The ACIDS of PINE OLEORESIN and ROSIN* Burean of Chemistry and Soils, U. S. Department of Agriculture
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BOUT 90 per cent. of rosin consists of acids (la, 5a). A better knowledge of the properties of these acids holds the key to a better understanding of rosin itself. It is the acid and consequent salt- (soap)-forming properties that make possible its use in soap and paper size, two major industrial outlets for rosin. The ester-forming properties extend materially its usefulness in varnish, another important industrial outlet. Rosin has often been hailed the world's cheapest and most abundant source of organic acids. To obtain these acids in a reasonable state of isomeric purity, however, is another matter. Contrary to prevailing belief, rosin is not primarily abietic acid, nor does abietic acid, strictly speaking, constitute the bulk of the acids present (20). The original oleoresin or "gum" apparently contains no abietic acid. What is the composition of fresh "gum" or oleoresin? How does it differ from the rosin obtained from it? What happens to the constituents of gum during the process of steam distillation in ordinary practice? What is the mechanism of resin acid formation? For a hundred years or more these questions have received study, principally abroad (75b). While much information has already been accumulated, a great deal remains to be done. No consideration will be given here to the changes which the limpid secretion fresh from the tree undergoes in the formation of the more viscous "guq," or to the various theories [Aschan (Z),Dupont (18)] relating to resin acid formation. In this paper i t is proposed to deal with this gum product only in the form i t takes after it has stood a while, the form in which it finds its way into the still for the preparation of turpentine and rosin, confining ourselves further to the non-"spirits" part, that is, the portion which is not volatile with steam a t atmospheric pressure, and its product of conversion, rosin. Like rosin, the non-volatile part of the gum consists principally of a mixture of acids (about 90 per cent.) (48). The rest, about 10 per cent., consists of neutral bodies only very vaguely characterized, small quantities of esters, sesquiterpenes, oxidation products, and components yet to he identified. Recently a long-
chain paraffin hydrocarbon, n-heptacosane, has been found in our longleaf (Pinus palustris) gum (3). It soon became obvious that the non-volatile part of the gum was relatively complex, that it did not lend itself well to separation into distinct entities, that nearly all the acid products isolated were characterized by instability, that the properties of these acid products varied over a wide range depending on the conditions of experiment, but that they all showed the same elemental composition (GoHso02). Space will not permit a complete review of the specific contributions of the numerous pioneer investigators beginning with Braconnot in 1808 (75b), who was probably the first to call attention to the acids of rosin. The literature that has accumulated since then on the composition of pine oleoresin, of rosin, and of the various acids alleged to have been obtained from these is voluminous. In most instances, because of the difficulties involved in their isolation, the acids reported in the early literature represented mixtures of isomers rather than individual compounds, and the miscellaneous nomenclature that developed, owing partly to the different species of pine involved, added further to the confusion. The great diiculties in isolating the individual acids account for differences in properties of even the supposedly pure acids reported by different authors, as may be seen from Table 2. One must interpret broadly, therefore, the reported findings of the various early authors, such as the products obtained by Baup in 1826 (6) from Pinus abies to which he gave the name "abietic" acid, the so-called "pinic" and "sylvic" acids of Unverdorhen (72), from both P. sylvestris and P. a&s, the "sylvic" acid of Trornmsdorf (1835) (70), the "pimaric" and "pyromaric" acids of Laurent (36), or even t&e more recently reported "pinabietic" acid of Aschan (lb) which is appareritly identical with the abietic acid of Levy. It is not until one comes to the later authors, Siewert (68), Fluckinger (a), and particularly Vesterberg (74), that one begins to recognize individual compounds such as dextro- and levo-pimaric acids, and to see a sharp distinction drawn between the primary or "gum" acids and their isomerized products, the rosin acids. Kohler (38) and Schulz (67) were probably the first to prepare abietic acid, the best known of these acids, in a high state of isomeric purity, and this acid is often referred to as the 1-abietic acid of Schulz. Tschiich (71) was one of the earliest investigators to attempt a
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* P~esentedbefore the Division of Agricultural and Faad Chemistry (Symposium on Naval Stores), St. Petasburg meeting of the A m e r i ~Chemical Society, March 25-30, 1934. t Senior chemst. i5
36 classification of these acids on the basis of chemical behavior. It is not until one reaches Kohler's scheme of classification (39), however [later modified by Duffour and Dupont (ax)], that one begins to get a rational understanding of the interrelationship of the various acids involved. The Kohler scheme, as a whole, seems to harmonize better with the information developed by later iuvestigators than the complicated scheme proposed by Tschirch (71), which depends upon fractional extraction with various agents, or the three-class system suggested by Aschan (la), some aspects of which will be taken up presently. In the Kiihler scheme two main classes are recognized (see Table 1): (a) the gum acids, termed "primary," " oleoresins," or "resin acids," and (b) the rosin or Colophonic acids, those acids to which the primary are converted by heat or by treatment with mineral acid. Each of these classes consists of a series of isomeric, isomorphous acids. The complications arising from their isomorphous character and their behavior on crystallization have beeu recently illustrated by Georgi (30). Resin Acids: In the French P. pinaster (13) and likewise in our American P. palustris (3, 47), two geueral groups of primary acids showing rather marked differencesin properties have been isolated, the pimaric and the sapinic acids, the latter predominating. To the latter class belong also the "pinic" and "alepic" acids obtained by Dupout (15) from the simpler oleoresins, P. halepensis and P. pinea, respectively.
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by the methods of Kable,* Schkateloff,* Kiihler,* Levy (44), Ruzicka (60), and Dupout and Uzac (23), or, as has recently beeu shown, by the action of ultraviolet light (77). It is transitions due to heat with which one is concerned when dealing with the ordinary steam distillation of gum. Despite their apparent complexity, the acids exhibit a simplicity of behavior, for with the exception of dextro-pimaric acid the primary acids are all convertible in part to the one 1-abietic acid of Schulz (23, 48). This does not occur directly, however, as the transitzon is cmnpliuzted by the formation of intermediate forms, all of which are doubtless present in rosin. In rosi~i,therefore, not all, or even the major part (20), of the total acid is abietic, but rather there is a mixture of acids in various stages of transformation, a complexity which doubtless accounts for the vitreous character of rosin. The course of transition of the primary acids to the products of isomerization (the acids of rosin) and the way in which these add to the complexity of the composition of rosin, has been illustrated diagrammatically by Vezes and Dupont (754 as in Figure 1.t De=tmpirnarie acid Lcvopimar'c nSapinic aeid s a p i . i c .cid
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n-pimarabietiel o-Sapinabietic8.Sapinabietic/l
Cuoeh~nged p P y r o a b i e t i e acid
