Volatile Oils
On this page
Direct answer
Peel a sweet orange and the fragrant mist that touches your skin is volatile oil — an odorous, steam-distillable plant essence that evaporates completely on filter paper, leaving no greasy stain, exactly the opposite of a fixed oil. These essential oils are complex mixtures of terpenoids and aromatic compounds, extracted by distillation, expression, solvent extraction or enfleurage, and they carry the carminative, antiseptic, counterirritant, flavouring and perfumery roles of the pharmacopoeia. Clove oil with at least 85 per cent eugenol and mentha oil rich in menthol are the two monographs Indian examiners trust most.
What you must remember
- Volatile oils evaporate entirely at ordinary temperature leaving no permanent grease spot; fixed oils leave a permanent translucent spot, saponify with alkali and become rancid — volatile oils do neither.
- Chemical classes with drugs: hydrocarbons (limonene in lemon peel, pinene in turpentine), alcohols (menthol in mentha, santalol in sandalwood, geraniol in citronella and palmarosa), phenols (eugenol in clove, thymol in ajowan), aldehydes (cinnamaldehyde in cinnamon bark, citral in lemongrass), ketones (carvone in caraway, menthone), oxides (cineole in eucalyptus), esters (methyl salicylate in wintergreen).
- Clove: dried flower buds of Syzygium aromaticum, Myrtaceae; oil contains not less than 85 per cent eugenol, used in dentistry as an antiseptic and analgesic.
- Extraction methods: water distillation, water-and-steam distillation, direct steam distillation, expression (citrus scarification and sponge), solvent extraction giving concretes and absolutes, enfleurage for jasmine and tuberose, supercritical carbon dioxide as the modern option.
- Stored cool, in the dark, in completely filled, well-stoppered amber containers because air and light cause oxidation and resinification.
- Mentha oil from Mentha species grown in India yields menthol, used as a carminative, counterirritant and in inhalations.
- Optically active in most cases — optical rotation is an official quality parameter in the pharmacopoeia.
- Distillation water (fragrant water) of rose and peppermint is itself an official preparation.
Choosing an extraction method, reasoned
The method follows the chemistry, and the reasoning is examinable. Most spice and leaf oils — mentha, eucalyptus, cinnamon, ajowan — tolerate heat and sit inside tough glands, so steam distillation wins: the oil immiscibilises with the vapour, co-distils below its normal boiling point and separates in the florentine receiver. Citrus peel oils are different; they sit in large, easily ruptured oil sacs just below the surface and they are heat-labile and full of delicate top notes, so expression — pressing or scarifying the peel — recovers the oil without ever boiling it. The most fragile floral perfumes, jasmine and tuberose, do not survive any heat and contain non-volatile odorants that distillation cannot carry at all; enfleurage layers them onto fat over days (or solvent extraction dissolves them into a concrete, from which alcohol washes the absolute). Supercritical carbon dioxide gives solvent-free, thermally gentle extraction and its use in ginger and spice oils is the modern add-on worth one mark.
Then quality: an essential oil monograph is judged by optical rotation, refractive index, specific gravity and, above all, content of the marker constituent — eugenol in clove, menthol in mentha, cineole in eucalyptus. Storage is the postscript examiners like: filled amber bottles exclude both the air that oxidises terpenes and the light that accelerates it.
The fixed-versus-volatile trap
The guaranteed viva question is to differentiate volatile from fixed oils, and the guaranteed error is answering only "volatile oils evaporate". Complete the answer: volatile oils cannot be saponified by alkalis, do not become rancid, are mixtures rather than glycerides, and distil with steam. Fixed oils are triglycerides — saponifiable, rancidity-prone, non-distillable — and leave that permanent grease stain on filter paper, the classic bench test.
Two quieter traps follow. First, "essential" here means essence, not essential fatty acids — a confusion that has appeared as a distractor in MCQs. Second, in the constituent list students attach eugenol to cinnamon; cinnamon bark oil's marker is cinnamaldehyde, while eugenol belongs to clove and to cinnamon leaf oil. Keep drug, marker constituent and use in one three-word chain — clove-eugenol-dental; mentha-menthol-carminative; eucalyptus-cineole-expectorant — and the entire chapter compresses to six chains.
Frequently asked questions
Differentiate volatile oils from fixed oils.
Volatile oils distil with steam, evaporate leaving no grease spot, are not saponifiable and do not go rancid; fixed oils are glyceride esters that saponify with alkali, go rancid and leave a permanent translucent spot on paper.
What is the chief constituent of clove oil and its pharmaceutical use?
Eugenol, not less than 85 per cent; used in dentistry as an antiseptic and local analgesic, and as a pharmaceutical flavouring agent.
Explain enfleurage and name two flowers needing it.
Cold fat absorbs the fragrance of freshly laid petals over successive days; jasmine and tuberose, whose odorants are too heat-labile for distillation, are processed this way (or by solvent extraction into concretes and absolutes).
Why are volatile oils stored in filled amber containers in a cool place?
Air and light oxidise terpenes and cause resinification; full stoppering limits the headspace of oxygen and amber glass filters the active wavelengths.
Name the source and marker constituent of eucalyptus and ajowan oils.
Eucalyptus oil from Eucalyptus globulus leaves, marked by cineole; ajowan oil from Trachyspermum ammi fruits, marked by thymol.