Natural resins are water-insoluble mixtures of compounds, many of which have a hydroaromatic structure. Mixtures of isomeric carboxylic acids, such as abietic and pimaric acids, which occur in rosin in nature in solvent-free form, in the form of tree sap or wood rosin such as pine oleoresin, where they are dissolved in terpenic hydrocarbons. They can also be present as fossil coal or copal resins, in old pine tree stumps, etc.

Commercially, the manufacture of wood pulp grade chemical cellulose using the kraft chemical pulping processes releases these resin acid compounds. The Kraft process is conducted under strongly basic conditions of sodium hydroxide, sodium sulphide and sodium hydrosulphide which neutralizes these resin acids, converting them to their respective sodium salts, sodium abietate, ((CH3)4C15H17COONa) sodium pimarate ((CH3)3(CH2)C15H23COONa) and so on. In this form, the sodium salts are insoluble and, being of lower density that the spent pulping process liquor, float to the surface of storage vessels during the process of concentration, as a somewhat gelatinous pasty fluid called kraft soap, or resin soap.

Kraft soap can be reneutralized in the presence of concentrated sulphuric acid to restore the acidic forms abietic acid, palmiric acid and their isomers which form the resin acid component of a pulping byproduct called tall oil. Other major components include fatty acids and unsaponifiable sterols.

Resin acids, because of the same protectant nature they provide in the trees where they originate, also impose toxic implications on the effluent treatment facilities in pulp manufacturing plants. Furthermore, any residual resin acids that pass the treatment facilities add toxicity to the stream discharged to the receiving waters.

For example, methane (swamp/marsh gas) is a hydrocarbon with one carbon atom and four hydrogen atoms: CH4. Ethane is a hydrocarbon (more specifically, an alkane) consisting of two carbon atoms held together with a single bond, each with three hydrogen atoms bonded: C2H6. Propane has three C atoms (C3H8) and so on (CnH2n+2).

Liquid geologically-extracted hydrocarbons are referred to as petroleum (literally “rock oil”) or mineral oil, while gaseous geologic hydrocarbons are referred to as natural gas. All are significant sources of fuel and raw materials as a feedstock for the production of organic chemicals and are commonly found in the Earth´s subsurface using the tools of petroleum geology. Oil reserves in sedimentary rocks are the principal source of hydrocarbons for the energy and chemicals industries.

Hydrocarbons are of prime economic importance because they encompass the constituents of the major fossil fuels (coal, petroleum, natural gas, etc.) and biofuels, as well as plastics, waxes, solvents and oils. In urban pollution, these components–along with NOx and sunlight–all contribute to the formation of tropospheric ozone.

Usually carbon backbone is represented as molecular graph in which only carbon atoms are represented as vertices and bonds as edges. Molecular graphs contain the structure of the hydrocarbon in which missing hydrogen atoms can be added in a unique way. Hydrocarbons are extensively studied in mathematical chemistry.

There are essentially three types of hydrocarbons:

1. aromatic hydrocarbons, which have at least one aromatic ring
2. saturated hydrocarbons, also known as alkanes, which don’t have double, triple or aromatic bonds
3. unsaturated hydrocarbons, which have one or more double or triple bonds between carbon atoms, are divided into:
* alkenes
* alkynes
* dienes

Plants produce resins for various reasons whose relative importances are debated. It is known that resins seal the plant’s wounds, kill insects and fungi, and also allow the plant to eliminate excess metabolites.

Formed in special resin canals, resin is typically exuded in soft drops from wounds, hardening into solid masses in the air. It may be obtained by making incisions in the bark or wood of the secreting plant, or extracted from resin-bearing plants by leaching of the tissues with alcohol. The hardening property has made resins traditionally useful as varnishes and adhesives.

Resin as produced by most plants is a viscous liquid, typically composed mainly of volatile fluid terpenes, with lesser components of dissolved non-volatile solids which make resin viscous and sticky. The commonest terpenes in resin are the bicyclic terpenes alpha-pinene, beta-pinene, delta-3 carene and sabinene, the monocyclic terpenes limonene and terpinolene, and smaller amounts of the tricyclic sesquiterpenes longifolene, caryophyllene and delta-cadinene. The individual components of resin can be separated by fractional distillation. Some resins contain high proportion of resin acids.

A few plants produce resins with different compositions, most notably Jeffrey Pine and Gray Pine, the volatile components of which are largely pure n-heptane with little or no terpenes. The exceptional purity of the n-heptane distilled from Jeffrey Pine resin, unmixed with other isomers of heptane, led to its being used as the defining zero point on the octane rating scale of petrol quality. As heptane is explosively flammable, distillation of resins containing it is very dangerous. Some resin distilleries in California exploded because they mistook Jeffrey Pine for the similar but terpene-producing Ponderosa Pine. At the time the two pines were considered to be the same species of pine; they were later classified as separate species in the mid to late 19th century.

Some resins when soft are known as oleo-resins, and when containing benzoic acid or cinnamic acid they are called balsams. Other resinous products are in their natural condition mixed with gum or mucilaginous substances and known as gum resins. Many compound resins have distinct and characteristic odours, from their admixture with essential oils.

The hard transparent resins, such as the copals, dammars, mastic and sandarac, are principally used for varnishes and cement, while the softer odoriferous oleo-resins (frankincense, elemi, turpentine, copaiba) and gum resins containing essential oils (ammoniacum, asafoetida, gamboge, myrrh, and scammony) are more largely used for therapeutic purposes and incense.

Certain resins are obtained in a fossilized condition, amber being the most notable instance of this class; African copal and the kauri gum of New Zealand are also procured in a semi-fossil condition.

Solidified resin from which the volatile terpene components have been removed by distillation is known as rosin. Typical rosin is a transparent or translucent mass, with a vitreous fracture and a faintly yellow or brown colour, non-odorous or having only a slight turpentine odour and taste. It is insoluble in water, mostly soluble in alcohol, essential oils, ether and hot fatty oils, softens and melts under the influence of heat, is not capable of sublimation, and burns with a bright but smoky flame. This comprises a complex mixture of different substances including organic acids named the resin acids. These are closely related to the terpenes, and derive from them through partial oxidation. Resin acids can be dissolved in alkalis to form resin soaps, from which the purified resin acids are regenerated by treatment with acids. Examples of resin acids are abietic acid (sylvic acid), C20H30O2, plicatic acid contained in cedar, and pimaric acid, C20H35O2, a constituent of gallipot resin. Abietic acid can also be extracted from rosin by means of hot alcohol; it crystallizes in leaflets, and on oxidation yields trimellitic acid, isophthalic acid and terebic acid. Pimaric acid closely resembles abietic acid into which it passes when distilled in a vacuum; it has been supposed to consist of three isomers.