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212
Acknowledgments Acknowledgment is due to the late Hans Tropsch for his earlier and parallel contributions in this field. Thanks are due to Gustav Egloff, J. C. Morrell, L. S. Kassel, and Jack Sherman for many valuable discussions and help in calculations and particularly to J. M. hlavity, W. J. hlattox, and M. W. Cox for continuous enthusiastic cooperation and assistance in experimentation.
Literature Cited Burgin, J., Groll, H., and Roberts, R . M., Natl. Petroleum News, 30, NO.36, R-1432 (1938) ; Oil GUSJ.,37, NO. 17, 48 (1938). Burk, R. E., “Polymerization”, A. C. S. Monograph 75, New York, Reinhold Pub. Corp., 1937. Egloff, Gustav, Proc. Am. Petroleum Inst., 111, 16, 137-8 (1935). Egloff, Gustav, “Reactions of Pure Hydrocarbons”, A. C. S. Monograph 73, p. 6, New York, Reinhold Pub. Corp., 1937. Egloff, Gustav, Schaad, R. E., and Lowry, C. D., Jr., J . Phys. Chem., 34, 1619 (1930). Egloff, Gustav, Thomas, C. L., and Linn, C. B., IKD.ENG.
CHEY.,28, 1283 (1936).
VOL. 32, NO. 2
(7) Ellis, Carleton, “Chemistry of Petroleum Derivatives”. Vol. 11, pp. 631-75, Reinhold Pub. Corp., 1937. (8) Frey, F. E., and Huppke, W. F., IND. ENG.CHEX.,2 5 , 5 4 (1933). (9) Grosse, A. V., J. Am. Chem. SOC.,59, 2739 (1937).
(10) Grosse, A. V.,Morrell, J. C., and Mattox, W. J., IND.ENQ. CHEM.,to be published. (11) Ipatieff, V. N., Bar., 34, 3589 (1901); 35, 1047 (1902); J . RuS3. Phys. Chem. Sac., 34, 182 (1902); 40, 500 (1908). (12) Kassel, L. S., J . Chem. Physics, 4, 276, 438 (1936). (13) Lazier, W. A., and Vaughen, J. V., J. Am. Chem. SOC.,54, 3080 (19321. (14) Marek, L. F., and Paul, R. E., IND. EXQ.CHEM.,2 6 , 4 5 4 (1934); Marek, L. F., McCluer, TVV. B., Ibid., 23, 878 (1931). (15) Pease, R. N.,and Durgan, E. S., J . Am. Chem. SOC.,50, 2715 (1928); 52, 1262 (1930). (16) Piteer, K. S., J . Chem. Physics, 5, 473 (1937). (17) Sabatier, P., and Mailhe, A., Bull. SOC. chim., [4] 1, 107, 341, 524, 733 (1907); Compt. rend., 146, 1376 (1908); 147, 16, 106 (1909); 148, 1734 (1909); Ann. chim. phys., [ 8 ] 20, 289, 302 (1910).
PRESENTED before the Dirision of Organic Chemistry a t the 96th Meeting of the Smeriortn Chemical Society, Milwaukee, Wis.
Reactions of Maleic Anhvdride with Abietic Acid and Rosin A. G. HOVEY AND T. S. HODGINS, Reichhold Chemicals, Inc., Detroit, Mich.
Contrast is made of results where rosin is used instead of the pure precipitated abietic acid previously discussed in the study of the structure of abietic acid, and of the addition products formed by qbietic acid with maleic anhydride. Although the existing theoretical data on the reaction of resinic acids with maleic anhydride which exist in abundance are valuable, commercial operations have to be carried out with rosin. Much of the theoretical work previously done was performed on freshly prepared abietic acid on which no oxidation had taken place if it could possibly be prevented. On the other hand, industry has to use rosin that may or may not have undergone a certain degree of oxidation.
formation of abietic acid occurs in rosin under the influence of heat or treatment with acids (11). Ruzicka and Meyer (33) obtained approximately 90 per cent yields of abietic acid by vacuum distilling rosin at low pressures and 200-210° C., and suggested that the formation of abietic acid from rosin involved isomerism. Much d E culty has arisen in the studies of the structure of rosin acids due (a) t o the fact that abietic acid is only one of many pos(Table I), (b) to sible isomers of the general formula CZ~H3002 the marked tendency of these isomers to oxidize, and ( c ) to their tendency to form mixed crystals. The presence of resene is said to act as a positive catalyst for oxidation (20). Until recently ( I d , 14, 15, SO) it was considered that pyroabietic acid was one of these isomers into which even the relatively stable abietic acid was changed by heating.
TUDIES of the composition of rosin and of the structure of the resin acids were stimulated by early success in improving on the properties of nature’s product by “liming” or by esterifying. The acidic characteristics of rosin, recognized over a century ago (6),have been widely utilized in the manufacture of driers and other rosin soaps for many industrial purposes. Although abietic acid is not an original constituent of the tree secretions and is apparently not present to an appreciable extent even in virgin rosin, the
The resin acids existent as primary constituents of oleoresins of conifers have been designated as sapinic acids (18). Fleck and Palkin ( I S ) suggested that, in addition to the three methods previously used for isomerization of these labile resin acids-i. e., acid treatment, heat without chemical agents, and ultraviolet light-isomerism could also be promoted by other catalysts. Although it was formerly thought that rosin consisted principally of abietic acid anhydride, Fonrobert (16, 17)
S
TABLE I. ISOXERS OF THE FORMULA C20H8002 Acid
LAbietic (avlvic) “l-Pimario“ (1Sapietic) d-Pimaria (alpha) Proabietic
M. P.. O C. 171-173 152 212-214 159-160
Specifio Rotation in Abs. Aloohol About -looo
Reference (1.9) Sohulz method
-282’ 740 11.5’
++
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INDUSTRI4L AND ENGINEERING CHEMISTRY
showed that the properties of the true anhydride were different from those of rosin. Furthermore, in the earlier work of Ruzicka and Meyer (SS), abietic acid was prepared by a method by which the addition of water to rosin is practically impossible-i. e., by vacuum distillation a t 200-210 O C. under less than 1 mm. pressure. Abietic acid was early recognized as a diterpene acid. The isoprene rule (the hypothesis that the carbon skeleton is resolvable into isoprene residues, in this case, four) wm of value as a guide in determining the structure but not nearly so valuable as the discovery by Vesterberg (36) that abietic acid might be dehydrogenated completely to the hydrocarbon, retene. The identification of the ring structure of abietic acid with that of retene constituted a great advance. Independent work in 1932 by Vocke ( S 7 ) , by Haworth (19), and by Ruzicka, de Graaff, and Muller ($9)resulted in the general acceptance of the skeletal structure for abietic acid (Figure 1). The only remaining point of uncertainty was the location of the two nuclear double bonds.
273
double bonds had not been ascertained. The exact location, of the double bonds is important from any study or use of the reaction of abietic acid or rosin with maleic anhydride. The addition reactions, now well known as the DielsAlder diene syntheses ( 1 , 7 , 9 ) ,are part of a numerous and im-. portant series; a diene or a dienic structure ( A ) unites with an olefin or olefinic structure ( B ) under suitable conditions to form a cyclic structure (C) which is generally called the adduct (Figure 2 ) . I
I
+ *\(y’‘
A
(4
(B) ( C) FIGURE 2. DIELS-ALDER SYNTHESIS
Another variation occurs when the diene is a cyclic compound instead of a simple straight-chain hydrocarbon; in this case bicyclic derivatives result (Figure 3).
HBC COOH \ / 10
+ FIGURE 1.
’
SKELETAL STRUCTURE OF ABIETIC
ACID
Haworth’s work was the comparison of the hydrocarbons resulting from the dehydrogenation of the resin acids and their derivatives with a sample of 1-ethyl-7-isopropylphenanthrene obtained by new methods for the synthesis of a wide variety of alkyl phenanthrenes; on the other hand, Vocke, after a study of oxidative degradation products, suggested that the carboxyl group is probably located a t CI along with the nuclear methyl group. Haworth’s contentions (19) in favor of the above nuclear structure were as follows: (a) It is constructed from isoprene units. (b) It is capable of forming structures which are consistent with the properties of the products formed. ( c ) It represents abietic acid as a tertiary carboxylic acid; this accounts for its resistance to esterification and the loss of carbon monoxide and hydrogen chloride by the action of heat on the acid chloride, and thus checks the earlier work of Levy (23). Ruzicka, de Graaff, and hiuller (Sd) attacked the problem from still another angle, since it had been previously found ( S I ) that alkyl phenanthrenes can be oxidized to phenanthrenecarboxylic acids by alkaline potassium ferricyanide. Upon such oxidation, retene yields phenanthrene-1,7-dicarboxylic acid together with a product of further oxidation. From so-called methylretene (actually homopimanthrene) upon application of a similar oxidation treatment, phenanthrene-1,7-dicarboxylic acid (the same identical oxidation product) was obtained as from retene. From this it was realized that the additional methyl group was not in the phenanthrene nucleus, that it must be in the side chain, that the supposed “methylretene” was really l-ethyl-7isopropylphenanthrene, and that in abietic acid the extra methyl group must be located a t C, along with the carboxyl group. As a result of the independent work of these three investigators, the skeletal structure of abietic acid seemed established beyond a reasonable doubt, but the location of the two
DERIVATIVES OF DIEXES FIGURE 3. BICYCLIC Benzenoid and thiophene rings appear to be inert. Phenanthrene fails to enter into a Diels-Alder reaction with maleic anhydride (8); this indicates the absence of an active diene grouping similar to that present in the central ring of anthracene. Considerable use of the addition of maleic anhydride to vegetable oils has been made in the examination of these products from the standpoint of analytical control; the test is termed “diene number” or “maleic value” (6). S o corresponding application of these tests seems to have been utilized for determining variations in rosins. When Ruzicka, Andersmit, and Frank (28) first noted that an addition compound (melting point, 227-228” c.; [ a ] D , - 2 5 O ) can be formed from abietic acid or fromits esters with maleic anhydride, it seemed to indicate that the double bonds were conjugated and in the same ring (Figure 4). Independently of Ruzicka, Arbuzov (9)started with abietic acid (melting point, 166” c.; [ a ] D , -78.9”), obtained from rosin acids by isomerization with hydrochloric acid, and reacted it for 4 hours a t 170” C. in a sealed tube in the presence of dry benzene with maleic anhydride. Upon purification, he also obtained the addition compound CZ4Ha205, melting a t 227” C. Arbuzov’s work, together with that of Ruzicka and co-workers, seemed to be conclusive evidence for the presence of conjugated double bonds in abietic acid. Ellis ( I O ) considered as “diene products of rosin” any rosinmaleic anhydride addition products and any of the esters of such diene products with polyhydric alcohols. All Ellis’ examples were prepared by fusion of the maleic anhydride with rosin, although most of the theoretical work on maleic anhydride additions had been effected in the presence of an inert solvent, and although the older Brsem (3)resins could also be prepared in the same way. Peterson (27), however,
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states that a hydrocarbon not containing conjugated double bonds, such as terpinolene, reacts with maleic anhydride and a compound containing the abietyl radical, such as rosin or abietic acid. H3C COOH \/
FIGURE 4. STRUCTURE OF ADDITION COMPOUND OF ABIETIC ACIDWITH MALEICANHYDRIDE
Considerable doubt was soon raised as to whether the addition of maleic anhydride to abietic acid was actually by the diene mechanism. Bacon and Ruzicka (4) compared the addition product of maleic anhydride and abietic acid with the maleic anhydride “Z-pimaric” acid addition. Regarding the two addition products, Wienhaus and Sandermann (38) independently showed that maleic anhydride and sylvic (abietic) acid, even in the presence of solvents, would not react a t room temperature but would react above 100” C.; and a t 130-140” C. they formed crystals of the addition product with the same melting point (227” C.) as Ruzicka’s earlier product (28). Bacon and Ruzicka (4) found that “Lpimaric” acid reacts quantitatively with maleic anhydride in benzene solution a t room temperature t o produce an addition product which melts a t 227” C., [ a ] =-29”; , its methyl ester melts a t 215” C., [a]=,-29”. This product is identical with those previously formed from abietic acid by Ruzicka, de Graaff, and Muller (32) and with those formed by Wienhaus and Sandermann (38). The identity of these addition products is surprising; to explain it, Bacon and Ruzicka assumed that abietic acid a t high temperatures is in equilibrium with a t least a small amount of “l-pimaric” acid which is continuously removed by the formation of the addition product with maleic anhydride. I n view of this possibility that the maleic anhydride was not really reacting with abietic acid but actually with “1-pimaric” acid, Bacon and Ruzicka said; “The possibility is not excluded that conjugation does not exist in abietic acid. On the other hand, the very ready addition of maleic anhydride to E-pimaric acid must be considered as conclusive proof for a conjugated system in the molecule” (i. e., I-pimaric). Bacon and Ruzicka suggested that the conversion of the more stable isomer (abietic acid) into the maleic anhydride adduct takes place by a high-temperature reversal of the more usual tendency of the “l-pimaric” acid to form abietic acid with the less stable substance being removed from the equilibrium as soon as it is formed by reaction with the maleic anhydride; this suggestion raised a question, not only as to whether the addition of maleic anhydride to abietic acid was a diene reaction, but also as to the existence of actually conjugated double bonds in abietic acid. When this observation of Bacon and Ruzicka raised questions as to whether there was conjugation of the double bonds in abietic acid and as to the exact location of these two double bonds, Fieser and Campbell (12) offered the most probable suggestion as to the location of the double bondsnamely, a t the A9,14 and A7,8 positions, respectively, in the nuclear structure (Figure 6). Their reasons for the location of the double bonds are as follows:
VOL. 32, NO. 2
1. The oxidation of abietic acid gives a certain quantity of isobutyric a,cid; the carbon atom carrying the isopropyl group must therefore be unsaturated. 2. Since abietic acid is produced by the isomerization of “Z-pimaric” acid and by treatment with acetic or hydrochloric acid under very mild conditions, it is probable that the reaction involves no more than the migration of a double bond or of the whole diene system to an adjacent position, possibly by the addition and elimination of a molecule of acid. The structure of the “1-pimaric” acid (Z-sapietic acid) would seem t o provide a satisfactory account of the significant observations by Ruzicka, Waldemann, Meier, and Hosli (5’4), confirmed by Wienhaus and Sandermann (58).
Sandermann (36) stated that, of the resin acids thus far invest’igated,only “Z-pimaric” reacts quantitatively at room temperature with maleic anhydride or quinone. The quantity of “Z-pimaric” acid in the presence of other isomers can therefore be determined by acidimetric or iodometric titration of the excess anhydride or quinone. The suggest’ionof Fieser and Campbell (12) that the two double bonds in “l-pimaric” acid are conjugated and in the same ring would seem to explain better the greater ease with which LiE-pimaricJJ acid reacts with maleic anhydride a t the lower temperatures; this is not possible with abietic acid, in which the double bonds are conjugated but not in the same ring. This suggestion is consistent with the contentions of Ruzicka, Bacon, Lukes, and Rose (29) that the double linkage is near the isopropyl group (Figure 5). Although it is possible that abietic acid may isomerize into ‘7-pimark” acid, the evidence, as pointed out by Sandermann (35), is that “Zpimaric” acid alone of the resin acids is capable of true diene reaction with maleic anhydride.
l
l
cH31
FIGVRE 5. STRUCTURE OF ABIETIC ACID (12) AT LEFT A N D “I-PIMARIC” (12) OR l-SAPIETIC (18) ACID AT RIGHT
Hasselstrom and Bogert (18) previously pointed out that the so-called 1-pimaric acid is a misnomer, that it has no structural connection with d-pimaric acid, that it should be called 1-sapietic acid to show its classification with the original labile sapinic acids, and yet that i t has structural similarity to the abietic acid class since it yields retene on sulfur treatment instead of pimanthrene as it would if it belonged to the pimaric acid class. Although Hasselstrom and Bogert (18) stated that the crystalline acid mixtures from fresh oleoresins contained only d-pimaric acid and I-pimaric (Isapietic) acid in varying proportions, Kraft (22) found a third which he called “proabietic acid”. It is possible that the numerous primary (sapinic) acids described in the literature, other than these three, may be mixed crystals containing varying amounts of the three acids and of abietic acid. Kraft (22) seems to have proved that “E-pimaric” acid is a doubly unsaturated tricyclic acid, in contrast t o his earlier hypothesis (21). His determination of the ultraviolet absorp tion maximum of 272.5 mp, according to Fieser and Campbell ( l a ) , indicates an analogy in the field of steroid chemistrynamely, that the double bonds are present in a conjugated system continued in a single ring of the molecule.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
On the other hand, although d-pimaric acid (18) exists in large proportions in- the sapinic acids, it possesses much greater stability than the other sapinic acids, which explains its presence in ordinary colophony. Fleck and Palkin (IS) state that d-pimaric acid can be distilled in vacuum and that its rotation is not altered even a t 250' C. The two possible skeletal structures of d-pimaric acid' according to Haworth (19) and Ruzicka (28, 38) are shown in Figure 6. There are indications that the two double bonds are not conjugated and that no addition product is formed with maleic anhydride. The structure on the left seems more probable on the basis of ozonization degradation products and apparently has been unchallenged by recent work.
CHa CHa STRUCTURES OF (E-PIMARIC ACID FIGURE6. SKELETAL
Cntil recently pyroabietic acid was considered the more stable isomer into which even abietic acid was convertible by heat. The suspicion of Fieser and Campbell ( l a ) of the real lack of stability of pyroabietic acid, based on spectroscopic data and on nitrogen derivatives, has been confirmed. Fleck and Palkin (14)recently found that the so-called a-pyroabietic acid, heretofore regarded as an isomer of abietic acid, can be resolved by way of the methyl ester into dehydroabietic acid, dihydroabietic acid, and tetrahydroabietic acid. Evidence is thus offered that the so-called a-pyroabietic acid is not an isomer of abietic acid but a product of dehydrogenation and disproportionation. Fleck and Palkin (15) recently isolated the dehydroabietic and dihydroabietic acid from pyroabietic acid, which had been prepared without the aid of catalysts. Thus, it appears that abietic acid and d-pimaric acids are the most stable of the isomers of the formula CzoH300z. These findings were also confirmed in the same year by Ruzicka, Bacon, Steinbach, and Waldemann (SO) and by Littmann (24) on material treated catalytically with palladium. Fleck and Palkin (15) recently isolated dehydroabietic and dihydroabietic acids as the lactone from the pyroabietic acid which they prepared from I-abietic acid in 3-4 hours instead of the traditional 100 hours by increasing the temperature from the customary 250" to 330' C. Probably one reason why the a-pyroabietic acid, which has so often behaved as a single compound, is not one compound is that both dehydrogenation and disproportionation take place; thus dehydrogenated molecules and molecules which have taken up the hydrogen a t the expense of the dehydrogenated molecule are formed. The result is a reproducible mixture, with an average molecular weight of C20H3002, which seems to act somewhat as an eutectic in physical properties and to give the impression of being a single pure compound. The condensation with maleic anhydride (15) gave no helpful clues as to the degree of unsaturation except to indicate the low reactivity or high stability already known t o characterize the so-called pyroabietic acids. 1 However, Rusicka i n a more recent article [Bull. soc. chim., [5] 4, 1313-14 (193711 suggests t h a t (a) t h e vinyl group is a t t h e 14 (angular) position instead of t h e 9 and ( b ) t h e double bond is a t t h e 6,7 position instead of the 7,s.
275
It is our contention that, although the existing theoretical data obtained by Ruzicka, Arbuzov, and others on the reaction of abietic acid with maleic anhydride is valuable, commercial applications of this reaction have to be effected with rosin. From the structures as developed for abietic, I-pimaric, and d-pimaric acids, the factors important in any investigation on the constitution of abietic acid are (a) the uniformity and purity of the starting material, (b) the presence of dpimaric acid in the starting material, and (c) freedom from oxidation. The presence of d-pimaric acid would appear to be important when the addition of maleic anhydride to rosin is concerned. In our experimental work on the addition of maleic anhydride to rosin, we have purposely disregarded the first two factors and, on one run, all three. The results of reactions with maleic anhydride, when there is the substitution of rosin for abietic acid, are obviously different from the results obtained by the investigators who used carefully purified abietic acid. Furthermore, the attainment of pure (Schulz) abietic acid is a laborious task, even for laboratory samples. Palkin and Harris (26) cite that fact, together with the susceptibility of abietic acid towards oxidation, t o show it is unlikely that pure abietic acid will soon be used for commercial operations. Furthermore, most of the theoretical work on the addition of maleic anhydride to abietic acid or to rosin has been by reaction in the presence of solvent, whereas most commercial runs are by fusion reaction, because of the fire hazard involved when solvents are used in standard direct-fired kettles. Materials Used in Studying the Reaction of Maleic Anhydride with Commercial Rosins The maleic anhydride had the following characteristics: Minimum purity, % Maximum ash, % ' Heavy metals hlinimum solidification point,
99 0 1 'Traces 60
C.
The oxidized rosin had the following characteristics, before and after standing in air for 3.5 years in approximately 0.5inch lumps; it is assumed that this would be characteristic of rosin which is badly oxidized : Original After 3.6 Yr. Acid number 181 170 Color6 X w-m Melting point C. 50-60 Saponification( number 199 6 Determined b y comparison with Rosin Standards Cubes of the U. S . Department of Agriculture.
... ...
The gum and wood rosins had the following analysis: Gum Acid number Color Melting point, C. Saponification number
164
N j3-60 180
Wood 172
N
50-60 196
The benzene conformed t o A. C. S. specifications.
Isothermal Fusion Reactions of lMaleic Anhydride and Rosin N AMERICAN GUMROSIN. A 1020-gram portion of N gum rosin was heated to 200' C., 294 grams of maleic anhydride were added a t 200') and the reaction was conducted under as nearly isothermal conditions as possible. It is not possible to carry out the reaction under completely isothermal conditions on account of the vigorous exothermic reaction a t the initial stage. The maleic anhydride was added as rapidly as possible without its boiling over. The starting point of the reaction, to, was considered as the mean between the time when the addition was started, t l , and when completed, t2.
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TABLE 11. ISOTHERMAL REACTION Sample No.
t
T
Min. 0
C. 200
1 2
7 14
240 250
3 4
21 26
200 201
5 6 7 8 9 10 11 12 13
31 41 51 81
202 200 200 200 200 200 201 200 Room temp.
Wt. of Sample
OF
Status of Reaction
Grams
Starting t o add maleic anhydride 11.106 Maleic anhvdride all added 9 , 8 9 0 Max. tern
