butylene oxide niix as a function of the time during which these components are in contact. These components on standing formed a bro\vn gummy sirup. 7’he supernatant liquid retained its catalytic activity. perhaps even having slightly greater activity, on standing for a day. ‘ I h e brown gummy 5irup which had formed during this same period of one day was not nearly as active as the supernatant solution; however. ‘lDI-80 did form a product in its presence on standing overnight. O n the basis of our results, it appears that these gummy products do not contain the active catalyst species. Earlier Xvork in our 1aboraror)- has shown. as indicated earlier, that these gummy products contained low molecular \veight polye thers. If, in fact, a zwitterion species is formed from an amine and olefin oxide, it would appear that this intermediate has an extremely loose association brt\veen the tkvo components, as \vas demonstrated by the following experiment. Triethylenediamine (6.6 grams) and propylene oxide (29.9 grams) Lvere mixed and allowed to stand in a closed vessel for 14 minutes. An aliquot of this solution (1.7 cc.) was pipetted into a flask, containing ‘TDI-80 (61 grams). Another aliquot (1.9 cc.) \vas placed in a flask, connected to a rotary evaporator, and pumped off a t room temperature bvith water aspirator vacuum. T h e solution added to the TDI-80 produced reaction in 14 minutes. T h e evacuated aliquot on treatment with TDI-80 gave no indication of reaction in 2 hours, but when this mixture was treated with additional propylene oxide (2.0 ml.), an exothermic reaction occurred in less than 5 minutes. If the zwitterion (I) is the active species, the association between amine and olefin oxide must be a weak one. Other possible explanations for the catalytic nature of this reaction which may be considered are : Formation of an unstable complex of olefin oxide and isocyanate due to the catalytic effect of triethylenediamine. Formation of an unstable molecular complex of triethylenediamine, olefin oxide, and isocyanate.
Miscellaneous Polymerizations and Copolymerizations. Attempts to extend the olefin oxide-triethylenediaminecatalyzed trimerization to aliphatic isocyanates (n-propyl isocyanate) or to phenyl isothiocyanate failed to give trimeric products. Similarly, attempted copolymerizations of these Compounds with phenyl isocyanate resulted in the isolation of triphenyl isocyanurate as the only product. Acknowledgment
T h e author thanks the Houdry Process and Chemical Co., Division of .4ir Products and Chemicals, Inc., for permission to publish this paper. He also acknowledges with gratitude the suggestions received from A. Farkas during the course of this work. literature Cited ( 1 ) .\mold, K. G , , Nelson, J. A , , Verbanc, J. J., Chem. Revs. 57, 47 (1957). ( 2 ) I3eitchnian. B. D. (to Air Products and Chemicals, Inc.), U. S. Patent 3,146,219 (hug. 25. 1964). 13) Ibzd.. 3.154.522 fOct. 27. 1964) ( 4 ) Ibzd., 311791626 (April 20, 1965). ( 5 ) Heitchman, H. D., Erner. \V. E. (to Air Products and Chemicals. Inc.), Ibzd.,3,168,483 iFeb. 2, 1965). ( 6 ) Heitchman, B. D., Krause, J. H. (to .4ir Products and Chemicals. Inc.). Ihid..3.179.628 (Aoril 20. 1965). ( 7 ) Rurkus.”J. ( c o knifed St& Rubber ’Co.), Ibid., 2,979,485 (April 11, 1961). ( 8 ) I h e r , W. I:. ( t o Houdry Process Corp.), Ibid., 3,010,963 (Kov. 28, 1961). ( 9 ) Farkas. X.. Mills. G. A , . Advan. Catalvsis 13. 393 11962). ( 1 0 ) Gilman, ‘L.. O’Connell, J. J., Hathaway, C. E., (Vurster, C. F., Technical Documentary Rept. No. ASD-TDR-63-396 (April 1963). ( 1 1 ) Jones, J. I., Savill, N. G., J . Chem. Soc. 1957, 4392. (12) Xicholas, L., Gmitter, G. T., J . Cellular Plastics 1, No. 1, 85 (1965). (13) Tsuzuki, K., Ichikawa, K., Kase, M., J . Org. Chem. 25, 1009 ( 1 960). ( 1 4 ) Ibid., 26, 1808 (1961).
RECEIVED for review October 26; 1965 ACCEPTED December 29, 1965
COLLOIDAL OXYCELLULOSE BY NITROGEN DIOXIDE TREATMENT OF LEVEL-OFF DEGREE OF POLYMERIZATION CELLULOSE A L A N
M . BELFORT A N D
R O B E R T B . W O R T 2
Avzcel Laboratory, Amprzcan Viscose Dtuiszon, F M C Gorp., M a r c u s Hook, Pa.
microcrystalline cellulose (FMC Corp., Marcus Hook, Pa.) is level-off degree of polymerization cellulose obtained from acid hydrolysis of high purity a-cellulose pulp. T h e term “level-off degree of polymerization” refers to the relatively constant value of degree of polymerization which results when cellulose is subjected to severe acid hydrolysise.g., 2 . 5 s hydrochloric acid solution at the boil for 15 minutes. Aqueous gels can be prepared from this product by mechanical attrition of high solids wet cakes. Centrifugation of gels yields a minor fraction consisting of cellulose microcrystals which measure approximately 0.1 micron in length and 0.01 micron in thickness. vrEL
A
T h e flow properties of fractionated microcrystalline cellulose gels were studied by Hermans (5). Shear stress was measured as a function of the shear rate with a Couette viscometer. T h e rheology of the fractionated gels is similar to the rheology of certain clay systems ( 3 ) :wherein each elongated particle is in contact cvith a few others. ’The greater stiffness and viscosity of these gels depend on the increased content of colloidal rigid particles. A continuing objective of this laboratory is to increase the colloidal nature of gels from microcrystalline cellulose. Mechanical attrition is one approach to produce colloidal gels, but it is inefficient. Chemical means Lvere sought to reduce VOL.
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NO. 1
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41
Nitrogen dioxide oxycellulose was prepared from level-off degree of polymerization cellulose powder. Colloidal dispersions prepared from neutralized aqueous slurries of the product are gels composed of negatively charged rigid rods less than 0.2 micron in length. Sodium carboxylate groups on the surfaces of the rigid rods rendered the oxycellulose less sensitive to electrolyte flocculation, stabilized the particles against flocculation over a wide pH range, and enabled dried and ground gels to b e reconstituted in water. Sodium borohydride chemically reduced alkali-sensitive carbonyl groups and produced an unexpected increase in gel strength. Low solids gels demonstrate high structural viscosity, high yield values, marked thixotropy, and synergistic viscosity behavior when blended with hydroxyethylcellulose and sodium carboxymethylcellulose.
the large requirements for mechanical energy in the liberation of colloidal cellulose particles. Certain chemical reactions on cellulose are specific, require little energy input, and can be controlled to effect changes only on exposed surfaces. Oxidation of cellulose with nitrogen dioxide is such a reaction. Oxidation of cellulose Ivith nitrogen dioxide was first reported by Kenyon and coworkers in 1942 (72). T h e predominant attack on cellulose is oxidation of primary hydroxyl groups of anhydroglucose units to carboxyl groups. The celluronic acid product also contains relatively small amounts of alkali-sensitive carbonyl groups produced by a side reaction (6). T h e only commercial form of this product is oxidized cotton gauze, an absorbable gauze and hemostatic agent for surgical packing ( 4 ) !which also has limited use as an cationic exchange resin for special chemical separations. Nitrogen dioxideoxidized cellulose disintegrates when treated with heat and alkaline reagents. T h e instability of oxidized cellulose to heat and to alkaline reagents has barred its application in fields now using such carboxylate materials as carboxymethylcellulose and alginic acid (4).Neve11 (8) reported that highly oxidized, fibrous oxycellulose reacted with dilute ammonia solution to form a clear gel which became liquefied in a few minutes. T h e work reported here describes the properties of nitrogen dioxide-oxidized microcrystalline cellulose ( 7) of low degree of oxidation (D.O.) and gels prepared from this oxidation product. Dispersions of the oxidized product are composed of colloidal rigid rods of cellulose I structure, the surfaces of which are altered by conversion of some primary hydroxyl groups to carboxyl groups. Experimental
Solution Oxidation. Microcrystalline cellulose powder was oxidized with nitrogen dioxide (Matheson Co., East Rutherford, N. J.) dissolved in anhydrous carbon tetrachloride. In a typical oxidation. 525 grams of microcrystalline cellulose was added to a solution containing 525 grams of nitrogen dioxide in 2625 grams of carbon tetrachloride. The reaction mixture, contained in a 5-liter round-bottomed flask fitted for stirring and with an ice water condenser, was.stirred for 6 hours at 28' C. The reaction mixture was filtered through a coarse sintered glass funnel and washed with methanol and water until free of nitrogen dioxide and carbon tetrachloride. T h e carboxyl content of the oxidized product was determined by the calcium acetate method of Yackel and Kenyon (72). Gaseous Oxidation. Microcrystalline cellulose powder and nitrogen dioxide vapors were allowed to react at 40' C. in a 300-cc. capped pressure bottle. Quantities of 3 grams of dinitrogen tetroxide liquid and 1.5 to 3 grams of microcrystalline cellulose powder were placed in a pressure bottle. The microcrystalline cellulose powder was contained in a short length of plastic tubing, closed at one end, to avoid contact with dinitrogen tetroxide liquid. The bottles were sealed and the liquid was vaporized to form nitrogen dioxide by placing the bottles in a 40" C. water bath. After the 42
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
vaporization step. the microcrystalline cellulose polvder was shaken out of the tubes and the bottles were rotated in the 40' C. water bath for 1 to 4 hours. The products were purified and the carboxyl contents determined according to the procedure described above. X-Ray Diffraction. X-ray diffraction patterns of the dried free acid form of the oxycellulose were obtained with a General Electric x-ray diffractometer. Laue potvder diagrams were prepared by the standard technique. Neutralization, Stabilization, and Dispersion. X 150gram portion of the oxycellulose product (D.O. 0.23) was mixed \vith distilled water with a planetary mixer to form a 9% solids slurry. T h e slurry. pH 2.7. was adjusted to p H 9.2 by slow addition of l.V sodium hydroxide solution. The neutralized oxycellulose slurry was treated with 0.75 gram of sodium borohydride (NaBHa, 5000 p.p.m. based on the oxidized product). The slurry \vas dispersed in Lvater to form a gel by vigorous mixing in a LVaring Blendor. Rheology. Gel consistencies of sodium oxycellulose (NaBHg-stabilized sodium salt of nitrogen dioxide-oxidized microcrystalline cellulose is abbreviated to sodium oxycellulose) were measured ivith a Brookfield R V T Synchro-Lectric viscometer (product of Brookfield Engineering Laboratories. Stoughton. Mass.) mounted on a Helipath stand and equipped with a 0.804-inch T-D bar set to revolve at 20 r.$:m. Gel consistencies were measured as the resistance of gels to the motion of the bar spindle. Readings of the torque rrquired to rotate the spindle were obtained as Brookfield units (B.U.) on the 0-500 scale. A Rao flow birefringence viscometer was used to measure shear stress as a function of shear gradient and to determine yield points of the sodium oxycellulose gels described above. The Rao instrument is of the Couette type. An outer cylinder is rotated and the torque transmitted through the gel to the inner cylinder is measured. Use of this cylindrical rotational viscometer to measure the flow properties of microcrystalline cellulose gels was described by Hermans (5). Effect of Temperature on Gel Consistency. The consistencies of sodium oxycellulose gels, adjusted to 25", 40°, 60°,and 75" C.. respectively, were measured with a Brookfield viscometer. Gels stored in a recirculating oven at 50' C. for 6 months were examined periodically for gel consistency and to observe gel stability. Syneresis was taken as an evidence of instability. Salt and pH Effects on Gels. Colloidal sols at 0.2570solids were prepared from sodium oxycellulose. Varying amounts of sodium chloride solution were added and after 3 days the sols were inspected visually to detect flocculation. Colloidal sols and 5% gels were also adjusted to various p H values ranging from 2.7 to 12.0 with hydrochloric acid solution or sodium hydroxide solution and observed for flocculation or syneresis. Compatibility with Hydrocolloids. T h e nature of gels of sodium oxycellulose containing varying amounts of soluble hydrocolloids was studied. Dispersions of the sodium oxycellulose were blended separately with hydroxyethylcellulose (Cellosize H E C W P 4400, a product of Union Carbide Chemical Co., New York, N. Y . ) and sodium carboxymethylcellulose (cellulose gum CMC-7MP, Hercules Powder Co., Wilmington, Del.). The dispersions, 2% total solids, contained 0 to 100 weight yo of hydrocolloid. After being allowed to stand for one day, viscosities of these sols were determined with the spindle attachments of the Brookfield viscometer before and
Results
Figure 1 . ticles
Electron micrographs showing rigid colloidal parLefi.
Right.
Sodium oiycellulo~e Froclionoted microcryrtalllne ~ellulme
3t
B
250
200
-
z
3 0
d
150
5
10090-
0
80-
0
2
>.
7060-
0
50-
L
Y)
z
40-
8
u
3025-
d ID
I
1.5
1
I
,
I
I
I
2.0 2.5 3.0 4.0 5.0 6.0 PERCENT SOLIDS
,
,
,
,
ao 10.0
I
15.0
Figure 2. Brookfield consistency of sodium oxycellulose gels a t various solid levels a t 25" C. togl10g plot Sodium oxycellulore gel NaBH4-treatedsodium oxycellulore gel
A. B.
after shaking. Spindle speed was kept constant at 50 r.p.m. and the spindles were varied to give midscale readings. Readi n p obtained on the 0 to 100 viscometer scale were converted to~centipoises. Compatibility of sodium oxycellulose in dispersions with various hydrocolloids was determined by examining 50: SO blends at 1.0% total solids. Flocculation or syneresis was tdkcn as evidences ofincompatibility. H E C and C M C blends w e i t diluted to 0.25% solids, so that the viscous nature of the blcnds would not mask incompatibility.
Microcrystalline cellulose reacted with a n equal weight part of nitrogen dioxide in anhydrous carbon tetrachloride solution a t 28O C. td yield an oxycellulose product of D.O. 0.23 in 6 hours. Oxidation levels up to D.O. 0.43 were achieved after 21 hours. Oxidation with 2 weight parts of gaseous nitrogen dioxide a t 4O0 C. yielded oxidized product of D.O. 0.21 in one hour. X-ray diffraction patterns of the oxycellulose from solution oxidation and of microcrystalline cellulase revealed that the oxidation reaction produced changes in the scattering factor of cellulose. T h e Laue x-ray powder diagrams showed that the changes are slight. The major portion of the oxidized product remains in the cellulose I structure. Roseveare and Spaulding (70) demonstrated that crystallinc regions of cotton cellulose were penetrated by concentrated solutions of nitrogen dioxide in carbon tetrachloride. The nitrogen dioxide solutions used in this report to oxidize microcrystalline cellulose were comparatively dilute. Our x-ray observations show that oxidation apparently occurred mainly a n the cellulose microcrystal surfaces. T h e degree of polymerization of the microcrystdline cellulose is approximately 220; of the oxycellulose product, about 165. Dry oxycellulase resembled microcrystalhe cellulase powder in respect to particle size and color. Water dispersions of the free acid form of the oxycellulose could not be reduced to colloidal particle size, even with severe mechanical attrition. Water slurries of the product thickened gradually as they were neutralized with sodium hydroxide solution. High shear mixing of a neutralized oxycellulose dispersion in a Waring Blendor produced colloidal particle distribution. Figure 1 shows electron micrographs of gels of sodium oxycellulose and fiactionated microcrystalline cellulose. T h e particles are similar in size and shape. Centrifugation studies showed that more than 95% of the sodium oxycellulose particles had a Stokes equivalent diameter less than 0.2 micron. Attrited microcrystalline cellulose gels have only 35% or less of a colloidal fraction with a Stakes equivalent diameter of less than 0.2 micron. Sodium oxycellulose gels are essentially totally colloidal. NaBH4 treatment of sodium oxycellulose gels increased their chemical stability and caused remarkable increases in gel consistency and thixotropy. NaBH4 was used initially with the purpose of reducing by-product carbonyl groups to hydroxyls. I t was soon discovered that a n unusual result accrued from the use of small quantities of this reagent. Figure 2 shows the effect of 5000 p.p.m. of NaBH4 (dry product basis) on the dilution curve o f a gel of sodium oxycellulose. On the average, Brookfield units were increased fivefold by the use of NaBHn. Figure 3 shows the development of gel consistency of NaBHItreated sodium oxycellulose gel after shaking. Gel consistency reached a maximum after 7 days. Pseudoplastic flow properties of 5% gels of sodium oxycellulose and of fractionated cellulose microcrystals arc shown in Figure 4. Data far the microcrystalline cellulose gel are from Hermans ( 5 ) . Shear stresses are higher for the sodium oxycellulose gel a t all shear gradients. Yield points of sodium oxycellulose and microcrystal cellulose gels, shown on the yaxis (shear gradient = 0 set.?), are 2112 and 208 dynes per sq. cm., respectively. T h e sharp rise in shear stress values a t low shear gradients (2 to 10 set.-') which is characteristic of the fractionated microcrystalline cellulase gel was not observed for the sodium oxycellulose gel. Over the range 2 to 80 sec.?, the sodium oxycellulose gel was extremely shear-thinning, dropping from an apparent viscosity of 36,300 cps. a t 2 sec.? VOL. 5
NO. 1
MARCH 1966
43
D- 7 days
'"I
300
I
200
ii
(3
GRADIENT (SEC:' )
3025
20
Figure 4. Consistency curves for sodium oxycellulose and microcrystal cellulose gels a t 25" C.
-
-
linear plots of shear stress vs. shear rates Yield points far sodium oxycellulose and microcrystal cellulose gels shown on ordinate axis
I5-
IO
I
1.0
1.5
2.0 2.5 3.0
I
I
4.0 5.0 6.0
PERCENT
1
1
8.0
1
10.0
3000
-
BEFORE SHAKING
SOLIDS
Figure 3. Brookfield consistency of sodium oxycellulose at 2 5 " C. Log/log plot immediately after shaking 6 . After standing 15 minutes C. After standing 1 d a y D. After standing 7 days
A.
to 1600 cps. a t 80 set.-'. Shear thinning behavior of the fractionated microcrystalline cellulose gel was demonstrated by a drop from 2500 to 750 cps. over the same range of shear gradient. Elevated temperatures caused only slight reversible loss in sodium oxycellulose gel strength. Consistency of 5% gels fell from 140 to 100 B.U. as the temperature increased from 25' to 75" C. T h e original consistency was recovered on cooling. After storage for 6 months a t 50" C.?no syneresis or irreversible loss of gel consistency was observed. Gels not treated with NaBH4 demonstrated loss of consistency during storage over the same period at 25' C., gradually approached the consistency of water, and became discolored. NaBH4treated gels withstood several freeze-thaw cycles and recovered full consistency when shaken after each thaw. Flocculation studies showed that dilute colloidal dispersions of sodium oxycellulose were stable against flocculation by sodium chloride in solutions containing u p to 222 mmoles per liter of sodium chloride. Microcrystalline cellulose sols were flocculated by sodium chloride concentrations of 0.3 mmole per liter. The dispersions were also stable and did not settle when treated with hydrochloric acid or sodium hydroxide solution over the p H range 3 to 12. Sodium oxycellulose gels were dried and granulated to powders which redispersed readily in water to fully reconstituted gels. Microcrystalline cellulose gels similarly treated require prolonged wet attrition to reconstitute. Vnexpected synergistic effects were observed in gel blends prepared by mixing sodium oxycellulose dispersions with 44
I&EC
PRODUCT RESEARCH A N D DEVELOPMENT
\
24
D 0 SODIUM
I
I
I
25
50
75
0
CELLOSIZE WP4400
OWCELLULOSE
WEIGHT W OF HYDROXYETHYLCELLULOS
Figure 5. Viscosity of sodium oxycellulosehydroxyethylcellulose blends at 1 .O% total solids a t 25" C. Semilog plats
solutions of H E C and C M C . Figures 5 and 6 show the relationship between structural viscosity and composition for blends of sodium oxycellulose with hydroxyethylcellulose (HEC) Cellosize WP4400 and with sodium carboxymethylcellulose (CMC) cellulose gum CMC-7MP. Brookfie Id viscosities of the 1% dispersion of sodium oxycellulose, the 1% solution of H E C , and the 1.257, solution of C M C were 24, 480, and 97 cps., respectively. T h e viscosities of 1 and 1.257, dispersions of sodium oxycellulose are the same (24 cps.) because the technique for measuring Brookfield viscosity is not sensitive to small changes in viscosity.
4oor-
~
1
BEFORE SHAKING
AFTER SHAKING
K 24 IO
0
25
50
SODIUM OXYCELLULOSE
75 100 CELLULOSE GUM CMC- 7 M P
WEIGHT % OF SODIUM CARBOXYMETHYLCELLULOSE
Figure 6. Viscosity of sodium oxycellulose-carboxymethylcellulose blends at 1.2570 total solids a t 25" C. Semilog plots
T h e lower dashed curves in Figures 5 and 6 represent the expected additive viscosities of H E C and C M C blends with sodium oxycellulose dispersions. T h e top and middle curves in Figures 5 and 6 represent the measured viscosities of the blends as a function, of percentage composition, before and after shaking. Viscosities of the HEC-sodium oxycellulose blends were higher than expected over the whole range of composition. T h e peak performance of the HEC-sodium oxycellulose blend was a t 50 weight yGcomposition. where the viscosities were 15 to 25 times greater than expected. A similar general increase in viscosities was demonstrated by all CMC-sodium oxycellulose blends. T h e maximum viscosities were observed a t the 50 to 75 weight yGC M C levels, where viscosities were 2 to 8 times greater than expected. Sodium oxycellulose effrcts on structural viscosity are greater with H E C than with C M C solutions. T'he upper two curves in Figures 5 and 6 show the thixotropic behavior which resulted when sodium oxycellulose was blended with H E C or C M C . The maximum thixotropic effect of the HEC-sodium oxycellulose blend in Figure 6 was a t about 80 weight 7 0 of H E C , where a twofold increase in gel viscosity was shown. T h e CMC-sodium oxycellulose blend demonstrated the greatest thixotropic effect a t 50 weight 7, of C M C , where a threefold increase in gel viscosity occurred, The positive thixotropic effect of sodium oxycellulose dispersions is greater with C M C than with H E C solutions. Discussion
T h e product described above is the first reported oxycellulose which forms stable, completely colloidal gels when neutralized and dispersed in water. T h e electron micrograph in Figure 1 shows an isolated portion of a sodium oxycellulose gel where the rigid rodlike particles are less than 0.2 micron in length and have axial ratios approximately 10 to 1. Gel studies with sodium oxycellulose showed that the particles wrre solvated with water and dispersed readily to form stable gels. The chemical change was predominantly on the surfaces of the rodlike particles; x-ray studies showed that the cellulose I structure remained essentially intact. T h e degree of oxidation, approximately 0.2, was enough to impart "waterloving" properties to the particles but did not produce solu-
bility. This behavior suggests that the mechanism by w.hich hydrophilic properties were imparted to sodium oxy-cellulose involved three steps: (1) the chemical formation of carboxyl groups, (2) the breaking up of hydrogen bonds as the carboxyl groups were neutralized, and (3) the association of the formally polar charged neutralized carboxyl groups with great numbers of water molecules to form sheath systems around the particles. Dispersions of colloidal size particles of sodium oxycellulose are aided in their dispersion stability by the action of the mutually repelling sodium carboxylate groups on the particle surfaces. Figure 1 offers evidence for this repulsive charge effect of the particles. If the electron micrographs are considered representative of the particle arrangement in the aqueous gel states. fractionated microcrystalline cellulose particles tend to align themselves and associate along their lateral dimensions, \where particles establish and maintain an extended gel network on the basis of the random contact of each particle with several neighboring particles. Sodium oxycellulose gels not treated to reduce the carbonyl content were sensitive to alkali treatment and gradually decreased in viscosity and discolored when stored at room temperature. NaBHd is a general reducing agent and has been used for many years to reduce carbonyl groups in cellulose (2). Parkinson ( 9 ) stated that NaBH4 treatment of celluronic acid from cotton linters reduced alkali solubility markedly. T h e sodium oxycellulose described in this work was stabilized against alkali degradation by NaBH4. In addition. NaBH4 treatment of sodium oxycellulose gels resulted in an unexpected fivefold increase of gel consistency. This is believed to be the first demonstration of gel strength improvement due to NaBH4 treatment of a cellulose product. The dramatic increases in structural viscosity and thixotropy which resulted when sodium oxycellulose gels were treated with NaBH4 cannot be explained by carbohydrate-borate complex theory. Complex formation of borate salts and boric acid with carbohydrate polymers has been known for years ( 7 7 ) . Mono- and dicomplex formations were shown with galactosides and mannosides ( 7 ) . Carbohydrate-borate complexes usually manifest a gel-stiffening effect similar to that shown above, but an essential feature of these complexes is that the carbohydrate material contains adjacent cis-hydroxyl groups (7). For example, microcrystalline cellulose gels alone or in combination with C M C solution showed no change in viscosity behavior when treated with NaBH4 and NaB02. A carbohydrate-borate complex in the reduced sodi tim oxycellulose system cannot be totally discounted on the basis of the evidence presented above. Sodium oxycellulose colloidal properties may be ascribed to a combination of the stabilizing effect of NaBH4 and the charge effect of the sodium carboxylatr groups on the rigid particle surfaces. Acknowledgment
T h e authors acknowledge with gratitude the generous service of George E. Raynor, Jr., who provided technical and editorial assistance in the preparation of this publication. Literature Cited
( 1 ) Rattista, 0. A . Fleck. L G., ( t o FMC C o r p . ) , U. S. Patent 3,111,513 ( N o v . 19. 1963). ( 2 ) Chrm. ProcPssin,q (Chicaqo) 25, 27 (0% 8. 1962). ( 3 ) Gabraysh. A. F.. E y i n g : H . >IMcKee. N.. Cutler, I.: Trans. SOC.Rhrol. 5 , 6; f 1961 ). VOL. 5
NO. 1
MARCH 1966
45
( 4 ) Hasek, R. H., Davy, L. G., Broadbrooks, K. J., “Modern Chemical Processes,” Vol. I, p. 182, Reinhold, New York, 1950. ( 3 ) Hermans, J., Jr.. J . Polymer Sci.,Part C, No. 22, 129 (1963). ( 6 ) McGee, P. H., Fowler, W. R., Jr., Unruh, C. C., Kenyon, LV. O., J . Am. Chem. SOC. 70, 2700 (1948). ( 7 ) .Malcolm, E. I$’., Green, J . LV., Swenson, H. A , , J . Chem. Soc. 1964, 4669. ( 8 ) Nevell, T. P.. J . Textile Inst. 42, T 91 (1951). ( 9 ) Parkinson, J. R., Tuppi 41, No. 1 >661 (1958).
( 1 0 ) Roseveare, IV. E., Spaulding, D. IV.. Znd. Eng. Chem. 47, 2172 (1955). ( 1 1 ) Whistler, R. L.. EkMiller, J . N., “Industrial Gums;” Xcademic Press, iYew York, 1959. ( 1 2 ) Yackel, E. C., Kenyon, 1%’. O., J . Am. Chem. SOL. 64, 121 (1942).
RECEIVED for review September 20. 1965 ACCEPTED DecernbFr 6 . 1965
PREPARATION AND HYDROLYTIC STABILITY
OF TRIALKYLACETIC ACID ESTERS MY R0N C00P ERSM IT H A ND A L FR ED J
J
S E RA F I NG
.
FUSC0
, Enjay
.
RUT K0W S K I
,
Enlay Chemtcal Laboratorws, Linden, S. J .
Chemical Co., Cranford, ,V. J .
Trialkylacetic acids (neo-acids) are difficult to esterify by conventional techniques. Laboratory experiments have shown that a standard method of esterification employing an acid catalyst and a water entrainer can b e modified to produce esters of neo-acids in high yields. The rates of esterification can be adjusted over a fairly wide range by varying the type and concentration of catalyst and by the selection of an entrainer with a suitable boiling point.
Once formed, neo-acid esters are about 20 times more resistant to acidic
hydrolysis and from 170 to 1300 times more resistant to basic hydrolysis than unhindered acid esters of approximately equivalent molecular weight.
associated with the preparation of esters of T trialkylacetic . acids has been described ( 2 ) 3, 8). The
0
enhanced hydrolytic stability of these esters once prepared has also been \vel1 documented ( 7 . 8). The acids discussed in this paper contain only methyl groups or mixtures of methyl and higher alkyl groups on the alpha carbon atom. The difficulty in preparation and the enhanced hydrolytic stability of these trialkylacetic acid esters may. in theory. be attributed to either a steric or an ionization effect. A s reported by Newman (8). the rate of esterification decreases sharply as the degree of acid chain branching increases. Since the ionization constants of organic carboxylic acids of the type reported are essentially equivalent-e.g., 1.76 X 10-5 for acetic acid and 1.77 X 10-5 for diethylacetic acid-this decrease in reaction rate is more closely relatable to steric effects than to those associated with ionization. Several mechanisms have been employed to describe the acid-catalyzed esterification of organic acids. The one that appears operable for the trialkylacetic acids and is by far the most common is the acyl-oxy process. As given by Equations 1 to 3. an acid such as trimethylacetic adid is protonated to form a conjugated oxonium ion. This species then undergoes an exchange-type reaction Lvith alcohol. The nucleophilic attack is oriented tokvard the positive end of the carbonyl dipole. Finally a proton is lost to form the ester.
RC-0
HE DIFFICULTY
0
0
RC-OH
f H-
0 I
R‘OH
46
+
-
RC-OH2
(1)
0 f
+ RC-OH,
“ +/R + RC-0 \H
,i
0
+ p R d - o R t + ;1 \H -,
(3)
The bulky nature of the alkyl group, owing to the a-carbon branching, has a profound inhibiting effect on the formation of the bimolecular complex shown in Equation 2 (8). Hence the rate of the over-all reaction is markedly decreased. Steric hindrance also affects the rate of hydrolysis of the ester group in a similar way. Carboxylic acids have been esterified by many techniques, the most commercially attractive being the Fischer method of esterification which uses an acid catalyst and an entrainer to remove. azeotropically. the formed water. Sterically hindered carboxylic acids have long been known not to be appreciably esterified by this technique. Hence, certain modified procedures were developed to prepare esters of hindered acids in high yields (,5-6 . 9 ) . The object of the present investigation was to determine if the Fischer method of esterification could be modified to permit a convenient and rapid esterification of sterically hindered trialkylacetic acids and to determine the rate of hydrolysis of the hindered esters once formed under acidic and basic conditions. I n this study, the trialkylacetic acids that were esterified include neo-pentanoic, neo-heptanoic. neodecanoic, and neo-tridecanoic acids. They are called “neo” because of their quaternary carbon atom and neo-pentane-type structure. Results
+ HOH
(2)
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
Esterification. Data relating to the esterification of neoprntanoic acid are given in Table I . Neo-pentanoic acid can
