[CONTRIBUTION FROM THE
DEPARTMENT OF CHEMISTRY,
COLUMBIA UNIVERSITY]
T H E SALT EFFECT IN THE REARRANGEMENT O F BENZIL a-CARBOXYLIC ACID F. H. WESTHEIMER* Received August 19, 1936 INTRODUCTION
In a paper which will be presented elsewhere, the author determined the mechanism of the rearrangement of benzil. He showed that the first interjmediate is the negative ion derived from the hydrate:
()--co-co-()
(yp0-0 OH
+
OH-=
I
--j
c)
@-cooI OH
The present paper deals with the rearrangement of a substituted benzil, namely, benzil a-carboxylic acid. The salt effect in this reaction was found to be quite large. Further, on the very probable assumption that the rnechanism of the rearrangement of bensil o-carboxylic acid is the same as thle mechanism of the rearrangement of benzil, the medium effect shows an interesting peculiarity. METHOD
The method was essentially that used in the case of benzil. The reaction mixture was sealed in ampoules which were placed in a 100' thermostat. The ampoules were removed at known times, and thereaction stopped by cooling, then acidifying. Immediate extraction of the organic matter, and subsequent titration, the method employed in the rearrangement of bensil, could not be applied here, for Graebe and Juillard' have shown that the acid, produced by the rearrangement of bensil a-carboxylic acid, easily lactonizes.
* National
Research Fellow in Chemistry. GRAEBEAND JUILLARD,Ber., 21, 2003 (1888). 339
340
F. H. WESTHEIMER
0 eO-CO-()
I
\ \OH COOK
O-c-cooH I
0-C-COOK
~~
2%
COOK
\
\OH COOH
II
0 Then, since both starting product and final product each have one carboxyl group, no separation could be made acidimetrically in the cold; heating to open the lactone is obviously out of the question as it would further the rearrangement. Graebe and Juillard noted, however, that the rearrangement product, the acid lactone, is easily decarboxylated. Blanks showed that this reaction is quantitat,ively complete on heating one half hour in the presence of 0.005 M hydrochloric acid.
0 I
c>
II
I
II
0
0
Subsequent to the decarboxylation, extraction with ether and titration in the cold with standard alkali would show how much of the starting product is still present; the lactone opens but slowly in the cold. EXPERIMENTAL
The thermostat was the same employed in the rearrangement of benzil, and described in detail elsewhere. Essentially, it consisted of a water-boiler, in which the pressure was kept constant by the method of Coffin.2 The thermostat maintained a temperature of 100.04 &0.03". The continuous extractors were the same as those employed in the study of benzil. The analytical method was as follows. The solution in the ampoule was chilled to stop the reaction, and an aliquot was taken. This was neutralized, and sufficient
COFFIN,J . Am. Chem. Soc., 66, 3646 (1933).
341
REARRANGEMENT OF BENZIL 0-CARBOXYLIC ACID
excess hydrochloric acid was added to make the solution 0.005 M . It was then refluxed for half an hour in an all glass apparatus, quantitatively transferred to an extractor, and extracted two hours with ether. After distilling off the ether, and adding water and alcohol as solvent, the cold solution of benzil o-carboxylic acid and lactone was titrated with standard alkali using thymol blue as indicator. Blanks showed that the results were accurate to 1per cent. Thrb benzil o-carboxylic acid was made according t o the directions of Graebe and Juillayd, and was recrystallized four times from benzene, melting a t 141.5’. Carbonate free alkali was employed throughout, and all salts were recrystallized from water. RESULTS
The bimolecular constants for the reaction between salts of benail o-carboxylic acid and hydroxyl ion have an average deviation of about 6 per cent., although there is often a slight, downward drift as the reaction proceeds. A typical example is given in Table I. In this tabulation, TABLE I TYPICAL VELOCITYDETERMINATION c+4
0.1859 0.1850 0.1841 0.1833 0.1826 0.1819
f 4 log o-
t
1.293 1.333 1.378 1.426 1.453 1.518
5.00 10.17 15.58 21.08 26.08
I
1.04 1.09 1.11 1.00 1.12
I
Average..
k X 10
1.07
I
DEVIATION
.03 .02 .04
.07 .05 .04 o r 4 %
cc. stands for the number of cubic centimeters of approximately 0.01 M potassium hydroxide used in the analysis, c for the concentration of poa for the concentration of potassium tassium benzil o-carboxylate, c hydroxide present a t any time; t is quoted in minutes; k is calculated according to the standard bimolecular equation using natural logarithms. In this particular case, potassium chloride mas added to raise the ionic strength to 3.50. At a constant ionic strength of 0.50, when the concentration of alkali was held constant and the concentration of benail o-carboxylic acid varied, the bimolecular constant remained the same. When, however, the concentration of benail o-carboxylic acid was maintained while the concentration of potassium hydroxide was varied, the (‘constant,” calculated according to a bimolecular equation, was found to increase with increasing molality of alkali. The data are recorded in Table 11. Since the reaction is between a salt oi benzil o-carboxylic acid and hy-
+
342
F. H. WESTHEIMER
droxyl ion, that is to say, between two negatively charged particles, the Brgnsted theorya would predict a large positive salt effect. Such was, indeed, found. Using 0.01 M benzil o-carboxylic acid and 0.09 M poTABLE I1 OF POTASSIUM BENZIL0-CARBOXYLATE RATEOF REARRANQEMENT 0.5 IONICSTRENQTR CONCENTRATION OF POTASSIUM BENZIL O-CARBOXYLATE
OF
0.0045 0.014 0.05 0.012 0.011 0.014 0.011 0.012
KOH
0.095 0.086 0.100
0.028 0.039 0.086 0.19 0.38
SALTEFFECTIN
1
I CONCENTRATION
THE
0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50
0.40 0.40 0.35 0.46 0.45 0.40 0.30 0.11
1.25 1.23 1.29 0.86 1.01 1.23 1.48 2.08
TABLE I11 REARRANQEMENT OF POTASSIUM BENZIL0-CARBOXYLATE CONCENTRATION OF
Ir
Kc1
“k” X 101
....
0.1 0.5 1.5 3.5
0.75 1.23 3.43 8.3
0.40 1.40 3.40
TABLE IV RATEOF REARRANQEMENT OF POTASSIUM BENZIL0-CARBOXYLATE 3.5 IONICSTRENQTH CONCENTRATION OF POTASSIUM BENZIL O-CARBOXYLATE
0.011 0.011 0.012
CONCENTRATION OF KOH
0.025 0.089 0.18
CONCENTRATION
OF
KCI
3.46 3.40 3.31
c
“k” X 101
3.50 3.50 3.50
6.71 8.3 10.7
tassium hydroxide, the change of rate with change in ionic strength is recorded in Table I11 and in Fig. 1. Thus, since there was such a large medium effect,.it was thought that the reaction might approach the bimolecular formula more closely in soluBR$NSTED, Chem. Rev., 6, 231 (1928).
343
REARRANGEMENT OF BENZIL O-CARBOXYLIC ACID
tions #ofhigher ionic strength. That this was not the case can be seen from Table IV. Since medium effects in concentrated solution are often quite specific, measurements were made on the rate of rearrangement of the sodium salt TABLE V RATEOF REARRANQEMENT OF SODIUM BENZILO-CARBOXYLATE CONCENTRATION OF BODIUY BENZIL O-CAWBOXYLATE
0.013 0.011 0.012 0.011
CONCENTRATION
OF
NnOH
0.087 0.038 0,089 0.39
CONCENTRATION
"k" X 102
or NarSO,
.... 0.85 0.81 0.70
0.1 2.5 2.5 2.5
0.77 2.67 3.20 6.89
Ionic strength FIG.1
of benzil o-carboxylic acid in solutions of sodium hydroxide and sodium sulfate. The results are recorded in Table V. Comparing Table V with Table 111, it is worthy of note that, a t an ionic stre:ngthof 0.1, the rate with sodium hydroxide is the same as the rate with potassium hydroxide. The rate constant, for 0.09 M potassium hydroxide and an ionic strength of 2.5, interpolated from Figure 1,is 5.55, while the constant for the sodium
344
F. H. WESTHEIMER
hydroxide-sodium sulfate system, from Table V, is 3.20. Furthermore, while, as is shown in Table IV, the increase in the bimolecular rate constant with a tenfold increase in alkali molality is 1.8-fold in the case of the potassium hydroxide-potassium chloride system at an ionic strength of 3.5, Table V shows that the increase for a tenfold change in alkali molality for the sodium hydroxide-sodium sulfate system, at an ionic strength of 2.5, is 2.6-fold. THEORETICAL
No consideration of these data is adequate without a review of the facts concerning the rearrangement of benzil itself. In that case, the reaction between benail and hydroxyl ion is strictly bimolecular. The concentration of bensil was varied tenfold, the concentration of hydroxyl ion more than four-hundredfold without producing changes in the bimolecular rate constant which were outside the experimental error. In buffered solution, the rate varied with the buffer ratio, but not with its concentration. The salt effect, on addition of potassium chloride, was quite small. The only valid conclusion to be drawn from these facts is that the ion 0-
is in equilibrium with benzil and hydroxyl ion, and that the slow step is the further reaction of this ion. Furthermore, the reaction is not a case of general base catalysis; the equilibrium is a true one. There is no valid reason to suppose that the rearrangement of benzil o-carboxylic acid follows any mechanism other than that followed by bend. The only reasonable assumption is that the reaction, here, too, is one between the substituted benzil and hydroxyl ion, forming the doubly charged ion 0-
aa intermediate. Since the salt effect is absent in the rearrangement of bensil, the large medium effect is not inherent in the reaction, but is the result of the substitution, and presumably is caused by the negative charge of the carboxylate ion in the close neighborhood of the seat of the reaction.
REARRANGEMENT OF BENZIL 0-CARBOXYLIC ACID
345
Benzil o-carboxylic acid can exist, it is true, in two forms. One of these is open-chain, the other cyclic; they exist in equilibrium in solution. The salts, however, are chiefly those of the open-chain variety.'
OH
If the ion of the cyclic form rearranged, the rate would vary with less than the first power of the hydroxyl-ion concentration. This equilibrium cannot, therefore, explain the deviation from the bimolecular constant with change of alkali concentration. On the basis of the reasonable assumption, already stated, that beneil and beneil o-carboxylic acid rearrange by the same mechanism, the large salt effect must be the cause of the irregularities. The salt effect is peculiar in two ways: first because of its magnitude, second because the deviation with changing hydroxyl-ion concentration persists even in solutions of high ionic strength. Considering first the magnitude of the salt effect, one notes that the Brgnsted equation holds in dilute solution only. The general equation is: In fA-fB-= a 2 z A ZB fx-
6- @A-
+ + flB-
flX-1
c
where the rate is proportional to the concentration of a complex, X. While in dilute solution, dris large when compared to c, and, in general, the first term of the equation is large as compared with the second; this situation does not obtain in concentrated solutions. Here the salt effect is, in general, linear with the concentration of the added salt. The Brgnsted equation would demand that, in a reaction between two negatively charged ions, the rate increase with increasing salt concentration in dilute solution, and further, that the increase between j t equal to 0.02 and jt equal to 0.1 be in the ratio 1.22.5 An inspection of Figure 1 shows that this requirement is met qualitatively, although no measurements in the extremely dilute range are available for quantitative comparisons. In the more concentrated range, most authors have found a linear salt effect such as that found in this reaction. However, in reactions between two negatively charged particles, the ratio of increase in rate was generally much smaller than is found here. 4
8
HANTZSCH, Bet., 49, 213 (1916). BR~NBTED, 2.physik. Chem., 102, 169 (1922).
346
F. H. WESTHEIMER
In the reaction between chloracetate ion and ethyl xanthate ion, for example, Kiss6found the ratio of increase, with added sodium chloride to be 2.5 when the salt was increased from 0.5 M to 4.0 M . Even the large salt effect caused by the addition of 6 M potassium thiocyanate in the dealdolization of diacetone alcohol’ is smaller than the one here recorded, for 6 M potassium thiocyanate depressed the rate less than sixfold. The fact that the rate varied more than linearly with hydroxyl-ion concentration, and that this effect could not be “swamped” out by the addition of large amounts of salt, is the more important consideration. Among the few good studies of a similar nature in which such an effect was found is the work of La Mer and Sandveds on the equilibrium between potassium ferricyanide and potassium iodide to produce potassium ferrocyanide and iodine, in the presence of salts. In solutions having an ionic strength of 0.5, a twofold change in the concentration of iodide ion, keeping the concentration of the ferricyanide constant, caused a variation of about 20 per cent. in the equilibrium constant. Increasing the ionic strength to more than two did not eliminate the variation in constant with change in iodide concentration, but the variation was sharply diminished. This is shown by the fact that a twofold change in iodide concentration caused a variation of 6 per cent. in the constant. In the present case, however, a change in the hydroxyl-ion concentration of tenfold changes the rate constant by a factor of about 2.1, while in solutions with an ionic strength of 3.5,this factor is still 1.8. That is, the deviations are ironed out too slowly by increasing ionic strength, and in no solution available will the rate constant be even fairly regular. The fact that the deviation from a bimolecular constant differs markedly with sodium hydroxide and sodium sulfate from the effect with potassium hydroxide and potassium chloride is sufficient evidence that the deviations are, after all, due to medium effect. ACKNOWLEDGMENT
The author would like to thank Professor L. P. Hammett for the help and advice he gave in this research. SUMMARY
1. The rate of rearrangement of the salts of benzil o-carboxylic acid have been measured in aqueous alkali solutions at 100”. The reaction is subject to a marked positive salt effect. 2. The rate varies with more than the first power of the hydroxyl-ion concentration, and this effect cannot be swamped out by increased ionic strength.
* KISS, Rec. trav. chim., 63, 903 (1934). AKERLOF,J . Am. Chem. Soc., 48, 3046 (1926). 60,2656 (1928).
* LA MERAND SANDYED,ibid.,
