338
INDUSTRIAL A N D ENGINEERIXG CHEMISTRY
Acknowledgment This work was carried out under a p a n t from the Columbus Union Oil Cloth Company of Columbus, Ohio, acknowledg-
Vol. 21,
30.
4
ment of which is cheerfully and gratefully made. The writers also wish to acknowledge help from Lawrence K. Scott.
Atmospheric Oxidation of Esters of Beta-Eleostearic Acid with Monohydric Alcohols' A. B. Miller and K. L. Rohrbach ARVSTRONG CORK COMPANY, LANCASTER, PA.
The propyl, isopropyl, butyl, and benzyl esters of peleostearic acid have been prepared. These esters oxidize rapidly and completely in a manner similar to the polyhydric alcohol esters, but do not dry as they do. Splitting forms simpler acids which have a tendency to add to the ester molecule. Hydrolysis splitting off p-eleostearic acid occurs to a limited extent. Associates of several molecules of the oxidized ester are dissolved, in most cases, in a dispersion medium of single oxidized molecules. The acids freed from t h e oxidized esters are largely di- and poly-basic. The constants of t h e end products vary, in general, with the number of carbon atoms in the straight-chain alcohol esters and are regularly higher or lower in t h e case of isopropyl and benzyl esters. This is attributed to the relative tendency of the esters to hydrolyze on oxidation, produc-
ing p-eleostearic acid. These tendencies to hydrolyze are reversed in alkali hydrolysis. There is little neutral and no acid hydrolysis. Since these esters form abundant strong acids on oxidation, hydrolysis may be inhibited by the acidic medium. Methyl eleostearate oxidized in the presence of peleostearic acid gels, the beta acid forming an addition product with the ester molecule. Stearic acid does not combine under these conditions. The work described in this paper checks the writers' hypothesis t h a t a n important function of t h e alcohol group in a n ester of p-eleostearic acid is the stability it imparts to the ester, thus determining t h e tendency to split off peleostearic acid, which through addition and aggregation of its long carbon chains actively influences gel formation.
..............
I
N THIS paper the conditions for drying are studied by
comparison of the non-drying and the drying derivatives of p-eleostearic acid. This study has been undertaken as a part of a general program on the atmospheric oxidation of tung 0il,~?3 the plan of which is to establish to some degree the chemical reactions and subsequent molecular structures which build the colloid systems, drying-oil gels. It has been known for some time that the esters of p-eleostearic acid (and other drying acids) with monohydric alcohols do not possess the capacity to dry as do polyhydric alcohol esters, such as the glycerides, occurring in drying oils. This phenomenon has been studied in connection with polymerization,4,6#6and the conclusions have been that, though a monohydric alcohol ester may form a dipolymer by union of double bonds. the low molecular weight of t,he resultine: polymer prevents gelation14and that there is no opportunity for splitting and reesterifying a dipolymer of the eleostearic acid to two molecules instead of one of a polyhydric alcohol (glyptal reaction).6 KO thorough study has been made of this phenomenon when the oils are oxidized, but it is generally considered that the reaction is similar.
-
Materials
TUNGOIL-American tung oil from the 1927 crop a t Gainsville, Fla., was used. It had an unusually agreeable odor and gave slightly larger yields of a-eleostearic acid than the foreign 1 Presented before the Division of Paint and Varnish Chemistry a t the 76th Meeting of the American Chemical Society, Swampscott, Mass., September 10 t o 14, 1928. 2 Paper presented before Division of Paint and Varnish Chemistry a t the 73rd Meeting of the American Chemical Society, Richmond, Va., April 11 to 16, 1927. 8 Miller and Claxton, IND.ENC. CHEM., 20, 43 (1928). 4 Nagel and Gruss, Z. angew. Chem., 89, 10 (1926). 5 Fonrobert and Pallauf, Chem. Umschau, 83, 1, 41 (1926). 6 Morrell, J . Chem. Soc., 101, 2082 (1912).
oil, though the method usedl gives far from a quantitative yield. The oil showed the following constants: 0.9371 1.4949 2.15 172.5 169.6 12.5
Specific gravity 2Oa/2O0 C. Refractive indek a t 20' C. Acid number Saponification number Iodine number (Hiipl) Brown heat test, minutes Ultimate analysis, per cent: Carbon Hydrogen Oxygen
76.6 10.8 12.6
P-~LIETHYL ELEOSTEARATE-This was prepared by the esterification of a-eleostearic acid (m. p. 44.6-45.6' C.) with c. P. absolute methanol in the presence of 3.6 per cent dry hydrochloric acid a t room temperature without agitation. It was purified by washing with dilute sodium carbonate solution and calcium chloride brine. Saponification yielded p-eleostearic acid (m. p. 65-66' C.) This ester had the following constants: DETERMINED THEORETICAL^ Iodine number 186.5 173.2 5.44 0.0 Acid number SaDonification number 171.0 190.5 a The theoretical constants are calculated on the basis of three double bonds for eleostearic acid a s recently determined by Boeseken and by Kaufman. Iodine number is calculated for only the two reactive bonds.
Another batch of the methyl ester was prepared for use in the hydrolysis experiments. It had an acid number of 8.4 and a saponification number of 171.0. P-PROPYL ELEOSTEARATE-The ester was prepared by esterifying a-eleostearic acid with n-propyl alcohol (b. p. 96-98' C.) in the presence of 2.02 per cent dry hydrochloric acid overnight at room temperature. The crude ester was washed with dilute sodium carbonate solution and calcium chloride brine, then centrifuged. Saponification yielded 0-eleostearic acid (m. p. 68-69 O C.)
.
7
Kametaka, J . Chem. Soc., 68, 1042 (1903)
339
INDUSTRIAL A N D ENGIA'EERING CHEMISTRY
April, 1929
T a b l e I-Data
on Esters during Oxidation at 8Z0 C. ~
N ~ T~~~ .
Iyl$iE
Hours &Propyl eleostearate: 1 0 171.8 (158. 2 ) 5 2 1 ... 3 3 61.8 4 6 53.6 5 24 31.4 6 48 21.8 &Isopropyl eleostearate: 1 0 159.6 (158.2) 2 1 ... 3 3 73.9 4 6 50.1 5 24 25.9 6 48 21.1 @-Butyleleostearate: 1 0 149.3 (151.7) 2 1 109.0 3 3 62.2 4 6 41.3 5 24 23.2 6 48 20.5 p-Benzyl eleostearate: 1 0 101.2 (137.8) 2 1 ... 3 3 62.7 4 6 49.8 5 24 38.8 6 48 35.3 a
ACID
NO.
SAPONIFICA- p~ TION N O .
OXY
ACIDS
ULTIMATE ANALYSIS
MOL.
~
WT.
0
70
70
92 13.3
... ... ...
1.4900 1.4780 1.4759 1.4773 1.4780
... ... ... ... 4463
11.1 (11.4)
1.4940
33.6
(0.0)
437 (320.4)
...
72:7
... ...
1.4901 1.4810 1.4788 1.4792 1.4805
20.550 24.9
17.7 3.0 15.1 30.9 57.2
3.4
6i:g
1118
8.9 (0.0) 2.1 5.7 22.8 42.5 49.1
143 (167.8) 170 207 234 268 314
6.4
3.2 (0.0)
427.5 (334.4)
1.4947
...
6;:75
...
3.8
5i:O5
1.4873 1.4795 1.4770 1.4787 1.4801
2.7 (0.0) 0.7 13.0
124 (152.1) 76 97.5 147.0 161.0 224.0
7.2
291.5 (368.4)
...
42:O 59.7
H
7.8
... ... ...
4.1
...
6.4
...
...
6.8
...
7.4
..
8618 si:9 6.55
..
..
2.31 (0.0)
..
... ...
5318
4.2
6i:o
...
,
..
712
... ...
... ... ...
1838
... ...
ioso:o
... .. .. .. ... .. .. ..
.... ....
....
.... ....
~~
REMARKS
~
75.6 (78.6)
5.5
1,o.0 )
~
CP. 354.5 (320.4)
199 (175.0) 177.1 156 212 282 285.5
23.0
~
12.4
4.0 (0.0)
185.5 (175.0) 179.5 199.0 252.0 299.5 340.0
0.4 1.0.1 19.8 46.3 48.3
O
1.4982
70 5.8
$
C
7.3
1:o. 0)
;
(10.0)
.... ....
.. ......
Light yellow oil Bleached
....
.... .... 9.3
28.3
Dark yellow, viscous oil
75.0 (78,6)
10.6 (11.4)
14.4 (10.0)
DarkIoil
.... .... ....
Bleached
62.4
....
.... .... .. .. .. ..
24.0
Yellow viscous oil
78.9 (78.9)
12.25 (11.5)
8.85 (9.6)
Dark-oil
.... .... ....
Bleached
....
.... .... ....
66.3
9.7
....
.... ....
.... .... ....
5448
63.9
9.3
27.4
Yellow viscous oil
1.5312
22.2
9.2 (9.9)
13.9 (8.7)
Dark oil
1.5251 1.5217 1.5197 1 5213 1:5220
...
76.9 (81.4)
.... .... ....
....
Bleached
...
... ... 16,iiO
....
.... .... .... ....
68.1
8,5
.... ....
23' 4
Dark viscous oil Dark viscous oil
Figures in parentheses are theoretical constants.
P-ISOPROPYL ELEOSTEARATE-This was prepared by esterifying a-eleostearic acid (m. p. 45" C.) with isopropyl alcohol (b. p. 81.8" C.) and 1.47 per cent dry hydrochloric acid in a three-necked flask a t 80-90" C. using a motor stirrer with mercury seal and a reflux condenser. The material was washed, but a n attempt a t distillation in vacuo resulted in a partial decomposition. Saponification yielded impure /?-eleostearic acid (m. p. 59-63.3" C.). Examination of the constants (Table I) shows that the material contains about 10 per cent uncombined p-eleostearic acid and some oxidized material. ,%BUTYL ELEOSTEAIIATE-This was prepared by esterifying a-eleostearic acid with n-butyl alcohol (b. p. 116" C.) and about 5.0 per cent dry hydrochloric acid at 100-115" C. using the same apparatus as used in the case of the iso-propyl ester. It was washed with dilute sodium carbonate solution and calcium chloride brine. Saponification yielded impure 8eleostearic acid (m. p. 53.2-55.6" C.). ,%BENZYLELEOSTEARATE-This was prepared by esterifying a-eleostearic acid with benzyl alcohol (b. p. 92-94" C. and 8 mm.) in the presence of about 5.0 per cent hydrochloric acid at room temperature overnight. The ester was washed with dilute sodium carbonate solution and calcium chloride brine and then centrifuged. Saponification yielded p-eleostearic acid (m. p. 67.5-69" C.). The material contained 17.2 per cent free benzyl alcohol which was practically all evaporated the first hour by the strong stream of air through the batch. P-ELEOSTEARIC ACID-This was prepared by allowing bottles of the alcoholic solution of a-eleostearic acid to be exposed to direct sunlight and the crystalline precipitate filtered and dried in the presence of carbon dioxide. It had a melting point of 66-68.5" C. P-ELEOSTEARIN-The precipitate was removed from lightstruck tung oil and repeatedly recrystallized from acetone. It had a melting point of 59-63.3" C., and acid number of 10.0, and a saponification number of 193.7. STEARIC ACID-V. S. P. acid havinga melting point of 54.7" C. was used. The other constants for these materials are given in runs S o . 1 of Table I.
Analytical Methods Standard methods were used wherever applicable. Iodine number was determined by the Hiibl method, dibutyl phthalate being used as a solvent in the case of the methyl esterbeta-acid gel; the sample was warmed in 10 cc. to solution. Oxy acids were determined by Fahrion's method. Molecular weight was determined cryoscopically using stearic acid. Ostwald pipets were used to measure viscosity. Hydroxyl groups were estimated on the basis of increase of weight on acetylating a t 30" C.8 Since previous work in this laboratory showed that these substances gave off little volatile matter upon oxidation, ultimate analysis was used t o determine the amount of oxygen absorbed. H-ion concentration was determined by an empirical method described in a previous paper.3 Total acids were freed in the usual manner, extracted with ether, which was driven off in vacuo, and then the water was removed by adding absolute alcohol and evaporating the constant-boiling mixture on a water bath in vacuo. Petroleum ether extractions were made in Soxhlet extractors over a 3day period. Combined alcohol was determined by the regular method of saponification with aqueous potassium hydroxide and distillation into a potassium carbonate brine; the percentage of alcohol in the alcohol phase of the distillate was estimated by means of refractjive indices. Alkali and acid hydrolysis were determined by the usual method; neutral hydrolysis was carried out by bubbling steam through toluene solutions of the samples in flasks fitted with reflux condensers. The solutions were titrated after being given this treatment various lengths of time. Experimental Procedure The apparatus described in the previous paper3 was used to oxidize the esters. It affords means for blowing air of constant humidity (10 per cent a t 25" C.) through a vigorously agitated batch. The air volume is held constant and the reaction is carried on in a thermostatically controlled bath.
* Ellis, J. SOC.Chem. I d . , 46,
193T (1926).
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 21, No. 4
been split a t open bonds. The original sample contained 18.75 per cent combined alcohol by calculation; analysis of the end sample yielded 16.48 per cent which, considering the addition of oxygen, points to little or no splitting of the ester (calculated on this basis, 15.92 per cent). p-ISOPROPYLESTER-AS in the previous batch, the oxidation is rapid and the iodine number OXlDlZfD A T B Z ' C reaches the same minimum. The acid number drops owing to combination of the free betaOXIDIZED AT 82'C acid, then rises as a result of splitting. The free beta-acid evidently contributes to the building of the molecular complexes which give the final high molecular weight and viscosity. The rise of saponification number indicates formation and addition of split acids t o the ester molecule up to 24 hours. The low p H of the r,m. L.. rs Trme h w r r end sample is probably one of the factors in the Figure 1 FIgure 2 dispersion of the end colloid. The oxy acids rise quickly and then decrease, probably owing to simplification producing petroleum ether-soluble acids; Product Application this is also reflected in the change of refractive index. The The heavy oils resulting from the oxidation of these esters molecular weight of the end sample corresponds t o an average were used as plasticizers in lacquers with ethyl acetate and aggregate of 2.5 ester molecules. Ultimate analysis gives an alcohol as solvents. The films were inferior in appearance average of 12.7 molecules of oxygen taken up by each molewith high tensile strength and low elongation. I n a lacquer cule of the oxidized ester. Its total acids had a molecular using butyl acetate, butyl alcohol, and toluene as solvents the weight of 435.1 with an acid number of 292.7, which would oxidized butyl ester was a satisfactory plasticizer. The film give a calculated molecular weight of 382.0 if they were dihad a tensile strength of 73 kg. per square centimeter and an basic acids. elongation of 25.5 per cent. Aging tests were not made. Acetylation of the end sample gives 1.49 per cent OH in the The oxidized butyl ester showed solvent powers for nitro- end sample, indicating enol or keto1 formation such as reported by Ellis in the case of linseed oil oxidation.8 The 3.84 cellulose. A good piece of linoleum was made using the oxidized per cent OH found in the total acids indicates combination methyl ester-beta-acid mixture. It matured in three-fourths the time required by a control a 80 15350 sample. Oxidation of Esters Data obtained a t different periods during oxidation are given in Tables I and I1 and Figures 1 t,o 4, and data on the oxidized esters and zo6 ,5200 their products are given in Table 111. P-PROPYL Esrm-This ester oxidizes with vigorous, complete oxidation. The iodine number drops exponentially to 21.8; acid number drops the first hour owing t o combination of the free acid, then rises to 48.3 by splitting; the low pH of the end sample (4.1) suggests acids of short carbon chains. After a small drop the saponification number rises steadily owing to addition of split acids, since this value increases far more than the acid number does, rising 154.5 0 and 42.5 points, respectively; oxy acids rise to 86.8 and then decrease slightly; the molecular weight doubles, pointing to the dipolymer formation of Nagel;4 refractive index lines up with the change of percentage of oxy acids; 15 per cent oxygen is taken up or an average of 9.6 atoms of oxygen in each molecule of the oxidized ester. The percentage of total acids is a trifle low (found 75.3 per cent, calculated 87 per cent) due most likely to the volatilization of short-chain acids rather than unsaponifiable matter, which analysis of the oxidized methyl ester3 proved to be present only in traces. Molecular weight calculated from acid number shows that the total acids are dibasic (molecular weight, 402 as a dibasic acid; found, 392). Extraction analysis indicates 41 per cent a quatra-polymer of the original ester dissolved in a material of low molecular weight, probably ester molecules which have
IO
20
30
TBms-heurr
Figure 3
40
50
0
IO
20
30
40
50
T m c Lours
Figure 4
of split acids with the ester, which are hydrolyzed by the alkali on saponification. This combination may be not SO much with hydroxy acids as a t open bonds, hydrolysis in either case resulting in hydroxyl groups in the total acids. Analysis of the extracts shows that the two fractions separated by petroleum ether have nearly the same molecular weight. There is approximately 16.5 per cent combined alcohol in the original sample; 11.4 per cent was found in the end sample, indicating hydrolysis producing p-eleostearic acid with volatilization of the alcohol (calculated for no hydrolysis, 14.92 per cent). The end sample shows small gel lumps of loose aggregation so similar to the oil that they could not be observed under a
IND VSTRIAL A1YD ENGINEERING CHE,WISTRY
April, 1929
hours and 40 minutes, which was more rapid than with the /%eleostearic acid alone, which took 29 hours and 30 minutes. The drop in iodine number indicates lack of thoroughness of oxidation, pointing to the fact that the beta-acid linked up with the methyl ester, hindering its otherwise rapid oxidation. Acid number drops to 37.7 instead of 58.8, the figure when the two components were oxidized separately. Saponification number is not affected in such an addition of the acid to the ester. The petroleum ether extraction gives the most striking evidence of this combination; it is only 9.3 per cent while the calculated is 29.7 per cent, showing that the oxidized methyl ester is less soluble because of the addition of beta-acid molecules. Paralleling the methyl ester-beta-acid batch, a run was made using stearic acid sufficient to combine a t one open
microscope, but only when the oil was poured. They doubtless account largely for the high-viscosity figure obtained on this oxidized ester. 6-BUTYL EsTER-The changes of the constants in this batch correspond closely with those in the others. Thew is slightly more splitting, as indicated by the saponification number and the low oxy acids even a t their maximum; the large amount of oxygen in the end product also indicates considerable splitting, since the percentage oxygen is decidedly in excess of that required by the drop in iodine number. The molecular weight is that of a material having an average association of four ester molecules. Ultimate analysis gives an average of 30 atoms of oxygen taken up by each oxidized ester molecule. The average molecular weight of the total acids based on acid number is 349 calculated as dibasic acids: Table 11-Oxidation
of 8 - M e t h y l Eleostearate w i t h B-Eleostearlc Acid a n d w i t h Stearic A d d a t 82' C.
1
METHYL ESTER(205 GRAMS) A N D BETAACID(195 GRAMS) Time
Iodine NO.
Acid h-o .
Saponification Mol. XO. wt.
P
~
~
b
~
METHYL ESTER(204 GRAMS)A N D STEARIC ACID(196 GRAMS) ~
soluble
1
Hours
a
34 1
: Time ~ m
Iodine
Acid
h-0.
NO.
Saponification Mol. h-0. wt'
Pe::ttRm soluble
Hours
Calculated for original sample. Calculated on basis of previous separate runs of methyl ester and beta acid, changes on stearic acid upon blowing neglected
453 wa3 found, indicating some higher polybasic acids. The results on acetylation are similar to those on the isopropyl ester. Analysis of the extract indicates little difference between the two fractions on the basis of molecular weight. Combined alcohol found in this end sample is 16.35 per cent; the calculated amount is about 16.1 per cent, showing that little or no hydrolysis of the ester takes place. 6-BENZYLEsTER-This ester resembles the others in its course of oxidation, though, being of higher molecular weight, it becomes more vjscous, sufficiently so that the iodine number drops no lower than 35.3. There is somewhat less than normal splitting and oomhination of split acids, particularly in the first 24 hours; the continuous increase of the oxy acids suggests less decomposition than in the case of the other esters. Considering the volatilization of the free benzyl alcohol in the first sample, the molecular weight has a little more than doubled as a result of oxidation. The material has taken up 9.5 per cent oxygen. There is only 6.02 per cent combined alcohol in the end sample (calculated 25 per cent), indicating considerable hydrolysis liberating 6-eleostearic acid, which is a big factor in the thickening. Since the course of the oxidation is similar to that of the glycol ester3and the liberation of beta-acid makes it analogous to the methyl ester-beta-acid mixture, it is difficult to see why gelation has not taken place as it has done in t$hesecases. This is probably due t o the peptizing effect of the split acids, shown by the acid number and pH in these cases. However, this material was the only monohydric ester which, upon microscopic examination, showed a mottled appearance, due probably to flocculation preceding gelation. Oxidation of Methyl Ester Mixtures
These runs (Table 11) were carried out to determine if a monohydric alcohol ester of eleostearic acid would gel when oxidized with ,%eleostearic acid, the idea being that the acid might form addition products a t open bonds of the ester, and that this, together with addition to its own double bonds, would build up a gel framework. A batch of P-methyl eleostearate was oxidized with enough p-eleostearic acid to combine with one open bond of the ester. The material gelled to a tough, elastic solid in 27
bond of the ester, to determine if a saturated acid would give rise to gel formation in the same manner as did the P-eleostearic acid. The batch was run 45 hours and on cooling remained a darkbrown paste with the smectic structure of solidified stearic acid. The iodine number checks that calculated for no combination, the same may be said of the acid number, while the saponification number is somewhat higher than the calculated 243. Jlolecular weight is slightly higher. Petroleum ether-soluble is much higher than calculated. This is probably due to the peptizing effect of stearic acid upon the oxidized methyl ester colloid. On a whole, the evidence points to little or no combination between the ester and stearic acid, suggesting that the combination of the ester and betaacid is not esterification a t hydroxy acids, since the saturated stearic acid would esterify more readily in this case.9 The union is more likely to be addition of carboxyl to open bonds of tlie ester and to some extent polymerization by fusion a t the open bonds. Hydrolysis of Esters The tendency of these esters to hydrolyze, liberating betaacid, was tested through a study of their alkali, neutral, and acid hydrolysis. In alkali hydrolysis using 0.104 potassium hydroxide and an equivalent amount of the ester in an alcohol solution a t 47" C. the hydrolysis constants determined a t 10, 15, and 30 minutes averaged as follows: methyl ester 0.838, butyl ester 0.471, benzyl ester 0.085, and glycerol ester 0.743. Neutral hydrolysis, with an excess of steam, of equivalent amounts of the methyl and glycerol esters in toluene solutions a t 100" C. gave the following increases in acid number: ESTER Methyl Glycerol
3 MINUTES 7 MINUTES 90 MINUTES Water soln. Toluene s o h . ... 3.1 6.5 9.3
3.3
...
4.3
4.0
The methyl and glycerol esters treated in alcoholic solution with 0.114 N hydrochloric acid a t 47" C. for intervals up to 3 hours showed no measurable tendency to hydrolyze. @Sudborough,J . Chem. Soc., 87, 1840T (1905).
342
I X D U S T R I A L A,VD ENGIXEERING CHEMISTRY
h
n (0
h
m
w
d
m w m
Vol. 21, No. 4
These experiments, though of a preliminary nature, point to several interesting features. I n saponification the monohydric alcohol esters show hydrolyzing tendencies opposite to those observed upon oxidation and the glycerol ester hydrolyzes a t about the same rate as the methyl. T h e same tendency was noted on saponifying the oxidized esters for determination of combined alcohol. The acid number of the methyl ester rises a trifle more rapidly than that of t h e glycerol ester in neutral hydrolysis, but the acid number of the water solution indicates that the acid is being split up. Acid hydrolysis tests failed to split either the methyl or t h e glycerol ester, which is significant when it is considered that oxidation produces strong acids such as formic and acetic in the system. General Relationships
-
m
B
d.
$ 3
h
0
0
U
c
a " b
t
5
a " 'c
i 8
e
e J
e c
a
Lc
w
u3
s
3
Y
The similarity in the courses of these ester oxidations will be noted immediately. There are, however, differences which may be attributed to the shape and size of the alcohol group. The benzyl ester, the one with the largest alcohol group, does not drop so far in iodine number as the other. The acid number of the end product rises as the number of carbon atoms increases in the straight-chain alcohols and is somewhat higher in the case of the isopropyl and benzyl esters. T h e saponification numbers of the isopropyl and benzyl esters run low. Refractive index increases with the number of carbon atoms in the straight-chain alcohols; it is higher in the other esters. The same may be said of viscosity. Oxygen absorption increases with the lengjh of the chain in the esters of the straight-chain alcohols and is lower in the case of the other two esters. Total acids decrease from the methyl t o the butyl esters. The color is darkest in the case of t h e benzyl ester, but lightest as the alcohol group lengthens in the other esters. The acid numbers of the total acids of the straight-chain alcohol esters rise as the length of the chain increases. These effects line up in general with the tendency of esters to hydrolyze as determined by the percentages of combined alcohol in the end samples. I n the case of the straight-chain alcohols the esters hydrolyze more readily the longer the carbon chain in the alcohol, the secondary alcohol ester (isopropyl) hydrolyzes more readily than a primary, and t h e aromatic benzyl alcohol ester hydrolyzes most readily. AS has been noted before, the tendency of these esters to hydrolyze upon oxidation is opposite to that of saponification and may be explained on the basis of their relative tendencies to produce acidic systems and hence slow up hydrolysis. It seems likely, however, that the chief cause of hydrolysis in these oxidizing esters will be found in special conditions characteristic of oxidized molecules and aggregates, a subject now under investigation. Though there appears t o be formation of dibasic acids by polymerization, the addition of the long-chain unsaturated 0-eleostearic acid is important in the thickening and gelation.1° This was observed in the previous paper3 in the cases of t h e glycol and glycerol esters. Though these esters do not gel, they thicken in the order of their tendency to hydrolyze, liberating 0-eleostearic acid. When the methyl ester is oxidized in the presence of beta-acid, combination takes place resulting in gelation, the effect of this being the same as if hydrolysis had occurred. Acknowledgment The authors express their thanks to A. Jones, director of research, for permission to publish these results, and to E. Claxton for valuable suggestions during the course of t h e work. 10
Eibner, Chem. Umschau, S6, 6 5 (1928).
