the last of the formic acid. The resulting sirup was cooled to 20' C. and seed crystals of glyoxylic acid monohydrate were added. The mixture was stirred from time to time. Within 24 hours the sirup changed to a mass of crystals. X-ray diffraction showed that the product was crystalline and not just an amorphous mass of solid material. No maleic, formic, or oxalic acid was found in the product. The glyoxylic acid was identified by making the semicarbazone ( 6 ) . T h e crystalline product contained 0.336 mole of aldehyde carbonyl (Z), which would correspond to a yield of 97.4y0 if all the aldehyde were glyoxylic acid and if the initial maleic acid were 100.0% pure. ANALYSIS.Calculated for C 2 H 2 0 3H20: . C, 26.09; H, 4.38; CO, aldehyde carbonyl (Z), 30.43. Found: C, 26.1 f 0.5; H, 4.0 f. 0.4; CO, 32.4. These results indicate that a little anhydrous glyoxylic acid was mixed with the monohydrate. Any impurities present are believed to be less than 3y0 of the weight of (crystalline product. No attempt was made a t further purification. Formic acid in the distillate from the vacuum stripping was identified by a Duclaux distillation. The first batch of crystalline glyoxylic acid (used thereafter as a source of seed crystals) was obtained by letting the sirupy monohydrate stand for several days in a flask at room temperature. The walls of the flask were scratched from time to time with a stirring rod, and the crystals finally appeared. Different batches of crystals were found to have slightly different x-ray diffraction patterns; this was probably caused by slight variation in the: proportion of water of hydration. The glyoxylic acid monohydrate was sometimes difficult to crystallize, especially in the summertime. I t was found helpful to keep the sirup with seed crystals in a closed container at a temperature not over about 20' C. while the crystals were growing.
Analytical Determination of Maleic Acid
As an aid in following the course of the ozonation of maleic acid, an analytical method was devised (7) for determining the concentration of maleic acid in the presence of both glyoxylic and formic acids. This method consists in measuring in a spectrophotometer the absorbance of an aqueous solution of known glyoxylic acid content [determined by the method of Smith and Mitchell (a), on the assumption that other aldehydes are absent] at a wavelength of 260 mp, and reading off the maleic acid concentration from calibration curves prepared with solutions containing glyoxylic, formic, and maleic acids in known concentration. (For the present purpose, calibration need be carried out only for solutions in which the molar concentration of formic acid is the same as that of glyoxylic acid.) Safety
Ozone is extremely toxic. The ozonation should be carried out in such a way that no one has to breathe an atmosphere containing more than 0.1 p.p.m. of ozone ( 9 ) . literature Cited
(1) Bailey, P. S., Chem. Revs. 58, 927 (1958). (2) Firdsall, C. M., Jenkins, A. C., Spadinger, E., Anal. Chem. 24, 662 (1952). (3) Eisenbraun, A. A , , Purves, C. B., Can. J . Chem. 38, 622 (1960). (4) Harries, C., Ber. 36, 1935 (1903). (5) Hendricks, R. H., Ph.D. thesis, St. Louis University, 1935; Univ. Microfilms, Ann Arbor, Mich., Publ. 183, 1935. ( 6 ) Mohrschulz, W., 2. Ekktrochem. 32, 434 (1926). (7) Moore, I$'. N., Pankhurst, R. G., Linde Division, Union Carbide Corp., unpublished work. (8) Smith, D. M., Mitchell, J., Jr., Anal. Chem. 22, 750 (1950). ( 9 ) Stokinger, H. E., Advan. Chem. Ser., No. 21, 363 (1959).
RECEIVED for review April 25, 1966 ACCEPTEDAugust 15, 1966
ESTERIFICATION RATES OF LONG-CHAIN HYDROCARBON DICARBOXYLIC ACIDS AND T H E STERIC ENVIRONMENT IN THE VICINITY OF THE ACID GROUPS R. H. QUACCHIA AND A. J.
D I M I L O
Aerojet-General Corf ., Sacramento, Gal$
long-chain, a,w-dicarboxylic acid hydrocarbons based on polymerization of butadienes are being used almost exclusively for the preparation of the new solid-rocket propellant binders, methods of characterizing these polybutadienes are necessary and are being developed. Reactivity of the acid groups with curing agents is one of the parameters which require close control to ensure reproducible propellant properties. An approach to characterizing the reactivity of the acidterminated polybutadienes was made by the use of esterification rates of these acids with excess 1-butanol in refluxing benzene. While the method was not fully developed as a characterization test for reactivity, the results of the investigation afforded an insight into the nature of the polymer structure in the vicinity of the carboxylic acid groups. ECAUSE
Experimental Rates of Esterification.
METHODA. RATE OF WATER FORMATION BY AZEOTROPIC DISTILLATION (4).A 3-liter three-necked flask containing 0.139 equivalent of acid-terminated polymer or aliphatic carboxylic acid, 110 grams (1.53 equivalents) of 1-butanol (Merck, reagent grade), and 1.3 liters of benzene (Matheson, reagent grade) was fitted with a thermometer, a 5-ml. (graduated in 0.1 ml.) Dean-Stark trap (Ace-Glass Co., No. 7735), and a Liebig condenser with drip tip and protected from moisture with a tube containing Drierite. The reaction system was dried overnight by azeotropic distillation using a 3-liter Glas-Col heating mantle with a Variac setting of 55 to maintain a gentle reflux. A 300-ml. flask containing 0.9525 gram (5 X mole) of p-toluenesulfonic acid monohydrate and 200 ml. of benzene was fitted with a Dean-Stark trap and a reflux condenser. The system was dried overnight by azeotropic distillation. VOL. 5
NO. 4 D E C E M B E R 1 9 6 6 351
A series of aliphatic carboxylic acids, acid-terminated polybutadienes, and acid-terminated hydrogenated polybutadienes was esterified and their rates were determined. The esterification was done with excess 1-butanol with p-toluenesulfonic acid in refluxing benzene. The kinetics of the reaction was determined by the rate a t which water was evolved and checked in some instances by titration of the acid with base. The results indicated that the acid-terminated polybutadienes and their saturated analogs have two types of acid groups which differ in reactivity. The difference in reactivity was shown to arise from the presence of some a-vinyl groups from 1,2-addition or allylic rearrangement of the final butadiene group. The evidence was based on comparison of the fast and slow rates for hydrogenated polybutadienes with the rates for model acids. The idea of steric hindrance is shown to b e quantitatively consistent with the data of Smith for the effect of substituents on the acid-catalyzed esterification of aliphatic acids.
The dried p-toluenesulfonic acid catalyst was added quantitatively to the dried acid-alcohol-benzene system and the formation of water with time was measured in the DeanStark trap with a cathetometer. A soft aluminum wire was used to scrape water from the sides of the condenser and DeanStark trap prior to each water measurement. METHOD B. TITRATION OF UNREACTED ACID GROUPS. A 5-ml. automatic buret, graduated in 0.01 ml., was used for the titration of 6-ml. samples withdrawn from the reaction solution described in Method A. Methanolic potassium hydroxide (0.1N) was used as the titrant, with phenolphthalein as the indicator for colorless acids and thymol blue for Butarez-CTL. CALCULATION OF RATECONSTANTS AND PER CENT CARGROUP. All the reaction rates were determined under pseudo-first-order conditions (10-mole excess of alcohol) a t 81' C. and produced >99% of the theoretical amount of water as based on acid titration values. A typical first-order rate plot of the data is given in Figure 1. The break in the normally linear plot is indicative of carboxyl groups of differing reactivity. The rate constant for the slower-reacting carboxyl groups is obtained from the slope of the line from the latter stages of the reaction. Extrapolation of this line back to zero time affords the amount of slow-reacting carboxyl group and by difference the amount of faster-reacting carboxyl group. A rate constant for the faster-reacting carboxyl group is obtained by replotting the initial data with a calculated infinity value from the amount of faster-reacting carboxyl group present. BOXYL
CONTROL EXPERIMENTS. Suitable control experiments were made a t the esterification conditions to eliminate the possibility
that water was being formed by other reactions. The reactions specifically considered and eliminated as side reactions under the conditions used were: formation of butene, dibutyl ether, butyl-Antioxidant 2246 ether (trademark of American Cyanamid Co., 2,2 '-dihydroxy-4,4 '-dimethyl-6,6 '-di-tert-butyldiphenylmethane), the ester of Antioxidant 2246 and carboxylic acid, and the Antioxidant 2246 ether. Esterification of a Mixture of Pelargonic and 2-Ethylhexanoic Acids. Method A was used with 15.824 grams (0.100 mole) of pelargonic acid, 14.264 grams (0.0988 mole) of 2-ethylhexanoic acid, and 148 grams (2 moles) of 1-butanol. The amount of water produced was 99% of theoretical. Esterification of NPGA. Method A was used, except that 2.53 moles of 1-butanol were used, to compensate for the possible total ester exchange and to assure pseudo-first-order rate conditions. Materials, Butarez-CTL, an a,w-dicarboxypolybutadiene obtained from the Phillips Petroleum Co. and used as received. Nine lots of this material were studied (Table I). Telagen CT-1, an a,w-dicarboxypolybutadiene obtained from The General Tire and Rubber Co. and used as received (Table I). Telagen CT-2, an a,w-dicarboxypolybutadiene obtained from The General Tire and Rubber Co. Analytical property ranges for three lots of this material used in esterification studies are given in Table I. Telagen CT-2 and CT-1 differ in method of preparation. Telagen CTH, a hydrogenated a,w-dicarboxypolybutadiene obtained from The General Tire and Rubber Co. The material used for the esterification was a sample made by combining equal amounts of two lots (Table I). Hycar-CTB, an a,w-dicarboxypolybutadiene prepared by B. F. Goodrich Chemical Co., was esterified without purification (Table I). Poly(neopenty1 glycol azelate), acid-terminated (NPGA). An a,w-dicarboxypoly (neopentyl glycol azelate) prepared by
1.9-
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Properfies of Materials Used in Esterification Studies Telagen C T H Telagen Butarez-CTL CT-7 C T-2 7 2 Table 1.
Molecular weight“ Equivalent weightb Acid content,c meq./eram Antioxidant 2246, ’%Unsaturation, mm./grarn cis, 70 trans. 97 Vinyi, ’% Pendant ethyl content,d ‘% Ash content, yo Q
From viscosity data.
b
...
5000-7000 2680-2950 0.349-0.373 0.94-1.16
2470 0.405 0.84
...
...
28 .8-35.4 37.3-40.7 27.2-30.6
0,003;. 03 By endgroup analysis.
... ...
...
2680-2910 0.378-0.399 1.12-1.33 38.2-42.0 28.4-38.0 21.7-23.4
...
6090 2510 0.397 1 .o 0.16
Hycar-CTB
...
3080 1850 0.540
2370 0.432 0.7
1 .o
..
0.30
... ... ...
... ... ...
26.8
30
52 .~ 18
30.7
:
0 . ooi-O.009 0.05 0.03 0 006 Correctedfor antioxidant. % of total unsaturation as vinyl beforepolymer hydrogenation.
Emery Industries, Inc., was used for esterification studies. The NPGA had an equivalent weight of 987 by the group analysis. Pelargonic acid (Emery Industries, Inc.) was purified by distillation (spinning band column). B.p. 129’ C./5 mm.; n v 1.4308. Stearic acid (Baker, 1J.S.P. grade) was used as received. 2-Ethylhexanoic acid (Matheson Coleman & Bell) was purified bv distillation (spinning ._ .,band column). B.p. 118’ C./10 mm:; n v 1.4234. 2-Ethvl-1-bromohexa.ne. DreDared from 2-ethvl-1-hexanol by the method of Kamin’an’d Marvel (3) for the‘preparation of alkyl bromides using 48% hydrobromic acid. B.p. 8688’ C./25 mm.; n’,” 1.4.514. 3-Ethylheptanoic ac id, prepared from 2-ethyl-1-bromohexane by the method of Gilman and Kirby (7) for the preparation of dl-methylethylacetic acid, except that the Grignard reagent was carbonated by use of powdered solid, instead of gaseous, carbon dioxide B.p. 107’ C./5 mm.; ny 1.4350. Discussion of Results
T h e first esterification runs with the carboxy-terminated polymers prepared from butadiene showed, and subsequent runs confirmed, that there were a t least two types of acid groups which differed in esterification rates by about a factor of 10 (Table IT). The variation of the relative amounts of the two types of acid groups was most pronounced for materials from different manufacturers when the method of polymer preparation was varied from anionic to radical polymerization or from a single manufacturer, in the case of Telagen CT-1 and CT-2, when a process change was made (Table 11). Lot to lot variations of the relative amounts of the two types of acid groups for a specific polymer were insignificant. For the anionically polymerized carboxypolybutadienes, the rates of esterification of leach type of acid groups, fast or slow, did not vary more than 25%. The differences observed between the two acid types could arise from steric hindrance to esterification by pendant groups in the vicinity of the carboxylic acid groups. These pendant groups would be vinyl g,roups which are formed by 1,2- or by the more unlikely 3,4-addition of the final butadiene group added before carboxylation of the anion. We prefer the designation ‘*3,4-addition” rather than “head to tail” because the latter designation becomes ambiguous with butadiene, which may also polymrrize in 1,4 or 1,2 fashion, and presupposes the mode of addition of the previous unit in the polymer chain. For instance, consider the following scheme
Table II. Rates of Reaction and the Distribution of Functional Groups in Acid-Terminated Polymers from Various Sources Fast Acid Slow Acid Group Group k, k, Polymer Preparation see. sec. Name Method X 706 yo x 10‘ 70 Butarez CTL Anionic 39 50 4 . 0 50
Telagen CT-1 Telagen CT-2“ Telagen CTH Hycar-CTB NPGAb
.4nionic Anionic Anionic, hydrogenated Free radical Condensation
40 48.5 21.4
37 13
a Process changed from preparation of CT-7. acelate) acid-terminated.
50 60 65
68 100 b
4.0 5.2 0.92
50 40 35
7.7
32
Poly(neopenty1 glycol
Table 111. Esterification Rate Constants for Various Acids with Excess I-CIHSOH in Refluxing Benzene Acid k, Set.-' X 705
Acid-terminated polybutadiene Fast-reacting group Slow-reacting group Pelargonic
39 4.0
Stearic 2-Ethylhexanoic 3-Ethylheptanoic Acid-terminated hydrogenated polybutadiene Fast-reacting group Slow-reacting group
18 24” 20 0.89 1.oa 3.6 21.4 0.92
a Determined in mixture of 50y0pelargonic and ~ 7 0 7 2-ethylhexanoic ~ acid. This determination gave the mixture composition as 49 and 57%, respectiDely.
Another possible method which can produce a pendant vinyl group is allylic rearrangement of the anion: -CH2CH=CHCH2e
+ -CHzCHeCH=CH2
The preparation of carboxy-terminated polybutadienes by the free radical process does not involve the addition of carbon dioxide to a preformed polybutadiene. The carboxyl groups are built into or derived from functional groups on the initiator or the chain transfer agent used for chain termination. For this reason the nature of the groups causing steric hindrance (if this is still the cause of two esterification rates) for the freeCH +-CH~CH=CHCHZCHCH~~ or -CHZCH=CHCH~~ radical-produced polymers cannot be known unless details concerning the production process are known. CH=CH2 A series of model acids was esterified under the same condi-CH 2CH z=CHCH zCH &He tions as the acid-terminated polybutadienes (Table 111). 1 Pelargonic, stearic, and 2-ethylheptanoic acids were chosen as CH=CHz models for the possible terminal configurations of the polyor ---CH~CH=CHCH~CHZCH=CHCH~~ butadienes. A comparison of the rate constants for the model
I
VOL. 5
NO. 4 D E C E M B E R 1 9 6 6
353
acids with those of the unsaturated polymeric ones would lead to the erroneous conclusion that the fast acid group was primary as in stearic and pelargonic acids and the slow acid group was the 3-ethyl type. T h e formation of a 3-ethyl acid required that the last butadiene unit be added in a 3,4- fashion. This type of addition is rare and unlikely to occur with the frequency required to explain the relative amounts of the two acid groups. Also, the rate constants for esterification of the saturated models differed from those of the unsaturated polymeric ones by a factor of 2 or more. A sample of hydrogenated Telagen was esterified and the rate of reaction determined (Table 111). The hydrogenated polymer also contains two acid groups with different reactivities. The rate constants for esterification match those of the model acids to a high degree. The fast-reacting acid group (k = 21.4 X 10-5 sec.-l) is similar to the acid group in stearic acid (k = 20 X 10-5 sec.-l). T h e slow-reacting acid group ( k = 0.92 X 10-5 sec.-l) is similar to the acid group in 2-ethylhexanoic acid ( k = 0.89 X 10-5 sec.-l). That the slow-reacting group is the result of steric hindrance by an 2-ethyl group is further borne out by comparison of the data with those of Smith ( 5 ) and by application of the Taft equations (6) for the correlation of esterification rates with substituents on the acid. Smith finds that the rate of esterification of butyric acid relative to 2-ethylbutyric acid is 60.1 a t 20’ C. This rate is calculated to be 27.5 at 80’ C. by application of the Arrhenius equation and activation energies of 10.0 and 12.4 kcal. per mole for butyric and 2-ethylbutyric acid. This agrees well with our value of 23.3 for the rate of esterification of the fast acid group relative to the rate of the slow one. For acid-catalyzed esterification ( 6 ) , log (klk,) = paAu AE, where k and kE = acid-catalyzed esterification rate constants for a straight-chain acid and its a-ethyl substituted analog, p a = the reaction constant, ACT = the difference in the substituent values, and AE, = the difference in the substituent steric factors. In calculating E, values, Taft assumed that to a first approximation the relative rates of acidcatalyzed esterifications were determined by steric factors alone. Thus for the substituents n-CzH7 (E, = -0.36) and (C2HJ2CH (Es = -1.98) (these substituents are used to compare butyric and a-ethylbutyric acids), log k / k E = AE, = 1.62 utilizing values given by Hine (2). The replacement of a-H by a-Et in butyric acid should cause the rate to decrease at 25’ C. The error in the stericf actor for (C*Hb)*CH to
+
is given as A0.29 (6), so the relative rates vary from 22 to 79. Smith’s data, which have been shown to be consistent with our own, give an experimental relative rate of 60 at 25’ C. for butyric and a-ethylbutyric acids. T o establish the validity of determining separate esterification rate constants of a mixture of acids, an experiment was run using a mixture of 50% pelargonic and 50% 2-ethylhexanoic acids. The rate constants for these two acids determined from the mixture were higher (1 1 to 30%) than those determined on the separate acids. These values are close enough to allow conclusions as to the nature of the steric effect, which is of the order of 2000%. Application of methods indicated above gave the composition of the mixture as 49y0 pelargonic and 51% 2-ethylhexanoic acid. This is within experimental error of the actual values. Conclusions
For anionically polymerized polybutadiene a considerable amount of 1,2-addition, allylic rearrangement of the anion, or both occurs. The amount of 1,2-addition or of allylic rearrangement which occurs on the last added unit before carboxylation of the polymer is reflected by the relative amounts of fast- and slow-esterifying groups in the acid-terminated poIymer. The speed a t which the acid groups esterify is very likely a function of the steric hindrance induced by the 2-vinyl substituent which results from 1,2-addition or allylic rearrangement of the last added butadiene unit. The relative amounts of the fast- and slow-reacting groups in the carboxylated polymers are not necessarily the relative amounts ol‘ 1,4- and 1,2-addition of units in the polymer as a whole, because, while the propagation of the polymer may be kinetically determined, the configuration of the final or end group may be equilibrium-controlled. These results have many implications in the use of acid-terminated polymers as units for the preparation of elastomers by condensation reactions. literature Cited (1) Gilman, H., Kirby, R. H., “Organic Syntheses,” A. H. Blatt, Ed., 2nd ed., Coll. Vol. I, p. 361, Wiley, New York, 1941. ( 2 ) Hine, Jack, “Physical Organic Chemistry,” pp. 276-80,
McGraw-Hill. New York. 1956. (3) Kamm, O.,’Marvel, C.’S., “Organic Syntheses,” A. H. Blatt, Ed., 2nd ed., Coll. Vol. I, p. 25, Wiley, New York, 1951. (4) Pratt, E. F., Draper, J. D., J . A m . Chem. Soc. 71, 2846 (1949). (5) Smith, H. A., Ibid., 61, 254 (1939); 62, 1136 (1940). (6) Taft, R. W., Jr., Zbid., 74, 3120 (1952).
RECEIVED for review March 21, 1966. ACCEPTEDOctober 20, 1966.
XANTHATION OF STARCH IN LOW-CO N CENTRAT IO N PASTES E. 6. L A N C A S T E R , L. T . B L A C K ,
H. F. C O N W A Y , A N D E . L
GRIFFIN, JR.
Northern Regional Research Laboratory, U. S. Department of Agriculture, Peoria, Ill.
xanthates have been crosslinked by oxidation or with heavy metals to give water-insoluble compounds that impart useful properties if incorporated as an integral part of papers and pulpboards (2, 3 ) . The particular advantage of papers prepared with xanthates is in a sixfold or more increase in wet strength. A continuous method of preparing the xanTARCH
354
I&EC PRODUCT RESEARCH AND DEVELOPMENT
thates has been developed ( 5 ) , but it requires that a heavyduty mixer blend the starch with a relatively concentrated alkali solution and the carbon disulfide. The xanthate mixture issues from the reactor as a plastic mass, which must then be dispersed in water before the xanthate is applied to the pulp slurry where the crosslinking is carried out.
