I
CARL J. MALM, LEO J. TANGHE, HARRIETT M. HERZOG, and MARY H. STEWART Eastman Kodak Co., Rochester,
N. Y.
Reactivities of lower Aliphatic Anhydrides toward Hydroxyl Groups of Cellulose Data given contribute to understanding of reactions taking place during manufacture of the lower aliphatic ester of cellulose
1,
anhydride was added. This solution and a solution of 0.064 gram of sulfuric acid in 50 grams of acetic acid were brought to 130' F. in separate containers, and then mixed, with good stirring. Concurrent esterifications were carried out with the ester, catalyst, and corresponding anhydride dissolved in propionic, n-butyric, and isobutyric acids. Constant molecular amounts of anhydride were maintained by using 80 grams of propionic anhydride and 97 grams of the butyric anhydrides. Samples from acetic and propionic acid solutions were precipitated from time to time into water, and from n-butyric and isobutyric acid solutions into 50% alcohol. The precipitates were washed to neutrality in distilled water, and stabilized by adding just sufficient sodium carbonate to the last wash to impart a pink color to phenolphthalein. Hydroxyl contents were determined by the carbanilation method (7). The esterification by acetic anhydride was fastest, followed in turn by n-butyric, propionic, and isobutyric anhydrides (Figure 1). The same order of reaction rates was found on repetition of this experiment. The slightly greater reactivity of n-butyric anhydride in nbutyric acid over propionic anhydride in propionic acid is in agreement with earlier work of Nadeau and Gould (9) on the esterification of n-butyl alcohol with these anhydrides, when sulfuric and p-toluenesulfonic acids were used as catalysts. They also showed that the esteriiication of n-butyl alcohol follows the antic-
HOMOGENEOUS solution esterification of the hydroxyl groups in a cellulose derivative is an orderly process, its rate depending on temperature, and the nature and amount of anhydride, catalyst, and solvent. Exact kinetiqs are difficult to apply because the various hydroxyls, primary and secondary, react simultaneously and a t different rates. The esterification of cellulose is a more complex process, because the reaction begins in a heterogeneous system. Some factors influencing the rate of esterification are: distribution of catalyst between liquid and cellulose, diffusion of catalyst and anhydride into the fiber to the site of the reaction, and dissolution of the ester as it is formed. These are in turn influenced by the physical state of the cellulose as measured by accessibility, viscosity, and similar factors. This report is concerned with the esterification of a partially substituted soluble cellulose ester, and of cotton linters, with acetic, propionic, n-butyric, and isobutyric anhydrides, using a variety of catalysts.
Esteriflcation in Solution
The esterification in homogeneous solution was studied using a commercial cellulose ester containing 1.38 acetyl and 1.15 butyryl groups per glucose unit. This was chosen in preference to a hydrolyzed cellulose acetate because of its wider solubilities. Sulfuric Acid Catalyst. Fifty grams of the ester was dissolved in 500 grams of acetic acid, and 62.5 grams of acetic
ipated second-order kinetics. The esterification of a soluble cellulose derivative may be considered as three concurrent second-order reactions, when three different hydroxyl groups-one primary and two secondary-are present, each with its own reactivity. Replacement of Combined Sulfate by Acyl. With sulfuric acid the esterification is further complicated because most of the sulfuric acid quickly combines with the cellulose and is then slowly replaced by acyl (6) :
+
Cell-0-S020H AczO 6 Cellulose derivative containing combined sulfate Cell-OAc Acetate of cellulose derivative
+ AcO-SO2-0H Acetyl sulfuric acid
Experiments were carried out to compare the effectiveness of various anhydrides in promoting this reaction. These were similar to the esterification experiments, except that the concentration of sulfuric acid was increased from 0.001 to 0.033M, and the temperature was reduced to 107' F. Samples were taken in pairs, one to retain the combined sulfate for sulfur analysis, and one to remove the combined sulfate (6), so that it would be more stable and more suitable for determination of viscosity. Although the esterification itself was practically over within a few minutes, the replacement of sulfate by acyl continued for several hours. Sulfate was replaced a t approximately the same rate by acetyl, propionyl, and n-butyryl,
4
0
Figure 1. With 0.00 1 M sulfuric acid catalyst at 130' F. esterification by acetic anhydride was fastest, followed by nbutyric, propionic, and isobutyric anhydrides
n-Butyrate lsobutyrote
f
s
28
Proplonote n - Butyrate lsobutyrote
27
b
241
'
4
'
b
'
I2
'
Hours of reottion
I6
'
20 '
'
24 '
'
Figure 2. With 0.0 1 M methanesulfonic acid catalyst at 130' F. nbutyric anhydride reacted slightly faster than propionic anhydride
20
24
Hours of reoction
VOL. 5 0 , NO. 7
JULY 1 9 5 8
1061
but much more slowly by isobutyryl (Table I). Degradation became much more severe in going from acetic to the higher acids of theseries. Conversion of Sulfuric Acid. Sulfuric acid reacts with acetic anhydride to form sulfoacetic acid, and with other anhydrides to form corresponding products. These are first-order reactions in the presence of a large excess of anhydride (8). In a control experiment in acetic acid solution without cellulose, the half life of the reaction was 34 hours a t 130' F. This is equivalent to conversion of 10% of the sulfuric acid to sulfoacetic acid during the first 5 hours. In the presence of cellulose the amount of this conversion xvould, however, be very small, because most of the sulfuric acid is combined 7t'ith the cellulose and
Sulfate Is Replaced at Approximately Same Rate b y Acetyl, Propionyl, and n-Butyryl, but More Slowly b y lsobutyryl
Table 1. Time, Hours
3). This order of reactivities was verified by repetition of the experiment, and was also observed in the esterification of n-butyl alcohol, which was followed analytically by the disappearance of anhydride. The synergism between zinc chloride and hydrochloric acid was recognized in the early patent literature. Nebel (70) degraded cellulose with hydrochloric acid in acetic acid prior to acetylation with anhydride and zinc chloride, but it is not evident that he realized the effectiveness of the hydrochloric acid during the acetylation step. Mallabar ( 3 ) found that zinc chloride was effective a t a much lower temperature upon addition of a small amount of hydrochloric acid. Dreyfus ( 7 ) studied this effect thoroughly and found that it could be extended to other metal halides (2). Upon addition of a small amount of hydrochloric acid, all the esterification rates were greatly increased, and the order of the rates was changed. M'ith 0.01M zinc chloride and 0.01M hydrochloric acid; acetic anhydride reacted fastest, followed in turn by propionic and n-butyric anhydrides at the same rate, and finally by isobutyric anhydride. Whereas hydrochloric acid by itself had only feeble catalytic activity, it exerted a great effect on the catalytic activity of zinc chloride (Table 11). Therefore, w-ith zinc chloride in the
thus cannot undergo this conversion. The concentration of anhydride becomes progressively lower as it is consumed in the esterification reaction, and reduced concentration of anhydride greatly slows the conversion of sulfuric to the sulfoaliphatic acid (8). Methanesulfonic Acid Catalyst. The esterification rates with 0.0134 methanesulfonic acid are shown in Figure 2. As with sulfuric acid catalyst, n-butyric anhydride reacted slightly faster than propionic anhydride. Zinc Chloride Catalyst. Similar esterifications were carried out with 0.0334 zinc chloride catalyst. The order was different than with sulfuric acid catalyst; n-butyric anhydride reacted fastest, followed in turn by propionic, acetic, and isobutyric anhydrides (Figure
.
.Icet 1-1 Sa 1.V.b
Isobutyryl 1.V.b 0.091 1.14 0.090 1.37 0.088 1.59 1.65 0.5 0.091 0.088 0.95 0.077 1.14 0.076 1.17 1 0.075 1.51 0.70 0.076 0.59 0.83 0.042 1.43 0.044 3 0.046 0.49 Toolowviscosity 0.56 0.021 0.018 0.018 1.34 7 to recover Intrinsic viscosit.y in acetic acid. All intrinsic visa Combined sulfate. per glucose unit. cosities in organic solvents, reported herein, are based on measurement of relative viscosity at concentration of 0.25 gram of ester per 100 ml. of solvent.
Table 11,
Catalyst, H~SOP MeSOaHC ZnCls ZnChd ZnClz HCl ZnCL HCl ZnCln HC1 ZnCL K salte ZnC12 K salte AlCla
+ + + +
IIf 0.001 0.01 0.10 0.03 0.01 0.01 0.01 0.01
Propionyl 1.TT.b
n-Butyryl
S"
S"
Solvent Acidb Acidb Acidb Acidb Acidb Acidb Acidb
Minutes for Esterification of Half and Three Quarters of Hydroxyls Initially Present Acetic Anhydride Propionic Anhydride n-Butyric Anhydride Isobutyric Anhydride 3/4 1/2 3/4 1/2 3/4 1/ 2 3/4 1/2 30 150 400 60 1500 *.. > 2900
30 60 30
200 400
200
30 45 30
200 315 150
45 90 45
700 1000 700
150
360
135
360
360
1100
See Figure 2 .
See Figure 3.
e
130
Salt corresponding to acid.
f
At steam
*
absence of added hydrochloric acid, the greater reactivity of n-butyric anhydride in n-butyric acid, as compared to acetic anhydride in acetic acid might be due to the formation of more hydrochloric acid by reaction of zinc chloride with nbutyric acid than with acetic acid. The following results indicated this to be the case. Zinc chloride was acidic toward crystal violet indicator in both solvents, but more base (potassium acetate or n-butyrate) was required to neutralize zinc chloride in n-butyric acid than in acetic acid. In acetic acid 0.10M zinc chloride was neutralized by 0.01M potassium acetate, while in n-butyric acid 0.03M potassium n-butyrate was required. With 0.01.M zinc chloride and the above respective concentrations of base in acetic and n-butyric acids, the esterification rates with acetic and nbutyric anhydrides were comparable. The times required for half of the hydroxyls to react were 80 and 70 minutes, respectively (Table 11). Other Catalysts, Aluminum chloride showed catalytic activity comparable to, and antimony chloride greater than, zinc chloride, but in neither case did n-butyric anhydride react faster than acetic anhydride. In acetic acid solution magnesium chloride was less active than zinc chloride; in the other acids it was not sufficiently soluble. Cadmium, mercuric, and ferrous chlorides were not used because of their low solubility in acetic acid solution. Magnesium perchlorate was very active in acetic acid solution, probably because of the formation of perchloric acid. I t was not used in the other solvents because of its low solubility. Pyridine and the potassium salts of the solvent acids were used as basic catalysts. With pyridine, acetic anhydride reacted fastest, followed in turn by n-butyric, propionic, and isobutyric anhydride. There was little spread among the reactivities of the various anhydrides with the potassium salts (Table 11). Pyridine was a considerably more active catalyst than the potassium salts. The uncatalyzed reactions were slow even at steam bath temperature. Acetic anhydride reacted fastest, followed in turn by propionic and n-butyric anhydrides at the same rate, and finally by isobutyric anhydride. This is a different sequence than observed in most of the catalyzed esterifications a t lower temperature. Effect of Solvent. I n all the above esterifications the solvent was the acid corresponding to the anhydride. A better comparison of the reactivities of the various anhydrides is obtained when the same solvent and catalyst are used throughout. For this purpose esterifications were carried out using ethylene chloride, dioxane, and P-methoxyethyl
Figure 3. With 0.03M zinc chloride catalyst at 130' F. n-butyric anhydride reacted fastest, followed b y propionic, acetic, and isobutyric anhydrides
< < < 5%
1
Propiohate n- Butyrate
28 91
'"B
2.6
5 . 8 24L
'
d
'
A
' Hours
acetate as solvents for the four anhydrides (Table 11). In every case acetic anhydride reacted fastest and isobutyric anhydride slowest. Surprisingly little difference was found between the reaction rates of propionic and n-butyric anhydrides. Where differences were detectable, n-butyric anhydride reacted faster than propionic anhydride. The rate of reaction varied considerably with the solvent, as shown by the different quantities of methanesulfonic acid required to give comparable reaction rates. The data indicate comparable rates of acetylation in acetic acid and in ethylene chloride, but much slower rates in @-methoxyethyl acetate and in dioxane. With zinc chloride catalyst the acetylation was much faster in ethylene chloride than in acetic acid. Competitive Esterifications. Reactions were carried out with equal moles of acetic and n-butyric anhydrides to determine the relative amounts of acetyl and n-butyryl introduced when various catalysts were used. These esterifications were carried out in an equimolar mixture of acetic and n-butyric acids to provide a common solvent for the reaction of both anhydrides.
12 ' ' 16' of reoction
'
20 '
'
24 I
'
The first set of experiments was made starting with the same cellulose ester which was used for the rate studies. This material contained only 0.47 hydroxyl per glucose unit available for esterification. A second material which had been hydrolyzed to contain 1.09 hydroxyls per glucose unit was used to give a better analytical response to variations in the amounts of acetyl and n-butyryl introduced. The products were isolated after 24 hours a t 130" F. and mole per cent acetyl was determined by the extraction method ( 5 ) . The composition of a cellulose acetate n-butyrate in terms of its weight per cent hydroxyl, h, and its mole per cent acetyl, A , is given by the equations: Acetyl per glucose unit = 5.79A(31.5 - h ) 6070 h(250 - A )
+
+Butyryl per glucose unit = 5.79 (100 - A)(31.5 - h ) 6070 h (250 A)
+
-
I n the competitive esterifications more acetyl than n-butyryl was introduced with all catalysts (Table 111). This is especially interesting in the case of zinc
Table 111. In Competitive Esterifications, All Catalysts, Including Zinc Chloride, Promoted Faster Reaction with Acetic than with n-Butyric Anhydride Analysis Mole % Weight .. Ac % OH
Catalyst, M
MeSOaH AlCla CsHbN
+ HCI ZnClz + ZnCh
K salt a
...
0.20 0.21 0.17 0.16 0.13
1.08
0.51 0.66 0.65 0.46
0.44 0.42 0.39 0.27
1.69
1.28
0.59
0.47
1.70
1.25
0.60
0.44
0.01 1.0
1.38 1.60 1.62 1.65 1.64 1.63 1.10 1.61 1.76 1.75 1.56
1.15 1.35 1.36 1.32 1.31 1.28 0.81 1.25 1.23 1.20
0.01 0.01
56.9
0.15
57.8
0.32
(Starting materialb) ZnClz &So4 MeS08H CbHsN
...
2.70 0.27 0.15 0.1% 0.30 0.55 6.9 0.85 0.08 0.28 2.06
0.03 0.001 0.01 0.1 1.0 0.03 0.001
Groups Added Ac/ n-Bu/ g.u. g.u. 0.22 0.24 0.27 0.26 0.25
54.6 54.2 54.4 55.5 55.6 56.0 57.4 56.7 58.8 59.3 59.2
(Starting materiala) ZnC12
Composition Ac/ n-Bu/ g.u. g.u.
...
...
0.1 0.01
With 0.47hydroxyl per glucose unit. With 1.09 hydroxyl per glucose unit.
VOL. 5 0 , NO. 7
JULY 1958
1063
chloride, as this catalyst promoted a faster reaction with n-butyric anhydride than with acetic anhydride when the corresponding acids were used as solvents (Figure 3). Discussion. As an approximate measure of the over-all reaction rates, the times required for the esterification of half and of three quarters of the hydroxyl groups initially present are given in Table 11. Sampling was not continued to the complete esterification of all hydroxyl groups. The last samples of most of these experiments, taken after 24 hours, had small amounts of hydroxyl which could be reliably determined by the carbanilation method. I n the esterifications with isobutyric anhydride the reaction apparently leveled off at higher hydroxyl contents than in esterifications with the other anhydrides. The slower esterification with isobutyric anhydride may be due to a steric effect originating with the branched chain. This may make it more difficult for the isobutyric anhydride molecule to approach a given hydroxyl group. Also after some isobutyryl groups have been introduced, these may make it more difficult for a molecule of any anhydride to approach a neighboring hydroxyl. Factors influencing the exact course of
Table
IV.
these rate curves may originate with the anhydride used, with the hydroxyl groups undergoing esterification, or with the medium (solvent, catalyst) in which the reaction is carried out. Acetic, propionic, n-butyric, and isobutyric anhydrides often failed to align themselves in that order in their rates of esterification (Table V). I n the case of zinc chloride catalyst this was traceable to different acidities given by the same amount of catalyst in different solvents. This is supported by the competitive reactions with acetic and n-butyric anhydrides, when zinc chloride catalyst was used. At the start of esterification the same array of hydroxyl groups is presented to all four anhydrides, but after esterification of half or three quarters of them, the distribution of the remaining hydroxyl groups may be different, depending on the anhydride which has been doing the esterifying. I n the case of sulfuric acid, its rate of combination with the cellulose, the rate of replacement of combined sulfate by acyl, and the rate of conversion of sulfuric to the sulfoaliphatic acid may vary with the solvent and the anhydride, and thus influence the acidity and the rate of esterification.
Esterification of Dewatered Linters, Using Sulfuric Acid Catalyst Effect of sulfuric acid varies with solvent and anhydride Intrinsic Viscosity Time, Hours
Ester Acetate
Propionate
p-Butyrate
Start 0.5 1.0 1.5 2.0 2.5 3.0 4.0 5.0 Start 1 2 3 4 5 6 7 Start 1 2 3 4
5
Isobutyrate
6 7 Start 2 4 5 6 7 8
9
Temp.,
O
F.
%
Acyl/
...
... 0.48
Outside
Inside
Acyl
40 40/50 50 50/60 60 60/70 70/80 80
48 50 56 63 70 74 80 85
11.4 14.6 21.4 28.4 31.7 35.9 41.3 43.8
0.64 1.02 1.49 1.73 2.08 2.61 2.88
40/50 50/60 60/70 70/80 80/90 90/100 100
50 64 80 90 94 100
11.7 17.1 25.0 35.0 45.1 50.3 51.2
0.37 0.58 0.94 1.50 2.28 2.80 2.94
4;;iO 50/60 60/70 70/80 80/90 90/100 100
53 64 75 86 96 100
10.5 18.5 26.4 35.1 47.7 54.1 55.7
0.52 0.82 1.26 2.06 2.65 2.92
50/60 70/80 80/90 90/100 100/110 110/120 120
52 73 82 90 98 107 116
11.5 15.0 20.1 27.7 31.8 41.9 48.6
...
...
...
...
... 44
... 42
...
...
...
...
G.U.
...
... 0.25
...
0.30 0.40 0.57 0.87 1.06 1.63 2.14
Cuprammon ium a 7.09 5.25 5.07 4.75 4.76 3.87 3.67 3.28 3.06 7.04 4.92 4.74 3.73 2.81 2.12 1.49 1.54 7.19 4.79 4.76 3.92 3.06 1.82 1.31 1.01 7.19 3.67 3.63 2.84 2.36 2.13 1.66 1.15
MeC12: MeOH (9O:lO)
3.20
1.15
0.82
a Based on relative viscosity of solution containing 0.1000 gram of cellulose per 100 ml. of cuprammonium solution. I n acetate appropriate weights of ester were calculated; other esters were previously treated t.0 yield regenerated cellulose.
1 064
INDUSTRIAL AND ENGINEERING CHEMISTRY
Esterification Starting in Suspension
I n the esterification of cellulose itself, the reaction begins in suspension, and the additional factors mentioned above must be considered. The cellulose in the following experiments was acetylation grade linters. I t was activated by soaking at least 1 hour in distilled water at 150" F. and was dewatered with changes of the appropriate acid. For each change the cellulose was agitated in an acetylation mixer for 30 minutes with 5 parts of acid-e.g., acetic acid-and centrifuged for 2 minutes. This left slightly less than 1 part of liquid adhering to the cellulose. Three changes in this way removed the water satisfactorily. The fibers then gave high and uniform sorption of catalyst and good reactivity toward esterifying agents (4). Sulfuric Acid Catalyst. An acetylation mixer was charged with 3.0 pounds of activated cellulose wet with 2.5 pounds of acetic acid. A precooled mixture of 8.6 pounds of acetic anhydride and 11.9 pounds of acetic acid was added, and the mixer contents were cooled to 40" F. The catalyst, 27.2 ml. of concentrated sulfuric acid in 1 pound of acetic acid, was then added. This amount of catalyst was 3.5y0 of the weight of the cellulose; it provided a concentration of 0.045 mole of sulfuric acid per kilogram of solvent initially present. The jacket temperature was maintained a t 40' F. for 1 hour, and raised 10' F. at the end of each succeeding hour until the esterifications were completed or until the products were too degraded to recover. Similar esterifications were carried out with the other anhydrides, in which the cellulose was dewatered with, and the sulfuric acid dissolved in, propionic, n-butyric, and isobutyric acids. The amount of anhydride was maintained at 50% excess over the theoretical amount by using 10.8 pounds of propionic and 13.2 pounds of the butyric anhydrides, respectively. The amount of aliphatic acid was adjusted to maintain an initial liquid-solid ratio of 8 to 1. The same outside temperature schedule was used in all the esterifications; the internal temperatures are given in Table IV. Figure 4 shows the amount of acyl introduced as the time of the reaction increased. When starting with linters, the esterification with acetic anhydride was fastest, followed in turn by propionic, n-butyric, and isobutyric anhydrides. As the relative rates of esterification with propionic and n-butyric anhydrides are different from what they were in solution esterifications (Figure l), one or more of the factors mentioned in the introduction to this report must be operative. Zinc Chloride Catalyst. The reactivity of the various anhydrides toward cellulose was also determined, using zinc chloride catalyst. An acetylation mixer was charged with 3.5 pounds of cellulose
A L I P H A T I C CELLULOSE ESTERS wet with 3.2 pounds of acetic acid. A mixture of 9.0 pounds of acetic anhydride and 5.2 pounds of acetic acid was added and the mixer contents were heated to 120' F. The catalyst, 1.75 pounds of zinc chloride, was then added. This corresponds to 0.74 mole per kilogram of solvent initially present. Similar esterifications were carried out with the other anhydrides, in which the cellulose was dewatered with propionic, n-butyric, and isobutyric acids. The amount of anhydride was maintained a t 36'% excess over the theoretical amount by using 11.5 pounds of propionic anhydride and 14.0 pounds of the butyric anhydrides. The amount of aliphatic acid was adjusted to maintain a n initial liquid-solid ratio of 5 to 1. The jacket temperature was maintained a t 120" F. except on the acetate, where it was raised to 170' F. after 8 hours. At the start acetic anhydride reacted fastest, followed in turn by propionic, n- butyric, and isobutyric anhydrides (Figure 5 ) . This is in contrast to the results in solution (Figure 3) where n-butyric anhydride reacted fastest, followed in turn by propionic and acetic anhydrides. This reversal of rates is probably influenced by different rates of diffusion of the various anhydrides to the site of the reaction. The esterifications of linters did not finish in the same order as their initial rates of reaction. The n-butyrate finished first, followed in turn by the propionate, isobutyrate, and finally acetate. The acetate never gave a really good solution, even though it was heated longer and to a higher temperature than the other esterifications. This reversal of finishing times is probably due to differences in the solubility of the surface layer of the various esters from linters. When the surface layer of the ester does not dissolve in its esterification bath, the anhydride must diffuse through it to the site of the reaction. The importance of the physical state of the starting cellulose on the properties of the derived ester, when the esterification is carried out with but little degradation, is illustrated by comparing the behavior of linters and regenerated cellulose. Regenerated cellulose, obtained by deacetylation of medium viscosity cellulose acetate, was esterified with these anhydrides, using zinc chloride catalyst. Here the surface layer of the acetate was readily soluble in the esterification bath, and it finished first, followed in turn by the propionate, n-butyrate, and isobutyrate. The relative rates of esterification, degradation, and sulfate replacement by the various anhydrides are summarized in Table V.
Degradation A certain amount of degradation is desirable in the production of a useful cellulose ester from linters; otherwise its solution or melt is too viscous for practical application. Cellulose acetate can be made to have extremely high viscosity, but the esters above, the acetate in the homologous series have lower viscosity under comparable methods of preparation. I n the course of this work on the rates of esterification, some interesting observations on degradation were made. During the rate studies of esterification in solution only methanesulfonic and sulfuric acids gave significant degradation during the 24-hour interval of esterification. The intrinsic viscosity in methylene chloride-methanol (90 to 10) decreased from 1.3 to 1.15 with 0.01M methanesulfonic acid, and to less than 1.00 with 0.001M sulfuric acid. In the experiments on the replacement of sulfate by acyl (Table I) the larger amount of catalyst caused considerable degradation, which became more severe on going from acetic to the higher acids as solvents. Similar trends have been found when cellulose tripropionate and tri-n-butyrate were dissolved in acetic, propionic, and n-butyric acids and degraded with sulfuric acid. Degradation with Free and Combined Sulfuric Acid. Two samples of cellulose propionate having initially 0.04 and 1.14% OH, respectively, were dissolved in acetic acid-acetic anhydride (90 to 10) and degraded with 0.065M sulfuric acid at 104' F. The sulfuric
Table
Reaction Esteriflcation
V.
Hours of reaction
Figure 4. With sulfuric acid catalyst esterification of activated linters with acetic anhydride was fastest, followed b y propionic, n-butyric, and isobutyric anhydrides
Hours of reaction
Figure 5. With zinc chloride catalyst esterification of activated linters b y acetic anhydride was fastest at the start, followed by propionic, n-butyric, and isobutyric anhydrides. Esterifications did not finish in the same order
Summary of Reaction Rates
Ester in solution"
Catalyst HZSO4 MeSOsH ZnCl2 ZnClz HC1 ZnClz K saltc CsHbN K salt Uncatalyzed MeSOsHd ZnClzd
+ +
Esterification
Linters ZnClz* ZnClzf
Relative Rates of Reaction with Anhydrides ProaIs@ Acetic pionic Butyric butyric I* 3 2 4 1 3 1
3
2
2 2
1
4 4
2
3
1 2 1
4
1 1
3
1
1
1
2
1
2
1 1
2 2
2 3
1
2
3
4
2
1
2 2
3 3 3 4
4 3
Regenerated cellulose Ester in solution Linters Ester in solution
ZnClp,f 1 2 3 4 Degradation HzSO4 1 2 3 4 Degradation &SO4 1 2 3 4 Replacement of H2SO4 1 1 1 2 sulfate by acyl Lowest numSolvent is aliphatic acid corresponding $0 anhydride, except as stated in d. ber indicates fastest reaction; comparisons are valid for various anhydrides, when all other conditions are constant. C Acetate and butyrate, respectively, in amounts to bring solution t o At finish of esterineutrality. d Solvent was ethylene chloride. 6 At start of esterification. fication.
VOL.'50, NO. 7
JULY 1 9 5 8
1Q65
1
I
7 t
-1
-
eo
0
30
acid could combine with the sample of low hydroxyl content only to the limited extent of the hydroxyls present. The other sample had sufficient hydroxyl for all the sulfuric to combine. The viscosities and the amounts of combined sulfate are given in Table VI. Almost immediately after additions of catalyst both materials became triesters. The sample with the larger amount of combined sulfuric degraded as much in 1 hour as the other did in 6 hours. In a similar experiment with 0.065M perchloric acid, both samples degraded alike. In this case also, almost immediately after addition of catalyst both
I
Acyl pw glucose unit
Figure 6. Degradation during esterification of activated linters with sulfuric acid becomes more severe in going from acetate to higher esters
Table VI. Time, Hours
Degradation with Free and Combined Sulfuric Acid 0.04y0 OH 1.14% OH Catalyst %S I.V. %S I.V.
...
(Starting material)
2.26
Low Combined Sulfate 5 min. 1 3 6
0.065M HzSOa
1 3 6
0.065M HClOi
Table VII. Ester Acetate
0.055 0.058 0.044 0.045
125
165 128 136 132 127 126 124 137 136 130 128 Isobutyrate
119 122 124 125 130
...
1.22 0.82 0.78
8.05 10.05 12.0
1.78 2.23 2.30 2.47 2.55 2.64 2.68 2.72 2.80 2.92
0.5 1.0 1.5 2.0 4.5 6.0
26.5 41.0 46.9 50.0 51.2 51.9
1.02 1.95 2.47 2.81 2.92 3.02
0.5
23.6 42.8 53.8 55.6 56.6 56.9
0.70 1.69 2.61 2.82 2.92 2.97
1.78
15.5 19.3 23.6 29.0 37.3 44.7 50.6 53.6 55.4 56.8
0.42 0.55 0.70 0.93 1.35 1.83 2.30 2.62 2.76 2.96
I 34
1.0 1.5 2.0 3.0 4.5
0.5 1.0
1.5 2.0 3.0 4.0 5.0 6.0 9.5
Jacket temperature raised t o 170' F.
1 066
1.38 1.07 0.60 0.34
32.4 37.5 38.2 40.0 40.7 41.6 42.0 42.4 43.1 44.0
0.5 1.0 1.5 2 .o 3.0 4.5
7.0 a
High Combined Sulfate
No Combined Sulfate 1.08 1 .os 0.63 0.65 0.35 0.41
6.0
124
n-Butyrate
1.93 1.58 1.26
2.17
Esterification of Dewatered Linters, Using Zinc Chloride Catalyst Inside Intrinsic Viscosity in MeCl1:MeOH Temp., Time, F. Hours % Scyl Acyl/G.U. (9O:lO) 142 130 126
Propionate
...
...
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
materials were triesters but without combined catalyst in either case. Degradation during Esterification of Cellulose. The loss of viscosity during the esterification of dewatered linters with the lower aliphatic anhydrides, using sulfuric acid catalyst, is given in Table VI1 and Figure 6. The intrinsic viscosities of the partially acetylated samples were determined by dissolving these samples directly in cuprammonium solution. The sample weights were adjusted according to the amount of acetyl present to give a constant weight of cellulose. As the highly esterified cellulose propionates and butyrates \yere not readily soluble in cuprammonium, all the samples of these series were deesterified prior to dissolving in cuprammonium. For this purpose the samples were milled to 20 mesh and treated with 0.5M sodium methylate in methanol a t room temperature. This gave regenerated cellulose not by saponification but by transesterification ( 7 1 ) , forming methyl esters of propionic and butyric acids. Lack of appreciable degradation during this method of de-esterification was shown by the constancy of intrinsic viscosity as the time of treatment with sodium methylate was increased. There was good agreement between intrinsic viscosities of regenerated cellulose samples obtained in this way, and of acetate esters dissolved directly in cuprammonium solution. Degradation became more severe in going from the acetate to the higher esters of the series. There was only a small difference between the propionate and the n-butyrate, but a big difference between the n-butyrate and the isobutyrate.
literature Cited Not completely soluble
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Dreyfus, H., Brit. Patent 308,322 (March 13, 1929). Ibid.,309,201 (Dec. 15, 1929). Mallabar, J. H. (to Non-inflammable Film Co.), Zbid., 239,724 (June 7, 1928). Malm, C. J., Barkey, K. T.. May, D. C., Lefferts, E. B., IXD. ENG. CHEM. 44,2904 (1952). Malm, C. J., Nadeau, G. F., Gcnung, L. B., IP\D. END.CHEM, AYAL.ED.
14, 292 (1942). Malm, C. J., Tanghe, L. J.. IND. ENG.CHEM. 47, 995 (1955). Malm, C. J., Tanghe, L. J., Laird, B. C., Smith, G. D., ilnal. Chem. 26, 188 (1954). hlurray, T. F., Kenyon, W. O., J . Am. Chem. Soc. 62,1230 (1940). Sadeau, G. F., Gould, A. J., Eastman Kodak Co., unpublished data. Nebel, W., U. S. Patent 1,478,137 (Dec. 18, 1923) Zeniplen, G., Ber. 59, 1254 (1926). RECEIVED for review July 29, 1957 ACCEPTED December 12, 1957 Division of Cellulose Chemistrv. 132nd Meeting, .4CS, New York, N. Y . ;'September 1957.
