2.7.7 Chemical Composition of Resins
The copious volume of information with regard to the ‘chemistry of resins’ is mainly attributed by the meaningful research carried out by Tschirch and Stock, who advocated that the proximate constituents of resins may be classified under the following heads, namely:
(i) Resin Acids
(ii) Resin Esters and their Decomposition Products i.e., Resin Alcohols (Resinols) and Resin Phenols (Resinotannols).
(iii) Resenes i.e., the chemical inert compounds.
However, it has been observed that in majority of the known resins these three aforesaid categories evidently predominates and thus the resulting product consequently falls into one of these groups. It is worth mentioning here that representatives of all the three said groups are rarely present in the same product.
Given below are some typical examples of resin substances that predominates the three classes suggested by Tschirch and Stock, namely:
A. Resin-Esters : Examples: Ammoniacum; Asafoetida; Benzoin; Balsam of Peru and Tolu; Galbanum; Storax;
B. Resin-Acids : Examples: Colophony; Copaiba; and
C. Resenes : Examples: Bdellium; Dammar; Mastic; Myrrh; Olibanum.
A few important and typical chemical constituents that have been duly isolated and characterized from various naturally occurring resins are discussed below:
(iii) Resenes i.e., the chemical inert compounds.
However, it has been observed that in majority of the known resins these three aforesaid categories evidently predominates and thus the resulting product consequently falls into one of these groups. It is worth mentioning here that representatives of all the three said groups are rarely present in the same product.
Given below are some typical examples of resin substances that predominates the three classes suggested by Tschirch and Stock, namely:
A. Resin-Esters : Examples: Ammoniacum; Asafoetida; Benzoin; Balsam of Peru and Tolu; Galbanum; Storax;
B. Resin-Acids : Examples: Colophony; Copaiba; and
C. Resenes : Examples: Bdellium; Dammar; Mastic; Myrrh; Olibanum.
A few important and typical chemical constituents that have been duly isolated and characterized from various naturally occurring resins are discussed below:
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* Tschirch. A, and L. Stock: Die Harze, Borntraegr, Berlin, Vols. 1 & 2, 1933-36.
1. Resin Acids
Synonyms Resinolic Acid.
The resin acids essentially contain a large portion of carboxylic acids and phenols. However, they occur both in the free state and as their respective esters. They are usually found to be soluble in aqueous solutions of the alkalies, thereby forming either soap like solutions or colloidal suspensions.
Resinates, i.e., the metallic salts of these acids find their extensive usage in the manufacture of inferior varities of soaps and varnishes.
A few typical examples of resin acids are enumerated below:
Out of all the six commonly found resin acids Abietic Acid shall be discussed here under:
Abietic Acid (Synonym Sylvic Acid)
Chemical Structure 13-Isopropylpodocarpa-7, 13-dien-15-oic acid; (C20 H30O2).
It is a tricyclic diterpene embedded with four isoprene units. It is studded with four methyl moieties and a carboxylic acid function. Besides, it also has two double bonds one each in ring-Band ring-C of the phenanthrene nucleus.
Preparation It is a widely available organic acid, prepared by the isomerization of rosin.* It may also be synthesized from dehydroabietic acid.**
The commercial grade of abietic acid is normally obtained by heating either rosin alone or with mineral acids. The product thus achieved may be glassy or partly crystalline in nature. It is usually of yellow colour and has a mp 85°C i.e., much lower than the pure product (mp 172-175°C).
Characteristic Features It is obtained as monoclinic plates from alcohol and water. Its physical parameters are: mp 172-175°C; [α]24D -106° (c = 1 in absolute alcohol); UVmax 235, 241.5, 250 nm (ε 19500, 22000, 14300). It is practically insoluble in water, but freely soluble in ethanol, benzene, chloroform, ether, acetone, carbon disulphide and also in dilute NaOH solution.
Identification It readily forms the corresponding methyl ester as methyl abietate (C21 H32O2), which is colourless to yellow thick liquid bp 360-365°C, d2020 1.040, and n20D 1.530.
Uses
1. It is used for manufacture of esters (ester gums), such as: methyl, vinyl and glyceryl esters for use in lacquers and varnishes.
2. It is also employed extensively in the manufacture of ‘metal resinates’ e.g., soaps, plastics and paper sizes.
3. It also assists in the growth of butyric and lactic acid bacteria.
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* Harris, Sanderson, Org. Syn. Coll. Vol. IV, 1 (1963); and Fieser and Fieser, The chemistry of Natural Products Related
to Phenanthrene (New York, 3rd. edn., 1949).
** A.W. Burgastahler, and L.W. Worden., J. Am. Chem. Soc., 83, 2587, (1961) E. Wenkert et al., ibid, 86, 2038, (1964)2. Resin Alcohols
In general, resin alcohols are complex alcohols having higher molecular weight. These are of two types, namely:
(a) Resinotannols: The resin alcohols which give a specific tannin reaction with iron salts are termed as resinotannols.
A number of resinotannols have been isolated from the plant kingdom. It is an usual practice to name them according to the resins in which they are found, such as:
Alocresinotannol – From Aloe species viz., Aloe barbedensis Miller, (Curacao Aloes); Aloe perryi Baker, (Socotrine Aloes); Aloe ferrox Miller, Aloe africana Miller,Aloe spicata Baper. All these belong to the natural order Liaceae.
Ammoresinotannol – From Ammoniacum i.e., the oleo-gum-resin from Dorema ammoniacum D. Don. (Family: Umbelliferae).
Galbaresinotannol – From Galbanm i.e., the oleo-gum-resin from Ferula galbaniflua Boiss et Bubse (Family: Unbelliferae).
Peruresinotannol – From Balsam of Peru i.e., the balsam obtained from Myroxylon balsamum var Pereirae (Royle) Harms (Family: Fabaceae);
Siaresinotannol – From Sumatra Benzoin (Benzoin, Styrax) i.e., the gum exuded from Styrax benzoin Dryander (Family: Styracaceae).
Toluresinotallol – From Balsam of Tolu i.e., the Balsam obtained from Myroxylon balsamum (Linn.) Harms. (belonging to the family. Leguminosae).
(b) Resinols: The resin alcohols that fail to give a positive reaction with tannin and iron salts are known as resinols. The following are some typical examples of resinols, for instance:
Benzoresinol – From Benzoin which is purely a pathological product obtained either from Styrax benzoin Dryander and Styrax paralleloneurus Brans. (Sumatra
Benzoin) or from Styrax tonkinensis Craib. (Siam Benzoin) belonging to family Styraceae.
Storesinol – From storax which is the balsamic resin usually obtained from the trunk of Liquidamber orientalis Mill. family Hamamelidaceae.
Gurjuresinol – From Gurjun Balsam that is the aleo-resin obtained from Dipterocarpus turbinatus Gaertn. F. belonging to family: Dipterocarpaceae.
Guaiaresinol – From Guaiacum Resin obtained from the heartwood of Guaiacum officinale Linn. and Guaiacum sanctum Linn. belonging to family: Zygophyllaceae.
3. Resenes
These are oxygenated compounds, but are not affected either by alkalies or acids. In fact, they are more or less neutral substances being devoid of characteristic functional groups, and, therefore, do not exhibit any characteristic chemical properties. Interestingly, they are immune to oxidizing agents and variant climatic conditions, a fact which essentially attributes the resins containing them one of their major plus points for the manufacture of varnishes. A few important examples of resenes are as follows:
Dracoresene – Derived from the scales of the fruit of Dragon’s Blood i.e., Daemonorops draco Bl. (and other species) belonging to the natural order (Arecaceae).
Masticoresene – Derived from Mastic-an oleo-resin obtained from Pistacia lentiscus Linn belonging to family: Anacardiaceae.
Fluavil – Obtained from Gutta-percha and also from the bark of various trees. Guttapercha is hard and has a very low elasticity. X-ray diffraction studies have
