Pharmacognosy · Short notes

Volatile Oils and Resins: Short Notes & Cheat Sheet

Volatile oil chemistry, where plants store them, extraction methods, important oils, and resins, oleoresins and balsams.

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Short notes

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How volatile oils are extracted

The method depends on how heat-stable the oil is and where the plant stores it.

Volatile oils

Volatile (essential) oils are odorous plant oils that evaporate at room temperature. They are mainly terpenes (mono- and sesquiterpenes) and phenylpropanoids.
Unlike fixed oils, they leave no permanent grease spot, are not saponifiable and are not glycerides.
They are stored in glandular hairs (mint family), oil cells, oil ducts called vittae (fruits of the Apiaceae such as fennel and coriander), and lysigenous glands (citrus peel).
Extraction: steam distillation (most oils), expression (citrus peel), enfleurage (delicate flowers such as jasmine), solvent extraction and supercritical CO₂.

Important oils

Peppermint (Mentha piperita): menthol — carminative and cooling.
Clove (flower buds of Syzygium aromaticum): eugenol, used as a dental analgesic and antiseptic.
Eucalyptus: cineole (eucalyptol), a decongestant. Cinnamon bark: cinnamaldehyde.
Fennel: anethole. Coriander: linalool. Ajwain (Trachyspermum ammi): thymol, an antiseptic and anthelmintic.
Lemon and orange peel: limonene. Turpentine oil from pine oleoresin: α-pinene, a counter-irritant.
Garlic contains the sulphur compound allicin; black mustard releases allyl isothiocyanate.

Resins and related products

Resins are amorphous solids that are insoluble in water, soluble in alcohol and burn with a smoky flame. They form by oxidation of terpenes.
Oleoresins are resin plus volatile oil (turpentine, capsicum and ginger oleoresins). Gum resins and oleo-gum-resins also contain gum (asafoetida, myrrh).
Balsams contain benzoic or cinnamic acid: benzoin (Friar's balsam), Tolu balsam and Peru balsam.
Colophony (rosin) is the residue left after turpentine is distilled from pine oleoresin.
Podophyllum resin contains podophyllotoxin, used for genital warts and as the starting point for etoposide.
Capsaicin (from capsicum) is a counter-irritant; ginger (gingerols, shogaols) is an antiemetic.

Cheat sheet

High-yield facts in one place. Cover the right-hand column and test yourself.

Oil → main constituent

Oil	Constituent
Peppermint	Menthol
Clove	Eugenol
Eucalyptus	Cineole (eucalyptol)
Cinnamon	Cinnamaldehyde
Fennel / anise	Anethole
Coriander	Linalool
Ajwain / thyme	Thymol
Lemon / orange	Limonene
Turpentine	α-Pinene
Camphor	Camphor (ketone)

Resin types

Type	Examples
Resin	Colophony, podophyllum resin, jalap
Oleoresin	Turpentine, capsicum, ginger
Oleo-gum-resin	Asafoetida, myrrh
Balsam	Benzoin, Tolu, Peru

Memory aids

Balsams are Benzoic or cinnamic: A balsam must contain benzoic acid, cinnamic acid or their esters.

Exam traps

Volatile oils are not true fats: they are not glycerides and cannot be saponified.
Citrus oils are expressed, not steam-distilled, because heat spoils their aroma.
Vittae are the oil ducts of Apiaceae fruits (fennel, coriander, cumin).
Colophony is what remains after distilling turpentine oil; it is not itself volatile.

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