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U. S. DEPARTMENT OF COMMERCE NATIONAL BUREAU OF STANDARDS
RESEARCH PAPER RP
Part of Journal of Research of the JXational Bureau of Standards, Volume 25, December 1940
PREPARATION OF BENZOIC ACID OF HIGH PURITY
By Frank W. Schwab and Edward Wichers
ABSTRACT A comparative study was made of methods for preparing benzoic acid of a purity not less than 99.99 mole percent. The methods stud~ed were fractional distillati'on in vacuum, recrystallization from water and from purified benzene, fractional freezing, oxidation of purified toluene with subsequent recrystalliZ'S;tion from water, and hydroly sis of purified benzoyl chloride. The best preparations were obtained by repeated crystallization from benzene, by hydrolysis of benzoyl chloride, and by fractional freezing. They had a purity of 99.999 mole percent as determined by the freezing range and by specific heat measurements. Th e free zi ng point of benzoic acid is tentatively given as 122.36 ± 0.01 0 C.
CONTENTS Page I. Introduction ___ __ _ ___ _______ ____ ___ _ ___ __ ____ ____ _ __ __ __________ 747 II. Criteria of purity __ __ ___ _ __ __ __ _______ _______ _ ___ __________ __ _ ___ 748
- Vaporizing and condensing behavior ____ ________ _ __ __ ____ ____ 748
- Crystallizing and melting behavior __ __________ ______________ 749 III. Stability of benzoic acid ______ ______ _______ ___ _______________ __ ___ 750 IV. Method s of preparation __ ______ ___ ___ ____ _ __ _______________ __ _ ___ 752
- From parent sub s tances by selected r eact ions __ ______ ______ __ 752 (a) Oxidation of toluene __ ___ ___ ___ ___ ___________ __ ____ 752 (b) Hydrolysis of purified benzoyl chloride ________ _ __ ____ 752
- By purification of a commercial product _ _____ ______ _______ __ 752 (a) Distillation _____ __ _ __ __ ___ __ __________ ____________ 752 (b) Fractional freezing _ _ __ __ ______ _ ______ _________ ____ 753 (c) Crysta llization from sol vents ____ _______________ ____ 753 V. Results and discussion ____ ___________ _____ _ ___ ___ __ ____________ __ 756
1. INTRODUCTION
Many properti es of substances are determined by comparative rather than by absolute methods. The accuracy of comparative meth- ods depends on the precision of the measurements and on the accuracy with which the properties of the standard substance used for com- parison are known. As methods of making comparative measur e- ments improve, an increasing demand is made on the accuracy with which the properties of the standard substance are determined. A high degree of accuracy in the original determination of the properties of this reference substance requires not only a high degree of uniformity of composition but also a close approach to ideal purity. Benzoic acid is commonly used as a standard substance for calo- rim etry and acidimetry, and "standard samples" of this substance have been issued by the National Bur eau of Standards for many years. Morey! made a stud y of the suitability of benzoic acid as a primary standard in acidimetry and alkalimetry. He found that it could be titrated with a standard alkali to a high degree of accuracy 1 George W. Morey, J. Am. Chern. Soc. M, 1027 (1912). 747
~ ----- - ------------------------------------------------,
748 Journal oj Research oj the National Bur eau oj Standards [Vol..e
and that this method afforded a rapid and accurate means of com- paring the purity of various pr eparations. Mor ey purified commercial benzoic acid by recrystallizing it from so l vents and fractionally subliming it in a vacuum. 2 Weaver 3 purified the acid in a manner simi lar to that used by Mor ey. Each fused the acid prior to titration. This treatment was designed to remove water that was absorbed during contact with air. When a redetermination of the heat of co mbu st ion of benzoic acid was recently und ertaken by th e H eat Measurements Section of this Bureau, with an anticip ate d pr ec ision of 0.01 perce nt, it became desirable to prepa,l'e b enzoic acid which would have a purity of at least 99.99 mole percent.
II. CRITERIA OF PURITY
Criteria of purity for a nearly pur e organic substance should be independent of known properties of the substance. Physical prop- erties are usually not known with sufficient accuracy to permit esti- mation of purity by comparison even though the identities of the com- pounds under consideration and of some of the impurities in it are known. Ind ependent criteria which are generally used depend on (1) vaporizing or condensing behavior and (2) freezing or melting behavior. If, like benzoic acid, the substance is a moderately strong acid, its acid value is also a useful ind ex of purity and is often so used. The acidimetric m et hod as commonly applied is not suitable, however, for determining differences in purity smaller than 0.01 percent.
- VAPORIZING AND CONDENSING BEHAVIOR The behavior on evaporation or condensation as a criterion of purity 4 is suitable only for substances that do not decompose sig- nificantly, and is generally utilized by measuring the boiling range und er co nstant pr es sure or by determining the initial and final vapor pressures during an isothermal vaporization or condensation. The method is most sensitive when the substance is distilled in an efficient fractionating column. The boiling points under the same external pressure, or the difference in vapor pr essure at the same te mp eratur e, of the first and la st fractions may then be compared. It is known that benzoic acid undergoes measurable d ec omposition at temperatur es 50° to 100° C below its normal boiling point (about 250 0 C). The identity of some of the products of decomposition is not known, but in the course of the present work it was found that one of the reactions at these temperatures is the slow formation of benzoic anhydride and water. Although the concel).trations of anhy- dride and of water are probably small under equilibrium conditions at 130 0 C or somewhat higher temperatures, transformation of the acid to these produ cts is rapid enough so that distillation in an efficient fractionating column for the desired separation is not practicable. It is obvious, th erefore, that this method cannot be readily applied under conditions most suitable for obtaining the highest sensitivity. , George W. Morey , J. Am. Chern. So c. 3t , 55 0 (1912).
- E. R. Weaver, J. Am. Chern. Soc. 35 , 1309 ( 19] 3).
- E. W. W as hburn, The problem of es tablishing the identity and pu ril y of a hydrocarbon obtained from petro- leum, J. Ind. En g. Chern. 22, 985 (1930). Also see Martin Shepherd, A crilical test for the purily of gases, BS J. Research 12, 185 (1934) RP 643. For a furth er discussion of this subject see W. S wi~to s lawski, Ebu- liometry /Chemical Publishing Co., New York , N. Y ., 1937).
750 Journal oj Research oj the National Bureau oj Standard's [Vol. .!
In the remainder of this paper all references to "purity" and "content of impurity" are based on determinations of the freezing range.
III. STABILITY OF BENZOIC ACID
Before it was possible to reach a conclusion as to the relative merits of various methods of pl,ll'ification, and of various tests of purity, it was necessary to investigate the stability of benzoic acid at tempera- tures above its melting point. Three samples of material currently issued by the Bureau as Standard Sample 3ge (containing 0. mole percent of impurity) were heated in sealed glass tub es (Pyrex)
at 200 0 C for 72 hours. One was filled with air and a second with
oxygen, each under a pressure of 1 atmosphere, and a third was evacuated. After this treatment the first contained 0.046; the second, 0.047; and the third, 0.025 mole percent of impurity, as estimated from the freezing behavior. The color of the sample heated in the evacuated tube was pale yellow; that of the sample heated under air, light brown; and that of the sample heated under oxygen, slightly darker brown. From these results it appears that of the slight amount of decomposition which occurs below 200 0 C more is to be attributed to oxidation than to dissociation of the acid into benzoic anhydride and water. It was concluded that any treatment of the acid involving heating
at temperatures not over 135 0 C for relatively short periods was not
likely to cause significant deterioration. More exact information about the formation of anhydride was obtained in another way. During the process of repeatedly measuring the freezing range of a very pure sample of benzoic acid, the acid was dried before freezing and stirred while freezing by passing through it air which had been dried by phosphorus pentoxide. The dried air was passed at a con- stant rate through the molten benzoic acid, which was maintained
at a temperature of about 130 0 C, for measured intervals between
successive freezings. A progressive but slow decrease in purity was observed. After freezing the sample five times, the total change was 0.005 mole percent. Since benzoic anhydride is a possible product of this treatment, it seemed reasonable that an increase in the partial pressure of the water in the air would reverse the reaction. It was found that a partial pressure of water in the ail' equal to 0.5 mm Hg (corresponding to 0.005 mole percent dissolved m the acid) 9 was sufficient to do this, and by this treatm ent the content of impurity was reduced from 0.005 to 0.001 mole percent. In another series of experiments a known quantity of benzoic anhydride (0.764 mole percent) was added to the benzOlC acid. The concentration of water in the liquid acid was controlled by passing through it air saturated with water at selected temperatures. The rate of hydrolysis at 130 0 C was measured by maintaining a known concentration of water during a measured interval of time, then remov- ing the excess of water from the liquid by passing through it air
- The relation be tw ee n the amount of water dissolved In benzoic acid and the partial pressure of water In the air with which th e acid is In equilibrium was determined approximately by observing the depression of th e fr ee zing point of th e acid when frozen successively in equilibrium with dry air and with air containing 4.6 and 17.8 mm of water vapor. The Indicated partial pressures of water were established by saturating air with water at 00 0 and at 20.0 0 to 20 .3^0 0, respectively. At these partial pressures the observed depres· sions of the freezing temperature were 0.04^0 and 0.15°0, which correspond to 0.05 and 0.20 mole percent of dissolved water.
Schwab] Wicher8 Benzoic Acid oj High Purity^^751
dried over phosphorus pentoxide and observing the change in freezing temperature. The results in table 1 show that the reaction may be represented by the equation
TABLE 1.-Rate of hydrolysis of benzoic anhydride ITemperatur e= 130 0 CJ
Minutes IMolecent of per· a IMolecent of:z;per· I a-x a
a-x I log_a a-x^ I
390 240 180 120 300
. . . .
Concentration of water =0.31 mole percent
. . . .
.
. 24 . .
. 1931 . . . 1614
k
. .0 011 . . I------~____~____-L____~____________~_____
. 0003
Since the concentration of water is held constant and the rate of the reverse reaction is negligible in comparison with the rate of hydrol- ysis, the reaction is of the first order. The velocity constant for this reaction, for a given concentration of water in the liquid, may be calculated from the equation
in which
1 a k= - ln ---,
t a-x
a=mole percent of benzoic anhydride at the beginning, x=mole percent of benzoic anhydride changed to resultants, t=time in minutes, k=velocity constant.
No attempt was made to measure the rate of hydrolysis accurately. However, the results show that the rate of re action is dependent on the concentrations of water and of anhydride. After the last obser- vation 0.18 mole percent of anhydride remained. Several milliliters of water was then added to the acid, and the mixture a.gain heated to 130 0 C. The excess water was removed by washing with dry air. After this treatment, the purity of the acid was 99.996 mole percent, which indicat es that substant ially complete hydrolysis of the an- hydride had taken place. Since the acid was kept at about 130 0 C for 50 hours before and 30 hours during this course of treatment, its final purity shows that, except for the formation of anhydride, benzoic acid is remarkably stable at temperatures not much above its melting point. As previously noted, the formation of the anhydride occurs only when the acid is in contact with very dry air..
Schwab] WichlfS Benzoic Acid oj High^ Purity^^753
then distilled into a second receiver in thc same way. A comparison of the freezing ranges of these three fractions showed no improvement in purity in the middle fraction and a slight decrease in the end frac- tions. Evidently the impurities in the original material did not differ enough in vapor pressure from benzoic acid to be removable by a simple process of distillation. In view of the difficulties of operating an efficient rectifying column at low pressures, above the melting point of the acid (122.4° C), this method was abandoned. E. R. Weaver of this Bureau has suggested that sublimation in a partial vacuum would be more successful than distillation, provided the temperature of condensation (in the solid phase) were properly regulated and the vapor space ventilated so that the vapor phase would never become saturated with any of the impurities. In view of the success of other methods of purification this procedure was not tried. (b) FRACTIONAL FREEZING
Fractional crystallization ran be accomplished either with or with- out the use of a solvent. The latter method, fractional freezing, was suggested by the use of the freezing range as a criterion of purity. This method was first applied by allowing the molten acid to freeze rather slowly in a Dewar flask (method 1). Solid and liquid were kept thoroughly mixed. When the substance was one-half to two- thirds frozen, the remaining liquid was drawn off by suction. The solid portion was then remelted and the process repeated. The increase in purity which resulted from the use of this method was somewhat slower than was expected. This indicated that a satis- factory approximation to equilibrium freezing was not realized or that the crystals could not be properly drained. The head fraction had an estimated purity of 99.998 mole percent. A different method of fractional freezing (method 2), used by R. S. Jessup, of the Heat Measurements Section, was more successful. He lowered a glass tube containing the molten material (3ge) very slowly from a heating coil. Freezing began at the bottom of the cell and progressed upward until the whole mass was frozen. The portion remaining liquid was stirred constantly. About 50 grams was frozen in 6 hours. The glass tube was cut so that the last portion of the solid to freeze (one-third to one-fourth) was discarded. The process was then repeated with the remainder. Purification by this method was more effective than by the first method of fractional freez- ing. The purity of the best preparation exceeded 99.999 percent. The substance was allowed to freeze very slowly and nearly ideal conditions for separation were evidently obtained.
(c) CRYSTALLIZATION FROM SOLVENTS
In recrystallizing benzoic acid from solvents, ethyl alcohol, which is sometimes recommended for this purpose, was found unsuitable because ethyl benzoate is readily formed. Water and benzene are satisfactory, with a slight apparent advantage in favor of the benzene in the rate of purification by successive batch recrystallizations. The benzene was carefully purified by distillation in a highly efficient rectifying column. In each case the solvent was saturated with
benzoic acid at about 100° 0, the solution cooled in the usual way
to about 15° 0, and the recrystallized acid separated on a fritted
--~- I
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glass funnel. One hundred grams of a saturated aqueous solution contains 0.25 g of benzoic acid at 15° 0 and 5.9 g at 100° 0; 100 g of a saturated solution in benzene contains about 7 g of benzoic acid at 15° 0 and about 78 g at 100° OY Benzene is a more convenient solvent but not as readily available in high purity as water. The results in table 2 show that purification under the conditions which were maintained in these experiments was more efficient from benzene than from water. After favorable experience with batch recrystallization, an apparatus was built for continuous recrystallization combined with extraction. 12 The apparatus, shown in figure 1, was essentially an elongated U-tube occupying the full height of the laboratory (about 10 feet), with a condenser connecting the upper ends of the two arms. An enlarged part near the bottom of the larger arm of the U served as a container for the solid benzoic acid. A fritted glass disk, sealed in place near the bottom of this container, supported the crystals. The funnel- shaped part of the apparatus below the disk was designed to minimize channeling. The solvent (benzene was used in the work here de- scribed) was introduced by suction into the small arm through the solvent inlet and circulated by gravity. The solvent was saturated at room temperature by dissolving benzoic acid from the bottom of the crystalline mass supported on the fritted disk. Light tapping caused the remainder of the crystalline mass to settle as this process went on. Solvent was added until the saturated solution reached the desired level in the boiler. The liquid level ~ the two arms was not the same because of the difference in density of pure solvent and saturated solution; however, the arms above the boiler were long enough to com- pensate for this difference. An electric heating element served to warm the boiler, from which nearly pure solvent distilled through a water- cooled condenser into the opposite arm of the apparatus. Evaporation was allowed to proceed at a regular rate while crystals separated in or just below the boiler. The crystals fell slowly through about 180 cm of the saturated solution and settled on the mass of solid already retained on the fritted disk. During this process the crystals were purified by washing and exchange (reaction) in a solution that was less concentrated with respect to the more soluble fraction than the solution in the boiler from which they separated. An inlet near the top of the container for the solid permitted the passage of a slow current of air up through the extractor arm, which was necessary to keep the crystals agitated and to prevent clogging. The air was saturated with benzene at room temperature to prevent loss of solvent by evaporation. When the apparatus was operated at atmospheric pressure, satisfactory sep- aration of crystals did not occur. It was found by trial to operate properly under a pressure of 215 mm. A receiver was connected with the boiler by a tube which was fitted to the extractor arm through a ground joint. Each time after the entire charge of benzoic acid had been dissolved and recrystallized in the apparatus (at the end of each cycle), a portion of solution was removed from the boiler; and thus a
11 Atherton Seidell, Solubilities of Inorganic and Organic Compounds, ed. 2, p. 133-135 (D. Van Nostrand Co., New York, N:Y ., 1919). 12 F. W. Rose, Jr ., of the American Petroleum Institute Project 6 at the National Bureau of Standards, made many helpful suggestions in the design and application of this method, which is an adaptation to solid material of the Fenske reflux extractor. (M. R. Cannon and M. R. Fenske, Ind. Eng. Chem. 28, 1035 (1936).
756 Journal of Research oj the National Bureau oj Standards [Vol. £
charge could be divided into as many fractions as desired, varying from the most soluble to the least soluble portions of the material. Each fraction was cooled to 15° 0, and the crystals which formed were filtered from the mother liquor. Separation per unit crysta llization was not as good in this apparatus as that obtained by batch crystal- lization. This was probably caused, in part, at least, by channeling and diffusion, which prev ented ideal separation.
V. RESULTS AND DISCUSSION
The estimates of purity of the preparations discussed in the fore- going paragraphs, together with others obtained in the course of this work, are collected in table 2. For a number of these, the purity was estimated both from the freezing range and from measurements of the apparent specific heat of the solid near its melting point.
TABLE 2.-Purity of various preparations of benzoi c ac i d
Purity (mole percent) Prepar ation (^) F. From rom freezlllg specific -h eat r ange measurements
Standard Sample 391.. __ _____ ___ __________ _____ __ ____________________ _______ _ Standard Sample 3ge : Untreated _______________________________________ __ _____________________ _ Recrystallized 4 tim es from wat er ______ _____ __________________ ___ ______ __ Recrystallized 8 tim es from water ___________ ____________ ________________ _ Recrystallized 4 times from benzene ____________________________________ _ Recrystallized 8 time s from benzene ____________________________________ _ Recrystallized from benzene(a) First cycle ____ ___ ____ in ___________________________________________ extractor: _ (b) Composite 3d to 8th cycles ___ ___ ________________________________ _ (c) Composite 9th, 10th, 11th cycles ____ ___ __________________________ _ (d) Composite 13th and 14th cycles _____________ ________ __ __ ___ __ ___ _ After fractionally freezing three times(a) Head fraction _______ ______________________________________ by method 1: ______ _ (b) Tail fraction _______________________________ _________ ____________ _ After fractionally freezing hy method 2:
(a) Once frozen ________ ____ ______________ __ _____________ ___ __________ ____ ___ ______ _ (b) Twice frozen ___________ __ ___ ___ __ _______ __ ______________________ _ 99. Heated(a) Inat (^) air (^200) ___^0 C _________________ for 72 hour s: _____ _________ __________ __ _____ ________ _ (b) In a vacuum ___________________________ _____________ ___ _________ _ (c) In oxygen _______________________________________________________ _ Hydrolysis of benzoyl chloride: Fraction(a) Vapor-phase 1: b hydrolysis _________________________________________ _ (b) Liquid-phase hydrolysis _________________ ___________________ ____ _ Fraction 2: 0
oxidati;~)ot~~i~;ldht;!:,~K~~~~~~~~~~=:::::::=:::::::: ::::::::::::::::::::::
954
- These values were obt ained by an earlier m et hod and are not as accurate as the other r es ult s. b Original product was recrystallized from water 7 times.
99.989,
99 .9 95,
99.998, 99.999,
o Original product was not recrystallized; h yd rog en chloride and water were removed by pa ss ing dry air through th e liquid. d Original product was recrystallized from water 4 times.
It will be seen that the best preparations obtained by crystallization from benzene, by hydrolysis of benzoyl chloride, and by fractional freezing are of substantially equal purity, as estimated from the freez- ing range, and that any of the three methods can be used to prepare benzoic acid of sufficiently high purity to meet very exacting require- ments. In using the substance two circumstances must always be
Schwab]
Wichers Benzoic Acid^ oj^ High^ Purity^^757
taken into account. One is that it is notably hygroscopic in the liquid phas e and the other that formation of anhydride occurs, very slowly, at temperatures near the melting point of the acid if the atmosphere in contact with it is very dry (much drier than is ever encountered without intentional and thorough drying). A valid objection may be raised to the use of the freezing (or melt- ing) behavior as a criterion of purity for a substance purified by frac- tional freezing. Quite apart from the fact that the freezing range of the substance purified by fractional freezing was substantially equal to those of the best preparations obtained by crysta llization from benzene and by hydrolysis of benzoyl chloride, the temperature at .any given stage of freezing was identical for the three preparations, within the reproducibility of the measurements (about 0.0020 0). This is independent evidence that these three preparations were iden- tical in composition, and hence were pure substances rather than mixtures of the rare type that has a constant freezing point. A brief comment on the freezing point of benzoic acid may be desirable. Estimation of purity from the freezing range is based entirely on sufficiently precise measurement of differences in tempera- ture, rather than on any exact knowledge of the actual temperatures of freezing. Since the freezing point of benzoic acid was not of direct interest in the work reported in this paper, the resistance thermometer and bridge were not kept under calibr ation in a manner to permit recording absolute freezing temperatures to 0.0010 0, althou gh differ-
ences in temperature of about 0.0001 0 0 during freezing were regarded
as significant. Further, the measurement of a freezing range involves the withdrawal of heat at a rate rapid enough to cause the observed temperature to differ somewhat from the true temperature of equilib- rium between the liquid and solid phases. These considerations limit the accuracy with which the freezing point of benzoic acid can be reported on the basis of the work with which this pap er is concerned. Th e value tentative ly assigned is 122.36 ±0.01° O.
WASHINGTON, September 27, 1940.
13 The average deviation was 0.002° C and the range was 0.005° C. The freezing temperature of the product obtained by fractional freezing fell between that of the other two.
