. Titration of the bromide in the sample gave the correct result within 0.2 per cent. Reagent grade lithium perchlorate was dried under high vacuum for 24 hr.
25
50
t(min1 FIG.1 FIG.2 FIG.1. Log of rotation versus time in minutes, showing first-order nature of the reaction. k=-- -2.303 d log LI 2 dt ' FIG.2. Dependence of pseudo-unimolecular rate constant upon lithium bromide concentration at 0.1348M ester. Curve 2 has 0.4721 M lithium perchlorate added. T = 249°C.
a t 150°C.The addition of the salt to a solution of the ester did not produce a measurable change in the rotation over a period of 1 week. PROCEDURE
Stock solutions of lithium bromide and lithium perchlorate were prepared in a dry box. The air inside this box came from the purified side of the air purifier used by this laboratory for its air-liquefaction process. The dry-box technique mas necessary because the salts are very hygroscopic and the addition of even a small amount of water causes a large drop in the rate. Solutions for individual runs were made by transferring portions of these stock solutions and the ester with pipets to a volumetric flask and diluting to volume with dry acetone.
862
A. R. OLSON, L. D. FRASHIER AND
F, J. SPIETH
The polarimeter used has been described previously (2). Light from an A-B4 mercury lamp was filtered through a concentrated solution of sodium chromate and a solution of neodymium and praseodymium trichloride6 to isolate the 5461 A. line. When log CY was plotted against time a straight line was obtained for runs which had proceeded to 90 per cent completion. The slope of the line was read from the graph. Analytical determination of the slope by the method of least squares was used as a check on a number of runs and was found to agree with the graphical method within less than 1 per cent. The average reproducibility was about 1.5 per cent. For completely new preparations the reproducibility was normally less than 3 per cent. RESULTS
The experiments summarized in table 1, in which the concentration of ester TABLE 1 Dependence of rate on lithium bromide concentration Concentration of ester 0.1348 M; T = 24.9'C.
-
h X 101 CONCENTMRON 01 LiBr
Y
x
Ah
EIprimenul
Wedated
0.2746 1.021 1.703 3.14 5.08 4.96 8.03 10.36 14.59
0.2858
lol
0.05179
0.2580 0.5179 1.295 2.590 2.590 5.179 7.768 12.95
pn C a d
+3.4 -0.5 -0.8 -3.1 -0.4 -2.7 +2.0 0.0 +1.0
1.028 1.717 3.24 5.10 5.10 7.87 10.36 14.44
Average deviation.. . . . . . . . . . . . . . , , . , . . . . . . , . , . . . . . . . . . . . . . . . . . .
I
1.5
,
was constant at 0.1348 M and the concentration of lithium bromide was varied, correspond to the pseudo-unimolecular specific rates shown in the second column of that table. If these rates are plotted against the total concentration of lithium bromide, the points of curve 1 of figure 2 are obtained. Conductance measurements (4)have shown that lithium bromide is a weak salt in acetone, Its ionization constant is of the order of lo4, Curve 1 of figure 2 conforms to the equation,
k = kl(B1')
+ h(LiBr)
(1)
if the concentrations of bromide ions and lithium bromide molecules are calculated from the total concentration of lithium bromide and the dissociation constant K L r ~=r 3.6 x IO+. By adjusting the values of k~ and kr the curve may be fitted within the experimental error, even though K L ~ isB changed ~ by as
863
RACEMIZATION OF DIMETHYL 1-BROMOSUCCINATE
much as 40 per cent. However, the values chosen for these constanta mmt also be used to calculate the data to be presented in table 4. Tnis limits the value of K L ~toB within ~ 10 per cent of the above value. Lithium perchlorate is a weak salt in acetone solutions, but conductance measurements in this laboratory indicate that it is fully ten times as strong as lithium bromide. A solution of the perchlorate in acetone produces no measurable change in the rotation of the ester in a period of 1 week. I n the presence of a large concentration of lithium perchlorate the ionization of the lithium bromide should be greatly repressed, 80 that a t large constant concentration of lithium perchlorate we can eet (Br)= a(LiBr) and therefore equation 1 becomes:
k
=
k'(LiBr)
(2)
That this is experimentally observable can be seen from curve 2 of figure 2. In these experimenb the concentration of lithium perchlorate waa constant at 0.4721 M . The data for these runs are presented in table 2. TABLE 2 Dspcndsncc of r d r on lithium bromide wnceniration at a constant lithium perchlorate concentration of 0.4781 M Concentration of ester = 0.1348 M; T = 24.9'C. k X 101 CONCLNTXATION on
LiBr Erperimenlll
;*:
~
6.179
0.450 0.914
I
1
Cdculated
0.462 0.903
I
I
Ah
por cbnl
-0.4 +1.2
The data for those experiments in which the concentration of lithium bromide was kept constant and the concentration of the ester varied are presented in table 3. It is evident upon inspection that the pseudo-unimolecular rate decreases slightly as the concentration of ester increases. The ester molecule possesses at least three dipoles, two connected with the ester groups and one at the carbon-bromine bond. In solvents with as low a dielectric constant as that of acetone, such dipoles may cause appreciable association with either an ion or a dipolar molecule. Association a t the carbonbromine bond with either a bromide ion or a lithium bromide molecule leads to the reaction under consideration. Association at the ester-group dipoles would lower the availability of the lithium bromide for the reaction and would therefore decrease the rate. Although we are here ascribing the effect to an association of the bromide with the ester groups, the possibility that the main effect is due to an association betwsetl an ester group of one molecule with a carbon-bromine dipole of another ester molecule should not be overlooked. In fact, such an association would be compatible with an ionization calculation based upon the total concentration of lithium bromide, with identical retarding effects on the bromide
864
A . R . OLSON, L. D. FRASHIER AND F. J. SPIETH
ion and the lithium bromide rates, and with the effect of perchlorate ions presently to be discussed. Olson and Simonson (5) found that the effects of salts on the rates of ionic reactions in aqueous solutions could be explained by assuming such associations, provided the fraction of the reactant that was so associated was calculated by a distribution or adsorption equation. This led to the specific rate equation
where (z) is the concentration of the salt,
K ( z ) is the fraction of the reactant 1 +K(x)
TABLE 3 Dependence of rate on ester concentration
T = 24.9"C. Ak
Y
0.0809 0.1348 0.2669 0.4044
8.14 8.03 7.23 6.73
8.17 7.87 7.23 6.66
122.9
0.596 0.552
0.4044
1
1718 1848
1 j
j
#er
CmI
-0.4 f2.0 0.0
-2.3 -2.0
LiBr, 0.002590 M; LiClO,, 1.0114 M 0.1348 0.4044
0.352 0.335
I
0.352 0.337
2841
Average deviation.. . . . . . . , . . , , . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I
0.0 -0.6 1.0
1 is the fraction of the reactant that is free, and k' 1 K(z) is the specific rate for the associated reactant. Since any bromide that is tied to the ester groups is prevented from reacting a t the asymmetric carbon atom, we must set k' equal to zero, and so we obtain
that is associated,
+
k=
ko
1
+ ks(ester)
where ko is the extrapolated rate at zero concentration of the ester and k3 is a constant. The reciprocal of k should give a straight line when plotted against
R.XCEMIZ.XTION OF DIMETHYL
l-BROMOSUCCINATE
865
the concentration of thc ester, The reciprocals of the experimental rates are listed in column 4 of table 3 and shown graphically in figure 3. The values of kcnled. for the first four runs listed in table 3 are obtained by setting ka equal to 0.74. The constant for the ion might be expected to differ from that for the molecule, but we have been unable to detect such a difference. The other calculated values will be discussed presently. We interpret 1/(1 k3(ester)) to be the fraction of the lithium bromide which is not associated at the ester-group dipoles and ka to be the constant for the association of lithium bromide with the ester-group
+
N
s
2
7
p
;
1.60
I35 110 00
0 50 0 25 Ester (mols/liter) FIG.3 FIG.4 FIG.3. Dependence of l / k upon ester concentration a t constant lithium bromide and lithium perchlorate concentrations. T = 24.9"C. Curve 1, 0.005179 M lithium bromide; curve 2, 0.002590 M lithium bromide, 0.1888 M lithium perchlorate; curve 3, 0.002590 M lithium bromide; 1.0114 M lithium perchlorate. FIG.4. Dependence of 6 upon lithium perchlorate concentration a t constant concentrations of lithium bromidenndester. Ester, 0.1348 M ;lithium bromide, 0.002590M. T = 24.9'C
dipoles. In the mass law expression
($1
(a - r ) ( b - 2) for A
=
K
+ B e X, where complete saturation is assumed.
-
if ( b - r ) ( b ) . Honcver, in some of our experiments using bromosuccinic acid in place of the ester, (x) \\as so large compared to ( b ) that the assumption of saturation led to a 50 per cent error in the rate. Therefore, for these interactions we are tentatively cssuming complete unsaturation even though it probably is not correct.
866
A. R. OMON, L. D. FRASHIER AND F. J. SPIETH
By keeping the concentrations of lithium bromide and ester constant and increasing the concentration of lithium perchlorate the series of runs shown in table 4 is obtained. The data in this table are plotted in figure 4. The effect of lithium perchlorate in repressing the ionization of lithium bromide already has been mentioned. This causes the very rapid initial drop shown in figure 4.The tailing off of the rate a t high concentrations of lithium perchlorate cannot be explained in this fashion. However, if bromide ion can associate at the dipoles of the ester molecule, it is reasonable to assume that perchlorate ion also can associate at these dipoles. The association of perchlorate ions at the ester-group dipoles would decrease the bromide association constant, ka, mentioned above, and thus increase the rate. This would account for the observed decrease in ester effect with increasing concentration of lithium perchlorate corresponding to the smaller slopes of the upper lines in figure 3. However,
TABLE 4 Dependence of rate on lithium perchtorate concentration Concentration of ester = 0.1348 M; concentration of LiBr O.Oo2690
-
EONCENTXITION
k X lo1
or LiC104
Expcrimentd
Cdculatcd
1.741 1.344 0.716 0.582 0.450 0.394 0.352 0.338
1.702 1.303 0.728 0.596 0.452 0.388 0.352 0.338
Y
0.7553
1.0114 1.1330
-
Ah
pn
0.007553 0.01511 0.09442 0.1888 0.4721
kl
!
M; T = 24.9%.
6.32;ks = 0.303;ka
-
cm1
+2.3 +3.1 -1.6 -2.3 -0.4 +1.5 0.0 0 .o
1.4 0.74;kd = 20.5;k, = 50;K ~ i s ,= 3.6 X lW4;K L I C L5O 5~ X IO-'
association of perchlorate ions a t the carbon-bromine dipole would block the approach of a bromide ion. This would decrease the rate and would account for the tailing off of the rate a t high lithium perchlorate concentrations. The equation taking these effects into account becomes kl(Br) "[lf
1
+ kALiBr)
+ ks(C1Oi)
All of the values of ko.lod. corresponding to the solid lines in figures 3, 4, and 5 have been calculated from the rate expression: (3)
L' '
1
+ 50(C10
RACEMIZATION OF DIMETHYL
LBROMOSUCCINATE
867
+
The factor 1/[1 20.5(CIO;)] can be interpreted as the fraction of the ester which does not have a perchlorate ion associated a t the carbon-bromine dipole and is therefore free to react with a bromide ion or a lithium bromide molecule. The factor 1/[1 50(C10;)] may be interpreted as the fraction of the ester which does not have a perchlorate ion associated a t the ester-group dipole. The concentrations of lithium bromide molecules, bromide ions, and perchlorate = 3.6 X 10-4 and ions have been calculated from the ionization constants K L I ~=~ 5, X 1 t 3 . The average deviation is within the experimental error.
+
AQUEOUS ACETONE
This series of experiments was run in acetone dried by a modified method of Conant and Kirner (6) using solid potassium hydroxide. Comparative rate
00 50 00
-56
0.0
0.I
0.2
0.3
Ester (rnols/liter) L i a r (rnols/liter) FIG.5 FIG.6 FIG.5. Dependence of k upon lithium bromide concentration at an ester concentration of 0.1348M.The upper curve is an expansion of the lower end of the lower curve. The water concentration is approximately 0.15 M. T = 24.9"C. FIG.6. Dependence of l/k upon ester concentration. Water concentration approximately 0.15 M. T = 24.9"C. Curve l,O.o2003M lithium bromide; curve 2, 0.001044M lithium bromide.
runs with 1-bromosuccinic acid to be presented in a later paper show that the concentration of water in this acetone must have been approximately 0.15 M . The series of experiments in which the concentration of ester was held constant and the concentration of lithium bromide varied is recorded in table 5 and plotted in figure 5. The curve in this figure conforms to the equation k = kl(Rr-) k2(LiBr) if the ionization constant of lithium bromide in this acetone is assumed to be 1.13 X lo-'. The value in anhydrous acetone was 3.6 X lo-'. The increase by a factor of 3.14 is assumed to be largely due to the preferential hydration of the ions. Two series of experiments in which the ester concentration was varied are
+
868
A . R. OMON, L. D. FRASHIER AND F . J. SPIETH
TABLE 5 Dependence of rate on lithium bromide concentration Concentration of eater = 0.1348 M : concentration of H,O N 0.15 M; T = 24.9%. k X lW CONCENTRATION OF
id
x
LiBr
Ah
Exprimcntal
Calculated
0.1587 0.2685 0.476 0.815 1.177 1.509 1.805 3.00 6.33 8.92 10.31 14.41
0.1523 0.2559 0.465 0.847 1.182 1.484 1.814 2.93 6.28 8.88 10.58 14.13
1w
pcr ccnl
0.06020 0.1044 0.2009 0.4018 0.6026 0.8035 1.044 2.003 6.010 10.02 13.07 20.03
+4.2 +4.9 +2.4 -3.8 -0.4 f1.7 -0.5 f2.4 f0.8
f0.4 -2.6 f2.0
Average deviation
2.2
k, = 2.79; kr = 0.197; ks = 0.41; K L ~ B=, 1.13 X lo-' TABLE 6 Dependence of rate on ester concentration Concentration of H10 N 0.15 M ; T = 24.9"C.
LiBr, 0.02003 M M 0.0541 0.0541
14.49 14.73 14.66 14.41 13.51
0.0809 0.1348
0.2696
0.0541 0.1348 0.2696
1
1.848 1.805 1.705
14.59 14.59 14.44 14.13 13.43
~
1.873 1.814 1.724
I
~
per
69.0 67.9 68.2 69.4 74.0
541 554 587
Cell
-0.7 $1.0 f1.5 f2.0 +0.6
1
-1.3 -0.5 -1.1
kl = 2.79; kl = 0.197; ks = 0.41; KLB,= 1.13 X lo-'
recorded in table 6. In the first series the lithium bromide concentration was held constant a t 0.02003 M and in the second series it was kept constant a t 0.001044 M . The reciprocals of the pseudo-unimolecular specific rates are
RACEMIZATION OF DIMETHYL
1-BROMOSUCCINATE
869
+
plotted in figure 6. The solid lines conform to the equation k = k o / ( l 0.41 (ester)). The association constant, 0.41, is the same for both series of experiments, even though the concentration of lithium bromide has changed by a factor of 19. We therefore cannot distinguish experimentally between the association of bromide ions and of lithium bromide molecules with the carbomethoxyl groups. The association constant has decreased by a factor of 1.8 from the value of 0.74 in anhydrous acetone. This indicates that the hydrated bromide ion has less tendency to associate with the ester-group dipoles than does the bromide ion solvated with acetone. The values of kcaled. have been calculated from the equation:
k = 2.79(Br'-)
+
0.197(LiBr) 1 0.4l(ester) Comparison of the specific rates with those found in anhydrous acetone shows that the specific rate for the bromide ion has decreased by a factor of 2.3 while the specific rate for the lithium bromide molecule has decreased by a factor of only 1.5. This may be due to a greater hydration of the bromide ion, with a consequent lowering of its energy. If the ion must lose some or all of this water of hydration to form the reaction complex, the activation energy will be increased and the rate decreased.
+
TABLE 7 Dependence of rate on ester concentration
Concentration of LiC10, = 0.6432 M ; concentration of LiBr = 0.00717 M ;concentration of HIO N 0.15 M ; T 24.9"C. CONCENTRATION OF ESTEP
M
0.0539 0.2696 0.4044
kl = 2.99; kr
1
k X IW
1
Experimental
I I
-
I
1.027 0.943 0.893 0.211; kr
-
Calculated
-1
k-1
pn C U I 1
1.021 0.940 0.895
974 1060 1120
0.41; kc = 6.5; KLLB. = 1.13 X
t0.6 +0.3 -0.2 K~mi04= 1.9 X lo-*
Three additional experiments of this type were performed in which the concentration of the ester was varied at constant lithium bromide concentration but in these experiments a constant concentration of lithium perchlorate was present. These experiments are recorded in table 7, and the reciprocals of the rates are plotted in figure 7. The series of experiments in which the lithium perchlorate concentration was varied while the concentrations of lithium bromide and ester were held constant is recorded in table 8 and plotted in figure 8. The values of koalEd. in the experiments with the added perchlorate have all been calculated from the equation: 2.99(Br-) 0.2ll(LiBr) k= (5) [I 0.4l(ester)][l 6.5(ClOi)]
+
+
+
870
A. R. OLSON, L. D. FRASHIER Ah'D F. J. SPIETH
FIQ.7 Fig. 7. Dependence of l/k upon ester concentration with 0.6432 M lithium perchlorate added. Lithium bromide, 0.00717 M ; water, approximately 0.15 M . T = 24.9'C. FIQ.8. Dependence of K upon the lithium perchlorate concentration at constant concentration of ester and lithium bromide.
TABLE 8 Dependcnce of rate on lithium perchlorate concentration Concentration of eater 0.1848 M; concentration of LiBr 0.01536 M; concentration of HrO Y 0.15 M: T 24.9'C.
-
-
k
x
LO'
Calculated
Erperimental
Y
$W C
12.78 12.53 6.95 5.50 3.96 2.72 2.20 2.13 1.850 1.748
0.0 0.0 0.01296 0.02593 0.06481 0.2707 0.5415 0.6613 0.9919 1 .m
12.62 12.62 6.78 5.33 3.98 2.69 2.22 2.10 1.869 1.820
c1.1 -0.9 +1.4 -1.0 +4.0
Average deviation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I I
kr = 2.99;kt
-
0.211;k,
6
0.41;kd
6.5;K
L
p
d
+l.3 -0.7 +2.5 +3.2 -0.5
-
1.7
~1.13 X lo-'; KLLOIO,1.9 X
le'
The values of the two specific rates are 7 per cent higher than those used in the absence of perchlorate. This discrepancy is due to a difference in ester preparations. The experiments in tables 5 and 6 were all made with the first preparation of ester, which apparently contained a mall amount of water or alcohol. This
RACEMIZATION OF DIMETHYL
87 1
I-BROMO~UCCINATEI
TABLE 9 Dependence of rate on lithium bromide concentration Concentration of ester = 0.1348 M; T = 36%. CUNCXNRATION
Ah
or LiBr
M x 101 0.05179 0.2590 0.5179 1.295 2.590 2.590 3 . w
Exprimatel
C.lcul.tcd
0.627 2.336 4.11 7.52 11.81 11.76 15.15
0.626 2.393 3.99 7.49 11.81 11.81 15.35
pn CItll
+0.3 -2.4 +3.0 +0.4 0.0 -0.4 -1.3
Average deviation... . . . . . . . . . . . . . . . . . , . . . . . . . . , . . , . . . . . . . . . . . . . I
ki = 15.03; kr = 0.747; kr
I
1.1
= 0.74; K L , B=~ 3.3 X lo-'
L i B r (mols/liter) FIQ.9 FIQ. 10 FIG.9. Dependence of k upon lithium bromide concentration at an ester Concentration of 0.1348 M . Curve 2 has 0.4721 M lithium perchlorate added. T 35'C. FIG. 10. Dependence of k upon concentration of lithium perchlorate at constant concentrations of lithium bromide and ester. Ester, 0.1348 M ; lithium bromide, 0.002590 M . T 35°C.
-
change is small compared with the total change from anhydrous acetone to the acetone used in these experiments. No change in the association constant for the ester groups is observed upon the addition of lithium perchlorate. There probably is a small change, but it is not great enough to be detected experimentally. The blocking by the perchlorate
872
A. R. OLSON, L. D. FRASHIER AND F. J. SPIETA
ion a t the carbon-bromine dipole has decreased to about one-third of its value in anhydrous acetone. The ionization constants for lithium perchlorate and lithium bromide have both increased by a factor of about three or four. TABLE 10 Dependence of rate on lithiurn bromide concentration at a lithium perchlorate concentration of 0.4761 M
Concentration of ester = 0.1348 M ; T = 35°C. k X 10' CONCENTRATION OF
LiBr
1
Experimental
M
Calculated
x 101
pn C r n l
1.295 2.590
0.553 1.076
~
~
0.538 1.076
+2.8 0.0
~
TABLE 11 Dependence of rate on lithium perchlorate concentration Concentration of ester = 0.1348 M; concentration of LiBr = 0.002590 M ; T = 35°C. k X IW C O N C L N ~ A T I O NOF
LiCIOd
Ak
Experimental
Calculated
4.04 3.03 1.689 1.417 1.076 0.937 0.807
3.90 3.01 1.721 1.414 1.076 0.925 0.807
p n Coll
M
0.007553 0.01511 0.09442 0.1888 0.4721 0.7553 1.1330
C3.6 +0.7 -1.9 +0.2 0.0 +1.3 0.0
TABLE 12 Heat of activation and collision frequency factors S
EWt
SW.-
ki ........................ kr.. . . . . . . . . . . . . . . . . . . . . . .
i
1.15 X 10" 0.165 X 10"
15,660 16,310 ANHYDROUS ACETONE AT
35°C.
The pseudo-unimolecular specific rates shown in the second column of table 9 correspond to the series of experiments in which the concentration of ester was constant a t 0.1348 M and the concentration of lithium bromide varied. A plot
RACEMIZATION OF DIMETHYL
I-BROMOSUCCINATE
873
of these rates against the t,otal concentration of lithium bromide gives curve 1 of figure 9. This curve has the same general shape as curve 1 of figure 2 for the reaction at corresponding conditions a t 24.9"C. Two experiments in which 0 . 4 i 2 1 M lithium perchlorate was added are listed in table 10 and shown as curve 2 of figure 9. The same straight-line dependence on lithium bromide concentration is observed as was noted in the corresponding runs a t 24.9"C. The series of experiments in which the lithium bromide and ester concentrations mere held constant while the concentration of lithium perchlorate was varied mas repeated a t 35°C. The results are listed in table 1 1 and shown in figure 10. Comparison of the constants listed in tables 4 and 1 1 shows that the ionization constant for lithium bromide has decreased from 3.6 X lo-' to 3.3 X lo-' and the value of the blocking constant, k4, has increased from 20.5 to 21.3. Actually, both sets of data could be calculated within the experimental error by using intermediate values for these constants. Therefore, the change in the ionization constant may or may not be real. The values of k 3 and k5 hare not been determined at 35'C. and are assumed to be the same as the constants determined a t 24.9OC. The values of k l and k, may be used to calculate heats of activation and collision frequency factors for the ion and molecule rates. The values are listed in table 12. The activation energies of the bromide ion rate and the lithium bromide molecule rate differ by only 650 cal. This agrees with a AH calculated from the small change in the ionization constant for lithium bromide, assuming the same critical comple? for the two processes. The larger frequency factor for the ion may be associated with a larger local concentration of the ion at the carbonbromine dipole. REFERENCES (1) (2) (3) (4) (5) (6) (7)
CONANT, J. B . , ASD KIRNER, W . R . : J . Am. Chem. SOC.48,232 (1924). CUNNINQHAY, C. M.: Unpublished work in this laboratory. DIPPY, J. F. J., JESKINS, H . O., AND PAGE,J . E . : J. Chem. SOC.1939, 1386. KRAUS,C . A., AND BRAY,W. C . : J. Am. Chem. SOC.36, 1315 (1913). OLSON,A. R., AND LONG,F . A.: J. Am. Chem. SOC.66, 1294 (1934); 68, 393 (1936). T. R . : J . Chem. Phys. 17, 1167 (1949). OLSON,A . R., AND SIMONSON, TIMXERMANS, J., A N D GILLO,L.: Roczniki Chem. 18, 812 (1938).
