412 WILLIAMS AND SUDBOROUGH : HYDROLYSIS, ETC., OF ESTERS

L.- The Hydrolysis and SaponiJication of Eaters of Saturated and Unsutumxted Acids.

By THOMAS WILLIAMS and JOHN JOSEPH SUDBOROUGH.

As the result of numerous experiments it has been proved (Trans., 1898, 73, 81 ; 1905, 87, 1840 ; 1907, 91, 1633 ; 1909, 95, 315, 975) that the presence of an define linking in the up-position with respect to a carboxylic group has a most pronounced retarding effect on the rate of esterification of an organic acid when the catalytic method is employed. The effect is so marked that a mixture of an up-unsaturated acid and its saturated analogue can be readily separated by the method of fractional esterification (Sudborough and Thomas, Trans., 1911, 99,2307). Further experi- ments on the direct esterification of such acids (this vol., p. 237) prove that in the absence of a catalyst the difference in the rates of esterification is not nearly so marked. When, for example, the two acids have much the same dissociation constants, the relative rates of esterification of it saturated acid and the corresponding up-unsaturated acid are as 40 : 1 when the catalytic method is used, but only 4 : l or 5 : l in the absence of the catalyst. Using the direct method, it has been shown that when the unsaturated acid is much strmger than its saturated analogue i t can be esterified more rapidly than the saturated acid, for example, methyl hydrogen maleate and methyl hydrogen succinate.

Some years ago (Ber., 1895, 28, 1263) V. Meyer put forward the generalisation that esters which are difficult to prepare are not readily hydrolysed. Such a broad generalisation cannot hold good, as esters are formed a t very different rates, according to the, method of esterification used, and, further, the esters are hydrolysed quite differently by different methods. Ethyl chloroc and dichloro-acetates are hydrolysed by dilute hydrochloric acid less readily than ethyl acetate (Kistia.kowsky, Zeitsch. physikal. Chem., 1898, 27, ZSO), but are saponified by alcoholic potassium hydroxide solution much more readily than the unsubstituted ester (Sudborough and Feil- mann, Proc., 1897, 13, 241).

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OF SATURATED AND UNSATURATED ACIDS. 913

One of us hae. drawn attention to the resemblance between esterification by the direct method and hydrolysis of esters by means of water or alkalis, and on the other hand the resemblance between catalytic esterification and hydrolysis by means of dilute acids (see article “ Hydrolysis ” in Thorpe’s Dictionary of Applied Chemistry, new edition).

We have studied the hydrolysis of the three following pairs of saturated and unsaturated esters : (1) Ethyl propionate and ethyl acrylate, (2) ethyl n-butyrate and ethyl crotonate, (3) ethyl P-phenylpropionate and ethyl cinnamate. The relative rates of hydrolysis by means of dilute hydrochloric acid and of saponifica- tion by means of dilute barium hydroxide have been determined atc 20°. The hydrolysis by dilute hydrochloric acid is a uni- molecular reaction, and the constants have been calculated by using the ordinary equation for such a reaction. Both ethyl cinnamate and ethyl B-phenylpropionate were dissolved in a mixture of alcohol and water, as they were not sufficiently soluble in water. Under these conditions an equilibrium was reached in the case of ethyl B-phenylpropionate, and the equation K = 1 / t . e / a . log e / e - x wits used for calculating K. X, x, and a have the usual meanings, and e is the equilibrium point expressed in C.C. of standard barium hydroxide. The experimental method adopted was as follows : For the acid hydrolysis a roughly O.05N-solution of the ester was mixed with an equal volume of O.05N-hydrochloric acid at 20°. The con- centration of the hydrochloric acid in the mixture was determined immediately after mixing by titration with standard barium hydroxide, and the concentration of the ester determined by adding an excess of barium hydroxide to the neutralised solution, allowing to remain for several hours in a closed vessel, and titrating the excess oP barium hydroxide by means of standard hydrochloric acid. For the alkali hydrolysis a 0’05N-aqueous solution of the ester (except in the case of cinnamic and 8-phenylpropionic esters) was mixed at 20° with an approximately equivalent solution of barium hydroxide, and a portion of the mixture removed from time to time and titrated with standard hydrochloric acid. The concentratio,n of the ester at the beginning was determined by adding an excess of barium hydroxide, keeping until hydrolysis was complete, and titrating the excess of alkali with standard acid. With ethyl cinnamate and ethyl 8-phenylpropionate, 0’04N-soh- tions of ester and of barium hydroxide in 50 per cent. alcohol were used.

The esters used had the following boiling points: ethyl propion- ate, 98-3O ; ethyl acrylate, 101-102°/765 mm. ; ethyl butyrate, X29-l2l0/762 mm. ; ethyl crotonate, 136--137O/762 mm. ; ethyl

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414 WILLIAMS AND SUDBOROUQH : HYDROLYSIS, ETC., OF ESTERS

8-phenylpropionate, 243--245O/ 767.5 mm. ; and ethyl cinnamate, 272%---272*8O/ 767.5 mm.

The results obtained are given in the following tables :

A. Hydrolysis with Dilute Hydrochloric Acid.

Concentration of barium hydroxide = 0-0418N.

Concentration

0*0495.N b= 23 -70

Ester. of HC1. Ethyl propionate . . .

b = 23.70

Ethyl acrylate . . . .. , 0.04995N b=23'90

0-0496N b = 23.75

Ethyl butyxate .. . . . . 0.04995.N b = 23 -90

0-0495N b=25*7

Ethyl crotonate ... ... 0'04995N b=23*9

Time in Titration. hours, t.

0 24 48 72

101 125

0 17 41 65 89

117 161

0 113 255 354 656 896

1203 0

162 402 570 738 938

1098 0

24 49 76 98

146 192

0 25.75 65 88

137.25 165.75 209.25

0 168 336 508.5 671 844

1056 1224 1392

b+x. 23-70 26-83 29'00 20.63 32.05 32-90 23T0 26.03 28-38 30.08 31.35 32.40 33.50 23-90 24-50 25'20 25.60 26'70 27.60 28-65 23.75 24.70 25-55 26-30 26-90 27-40 27'75 23 '9 25.75 27-43 28.95 29 -98 31-70 32.90 23'70 25.70 28.27 29.35 31-22 31-97 32.85 23 -9 24.52 24.88 25'14 25.48 25-78 26'13 26'68 26-82

a - x. 11.18 8-05 5 *a8 4.25 2-83 1.98

11-03 8.70 6.35 4-65 3-38 2.33 1 '23

10.80 10.20

9-50 9 '10 8 *OO 7.10 6 'OF

10.70 9'80 8-90 8-15 7 -55 7 *05 6.70

11-35 9.50 7 -80 6 '30 5.30 3.55 2-35

11.85 9.85 7 -28 6 -20 4'33 3 *58 2.70

11-16 10.54 10.18 9'92 9.68 9 -28 8 -93 8.40 8-26

l / t log ala - x.

0.00598 0.00581 0 -00583 0-00591 0.00600

0 *00606 0.00584 0.00577 0.00577 0 -00577 C.00591

0.000219 0-000218 0~000210 0~000199 0 *000203 0 '00021 0

0.000235 0-000199 0 000207 0*000205 0'000193 0*000185

0.00322 0.00332 0.00336 0.00337 G -0034 6 0.00358

0.00308 0.00326 0 -00320 0.00318 0*00318 0'00307

0'000145 0'000119 0.000105 0~000099 0 -000095 0 '000092 0~000102 0-000094

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OF SATURATED AND UNSATURATED ACIDS. 415

Ester. Ethyl crotonate.. . ,

Ethyl cinnamate .

Ester. Ethyl 8-phenyl-

propionate

aoncentration of HCl.

,.. 0-497N b = 127.75

b=127'55

.. 0.508N b=130'50

b = 130.30

Concentra- tion of hydro- chloric acid.

0.0510N b=24'40

0'0511N b = 24.45

Time in Titration, hours, t. b+z.

0 127-75 72 128.80

144 129.80 240 130.80 408 132.05

0 127.55 72 128'68

144 12950 265 130.65 312 130.95 432 132.00

0 13050 120 130'97 216 131.30 284 131-70 431 132.15 677 132.90

1008 133.55 0 130.30

120 130.77 216 131.10 384 131-40 431 131.75 725 132.65

1008 133-10

f, in hours.

0 24 50 5 96

144 213

0 28 50 90.75

163 ' 5 306

282

b+x. 24.40 24-90 25'42 26.20 26.85 27.27 27-72 24-45 25-00 25'42 26 *05 26.90 27-80

a - x. 6.33 - 5.28 0'00110 4'28 0'0011i3 3.28 0*00119 2-03 0 -001 21 6-33 - 5-20 0*00118 4.38 0'0011 1 3.23 0 -0011 1 2-93 0.00107 1-88 0'00122 5 -37 - 4.88 0*000344 4 '57 0 -000324 4-17 0'000382 3-72 0.000370 2 97 0 *000380 2.32 0'000361 5.37 - 4.90 0-000332 4.57 0-000324 4.27 0'000346 3.92 0*000317 3 '02 0.000344 2.57 0.000317

l/tloga/a - x.

e - It. 4'28 3'73 3 21 2.43 1 *80 1 '36 0.90 4 '21 3 $6 3-24 2.60 1 7 6 0 936

K = llt. eln logele - x.

0'00100 0*00104 0'00109 0'00113 0'00101 0.00105

0.00095 0 00100 0~00101 0'00102 0 '00099

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B. Saponification with. Dilute Barium Hydroxide Solution. Barium hydroxide solution = 0.02525N ; hydrochloric

acid = 0-02355N. t, 1 b ( a - z )

Ester. i i i hours. a - x. b - a. ~ = ~ ' ' g ~ ) Ethyl propionate . . . 0-0 19-80 21 '45 -

0.083 12.40 14-05 0'142 0.25 7 -30 8.95 0.130 0.50 0-75 1 '00 0.0 0.88 0.264 0 -50 0 '75 1 -00

4'35 3'00 2.20

19-80 12-25 7 *25 4.45 3.15 2'35

6 -00 4.65 3-85

21-45 13.90 8'90 6'10 4'80 4'00

0.127 0.125 0 *126

0-140 0.130 0,124 0'120 0-119

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416 WILLIAMS AND SUDBOROUQH :

Ester. Etliyl acrylate ......

Ethyl butyrate . . . . . .

Ethyl crotonate ...

Ethyl B-phenyl- propionate.

Ethyl cinnamate ...

t. i n hours.

0 'G 0.17 0.33 0 -52 0.77 0.0 0.083 0.27 0.52 0.77 0'0 0.083 0.267 0'55 1-15 0.0 0.083 0'28 0'61 1 *08 0 -0 0.28 0.60 1'57 2.62 0.0 0.27 0 *67 1 . T 1 2.70 0.0 0.25 0 -5 1'0 2'0 3.0 0'0 0.25 0.5 1.0 2 0 4'0 0.0 0.5 1'0 2-0 3 -0 4'0 0'0 0.5 1 *o 2 -0 3-0 4-0

n - x. 19-15 8 $0 5 '40 3 *70 2'40

19'15 12.00 6.55 3 70 2.40

19.75 14-90

9.79 6 -35 3-52

19.76 14.75 9.40 5.93 3.57

20.30 15-45 12.70 8-17 5.70

20 *30 15.85 12-25 7-67 5-60

16'62' 13'42 11 '34 8.87 6 22 4'72

16-62 13.62 11-47 8-87 6-17 3.52

16-50 14'80 13.35 11.35 9'SO 8 '80

16-50 14.75 13-45 11-35

9 *95 8.85

* Ba(OH),=0*0249N. f Ba(OH),= 0.0200N. t BdOH),=O.O197N.

HYDROLYSIS, Em., OF ESTERS

21 *45 11-10 7.70 6-00 4 -70

21-45 14'30 8 -85 6-00 4 *70

20.85 16-00 10-89 7'45 4 '62

20.85 15-85 10-50 7 -03 4-67

20-85 lt1*00 13.25 8.72 6 *26

20.85 16'40 12'80 8 *22 6-15

16-75 13-55 11-47 9.00 6.35 4-85

16.75 13.75 11-60 9 -00 6 -30 3 -65

16.70 15-00 13.55 11 -55 10 .oo 9 -00

16.70 14-95 13-65 11.55 10.1 5 9 *05

HCl= 0 '02387N. HC1=0*02387N. RCl= 0.02355iV.

- 0'134 0.137 0'136 0'138

0.140 0-133 0.136 0.138

0.0792 0,0774 0.0756 0.0744

0.0756 0*0786 0.0750 0.0780

0.0224 0 *0203 0 *0193 0.0197

0.0205 0'0199 0.0194 0-0194

0,0246 0.0231 0.0223 0.021 5 0.021 5

0.0246 0'0246 0.0223 0.0215 0.0238

0'00600 0'00600 0.00600 0*00600 0.00564

0.00696 0.00600 0*00600 0*00564 0 '00564

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OF SATURATED AND UNSATURATED ACIDS. 4 L7

These numbers give the following values for the constants of hydrolysis, using N-hydrochloric acid :

Ethyl propionate .......................... 0.1 19

,, butyrate ........................... 0.0657 y 7 crotonate ........................... 0.00225

8-phenylpropionate ............... 0*0201 ,, ciunamate ........................... 0 -000 ti72

and the following comparative saponification values for standard barium hydroxide :

.............................. y 7 acrylate 0-004 16

Ethyl propionate .............................. 5 *44 ................................. , , acrylate 5 '78

,) butyrate 3.213 ,, crotonate ............................... 0'842 , B-phenylpropionate 0.964 7, cinnaniate 0'255

The numbers for ethyl /3-phenylpropionate and ethyl cinnamate are not directly comparable with the values obtained for the other esters, as these esters were dissolved in a mixture of alcohol and water, and not simply in water. The numbers at once indicate that in the case of the three

pairs of compounds examined the ester of the saturated acid is hydrolysed by dilute hydrochloric acid about thirty times as readily as the corresponding unsaturated ester, but that when hydrolysed by dilute barium hydroxide the difference is not nearly so marked, the ratio of saponification of a saturated ester and its unsaturated analogue being about 4 to 1, except with ethyl propionate and ethyl acrylate, where the ratio is 1 : 1.1. This may be due in part to the fact that acrylic acid is a much stronger acid than propionic, the dissociation constants being respectively 0'0056 and 0*00134. The values for the dissociation constants of the other acids are : butyric, 0*00149 ; crotonic, 0*00204 ; /3-phenylpropionic, 0.00227 ; and cinnamic, 0,00355.

These results indicate that the relationship between hydrolysis of esters by acids and catalytic esterification is marked, and also that between direct esterification of an acid and hydrolysis of the esters by alkalis.

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THE EDWARD DAVIES LABORATORIES, THE INDIAN INSTITUTE OF SCIENCE,

ABEI~YSTWYTH. BANGALORE.

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