INDUSTRIAL AND ENGINEERING CHEMISTRY
304
Vol. 34, No. 3
LITERATURE CITED
which will afford the maximum conversion. The maximum per cent conversion may be found from Figure 7 . The slope of the curve in Figure 8 is 1.14 X lo4degrees, and the activation energy is 52,000 calories per mole.
(1) Hass, H. B., Hibshman, H. J., and Pierson, E. H., IND.ENC. CHEM., 32, 427 (1940). ( 2 ) Hass, H. B., and Hodge, E. B., U. S.Patent 2,071,122 (Feb. 16, 1937); Canadian Patent 382,346 (March 21, 1939). (3) Hass, H. B., Hodge, E. E., and Vanderbilt, B. M.,IXD.ENG. CHEM.,28, 339-44 (1936). (4) Hass, H. B., Hodge, E. B., and Vanderbilt, B. M., U. S. Patent
ACKNOWLEDGMENT
1,967,667 (July 24, 1934); Brit. Patent 443,707 (June 30,
1937) ; Canadian Patent 371,007 (Jan. 4, 1938). (5) Hoskins ICZfg. CO.,Chrome1 Catalog L, Chicago, 1937. (6) Landon, G. K., U. S. Patents 2,161,475 (June 6, 1939); 2,164,774 (July 4, 1939). (7) Kolbe, J. prakt. Chem., 5, 427 (1872). (8) Rinelli, W.R., and Willson, R. S., IND.ENG.CRBM.,ANALED., 12, 549 (1940). (9) Whitwell, J. C., IND. ENQ.CHEM.,30,1157 (1938).
The financial assistance from the Commercial Solvents Corporation, in cooperation with the Purdue Research Foundation, without which this project could not have been undertaken, is appreciated greatly. Thanks are due W. E. Fish and John Hession for their kind assistance in the design and construction of the apparatus.
Studies in Esterification PREPARATION AND PROPERTIES OF STARCH PROPIONATE Darrel
E.
M a c k ' and
R.
Norris Shreve
Purdue University, Lafayette, Ind. tarch tripropionate can b e made by refluxing, w i t h vigorous agitation, starch, propionic acid, and propionic anhydride. There i s some anhydride formation within the starch during the reaction. The acid esterifies about 8.7 times a s fast as the anhydride. The rate equation
S
e
Cl
= -i0g-
K
+
czx 1 I-x
can be used to f o l l o w the reaction, where K i s the rate constant, C l and C2are functions OF the starting concentrations, x is the fraction esterified, and 0 i s the time. The finished product i s soluble in many organic solvents, insoluble in water, and can b e used to give useful protective coatings on metal, wood, paper/ cloth, etc. The sheets prepared of plasticized starch propionate w e r e too weak for practical use.
HE low price of starch at $0.037 per pound (d), together T with the entrance of propionic acid and anhydride into the field of low-price organic reagents, makes the study of the propionic acid ester of starch of considerable interest as a possible cheap film-forming agent. That starch esters have many potential uses is evidenced by the recent appearance of starch acetate on the market ( 3 ) . The product investigated was a propionate corresponding t o a completely esterified starch. Although the less completely esterified starches probably have useful properties they were left for future investigation. 1
Present address, Lehigh University, Bethlehem, Penna.
A11 the reactions were run without catalysts; the possibility was eliminated that they would change or degrade the products in any way, and all runs were thus comparable. It is possible that the various catalysts used by other investigators of starch esters accounts in large part for the discordance of results when their data are compared. Sutra (7) and Reich and Damanski (8) noted that sulfuric acid or phosphoric acid catalysts may degrade the starch. Higginbotham and Richardson ( 2 ) stated that sulfur dioxide plus chlorine catalysts degrade the product, but that pyridine has no effect. The amount of pyridine needed and the difficulty of its recovery make this cat'alyst economically impractical. The writers' own observations show that sodium propionate also tends to decompose the starch. E X P E R I M E N T A L PROCEDURE
The apparatus (Figure 1) consisted of a round-bottom flask of about 100 ml. capacity, fitted with condenser and variablespeed stirrer. It was heated by a controlled-temperature hot oil bath. The reagents were propionic acid (99f per cent), propionic anhydride (98f per cent), and Amaizo Common starch analyzing as follows ( 1 ) : sulfur dioxide, 0.003 per cent; acid, 0.18; total protein, 0.37; ash, 0.10; water-solubles, 0.10; and insolubles. 0.22. The viscositv of the starch was < 100 cc. and its pH was 4.'7. To make a run, the starch was dried to constant weight at 110' C. The reagents were introduced into the flask, the stirrer was turned on, and the oil temperature raised to give the desired reaction temperature. When the run was completed, the flask contents were poured into an evaporating dish, the flask was washed with acetone, and the contents and washings were evaporated to dryness. To make sure that the reaction did not continue during the rocess, drying was carried out at a low temperature (40-50' C.f The residue was then ground in a mortar and dried a t 110' C. t o constant weight. This product was analyzed for ester content according t o the following procedure: One hundred milliliters of c. P . butanol plus exactly 10 ml. of about 4 per cent sodium hydroxide in butanol were heated to boiling in a stirred flask fitt'ed with reflux condenser. One-half gram of sample was then introduced and boiled for about 1.5 minutes. (Less boiling was not sufficient to saponify the ester completely. Longer boiling tended to decompose the liberated starch with the formation of a brown color which masked the end
305
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1942
point.) Fifty milliliters of water were then added, and the solution was titrated with standard acid using phenolphthalein indicator. A blank run was made for each analysis using about as much starch as the ester sample would contain. It was necessary t o add starch to the blank as it also reacted to a slight degree with the caustic. Calculation gave the acid value of the starch propionate. Since the products were all carefully evaporated to dryness and no loss of solids occurred, the yield based on starch content should have been substantially 100 per cent. I n most of the runs, and in all the runs where anhydride was present, the starch as ester in the product was always less than the starch as such in the beginning. As no solid product was lost during the manipulation, the starch itself must have lost weight during esterification, probably through anhydride formation within the starch molecule. Radley (6) speaks of this effect in the formation of dextrin. The amount of water split out, exclusive of the esterification, was calculated as follows : EXAMPLE 1, Rux 100: Acid value = 0.684 gram acid/gram ester Starch at start = 9.5 grams Total dried solids = 18.5 grams Acid reacted with starch = (0.684) (18.5) = 12.65 grams Water of esterification = (12.65) (18)/74 = 3.07 grams Starch in product = 18.5 3.07 - 12.65 = 8.92 grams Loss in weight of starch = 9.50 8.92 = 0.57 gram Moles water lost per molestarch = (0.57) (162)/(18) (9.5) = 0.54
+
ables kept constant, the effect of stirring was investigated. The results are given in Table I1 and Figure 3. These figures show that there is almost a 50 per cent variation in the reaction speed with a change in stirring. As would be expected, there is less variation a t the low and high speeds and considerable change a t medium speeds. The logical explanation for this peculiar effect is that, as the starch granules begin to react, their surfaces become partially esterified. A partially reacted starch (30 to 80 per cent ester) is gummy and swells, but does not dissolve in the reagents. The result of stirring would be to abrade off this gummy film which tends to keep the rest of the starch from reacting. The best type of stirring would, then, be that which gives a maximum of turbulence. It would rub off the interfacial film and keep fresh surfaces continually exposed.
VARIABLE - S PE ED MOT0 R
-
With this information the per cent esterification can be calculated. The loss in weight of the starch makes the acid value higher than it would be if the starch did not lose weight, so it must be corrected to a no-weight-loss basis. 2: Cor. value = (0.684) (18.5)/(18.5)
EXAIMPLE
+
(0.57) = 0.664 gram acid/aram - theoretical ester actual acid value 0.664 = yoesterification = acid value for 100% ester = -0.670 ”’%
The water split out in this manner is given in Table I and plotted in Figure 2. It corresponds roughly t o 1 mole of water lost for every 6 moles of acid added.
TABLE I. WATERLOSTDURING ESTERFORMATION Run No. 71 72 73 74 75 77 7s
SO
81 82 83 84 85 86
Per Cent Ester 76 88.5 77.5 71 72.5 95.5 72 97.5 88 59.5 99.5 31.5 85 95.5
Moles Water Lost/Mole Starch 0.19 0.00 0.38 0.10 0.286 0.665 0.475 0.35 0.285 0.20 0.46 0.14 0.475 0.706
R u n No. 87 88 90 91 92 93 94 95 96 98 100 102 103
Per Cent Ester 94 73 64 31 93 60.5 93 87.5 86 96 99 79 9s
Moles Water Lost/Mole Starch 0.308 0.285 0.34 0.17 0.41 0.465 0.62 0.475 0.315 0.10 0.54 0.475 0.64
At first it was thought that the starch was not thoroughly dried; but the following experiments showed that it was dry (one per cent accuracy) : Dried in oven at llOo C . for 24 hr. Dried under v a c u u m a 110’ C. for 24 hr. Ground in ball mill 48 hr., dried in oven at 150° c. for 24 hr.
No weight loss No weight loss No weight loss
EFFECT OF STIRRING At first it would seem that the material need only be kept in suspension to accomplish the desired esterification, but as the acid value continued to vary widely with all other vari-
TEMPERATURE Figure
CONTROLLER
1. Esterification Apparatus
To keep conditions of stirring as uniform as possible, subsequent runs were made in the same apparatus, the stirrer was run at a uniform speed, and the total amount of solution was the same (50 ml.). It was even found necessary to keep the depth of the stirrer constant for all runs.
TABLE 11. VARIATION OF ESTERIFICATION WITH STIRRING [lo grams starch R u n No. 71 72 73 74 77 75
78
Jacket Tfmz., 165 165 165 165 190 165 165
(5% HzO), 10 grams acid, 40 grams anhydride] Reaction Stirrer Time Per Cent Tyz., Speed, R. P. M. Hour; Eater 149-146 151-148 151-148 151-148 152-150 150-146 151-148
875 1250 675 400 675 1700 675
24 24 24 24 24 24 24
76 88.5 77.5 71 72.5 95.5 72
306
VoL 34, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
EFFECT OF TEMPERATURE The highest temperature used was the reflux temperature of the anhydride-i. e., 156" C. Two experiments were made to determine whether the starch decomposes a t this temperatu're. A sample was heated to 160" C. for 24 hours in the oven; no weight loss occurred. Several days of heating still gave no weight loss, but the starch turned slightly brown. Another sample was boiled for 24 hours with dibutyl ether (boiling point 138" C.) as a water-removing agent. At the end of this time no water had separated, as shown by no drop in the boiling point of the ether, and the dried starch had not lost weight. Variation of esterification with reaction temperature is given in Table I11 and in Figure 4. A higher temperature is favorable to a faster reaction, as would be expected. The limiting temperature was that of reflux, which would be the maximum temperature obtainable under ordinary conditions. This is probably about the optimum temperature anyway, since the starch would tend to decompose above 160"
no appreciable progress, even over extended periods; the red t was that the starch gradually decomposed under the prolonged treatment. This left only the anhydride as a practical water-removing agent; and for this case the three variables were acid concentration, anhydride concentration, and starch or ester concentration. If the latter were an insoluble substance, its concentration would not affect that of the acid and of the anhydride except as the water formed by the reaction changed them. The question remained as to whether the starch ester behaved as if it were in solution or only as a dispersed colloid acting as a solid. Several runs were made, varying the amount of starch but holding the acid and anhydride concentrations the same. These experiments showed that the original starch concentration at the start had little effect on the reaction speed. I n other words, i t behaved a t all times as if it were a solid and never as a solution, as far as the concentrations were concerned. These data will be shown later.
c.
TABLE 111.
VARIATION O F
REACTION WITH
100
$u
TEMPER.4TURE
[lo grams starch (5% HzO), 10 grams acid, 40 grama anhydride, 24 hours, 075 r. p. m. stirrer speed] % Ester Run S o . Reaction Temp., a C. 63 58 138 77.5 73 152 75 152 73 73 78 151 68 96 145 109 117 22.5 40 0
00
cld L
a0 u)
w
ro
...
STIRRER SPEED
r
EFFECT O F C O N C E N T R A T I O N O F R E A G E N T S To be able to run the reaction without anhydride would be an advantage. If this was done and the water was allowed to build up, the reaction proceeded slowly up to about 50 per cent ester or about 2 per cent water. At this point the starch ' began to hydrolyze, as shown by a considerable gain in weight which could be accounted for only by the addition of water.
ao
dxdB - rate of esterification
60
A + 3Sx A + B
0-
(1
5 2
fraction unesterified acid concentration at any time (mole fraction) anhydride concentrationat any time (mole fraction)
40
Since the two reactions are going on a t the same time, the total rate of esterification will be the rate by the acid,
W U
D1
2
=
-=
r
z
- z)
BA-:F
w CIL
c
Since either the acid or the anhydride may enter into the reaction, the speed with which it proceeds is probably dependent upon both concentrations. This can be expressed mathematically as follows: S = moles of starch present at start A = moles of acid present at start B = moles of anhydride present at start x = fraction of esterification at time 8, in hours
100
z
Figure 3. Variation of Esterification with Stirring
20
(%),= (W(1 - 4 (-)A + 3Sx 02
0.4
(1)
0.6
MOLES WATER PER MOLE STARCH
plus the rate by the anhydride,
Figure 9. W a t e r Lost fromstarch d u r i n g Ester Formation Also, its physical characteristics changed. It was soluble in the reaction medium a t about 50 per cent ester value, whereas the regular product did not dissolve until about 90 per cent ester was formed. The slowness of the reaction darkened the product considerably. If the anhydride was omitted and a water-removing agent, such as dibutyl ether or naphtha, was used, the reaction made
dx =
(sB) - + + p 4 k 1
ki)
klA
(3)
k21.
This integrates to: A + B
= 38S(ki
- kz) + kiA + kzB
+
3Sz(ki - kz) i- kiA kzB (1 - x)(hA kzB)
+
(4)
March, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
To test the accuracy of Equation 4,the data of Table I V were taken, and the k values of Equation 4 were determined by trial and error to be: kl = 0.130, k2 = 0.015. Equation 4 then reduces to : (2.3)(A
+ B)
e = (0.345)s + (O.13)A + (0.015)B log
+
+
( 0 . 3 4 5 ) s ~ (0.13)A (0.015)B (0.015)Bl (1 - x)[(0.13)A
+
(5)
307
down decomposition and to speed up the reaction, some anhydride was always present. The proportions of the reagents of series 111 were made so that most of the anhydride would be used up when the reaction was completed. This would be somewhat slower than it would be in 100 per cent acid, a condition which the authors could not obtain. I t s speed in 100 per cent acid can be easily calculated, however, and is equal t o
g
= kl(1
- 5)
Integrated this gives e = - l1n - = - 1 kl
TE M PE RAT u R E,
06.-
e where
The results in Table IV and Figure 5 show that Equation 4 fits the data quite well. The rate of esterification by the acid was about 8.7 times as fast as that by the anhydride, as shown by the fact that
8.7
OF ESTERIFICATION WITH TIME TABLEIV. VARIATION
Per Cent Ester Series I: S
-
Actual Time, Hr.
Calcd. Time, Hr.
0.587, A = 0.190, B = 0.280
88
90 91 100 108)
Series 11: S = 0.833, A 95.5 94 73 64 31 99 81
-
=
c, =
c1 -log IC
0.0554,B = 0.3570 41 42.5 47 39.5 30 23 22.5 19.5 12.5 9.2 71.5 58 27.5 27.5
-
1
(7)
+
C*x 1 1-2
(2.3)M (2.66)s A
+ B)
+ + (0.115)B
c* = A
(2.66)s
+ (0.115)B
For example, for the three series of data the equations are:
e
Series 111: S = 0.030. A 3 0.498, B 0.125 92 93 21.5 22.3 93 60.5 8.2 8.5 93 22.0 28 94 96 27.0 98 29.5 102 97 8 13.5 40 32.0 9s 103 Run made with half the amount of starch (+ on Figure 5,series I). b Run made using one third the amount of starch (+ on Figure 6, aeries 11).
+
22.0 log
0.72 1 1 - x
= 23.0 log
2.32 1 1 - x
Series I:
-
Series 11:
-0
Series 111:
+1 - 0 = 18.4 log 0.172 1-x
=
~
+ -
From Equation 8 the time required for any degree of esterification can be calculated. All that is necessary is to take enough points t o determine k for the equipment; then the reactant ratio can be varied a t will. HANDLING
86 87
2.3 0.130 log
IE
The fastest reaction would be run in acid with no anhydride present. Since an anhydrous medium is necessary to keep
Run No.
-x
When x = 0.99, 0 = 35 hours. Using the conditions of series I11 in Equation 5, when 2 = 0.99, 0 = 38 hours. Therefore very little time is to be gained by attempting to maintain 100 per cent acid concentration, although it would be cheaper to use no anhydride. The rate will vary, depending upon the equipment, but the ratio of the constants kl/k2 = 8.7/1 should hold. This enables Equation 4 to be reduced to the general form:
Figure 4. Variation of Esterification with Temperature
ki - = - 0.130 kf 0.015
1
OF
THE COMPLETED REACTION
The procedure used for studying the reaction obviously wastes all the excess reagents, so this method would not be used in manufacturing the ester. There are two possible ways of removing the ester from the acid. I n both cases the reaction product, which consists of starch ester and acid, is evaporated under vacuum to about 50 per cent acid content (ester basis). This gives a semisolid mass which is fluid when heated to 80-100° C. 1. The hot sirup can be dried in a thin sheet at 110' C. This gives a product similar in a pearance to flake shellac. It is difficult to dry out all the acifif the sheets are thick. The excess acid can be recovered in an activated carbon recovery system. 2. The sirup can be run into cold water with rapid agitation which precipitates the ester as porous granules. These are washed free of acid with water and dried at 110' C. All the acid is lost, and 2 to 5 er cent of the product forms a fine suspension which goes througt a coarse filter and plugs a h e one. It can be settled out on long standing and thus saved.
308
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure
5. Variation of Esterification with Time
PROPERTIES OF STARCH P R O P I O N A T E The products could be divided roughly into three groups: ( a ) a water-soluble ester which analyzes 0 to 30 per cent esterification; (a) a product partially soluble in water and organic solvents, analyzing from 30-90 per cent esterification; and (c) a n ester soluble in organic solvents, analyzing from 90-100 per cent esterification. The first and third groups were fairly homogeneous products, but the second was a mixture of various ester values. This was evidenced by the fact that some organic solvents dissolved part of the material and left behind a fraction soluble in water. There vas also considerable swelling without complete solution in either water or organic solvents. The material continued to give a blue color with iodine up to about 70 per cent ester, where the color disappeared without changing. Microscopic examination showed that the granular structure of the starch was completely destroyed in the higher esters. No particles could be seen. The medium ester values had a moderate number of granules, and the lower esters were similar to the original starch. The granules were not ruptured or slvollen during the process but seemed to dissolve slowly. STARCH P R O P I O N A T E OF 93-1 00 PER CENT ESTERIFICATION During the reaction this material gradually became brown. This color depended largely on the time of heating. The following table shows the extent of the color in the completed reaction: Heating Time, Hourb 10 20 30 40 60 60
Vol. 34, No. 3
Color Off-white Light amber Amber Dark amber Brown Dark brown
The darker colors could all be bleached to light amber by adding (ester basis) about 0.5 per cent hydrogen peroxide (30 per cent) to the ester in solution. Benzoyl peroxide bleached the material somewhat but not so well as the hydrogen peroxide. The finished product, bleached and ground t o a powder, was an odorless, nonhydroscopic, amorphous substance which showed no particles when inspected in solution under the microscope. It softened at about 200" C., melted a t about 230°, and slowly decomposed a t about 260". It was insoluble
in water, and n-as unaffected by boiling for an hour w t h 2 per cent sulfuric acid or 2 per cent sodium hydroxide. I t reacted slowly over a prolonged period (2 weeks) with cold 2 per cent caustic to give a dark resinous mass. It was unaffected on prolonged contact with cold 2 per cent sulfuric acid. Its solubility in some organic solvents is shor\-n in Table IT.
TABLE
1 ' .
SOLUBILITY OF STARCH PROPIONATE IN COLD
SOLVE~VTS
Water Insol. Insol. 2% &So4 Ins01.~ 2% NaOH Methanol Insol. Ethanol Insol. Isopropanol s. S.5 %-Butanol s. s. Amyl alcohol s. s. 2-Ethylhexanol s. s Diethyl ether Insol. Insol. Isopropyl ether Insol. Dibutyl ether Ethylene glycol s. s. Glycerol s. s. Gasoline s. s. Kerosene s. s. s. s. Lube oil s. s. Turpentine e Discolors on long standing. 5 grams in 100 cc. of solvent.
Castor oil Linseed oil (raw) Benzene Toluene Xylene Dimethyl phthalate Dibutyl phthalate Acetone Mesityl oxide Carbon tetrachloride Ethylene dichloride Tetrachloroethane Acetic acid Propionic acid Methyl acetate Ethyl acetate Butyl acetate
Swells Swells Dissolve@ Dissolves Dissolves Dissolves Swells Dissolves Dissolves Dissolves Dissolves Dissolves Dissolves Dissolves Dissolves Dissolves Dissolves
* Swells slightly.
Several of the more common resins were tested for compatibility with the starch ester. Equal amounts of resin and ester were dissolved in asuitable solvent and allowed to eraporate t o dryness as a thick film. Visible separation of the constituents or opaqueness were used as criteria of incompatibility: Dammar resin Ester gum Shellac Rosin Gum mastic Kauri gum Cellulose acetate Celluloid
Partially compatible Partially compatible Compatible Compatible Compatible Compatible Incompatible Incompatible
March, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
A coating of pure starch propionate shrinks enough on drying to crack, although its hardness is comparable to any of the common fossil resins or to shellac. To eliminate this fault and to give flexibility to the films where it was desired, a few standard plasticizers were tried. Castor oil separated out on drying the film. Butyl stearate and butyl lactate did not accomplish the desired effect up to 50 per cent plasticizer (based on ester), since the resulting films were too brittle. Tributyl phosphate gave a film which was too soft and at the same time brittle. Triacetin and tributerin tended to dissolve out in water and leave the film brittle. Dimethyl phthalate was good for a while but slowly evaporated. Dibutyl phthalate, dibutyl Cellosolve phthalate, and tricresyl phosphate all gave good results. The films did not change after soaking in water, did not embrittle on aging, and were not sticky. Of these three, dibutyl phthalate seemed to be best and least of it was required for a good effect. Varying amounts had to be used, depending on the use to which the films were put. The films were not strong. Some of the thinner ones were so delicate that a special technique had to be used in their preparation: A thick solution of sugar, water, and a little soap (to aid spreading) was used to glue a sheet of unglazed paper to a glass plate. It was given a coating of the sugar solution and dried. The starch propionate solution was brushed on until enough coats were added to give the desired thickness. After drying, the paper and film were easily peeled from the glass plate. Soaking in water served to dissolve the sugar solution and release the film from the paper. For these thin or fragile films 35 per cent (ester basis) of dibutyl phthalate was used. This concentration was also used to make
309
coatings on cloth or any material where great flexibility was demanded. Although quite soft (rubbery), they did not stick together unless heated. For paper coatings where a reasonable amount of flexibility but no crumpling was involved, 20 per cent plasticizer was used. Nonflexible coatings of rigid materials, such as wood or metal, required only 5 per cent. ACKNOWLEDGMENT The authors wish to thank the American Maize Company and the Tennessee Eastman Corporation for materials generously 'given. LITERATURE CITED (1) Goodman, A. H., private communication, April, 1941. (2) Higginbotham, R. S., and Richardson, W. A,, J. SOC.C h e m Ind., 57, 234-40 (1938).
(3) Niacet Chemicals Gorp., General Catalog, 9th ed., 1939. (4) Oil, Paint Drug Reptr., 139,No. 17 (April 28, 1941). . (5) Radley, J. A., "Starch and I t s Derivatives", p. 155, New York, D. Van Nostrand Co., 1940. (6) Reich, W. S., and Damanski, A. F., Compt. rend., 196, 1610-13 (1933). (7) Sutra, R., Ibid., 196,1608-10 (1933). BABEDupon a thesis submitted by D. E. Mack to the faculty of P u r d u e University in partial fulfillment of the requirements for the degree of doctor of philosophy.
Monomercuration of Benzene enzene has been mercurated b y a method found t o produce exclusivelya monomercurated product. Optimum conditions have been established whereb y a 92 p e r cent of crude phenyl mercuric chloride of high degree of purity has been obtained. The same method for direct mercuration has been incorporated w i t h the use of a high-boiling solvent to eliminate the use of a Fressure vessel. The crude product w a s found difficult t o purify. The solubility of mercuric acetate in glacial acetic acid from 25" t o 100' C. has been determined.
B
A
ROMATIC mercuration, a typical substitution reaction, was recently classified as a unit process by Kobe and Doumani (6), who reviewed the general importance of the aromatic mercury compounds. These compounds are important intermediates for organic syntheses because of the ease with which the mercury can be replaced by halogens and particularly by other metals, such as arsenic, tin, selenium, etc. The aromatic mercurials of type R-Hg-X, in which X represents a wide range of anions, possess useful properties as antiseptics, germicides, and fungicides, and are f
9
Present address, University of Texas, Austin, Texas. Present address, Union Oil Company, Wilmington, Calif.
Kenneth A. Kobe' and Paul
F. Lueth, Jr.*
University of Washington, Seattle,
Wash.
incorporated in pharmaceutical and industrial preparations. The direct mercuration of benzene has been difficult, and the development of a rapid industrial process which gives high yields of monomercurated benzene is essential for the development of processes based on this compound as an intermediate. The direct mercuration of benzene has been studied by numerous investigators. I n general, benzene is heated with the mercuric salt, usually either the oxide or the acetate, in the presence of glacial acetic acid or acetic anhydride. Because relative amounts of these constituents, as well as time and temperature, are important variables, the yields of monomercurated product reported have varied widely. Critical data on the direct mercuration of benzene by previous workers are summarized in Table I. I n all cases the reactants were intimately mixed together a t the beginning and allowed to remain in contact for the requisite length of time. Under these conditions it was generally found that the yield of monomercurated product was decreased by the formation of varying quantities of the dimercurated product, or by the reduction of the original mercuric acetate to mercurous acetate or metallic mercury. Certain modifications were employed in some cases. Maynard (6) used ethanol in the reaction mixture to esterify
