T H E VELOCITY O F SAAPOXIFICATIOXO F FATS AND OILS BY POTASSIGX H T D R O X I D E I X D I F F E R E X T SOLVENTS1 BY ERNEST ANDERSON A N D H. I,. BROWIS
There has been considerable controversy as to the exact reactions which take place during the saponification of glycerides.* The theory usually accepted is t h a t the alkali acts as an accelerating agent, and that no saponification can take place in the absence of all traces of ~ v a t e r . ~ All investigators agree t h a t such reactions as t h a t between ethyl acetate and aqueous potassium hydroxide are bimolecular, but the various investigators hold different x-iews as to the order of the reaction in the saponification of glycerides. The majority hold t h a t this reaction is bimolecular, * some, however, hold t h a t the saponification of certain glycerides is a tetramolecular reaction. I n this in\-estigation the points studied were: ( I ) The order of the reaction. 1 2 ) The relative \-elocity of saponification of different fats and oils under the same conditions. 13) The influence of the solvent on the yelocity of the reaction. (4) The temperature coefficient for the reaction. In addition to the above points the investigation throws light upon the influence of variation in the concentration of alkali and oil; upon the equilibrium finally reached, and other minor points. Method of Proeedure There are two accurate methods by which the velocity of saponification may be determined. I n this investigation Contribution from the Massachusetts Agricultural College.
* See Lewkou-itsch: Oils, Fats and LVartes, 1913 Ed., 76, for a list of references t o these controversies.
Lewkowitsch: Am. Chern. Jour., 43, 432 (1910). Geitel: Jour. prakt. Chem., 5 5 , 429 (1897); 57, 113-131 (1898). Kellner: Chem. Zeit., 33, 453, 662, 993 (1909); Jour. Chem. SOC., 96, 357, 548, 759; Balbiano: Ber. deutsch. chern. Ges., 36, 1571 (1903); 37, 155 (1904);Marcusson: 39, 3466 (1906).
196
Ernest .-indersol.z and H.L. Browiz
the volumetric method was used. It is very accurate though possibly not so accurate as the conductivity method used by Walker,l Stieglitz and Derby.? The exact procedure followed in the investigation is given below. A certain volume of oil, usually j cc or I O cc, , ~ placed was made up to IOO cc with the desired s o l ~ e n t and in the thermostat. At the same time an equal volume of potassium hydroxide in the same solvent was placed in the thermostat. After the two solutions had reached constant temperature, they were mixed. At suitable intervals, which depend upon the velocity of the particular reaction, I O cc of the reaction mixture were withdrawn, and run into an excess of N/IO hydrochloric acid, the time being accurately noted. The excess hydrochloric acid was then titrated with N,'Io barium hydroxide, using phenolphthalein as indicator. This titration was made in jo cc of ethyl alcohol so that the final mixture would, in all cases, be clear, thus making possible a delicate end point.
The Results Obtained in the Investigation
I. The saponification of fats and oils by potassium hydroxide is a bimolecular' reaction under the conditions of this investigation. The velocity of saponification of five different oils was determined as follows: Five cubic centimeters of the oil were saponified by 2 0 0 cc of 2 I O normal amyl alcoholic potassium hydroxide a t 25' C. Substitution of the velocity data Chem. News, 94, 238 (1906). Am. C,hem. Jour., 31,449 (1904). 3 The solvents used were methyl alcohol, ethyl alcohol and amyl alcohol. I n each case they were purified by treatment with a small amount of silver nitrate and considerable solid potassium hydroxide. After standing 24 hours they were filtered and distilled. Lewkowitsch (Oils, Fats and Waxes, 1913 Ed., Vol. I, 103-4), gives the various methods used in purifying alcohol for use in work such as described in this article. 4 Geitel: Jour. prakt. Chem., 5 5 , 429 (1897); 57, 113-131 (18g8), studied the hydrolysis of triacetin by dilute hydrochloric acid and the saponification of cottonseed oil by alcoholic potassium hydroxide, and decided that they were bimolecular reactions.
L'elocity o j Sapotfi$catio?i o j Fats aizd Oils, Etc.
197
in the various reaction formulas gave constant values with only the bimolecular formula, hence the reactions were all bimolecular. See Table I . Two cubic centimeters of methyl acetate were saponified under the same conditions, and gave a similar constant. This is a well known bimolecular reaction.
TABLE Evidence that the Reaction is Bimolecular. The constants in this table were obtained by substituting the experimental data from j cc oil and zoo cc 2 / 1 0 normal alkali (amyl alcohol) in the bimolecular reaction formula
_
_
~ ~
~~
~
~
~ _-_ ~ _ _
______~
Butter fat
l
Castor
Cottonseed
_ _~
~.
~-
~~~~~
Olive
Croton
Methyl acetate
I
0.001750
0.001529 0 . 0 0 1 7 2 0 0.00I 633 0.001789 0.005496 0.00171 I 0.0016I 6 0.001658 0.00j 363 0.001635 0.001633 0.001609 0.001611 0.0056I 0 O.OOIjg5 0.001614 0.001j 5 2 0.001575 0.005357 0.001572 0.001627 0.00I 606 0.001503 O.OOj303 0.00 I 5 5 5 0.00I 7 98 0.001521 0.00I 464 0.005 335 0.001556 0.001744 0.001535 0.001445 O.OOIj79 0.00I 768
~0.001801 o.oo1j37
0.001746 0.001773 0.001756 0.00I 7 69 0.001798
l
-
-
-
0.001466 _
Average o.oo17jo
_
0.001j57
-
_
0.001698
-
~ 0.001605
0.001578 0.005411
The average saponification value of the oils used is about Therefore, although it would require only 0 . 9 1 gram of potassium hydroxide to completely saponify the average oil, there were 2 . 2 4 grams of potassium hydroxide present. Determinations were also made in which 5 cc of oil were saponified by 2 0 0 cc of 4;I O normal amyl alcoholic potassium hydroxide. In this case there were 3 . 5 7 grams of potassium hydroxide in excess of the 0.91 gram actually required t o saponify the 5 cc. of oil. The results are given in Table 2 . The reaction proceeds quite rapidly, but the results are as nearly constant as could be expected. 200.
The experimental data and method of calculation for each table are given under the corresponding heading a t the close of the paper.
Ernest A n d e r s o n and H. L. Brown
198
TABLE 2 Evidence that the Reaction is Bimolecular. The constant in this table was obtained by substituting the experimental data from 5 cc castor oil and 2 0 0 cc 4/10 normal amyl alcoholic potassium hydroxide in the bimolecular reaction formula -
~-
_ _ ~ _ -~ ~
~
-
Castor oil
0.002175 0 002222
0.0020j2
o 002034 0
Average o
002125 002
149
m'hen I O cc of castor oil were saponified by 2 0 0 cc of normal amyl alcoholic potassium hydroxide, there was present about o 42 gram of potassium hydroxide in excess of the amount actually required by- the oil. The constant obtained in this case is given in Table 3 . 2
IO
TABLE 3 Evidence that the Reaction is Bimolecular. The constant in this table was obtained by substituting the experimental data from I O cc castor oil and zoo cc 2/10 normal amyl alcoholic potassium hydroxide in the bimolecular reaction formula
-
~ -_-~ _-__ - __~
Castor oil
0.001549 0.001541 0.001j j 7 0.00158 I 0.001554
0.001532 0.0015 I 7
0.001536 Average 0.001546 The results given in Tables I , 2 and 3 seem to show conclusively that the reactions are bimolecular under the three concentrations investigated. The constants for the glycer-
ides compare favorably with t h a t obtained for methj-1 acetate which is without a doubt a bimolecular reaction. 11. The fats and oils investigated are all saponified a t approximately the same rate.' The data given in Table 4 are the average of the constants obtained by saponifying j cc of the oil or fat in 2 0 0 cc of 2 / I O normal amyl alcoholic potassium hydroxide It is evident t h a t butter fat is saponified a little more rapidly than the other four glycerides. TABLE 4
The Saponification Constants for Different Oils and Fats under the Same -Conditions are Approximately the Same ~~--
~
Butter fat -
0.001770
Croton
Cottonseed -
-
-
0.001550
Olive -~
~-
~-
o.oo16oj
0.0015j8
Castor
~-~
0.001692
The mean molecular weights of the fatty acids present in the above four vegetable oils are approximately the same. If arranged in the order of their molecular weights i t will be seen t h a t the velocity increases with the molecular weight.
TABLE 5 The Velocity of Saponification Increases with the Molecular Weight ~~-
-
Oil -
~~
Cottonseed
Croton
Olive
-
Mol. wt. Vol. const.
~
2i4 0.0015jo
*i9 0.001578
The relation indicated in Table
_
- __ _ ~ -
-~
Castor
_
291 0.001605 j
-
2 93 o 001692
ma>- or may not be due
So far as the authors are aware, no one has compared the velocity constants of saponification of different fats and oils up to the present time. The results obtained here v-odd be expected from the work of Thum: &it. angem-, Chem., 1890,~Sz. and Henriques: Jour. SOC.Cheni. Ind., 1898,673and Sjg;Urbain, Saigon and Feige: Bull. SOC.Chim.. 31, I 194, deril-ed the conclusioii from their esperiments with thr castor bean ferment, that the saponillcution of the yarious glycerides proceeds a t the same rate. On the other hand.H d l e r : Compte.; rentlus 143, 660 ;1906), is of the opinion that glycerides of the Irnrel- iatty acids are more rendil!- li~-drolyzedthan t1io.e of higher fatty ncitl-. See preceding note for the conclucioiis o f Haller,
Ernest d n d e r s o u aizd H .L.Browiz
2 00
t o a coincidence. It would of course be possible t h a t experimental error was the cause of this apparent relation. 111. The velocity of saponification reactions is different in different soh-ents. The evidence supporting this statement was obtained b y saponifying castor oil in three different alcohols. Three different concentrations of oil and potassium hydroxide were used in each alcohol. The proportions were as follows: (a) Five cubic centimeters of oil with 2 0 0 cc of 4 I O normal alcoholic alkali. ( b ) Five cubic centimeters of oil with 2 0 0 cc of 2 I O normal alcoholic alkali. (c) Ten cubic centimeters of oil with 2 0 0 cc of 2 I O normal alcoholic alkali. The constants obtained when using j cc of oil with 2 0 0 cc 4 I O normal alcoholic potassium hydroxide are given in Table 6.
TABLE6 Relative Speed of Saponification of Castor Oil by Alkali in Different Alcohols _ _
4/10 ~
LIethyl -~
1
Ethyl ~
-
~
0,001I73
0.002222
0 . 0 0I I35 0 . 0 0 1 I 65
0.002034 0.002052
o.001179
0.002034 0.002 I 2 j
-
Average ,o.0001020 Ratio 1 I ,
0.001 2 14 0.001166 0.00120j 0.001177 11.5
~~
~~
0.00010j I
0.0001010 0.0001010
~
Amyl
0.0001079 0.0001 104 0.000I002
Korrnal
-
0.002149 21
1 The influence of the solvent on the speed of chemical reactions in general is very great. Menschutkin: Zeit. phys. Chem., 6, 41 ; Patterson and Montgomerie: Jour. Chem. SOC.,101, 2 1 0 0 ; Dimroth: Liebig’s Ann., 399, 91;Halban and Kersch: Ber. deutsch. chem. Ges., 45, 2 4 1 8 ; Halban: Zeit. phys. Chem., 67, Some investigators vary the composition 129; Jour. Chem. SOC.,96 11, 722. of the solvent in the saponification of fats by the addition of other solvents. Thus Geitel, Hehner, Duclaux, and others, have added ether to the mixture. Henriques: Jour. SOC.Chem. Ind., 1896, zgg and 476, dissolves the fat in petroleum ether, and mixes this solution with alcoholic soda solution.
Ircloiity
0.t
Sapoii?iiatio?? oj Fats a n d Oils, Etc.
201
U'hen 5 cc of oil were saponified by 2 0 0 cc of 2 I O normal potassium hydroxide, the constants given in Table 7 were obtained . TABLE T Relative Speed of Saponification of Castor Oil by 2 1 IO Kormal -Alkali in Different *Alcohols LIethyl
0.00006333 o.oooo63S~ o 00006218 0.000063j7 0.000062~4 0.00006301 -
Xrerage o.00006322 Ratio I
Ethyl
-Amyl
0 . 0 0 0 S 12j
0,001623
0.000786j 0.0007S94 0.0008039 0 .o o o s o ~ ~ 0.0008044
0.00160 j
0,000s002
0,001623 2j.6
12.6
0,001615 0.00166j 0 , 0 0 164j o.001603 0.001607
TT'hen I O cc of oil were saponified by 2 0 0 cc of 2 I O normal potassium hydroxide, the results given in Table 8 mere obtained. TABLE 8 Relative Speed of Saponification of Castor Oil by 2 , IO Normal *Alkali in Different -Alcohols
~-
LIethyl
Ethyl
Amyl
0.00007046 o.0000~115 0.00006864 o 0000695S 0.00007194 0.00006918
0.0008002
0.001 549
0.0008 I 19
0 , 0 0 1j4I 0 , 0 0 1jj7 0.001 581 0.001 j32
O.OOOOjO74
0,0007939 0.0007987 0.000799I 0.0007943 0.0007968
-
Average 0.00007023 Ratio I
O.OOOjg93 11.3
0.001jj4 0.001 517 0.001 j36 0.001j46 20.I
T h e average ratio obtained'frorn the above tables is I : 11.8: 22.8. T h e reaction is much slower in methyl alcohol than in either
of the other alcohols and the ratios obtained are fairly constant. If the constants for ethyl and amyl alcohols are compared, the ratios are inore nearly constant as is shown in Tables 9, I o a n d 1 1 . Tables 9, 10 and 1 1 . Relative Speed of Saponification of Castor O i l in Different Aleohols TABLE 9 Five Cubic Centimeters of Castor Oil in 4 1 I O Kormal Alkali -~
Average constant Ratio
Ethyl
dmyl
0.OOIIjj
0.002149
I
I
TABLEI O Five Cubic Centimeters of Castor Oil in
Average constant Ratio
o
.S3
Sormal -1lkali
2/10
0.001623
000S002 I
2
TABLE 11 Ten Cubic Centimeters of Castor Oil in
2/10 ~
Sormal Alkali- _ _ _
__
Ethyl
.lrnyl
Average constant 0.0007993 O.OOIj49 Ratio I I 9 As a check on the influence of the soh-ent upon the velocity of saponification the results obtained by saponifying z cc of methyl acetate by 2 0 0 cc of 2 I O normal potassium hydroxide in the different alcohols is given in Table 1 2 . If the ratio of the constants in ethyl and amyl alcohol are determined the result is the same as for castor oil. See Table 13. Tables 12 and 13. Relative Speed of Saponification of Methyl Acetate in Different Alcohols TABLE1 2 Methyl
Ethyl -~
~
Average constant Ratio
o 0001769 I
o 002795 I j
8
Amyl ____
0.005339
30. I
T’slocity of Sapoyz&-atioii
o j Fats aizd Oils, Etc.
TABLE13 ~
~
-
~
~
__
~~
~
~~~~~~
~
Ethyl __
-
~~
~~~~
203
~~
Amyl
~~~
Average constant Ratio
0.0027gj
0
I
005339 1.9
From the various data it would seem that the velocity of saponification is about ten times as great in ethyl alcohol as in methyl alcohol, and about twice as great in amyl alcohol as in ethyl a1cohol.l It might, therefore, be possible t o calculate the velocity of an>- saponification reaction in these alcohols if the velocity of that reaction in one of the alcohols is known. However it will be seen later t h a t the water content of the alcohol is a factor which would have t o be considered when calculating a velocity constant. IV. An increase of approximately one percent in the amount of water present causes an increase of about four percent in the velocity of saponification of castor oil a t 2 j O C. The evidence supporting statement IV lies in the results obtained with methyl and ethyl alcohols of different strengths. Only one strength amyl alcohol was used because t h a t alcohol dissolves only a small amount of water. The results of methyl and ethyl alcohol check each other as closely as could be expected. The results are given in Tables 14 and 15.
Tables 14 and 15. Relation between the Percent Water i n the Alcohol and the Speed of Saponification of Castor Oil TABLE14 _
_
-
~
~
Methyl alcohol ~~
-
~
Constant ~~
g21/27c by weight
o ooo16zo
by weight
0.0001757
gI
The velocities of other chemical reactions in these three solvents would not have the same ratios found here. See Patterson and blontgomerie: Jour. Chem. Soc., 101, 2 1 0 0 (1912).
Ernest A n d e r s o n and H . L. Brown
204 _
_
_
~ ~~~
~
_
TABLE15
Ethyl alcohol ~~~
_
Density of alcohol
gr1/2% by 891/270by 871;270 by 84l/2% by
Constant -
~
weight weight weight weight
o 8171 o 8238 o 8289
~~~
0
~
000952
0 001022
o 001031 0 0012g8
0.8370
The data in Table 14 show an increase of 4 4 percent in the velocity constant for an increase of one percent in the water content of methyl alcohol. More data were obtained for ethyl alcohol than for methyl. The data in Table 15 show an average increase of 3 . 8 percent in the velocity constant for every increase of one percent in the water content of the alcohol. V. Doubling the concentration of alkali causes an increase in the velocity of reaction, but the increase is not the same in alcohols of different strengths. The evidence supporting Statement V was obtained by using two different concentrations of alkali in the three solvents, methyl, ethyl and amyl alcohol. It is evident t h a t the water content of the alcohol is connected with the effect produced by variation in the strength of the alkali.
TABLE16 Relation between the Strength of the Alkali Csed and the Speed of Saponification of Castor Oil in Different Alcohols Methyl Alcohol ~ ~~
I
Constant with z normal alkali
Strength of alcohol __
-~
95% 921/2% 911/2%_' l ~
~
~
I
~
~
~~~
-
~~
61
0 0001020
::::$;
2%
20%
~
0001338 _ _ -~ Ethyl Alcohol 0 001177 o 0008002 o 001031 0.0009487 ~ _ _ _________ Amyl L4kohol (Density 0.8140) o 001623 0.002149 1 0
-~-
~
~~
o 0000632 o 0001401
g1~/2% 1 8g1/z?C I -
Constant wlth 41 I O
IO
Increase in velocity due to doubling alkali
13%
--7 ~
47 1 % 7.7% 32.4%
Velocity of Saponificatioz of Fats and Oils,Etc.
Strength of alcohol
Constant with 5 cc castor oil
~
Constant with cc castor oil
IO
20j
Increase in velocity due to doubling the oil ~
~~
-
Jones: Zeit. phys. Chem., 31, 114 (1899), has shown that the ionization of salts is different in different alcohols.
206
Ernest d n d e r s o u a n d H . L. Brown
the amount of oil is increased. This fact is possibly due to the formation of considerable glycerol during the reaction, which might cause a decrease in the velocity due to the change in solvent. It is well known that products of reactions do occasionally influence the velocity of the reaction. The glycerol might increase the velocity in one solvent and decrease it in another solvent. VI. An increase of 10' C in the temperature a t which the reaction is carried out causes castor oil t o be saponified 2 . 3 6 times as fast by ethyl alcoholic potassium hydroxide.' Castor oil was saponified by ethyl alcoholic potassium hydroxide a t I j " C and 2 j 0 C, all other factors being exactly the same. The results are given in Table 18.
TABLE 18 Relation between the Temperature and Speed of Saponification of Castor Oil by Ethyl Alcoholic Potassium Hydroxide ______-~~ _ _ ~ _ _ _ _ _ _~~~~~ ~ ~
'
Constant a t
Ij
0
0.0004527 0.0004456 0.0004538 0.0004450 0.0004478 0.0004479 0.0004424 0.0004479 0.0004386 0.0004437 0.0004482 -
Average
0.0004417
c
~
'
~- _. ~~~~
Constant a t
~
2j
0
c
0.001073 0.001077
0.0010j6 0.0010j4 0.001062 0.001018
0.001047 0.001069 0 001066 0,001047 I
0.001015
0.001013 0.001053
The temperature coefficient, K 2 5 O K 15", for this reaction is 2 .36, which is about the average coefficient for ordinary reactions. VII. The reaction does not go to completion but only t o equilibrium. Trautz and Volkmann: Zeit. phys. Chem., 64, j3; Jour. Chem. Soc.; 94 11, 824 (1908); Harcourt and Esson: Jour. Chem. SOC.,102 11, 923 (1912).
I-elocifj, o j Supoizi$catio?i o j Futs aud Oils, Etc.
207
The correctness of Statement VI1 is easily seen from the difference in the end titrations when oil is saponified by s-arying amounts of alcoholic potassium hydroxide. The more concentrated the alkali, the further the reaction is carried along. The figures gil-en in Table 19 are the number of cubic centimeters of tenth normal potassium hydroxide absorbed by I O cc of the reaction mixture.
TABLE19 Relation between the Concentration of -Alkali Present in the Solvent and the AAmountUsed up at Equilibrium Cubic centimeters of Cubic centimeters of z I O normal KOH 4 I O normal KOH used up by oil in I O used u p by oil in I O C C of the mixture cc of the mixture ~
~
~~~~
~
Amyl alcohol Ethyl alcohol Methyl alcohol
~
7.64 7 .j I 8.64
Difference
~
9.48 9.07 10.23
1.74 I .56 1.59
Summary of Results I . The saponification reactions under all conditions studied are bimolecular. 2 . The glycerides studied are saponified a t approximately the same rate. 3. An increase of I ~ in C the water content of the alcohol causes an increase of about 4 5 in the velocity of the reaction. 4. The yelocity of saponification of glycerides in amyl alcohol of density o 8140 ( I j 5 0 " ) is about twice as great as it is in ethyl alcohol of density- o 8171(15 50'1, and about ten times as great in this ethyl alcohol as it is in methyl alcohol of density o 8100 (15 5 0 " ) . 5. The temperature coefficient of the reaction K 2 5 O 'K 15 " is z 36 in the case where castor oil is saponified in ethyl alcohol. 6. The x-elocity is greatly accelerated b y increasing the
amount of alkali, but no definite ratios could be determined from the data. j . n'hen amyl alcohol of a density o 8 1 4 is ~ used as the solvent, an increase in the amount of oil causes a decrease n'hile in the case of methyl in the velocitj- of reaction. and ethyl alcohols there seems to be an increase in the 1-elocity due to the increase in amount of oil. Experimental Data and Calculations
9 s described abox e, in carrying out a determination, the oil dissolved in the soh-ent is mixed with the alkali dissolved in the sol\-ent. Xt intervals, the volume of I I O normal alkali remaining in I U cc of the mixture is determined as follows : Ten cubic centimeters of the mixture are run into excess, either 20 cc or 40 cc of I I O normal HC1. The excess acid is then titrated with I I C normal BaiOHj? The difference between the volume of I I O normal HC1 used and I I O normal Ba(OH)? used gives the 1-olumes I I O normal alkali present in I O cc of the mixture. This value is given in the second column in the experimental tables below. The second order reaction formula used in all of the calculations is 2.302 j T,(T, - Te) K = T, t log T~(TFT:)
K
Velocity constant. I I O normal alkali present in I O cc of the mixture a t first titration. T, = Volume of I I O normal alkali present in I O cc of the mixture a t the end of the reaction. T, = Volume of I I O normal alkali present in I O cc of the mixture a t time t. t = Time in minutes after the first titration. I n the experimental data below To is the first value and T, the last value under the second column. =
To = Volume of
1Flocitj, ot Sapoiz
