INFLUENCE
OF REACTION CONDITIONS ON THE DIMERIZATION
OF ABIETIC ACID AND ROSIN R.
G.
SlNCLAlR
A N D
D.
A.
BERRY
Battelle Memorial Institute, Columbus, Ohio 43201 W .
H .
SCHULLER
A N D
R .
V .
LAWRENCE
Naval Stores Laboratory, U. S . Department of Agriculture, Olustee, Flu. 32072 The acid-catalyzed dimerizations of abietic acid, isopimaric acid, and gum rosins were studied. Catalysts include sulfuric acid, aluminum chloride, zinc chloride, and boron trifluoride. The best results were achieved using chloroform solutions of pure abietic acid with sulfuric acid as catalyst. Yields of dimers have been as high as 90%. The yield has been limited by a side reaction involving disproportionation of starting materials. A tentative mechanism for the dimerization is postulated.
THE
POLYMERIZATION of rosin involves dimerization of constituent resin acids, such as abietic and isopimaric acids, and other resin acids which are their double bond isomers. The resin acids are principal components of the three types of rosin: gum rosin, wood rosin, and tall oil rosin. The primary objective of the research described was to optimize reaction conditions to obtain a maximum yield of resin-acid dimers. Individual reaction parameters, such as choice of catalyst, solvent, and concentration, were studied separately, and the relative importance of each was described. A variety of reaction conditions have been studied for “dimerizing” or “polymerizing” rosin acids. This work has been the subject of many patents issued during the last 40 years. The patents of Rummelsburg (1936, 1938)
0 DIHYDROABIETIC ACID
: A
COOH
ABIETIC ACID
COhH LEVOPIMARIC ACID
DEHYDROABIETIC ACID
: t i COOH ISOPIMARIC ACID
are typical. The reaction conditions vary in the patent literature, the principal point of departure being the heterogeneous mixing of rosin with certain acidic catalysts. The research described was directed toward optimizing the dimerization reaction conditions. The highest yields of dimer are obtained only when certain reaction influences 60
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 1, March 1970
are closely controlled. These are not easily distinguished by review of the patent literature. There has been very little information reported on the characterization of the products obtained from the dimerization reactions. Acid number, color, and softening point by the ball and ring method are the usual criteria for the characterization of dimerized rosin products. This is deemed sufficient in light of the conventional uses of dimerized rosin. Two reports by French authors, Brus (Brus et al., 1953) and Morrillon (1964), directed attention toward the structures of the products obtained. Their isolation procedures are somewhat tedious and result in less than 10% yields of dimers from crude reaction products. Bardyshev and Strizhakov (1968) used these procedures to reach further conclusions on the structure of resin-acid dimers. Chang (1968) reported a gel permeation chromatography (GPC) method for the assay of rosin materials and found 1 to 13% dimers in certain “polymerized” resin acids. His major concern, however, was to establish an analytical method rather than correlate yield of dimer with experimental reaction conditions. Leonard (1965) reported on the use of gas chromatography (GC) for correlating crystallization time us. per cent resinacid dimer content. This paper correlates numerous reaction factors and the yield of resin-acid dimers. I t was felt that dimerization of pure abietic acid, one of the principal components of gum rosin, would provide a simpler chemical model than gum rosin itself, which is a complex mixture-for example, a typical gum rosin analysis reveals approximately 7% neutral compounds, 8% dehydroabietic acid, 20% isopimaric acid, a sizable percentage of the remainder being abietic-type acids. Experimental
I n a typical experiment, pure abietic acid was mixed under selected conditions with catalysts. Crude products, freed of catalyst by extraction, were analyzed by gel per-
meation chromatography or gas chromatography. These analyses provided relative estimates of dimeric and monomeric size species without regard to acid functionality. The latter was determined by alcoholic KOH titration of aliquots of the crude dimerization products. Preparation of Pure Abietic Acid. Levopimaric acid [a]Zi -276 (ethyl alcohol) was acid-isomerized to abietic acid by the method of Schuller et al. (1967). After one recrystallization from aqueous ethyl alcohol the preparation was 99% or better abietic acid by GC with [a]: -103 (270 in ethyl alcohol). Dimerization of Abietic Acid by Sulfuric Acid. A solution of 1.5 grams of pure abietic acid in 20 ml of C.P. CHCls was mixed with 0.55 ml of concentrated (98%) sulfuric acid. The reaction flask was warmed by a 44°C oil bath, and heating and stirring were continued for 5.0 hours, after which time the reaction was quenched with 30 ml of water. The initial reaction period was accompanied by striking color changes (more easily observed in sequence at lower temperatures) proceeding from green to yellow to dark red to purple. The quenched products yielded dark yellow to brown solutions, which were washed with 5% NaCl solution until the aqueous wash was neutral. The CHCL was stripped on a flash evaporator, the crude product dried in vacuo at 60°C for 16 hours, and the resulting residue weighed and analyzed by titration and GC and/or GPC. The products had color vslues of H or I as determined by USDA rosin color standards and a softening point by the ASTM ball and ring method of approximately 162°C. Mass spectrometry yields m / e = 632 a t several points in the dimer portion of the gas chromatogram. The above dimerization has been scaled up and carried out successfully on 50-gram quantities of abietic acid. Gaseous BF3, BF ,-etherate, zinc chloride-aluminum chloride, and other acidic catalysts can be mixed with the abietic acid solution in lieu of sulfuric acid. Product workup is similar to that described above. Gas Chromatography Analysis. A 30- to 50-mg quantity of crude dimerized product was dissolved in approximately 3 ml of ethereal diazomethane (10 ml of such solution is equivalent to 1 gram of abietic acid). The methylation gave quantitative yields in less than 5 minutes as shown by titrations with base before and immediately after methylation. The ether was evaporated and the oily residue taken up in 0.5 n i l of benzene. A 1 0 - ~ 1aliquot was injected into a Varian Aerograph, Model 1720, gas chromatography setup as follows: 2-foot stainless steel column of 3% SE-52 silicone on Gas-Chrom Q, cured in air a t 325" to 35WC overnight, injector and detector heats a t 330" to 350"C, column temperature 200°C isothermal for 7.5 minutes, then programmed a t 20°C per minute to 330" C. Titration. A weighed 0.5- to 1-gram quantity of dimerized product was dissolved in ethyl alcohol, previously adjusted to the neutral point o f phenolphthalein indicator. The solution was titrated back to the end point with standard 0.1N alcoholic KOH. Gel Permeation Chromatography Analyses. The instrument employed was a Waters Model 100 gel permeation chromatograph modified by substitution of a Milton Roy Minipump and an R-4 refractometer for the pump, and the refractometer supplied with the instrument. A Hallikainen Model 1053 Thermotrol controller was added to control the refractometer cell temperature. Analyses were
performed in tetrahydrofuran on two 60A columns a t 23"C, using 0.25-ml injections, and a flow rate of 0.5 ml per minute. The detector response to monomer and dimer was calibrated. Accuracy of the analysis was estimated to be *5% based on analyses of synthetic samples. Precision was better than + l % and agreement with GC analyses was +570 except for badly degraded samples. Discussion of Experimental Results
The best dimerization results were obtained using sulfuric acid catalyst. The most important variables in the sulfuric acid-catalyzed dimerizations of abietic acid were: relative amounts of sulfuric acid, abietic acid, and chloroform, acid strength of the catalyst, and solvent used during dimerization. One of the most important reaction variables is the relative amounts of abietic and sulfuric acids. A series of dimerizations was performed in which only the relative concentrations of abietic acid, sulfuric acid, and chloroform were changed. I n all cases pure abietic acid was dimerized at 44°C for 5.0 hours. A maximum dimer yield was observed at a narrow range of values for the ratio of sulfuric acid to abietic acid. I t was noted that 7.5 grams per dl solutions of abietic acid in chloroform were best dimerized a t about a 2.1 to 1 molar ratio of sulfuricabietic acid. Further extension of this type of work revealed that this optimum ratio varied with the concentration of abietic acid in chloroform. Thus, 30 grams per dl solutions have an optimum molar ratio of 1.3, and 3.75 grams per dl solutions have an optimum ratio of 3.0. This is illustrated in Figure 1, where the optimum ratios appear as distinct maxima of the dimer yield. I n all cases, analyses of the best dimerization products revealed 80 to 85% dimer and 80 to 85% acid contents.
I
I
I
I
I
I
Legend
e 30 g abietic acid/dl of CHCI, B 15 g abietic acidldl of CHCI, 0 3.75 g abietic acidldl of CHCI,
0 0.5 I .o Milliliters of Concentrated H,SO,/I
1
1
20
5 G of Abietic Acid
Figure 1 . Relation of yield of resin-acid dimers to concentration of sulfuric acid used in dimerization Ind. Eng. Chem. Prod. Res. Develop., Vol. 9,No. 1, March 1970
61
As shown in Figure 1, the dimer content drops off sharply a t other than the optimum amount of sulfuric acid. Not shown is the fact that at higher than optimum amounts of sulfuric acid, the acid percentage also drops off sharply. The optimum amounts of sulfuric acid can be taken from the data of Figure 1 and plotted as a function of the abietic acid concentration. This is shown in Figure 2, which serves as a useful guide to predict the relative amounts of chloroform, abietic acid, and sulfuric acid for optimum dimerization. The use of an 89 weight %b sulfuric acid catalyst in a similar manner resulted in somewhat lower dimer and acid percentages than those obtained with concentrated (98%) sulfuric acid. Figure 3 reveals the shape of the curve with 89% sulfuric acid. The best results were obtained a t approximately a 2 to 1 molar ratio of sulfuric to abietic acid, which produces the best compromise between maximum dimer yield and minimum loss of acid functionality. The lower curve shows the loss of acid functionality (per cent neutrals = 100 - per cent acid) with increasing amounts of catalyst. Detailed examination of the influence of acid strength was conducted in a similar manner. The single reaction variable in this study was the amount of water (or the per cent sulfuric acid). Figure 4 shows the curve for dimerization results using 5 mmoles of abietic acid in 20 ml of chloroform, 10 mmoles of sulfuric acid, and a variable amount of water which was premixed with the sulfuric acid to give the indicated acid strength. The highest dimer yield and the greatest loss of acid functionality occur a t the highest acid strength. The use of a sulfuric acid equivalent to 101% did not alter the results obtained with concentrated sulfuric acid. A very general, chemical kinetic scheme, consistent with the above, is one in which abietic acid (AA) becomes protonated in a fast reversible step, followed by a slow step as the first step in dimerization.
(AA)
+ (H')
Molar Ratio, H2S04/Abietic Acid 9c
8C
3.4 0.9
1.9 2.8 3.7
Legend Percent dimer Percent neutrals
7c
6C . I -
f
$
5c
n l n L
2
.-
4c
n 30
2c
IC
0
c
I
0.5
I
I .o
I
2.0
Milliliters of 89 Percent H2S04/1.5G of Abietic Acid Figure 3. Relation of yield of resin-acid dimers and by-product to concentration of sulfuric acid used in dimerization
Fast Fast
(AA - H A ) Slow
+
(AA)
(AA - AA')
Rate = h(AA) (AA I
I
I
I
-
Fast
-H
-
(Dimer)
H') I
1
I
I
H2SO4, percent ML of con. HS04 (Optimum 1
Figure 2. Optimum amounts of chloroform, sulfuric acid, and abietic acid for optimum dimerizations
62
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 1, March 1970
Figure 4. Relation of yield of resin-acid dimers and by-product to acid strength of sulfuric acid used in dimerization
Table 1. Yield of Dimers of Abietic Acid Using Different Solvents during Reaction
Reaction Product, i'C Solvent
Reaction Time, Hours
CHC1,h n-Heptane/Et20,50/50* Toluene
15 15 4 4
CHCl:
5 16 64 15 15 15 5
Et20 CHCl,' Glyme' Concd. H2S04C
HzSO,, %
HzSO4, M1"
89 89 89 89 89 93 93 93 98 98 98
2.0 2.0 2.0 0.50 0.62 0.06 0.60 0.59 0.55 0.06 20.0
" P e r 1.5 g abietic acid dissolved in 20 ml of indicated solvent. '",SO, Analyses by gel permeation chromatography.
Thereafter, fast proton and double-bond shifts lead to isomeric dimer species. The scheme is generally consistent with the experimental results in which the rate is dependent on the amounts of both sulfuric and abietic acids. I t could be argued that the heterogeneous reaction is diffusion-controlled and the lower yields reflect less contact of reactants when smaller amounts of sulfuric acid were employed. However, it has been observed experimentally that rate of stirring has no discernible effect on the outcome of the dimerizatiion. Well-stirred and nearly static reaction systems, otherwise identical, yield identical results within experimental error. Table I shows some results demonstrating the influence of solvent on the dimer yields. The acid percentages were derived from titration data using the equivalent weight of abietic acid. The material balance reflects the per cent of material product received from the basis of abietic acid charged into reaction. I t remains close to 100% in all of the data. The best solvent was chloroform. Solvents containing oxygen functions, such as ether or glyme, lower the acid strength of the catalyst and the yields obtained. Table I1 compares solvent effects a t constant, optimum conditions of acid strength, balance of H2S04/AA, etc. Chloroform is again the best solvent. Ethylene dichloride (CH2C1CH,C1) and benzene were nearly as good except for lower acid percentages of the products. Petroleumtype solvents generally derive lower yields and acid percentages, and more complex products which are somewhat blackened and poorly soluble in most solvents. The solvent has easily discernible influence on the dimerization, not correlatable with dipole or dielectric strengths. At optimum conditions the reaction is completed in approximately 5 hours. Figure 5 illustrates that the reaction is nearly complete in 2 hours, and from 2 to 7 hours little change is noted. If a greater than optimum amount of sulfuric acid is used, side reactions as well as dimerization are accelerated, and the reaction is complete in approximately 30 minutes. However, the dimer yield is only 50 to 60%. If a smaller than optimum amount of sulfuric acid is used, the dimerization rate becomes very slow. Table I11 shows comparisons of different reaction temperatures employed with otherwise optimized reaction conditions. From approximately 25" t o 50" C essentially constant results were obtained. Above 50" C degradation becomes a problem and below room temperature the rate is noticeably slower. For a given extent of conversion
Temp., C, rt3
Dimerd
Acid
Material balance
73 10 57 48 48
