THE VELOCITY OF SAPONIFICATION OF CERTAIN ESTERS BY TENTH-NORMAL POTASSIUM HYDROXIDE I N DIF’FERENT SOLVENTS AT 25 0 C1 BY ERNEST ANDERSON AND H. B. PIERCE’
Purpose of the Investigatio.lz.--This. investigation was undertaken for two purposes; first, to determine whether or not the ratio of the velocities of saponification of any ester A in the four sojvents used is the same as the ratio of any other ester B ; second, to determine whether or not the velocities of saponification of the esters of any two alcohols with the same acid always have the.same ratio irrespective of what the acid may be, and conversely whether or not the esters of any two acids with the same alcohol always have the same ratio irrespective of what the alcohol may be. It is evident that if these two questions could be answered in the affirmative the whole problem of determining velocities of saponification would be rendered extremely simple. For instance, if the ratio of the velocities of saponification in the different solvents were known it would be possible to calculate the velocity of saponification of an ester in any of the solvents after determining experimentally its velocity of saponification in one solvent. Furthermore, by tabulating the acids along a vertical column and the alcohols along a horizontal column and determining the velocity of saponification of all the esters of one alcbhol with the various acids and all the esters of one acid with the various alcohols, it would be easy to calculate by simple proportion the velocity of saponification of any ester in the table. As an illustration consider the following table which gives the velocity of saponification of certain ester3 by 1 / 1 0 normal potassium hydroxide at 2 5 ’ C in a solution of 90 percent methyl alcohol. Contribution from the Massachusetts Agricultural College, The authors wish t o thank Mr. C. Wies and Mr. 0. M. O’Neil for assistance in some of the experimental work. 1
2
Velocity o j Saponification o j Certain Esters, Etc.
HI Formic Acetic Benzoic
Methyl
Ethyl
n-Propyl
-
0.0435 0 . I45 0 03447
-
0.03442 o 0466
'
-
Ber. deutsch. chem. Ges., 14, 1361 (1871). Liebig's Ann., 228, z j I (1885). 8 Jour. prakt. Chem., 35, 1 1 2 (1887). Zeit. phys. Chem., I , I I O (1887). Ibid., 2, 194 (1888). Jour. Am. Chem. SOC., 31,403, 886 (1909). Proc. Roy. SOC.,79, j 6 4 (1907). * Zeit. phys. Chem., 7, 290 (1891). Ber. deutsch. chem. Ges., 29, I I O (1896); 31, 1844 (1898). Proc. Chem. SOC., 13, 241 (1897).
'
0.0345 -
45
Ernest Anderson and H . B. Pierce
46
velocity of saponification of polybasic acids and especially on the relation between the saponification constant and the strength of the acid. Goldschmidtl made measurements on the saponification of acetoacetic ester. Findlay and Turner,2 Findlay and hick man^,^ and Dean4 studied the effect of hydroxyl and alkyloxyl groups. Williams and Subdoroughs compared the saponification of esters of saturated and unsaturated acids. Meyer and Kellas6 and recently McCombie and Scarborough7studied the saponification of derivatives of ethyl benzoate. Bischoff and Hedenstroms and McCombie and Scarboroughs have made exhaustive examinations of the velocities of saponification of aryl and benzyl esters of dibasic fatty acids as well as of the phenyl and substituted phenyl esters of monobasic acids. BrussofflO studied the velocity of reaction of alcoholic potassium hydroxide on various alkyl iodides. Korschumll has determined the velocity of saponification of a large number of pyrrole derivatives. The influences of the solvent on the velocity reactions is extremely great as has been shown by many investigators. Furthermore one reaction may be hastened by one solvent and retarded by another solvent while a second reaction may be affected just the reverse by the two solvents. Menschutkin12 has done some comprehensive work on the influence of the solvents. Patterson and Montgomerie13 have brought together much of the data of earlier investigators. Reference should also be made to the monograph by Protze.14 More 1 Ber.
deutsch. chem. Ges., 32, 3396 (1899);33, 1150 (1900). Jour. Chem. Soc., 87, 747 (1905). * Ibid., 95,roo4 (1909). 4 Am. Jour. Sci., 35, 605 (1915). 6 Jour. Chem. SOC.,101, 412 (1912). 6 Zeit. phys. Chem., 24, 221 (1897). 7 Jour. Chem. SOC.,107,156 (1915). 8 Ber. deutsch. chem. Ges., 35, 4094 (1902). 9 Jour, Chem. SOC.,105, 1304 (1914). 10 Zeit. phys. Chem., 34, 129 (1900). 11 Bull. SOC.chim. Paris, 19,164,221 (1916). 1 2 Zeit. phys. Chem.. 6,41 (1890);34, 157 (1900). 1s Jour. Chem. SOC.,101, 26 (1912). 14 “Ueber den Einfluss des Losungsmittels auf dieReactionsgeschwindigkeit” (Berlin, 1912). 2
Velocity of Saponi$catio% of Certain Esters, Etc.
47
work has been done on the influence of the solvent on the velocity of esterification and on the hydrolysis of esters1 than on the velocity of saponification. However, Findlay and Turner, and Findlay and Hickmans3 have compared the velocity of saponification of esters in water, and in alcohol containing varying amounts of water. Anderson and Brown4 compared the velocity of saponification of fats and oils in three alcohols.
Preparation of Reagents and Method of Prooedure The solvents used were water, methyl alcohol, 90 percent by weight; ethyl alcohol, 90 percent by weight, and isoamyl The three alcohols were alcohol, density 0.8166at 20°/15'. purified by dissolving I g AgN03 in 5 cc water and adding to 2 liters of each of the following: 95 percent commercial methyl alcohol (Columbian spirit), 95 percent commercial ethyl alcohol, and commercial isoamyl alcohol. After standing 24 hours with occasional shaking 20 g commercial NaOH, dissolved in a small volume of water, were added to each of the alcohols and again allowed to stand 24 hours. The solutions were then filtered to remove most of the precipitate and the alcohols distilled, care being taken to leave considerable alcohol in the distillation flask. The methyl and ethyl alcohols were then diluted with water respectively to the densities, 0.8235 a t 15'/4' and 0.8240 at I ~ O / I ~ O these , corresponding to 90 percent by weight. When necessary, the amyl alcohol was If amyl diluted with water to density 0.8166 at 20'/15'. alcohol has more water than this in it, water will separate when sufficient KOH is added to make a 2/10 normal solution. Next sufficient KOH was dissolved in each of the alcohols to make the solution slightly more than 2/10 normal. The Cf. Lapworth: Jour. Chem. SOC., 93, 2163, 2187 (1908); Goldschmidt and Udby: Zeit. phys. Chem., 60, 728 (1907); Goldschmidt: Zeit. Electrochemie, 15, 4 (1909); Sudborough and Turner: Jour. Chem. SOC., 101, 237 Kolhatkar: Ibid., 107,921 (1915). (1912); Jour. Chem. SOC.,87, 747 (1905). Ibid., 95, 1004(1909). Jour. Phys. Chem., 20, 195 (1916).
48
Ernest Anderson and H.B. Pierce
solutions were allowed to stand until the slight amounts of Kd.203 had settled out and then filtered rapidly. The solutions were then titrated with 1 / 1 0 normal HC1 using phenolphthalein; diluted to 2 / 1 0 normal and again accurately standardized. Large amounts of the alcohols were purified at first and sufficient alcoholic KOH prepared at once to last throughout the investigation. A carbonate free water solution of KOH was prepared by adding carefully to a water solution of KOH, small amounts of Ba(0H)t solution until all carbonates had been precipitated and minute traces of Ba(0H)z were in solution. The solution was then allowed to settle, filtered from BaC03, standardized with 1 / 1 0 normal HCl and diluted with COz free water until 2/10 normal. The esters used, except ethyl propionate, ethyl butyrate and methyl salicylate, were prepared according to the directions given in Beilstein’s Handbook, and were purified until they had the correct boiling point. The three esters mentioned were obtained from Eimer and Amend. The first two were further purified until they had the correct boiling point, but methyl salicylate was used just as it came and was synthetic oil of wintergreen. In a few cases the esters evidently still contained some of the corresponding alcohol with them since they required less than the theoretical amount of alkali. The exact procedure followed in the investigation is given below. A volume of ester slightly less than would be saponified by 2 0 0 cc of 1/10 normal KOH was placed in a IOO cc volumetric flask and made up to the mark with the desired solvent which had already been brought to the temperature of the thermostat. This solution was then poured into a 400 cc flask and set in the thermostat. The volumetric flask was washed, dried, and filled to the mark with IOO cc of the corresponding 2/10 normal KOH solution. After the two solutions had reached constant temperature they were mixed. By this procedure the ester was in every case being saponified by 1 / 1 0 normal KOH. At suitable intervals, which depended on the velocity of the particular reaction,
Velocity of Sapo.tzi$cation of Certaiw Esters, Etc.
49
cc of the reaction mixture were withdrawn and run into cc of 1 / 1 0 normal HCI, the time being accurately noted. The excess HC1 was then titrated with 1 / 1 0 normal Ba(OH)2 using phenolphthalein as indicator. In the case of some esters it was necessary to titrate in 25 cc of COz free ethyl alcohol so that the resulting solution would be clear. Wherever amyl alcohol was the solvent it was necessary to titrate in ethyl alcohol. Furthermore because of the viscosity of amyl alcohol it was found best to wash the I O cc pipette each time with ethyl alcohol. The burette measured I O cc and was graduated directly to 0.02 cc and could be read to 0.002 cc. IO
IO
TABLEI Saponification Constants of Certain Esters by 1/10 Normal Potassium Hydroxide in Different Solvents Solvent Ester Water
Ethyl bromide rt-Propyl bromide z-Butyl bromide i-Amyl bromide Methyl iodide Ethyl iodide n-Propyl iodide i-Butyl iodide ;-Amyl iodide Methyl formate Ethyl formate Methyl acetate Ethyl acetate n-Propyl acetate &Butyl acetate i-Amyl acetate Benzyl acetate Phenyl acetate Ethyl propionate Ethyl butyrate (n) Methyl benzoate Ethyl benzoate Methyl salicylate
Ethyl Alcohol 0.0443 0.0417
Amyl Alcohol 0.0422(?) 0.0419(?)
0.0412
0.0415 (?)
0.056
0.0416
0.0372
0.0332
0.0470
0.0433 0.0424( ?)
0.0429
( ?)
0.0413
0.0437 (?) 0,0420 ?)
0.02315 0.0~302
0.0265 0.0269
0.02298
0.0272
0 0227 2
0.0282
0.0424
-
.
-
(
-
0.02305 0.0440(?)
0.0269 0 . 0269 0.0436 (?)
O.OZIj5
0.024
0.0384
0.0223
0.0336 ( ?)
0.0210( ?)
0.0332
0.0317
0.0430(?)
0.0410
0.02297
Ernest Anderson alzd H . B. Pierce
50
In the case of all the rapid reactions the time was taken with a stop-watch reading to fractions of a second. Duplicate determinations were run on each ester and where these did not check, other determinations were made. Results Obtained The above table gives in a compact form th6 constants obtained for 23 esters in the four solvents used. On account of the slight solubility of most of these esters in water, only three were examined in that solvent. The formates are saponified with extreme rapidity and could be determined in only the slowest solvent, methyl alcohol. Similarly some of the esters were saponified so slowly that they were examined in only the fastest solvents, amyl and ethyl alcohol. The constants given in this table are in most cases the average of several determinations.
Conclusions from the Table It was found very difficult to get a satisfactory constant in the case of some of the reactions, especially where they were very slow. In general ethyl alcohol gives the most satisfactory constants and amyl alcohol the least satisfactory. Those constants after which questions have been placed are only approximations, and should be redetermined. However, they probably are very near the correct value. In most of these questionable cases our results give no constant but decreasing values as will be seen by reference to the determinations at the close of the paper. It is possible that the effect of the salt formed during the reaction1 may be greater in the amyl alcohol which has less water present than in the ethyl alcohol. Ostwald2 and many later investigators have shown that the salt formed during the reaction does not affect the velocity of saponification by a strong base, but decreases very much the velocity of saponification by weak bases. The ionization of potassium hydroxide in the amyl alcohol used may be so small 1
Arrhenius: Zeit. phys. Chem., I , IOI (1887); 2, 284 (1888). Jour. prakt. Chem., 35, 112 (1887).
Velocity of Saponi$catio.tz of Certain Esters, Etc.
51
that the salt formed may affect very much the velocity constant. It is evident from the following table that the ratios of the constants of one ester in any two of the solvents may be quite different from those of another ester in the same two solvents. It is not possible, therefore, to calculate with any degree of exactness the velocity of saponification of an ester in one of these solvents from the known velocity in a second solvent and the ratio of the velocities of a second ester in the two solvents unless the two esters are very closely related. Findlay and H i c k m a d have investigated this point. Their ratios for eight esters in the two solvents, water and 30 percent ethyl alcohol, vary from 1.4to 2.4,and they assume that the ratio is'not the same for different esters.
TABLG 2 Ratio of constants in solvents Ester
Ethyl bromide n-Propyl bromide &Butyl bromide &Amyl bromide Methyl iodide Ethyl iodide n-Propyl iodide Methyl acetate Ethyl acetate n-Propyl acetate &Butyl acetate &Amyl acetate Benzyl acetate Ethyl propionate Ethyl butyrate Ethyl benzoate
0.44 0.47 -
2.1 2.2
2.4 3.0 2.3 2.2
2.6 2.7 0.6
An examination of Table I will show that variation in the alcohol radical of the ester affects the velocity of the reaction 1
Jour. Chem. SOC.,95, 1004(1909).
52
Ernest Anderson and H . B. Pierce
very differently, depending on the acid radical with which they are combined. Thus while all the acetates, except phenyl, have practically the same constant in any alcohol, the iodides and bromides have decreasing constants as the alcohol radical increases in mass. Methyl iodide is saponified more than ten times as fast as ethyl iodide. This has long been known from investigations on the influence of various factors, reference to which was given earlier in the article. Experimental Data and Caleulations As already described, in carrying out a determination, the ester dissolved in the solvent is mixed with the alkali dissolved in the solvent. A t intervals, the volume of 1/10 normal alkali remaining in I O cc of the mixture is determined as follows: Ten cc of the mixture are run into I O cc of 1/10 normal HC1. The excess acid is then titrated with 1/10 normal Ba(OH)2. The difference between the volume of 1 / 1 0 normal HC1 used and 1 / 1 0 normal Ba(OH)2 used gives the volume of 1 / 1 0 normal alkali present in I O cc of the mixture. This value is given in the second column of the experimental tables. The second order reaction formula used in the calculations is
K = velocity constant. To = volume of 1 / 1 0 normal alkali present in I O cc of the mixture a t first titration. Te = volume of 1 / 1 0 normal alkali present in I O cc of the mixture at the end of the reaction. T t = volume of 1 / 1 0 normal alkali present in I O cc of the mixture at time t. t = time in minutes after the first titration. In the experimental data To is the first value and T, the last value under the second column. In all cases the concentration of the ester was slightly less than 1 / 1 0 normal while the alkali was very close t o that value at the beginning. The exact strengths of the various
Velocity of Sapo.tzi$cation of Certain Esters, Etc.
53
solutions are given below. No corrections for strengths of solutions were applied to any of the volumes. HC1 = 0,994 1 / 1 0 normal. Ba(OH)2 = 0.988 1 / 1 0 normal. Amyl alcoholic KOH = 0.965 1 / 1 0 normal. Ethyl alcoholic KOH = 1.0011 / 1 0 normal. Methyl alcoholic KOH = 1.001 / 1 0 normal. Water solution of KOH = 0.998 1 / 1 0 normal. These were the strengths after equal volumes of alkali and ester had been mixed. In Water Solution Methyl Acetate 1.4 cc
t
Tt
1-45 2.15 2.97 3.85 4.60 5.31 6.15
5.975 3.805 3.295 2.930 2.680 2.505 2 ,405 2.300 I .860
0
E
Ethyl Acetate 1.6cc
K
t
Tt
K
2.67 3.60 4.43 5.33 6.30 7.31 8.30 9.30 10.35
7.25 4.495 4. I 1 0 3.870 3.680 3.510 3.385 3.315 3.210 3.130 2.830
0.06595 0 * 06594 0.06534 0 * 06434 0.0643I 0.06348 0.06075 0.06227 0.063 17
0
0.1105 0.1147 0.1148 0.1132 0.1176 0.1125
0.1119
E
I n Water Solution
--
In Methyl Alcohol Solution
-
%-PropylAcetate z cc
t 0
3.45 4.25 5.11
5.97 6.80 7.70 8.63
E
Ethyl Bromide 1 . 3 cc
K
I ' T i
6.480 3.775 3.515 3.295 3.145 3.010 2.880 2.750
I .5m
t
Ti 0
04649 0 04.594 0.04488 0.04296 0.041 8 I 0 * 041 7 7 0.04055 0*
3
56 98 I49 259 400 513 I118
I359 I593
E
9.932 9.859 9.848 9.812 9.634 9.462 9.288 8.534 8 * 329 8.032 3.117
K
846 0.041281 0.041240 0.041740 0.041846 0.041
0.042001
0.042227
0.042509 0.042768
Ernest Anderson and H . B. Pierce
54
In Methyl Alcohol Solution %-PropylBromide 1.5 cc
;-Butyl Bromide
t
9.942 9.894 9.872 9.812 9.742 9.682 9.298 9.178 9 . I20 3.880
0
59.6 97.7 258.6 400.9 490.3 1118.0 1239.0 I593 0
-
E
~
0.041294 0 . 0 4 1 2 I4 0.058492 0.058584 0.059198 0 . 0 4 1 046 0 . 0 4 1 I35 0.059612
I
t
Tt
0
9.823 9.814 9.788 9.759 9.686 9.676 9.355 9.317 5.376
34 123 234 376 465 1314 I509
E
t
Tt 9.952 9.924 9 * 898 9 * 757 9.722 9.642 6.165
0
237.9 379.6 1098.0 1319.0 1574.0
E
T1
E
0 .057 5 5 9
0.056963 0.056406 0.0~8656 0.057459 0.058833 0.058373
Methyl Iodide
I
K
t
Tt
0
0 -053296
40.5 172.5 271.3 321.8 375.5 414.5 457.5
9.658 9 . I02 7.704 6.858 6.508 6.212 5.994 5.796 I .384
0.053935 0.054897 0.054840 0.06555 1
9.816 9.274 9.138 9.044 8 933 8 * 748 8.638 9
2 * I20
1 . 2 cc
1.'
K
0.0~1807 0 . 0 3 I 8I 3 0 . 0 3 I9 I9 0 . 0 3 I892 0 . 0 3 I 874
0,031878 0 . 0 8 I 862
n-Propyl Iodide 1.8 cc
Ethyl Iodide 1.4cc
505
K
--
E
0
cc
~
&Amyl Bromide 1.6 cc
217 268 312 367 460
2
I
K
Tt
t 0
92 123 I83 243 303 363
E
9.902 9.820 9.816 9.752 9.712 9.668 9.590 2.512
K
0.041096 I93 0.041257 0 . 0 4 1094 0.041089 0.041
0 . 0 4 1 2 12
Velocity of Saponi$cation of Certain Esters, Etc. &Amyl Iodide
;-Butyl Iodide z cc
2.5
55
cc
~
t
Tt
I I
9.176 . 9.116 9 10.5 9.050 8 978
0 2 I2
272 332 411
K
t
0.055I41 0.054758 0.057183 0 * 06947I
215 275 335 396
'
K
Tt
1
0
9.858 9.765 9.742 9 738 9.700
i
0.068450 0.069084 0 * 057729 0.068602
9
472
E-
t 0
1.68 2.58 3.43 4.38 5.38 6.28 7.28 8.13 9.37
E
1
Tt
4.183 2.378 1.958 1.720 1.575 1.397 1.302 1.252 1.160 1.116
K
/
5 * 115 3.738 3.008 2.686 2.432 2.278 2,138 2.034 I .6go
0
0.1508 0.1562 0 . I559 0 . I474 0 * I554 0 .I555 0.1469 0.1581
0.79 1.66 2.42
3.26 4.02
4.88 5.73
E
0
35.25 69.00 104.80 123.50 245.00 299.50 390* 30 451.oo 481.50
E
0.1498 0.1512 0.1444 0.1428 0.I404
0.1409 0.1421
0.850 Ethyl Acetate 1.8cc
Methyl Acetate 1.4cc
t
K
Tt
1
Tt
K
0
8 * 734
7.738 6.980 6.338 6.028 4.682 4.220
3.746 3 * 438 3.298 0.598
t
31.5 64.5 94.5 124.5
'034479 0.034576 0.034483 0.034553 0.034478 0.034576
211.5
0.034420
242.0
0.034457 0.034487
280.5 301 * 5 332.0 361.7 421.5 474.0
0
181.5
E
Tt
8.618 7.812 7.088 6.518 6.078 5.394 5.146 4.858 4.586 4.444 4.264 4so52 3 * 768 3.534 I. I22
K
Ernest Anderson and H . B . Pierce
56
n-Propyl Acetate
.
t
-0
2
cc
Tt
&Butyl Acetate 2.4 cc
K
8.244 6.788 6.612 5.592 5.280 4.816 4.622 4.518 3.000
100.0
114.0 239.3 290.5 396.8 449.5 475.0
E
t
T:
0
8”. 202
E
Tt
0
128.0 150.5 171.5 202.3
258.5 291 * 5 314.0 330.0 347.0
E
5 * 406 5 ’ I44 4.872 4.346 4.088 3.822 3 * 744 0.292
147.8 168.0 198.0 255.3 287.0 325.8 342 * 3
K
Tt
t 0
9.944 7.208 6.912 6.628 6.308 5 798 5.528 5.352 5.254 5.156 2.696
0‘034534 0.034553 0.034420 0.034479 0.034560 0.034550 0.034515
Benzyl Acetate 2.8 cc
&Amyl Acetate 2.5 cc t
K
33.5 53.0 68.5 108.5 138.5 163.o 209.0 223.3 236.8 256.8
E
9.766 8.666 8.078 7.656 6.816 6.292 5.942 5.364
0.034477 0.034594 0.03465I 0.034571 0.034590 0.034555 0.034560 0.034541 0.034518 0.034532
5,200
5 .og6 4.896 0.766
Ethyl Propionate
Phenyl Acetate 2.5 cc
2
cc
I
t 0
66.0 397.3 451.0 478.5 E
Tt
8.552 8.392 7.682 7* 578 7.534 2.615
K
t 0
0.044938 0.04492I 0 * 044940 0.044896
162.2 195.5 241.5 262.5 294.0 335 - 0 363.0 458.0
E
Tt
9.805 7.040 6.640 6.172 5 * 990 5.730 5.435 5.242 4.720 I .ooo
K
0.032826 0.0~2850 0.03286I 0.032860 0.032869 0.032860 0,032867 0.0~2845
Velocity of Saponification of Certain Esters, Etc. Methyl Benzoate 2.3 cc
Ethyl Butyrate 2.4cc
I
Tt 0
272 317 374 424 467 511
E
Ti
K
9.712 8 * 904 8 * 596 8.488 8.032 7.874 7.716 1.375
0.046425 0.046965 0.0465 IO 0.046787 0.046901 0.046436
t
K
._
I
9 * 764 7.362 6 * 996 6.672 6.356 6.074 5.850 5,634
22 I
57
