Pharmacognosy | Volatile Oils

Volatile oils

• Other names: “ethereal oils,” “essential oils,” “essences,” “olea aetherea”

• odoriferous principles in plants

Properties of volatile oils (3)

Optically active,

with ↑ refractive index,

immiscible with water, soluble in organic solvents

Storage of volatile oils

Cool, dry place; tightly stoppered; preferably full, amber container

Function of volatile oils (2)

1. Act as insect repellants (prevent destruction of plant);

2. attractant (aid in fertilization)

Drying agent for purifying essential oils

Anhydrous calcium sulfate

Principal component of essential oils

Terpenes

Rutaceae (citruses): Volatile oil location

Schizogenous/lysigenous ducts

Pinaceae (pines): Volatile oil location

Schizogenous/lysigenous ducts

Apiaceae/Umbelliferae (celery family): Volatile oil location

Oil tubes called vittae

Piperaceae (peppers): Volatile oil location

Modified parenchyma cells

Lamiaceae/Labiatae (mints): Volatile oil location

Glandular trichomes

Rose: Volatile oil location

Petals

Cinnamon: Volatile oil location

Bark

Fixed oils vs volatile oils: distillation

Fixed oils cannot be distilled;
volatile oils can be distilled from natural sources

Fixed oils vs volatile oils: glycerol

Fixed oils with glycerol;
volatile oils without glycerol

Fixed oils vs volatile oils: grease spot

Fixed oils leave permanent grease spot on paper;
volatile oils leave no permanent grease spot

Fixed oils vs volatile oils: saponification

Fixed oils are saponifiable;
volatile oils are not saponifiable

Fixed oils vs volatile oils: deterioration

Fixed oils become rancid (rancidification);
volatile oils oxidize and resinify (resinification), does not become rancid

Fixed oils vs volatile oils: composition

Fixed oils: triglycerides (with some free fatty acids or sterols); volatile oils: terpenes (mono, sesqui, di, tri) and aromatics (phenylpropanoid)

Fixed oils vs volatile oils: nutritive value

Fixed oils have nutritive value;
volatile oils do not

Terpenes

Acyclic/straight-chain or cyclic but NOT aromatic; came from mevalonate pathway

Aromatic volatile oils

Came from phenylpropanoid from the Cinnamic acid in the Shikimic acid pathway

Isoprene unit

C5H8

2 isoprene units

C10H16; Monoterpene

Monoterpene — most widely distributed

Limonene

3 isoprene units

C15H24; Sesquiterpene; 1 ½ MT

Sesquiterpene — example

Quinghaosu (Artemisia annua)

4 isoprene units

C20H32; Diterpenes; 2x MT

Diterpene — example

Paclitaxel (antineoplastic)

6 isoprene units

C30H48; Triterpenes; 3x MT

Triterpene — example

Neem (Azadirachta indica)

8 isoprene units

C40H64; Tetraterpenes; 4x MT

Tetraterpenes — distinction

Not volatile oils anymore; known as pigments

Carotenoids

Carotene (orange); xanthophyll (brown)

Polyterpenes

x n units = CnHn / (C5H8)n

Examples of polyterpenes

Rubber; Gutta Perscha

Formation of volatile oil

Ordinarily formed in special cells or group of cells

Methods of obtaining volatile oils (7)

1. Distillation,

2. enzymatic hydrolysis,

3. expression,

4. extraction, maceration, digestion,

5. enfleurage,

6. percolation with volatile solvents,

7. destructive distillation

Heat-sensitive oils

usually, expression is used for those oils

Distillation — equipment

Clavenger Apparatus (small scale)

Distillation — Principle

Difference in boiling points

Distillation — Cohobation

Repeated distillation in such a way that the volatile oils will be purified

Distillation — Cohobation Advantage

Better volatile oil product if repeated distillation is used

Distillation — types (3)

1. Water

2. Water and steam

3. Direct steam

Water distillation

Applied to dried plant material not injured by boiling

Water distillation — Example

Turpentine from pine trees

Water and steam distillation

Applied to dried and fresh materials that may be injured by heat

Water and steam distillation — process

Steam passed through the macerated mixture (indirect)

water and steam distillation — examples (2)

Cinnamon, Clove

Direct steam distillation

Applied to fresh materials (own moisture); no maceration needed

Direct steam distillation — process

Steam forced thru the fresh herb, carries oil droplets thru vapor pipe

direct steam distillation — examples (2)

Peppermint, Spearmint

Enzymatic hydrolysis

Used for glycosidic volatile oils

enzymatic hydrolysis — example

Sinigrin → (myrosin) → allyl isothiocyanate

Expression

Best method used for citrus volatile oils whose aroma is injuriously affected by heat

Expression — types (3)

1. Sponge proccess

2. Eucelle method

3. Machine process

Sponge process

Rind is used

Eucelle method

• Whole fruit

• Citrus oil glands are punctured/pierced as the fruits are rolled over a pan with spikes

Eucelle method — examples (2)

Lemon oil, orange oil

Machine process

Uses either sponge or Ecuelle after centrifugating

Citral

Most important constituent of volatile oil;
upon H+ forms p-cymene

Extraction

• Soxhlet apparatus

• Volatile solvent systems like benzene, ether

Extraction — Advantage

Fast way; quality is preserved (controlled temperature at 50°C)

Extraction — Disadvantage

Expensive

Extraction: Types (2)

1. Maceration

2. Digestion

Maceration

Use of inodorous, bland fat (pomade)/fixed oil

Digestion

Moderate heat, same as maceration

Enfleurage

Suitable in obtaining small amounts of volatile oil in plant parts like flower petals

Pomade

• Sx impregnated on a layer of bland fat (spread lard 60% and beef tallow 40%)

• Oil isolated by alcohol extraction

Enfleurage — use

Resulting fatty products impregnated with the floral odor

Enfleurage — distinction

Very tedious process, but produces the best quality volatile oil

Percolation with volatile solvents

Chief solvent is highly purified petroleum benzene

Percolation with volatile solvents: Products (2)

1. Concrete

2. Absolute

Concrete

• Resulting product after removal of volatile solvent by vacuum distillation

• Alcohol + Volatile oil + some waxes

• Volatile solvent is removed by vacuum distillation

• Contain the plant or flower waxes

Concretes — Removal of waxes

Odorous portion is dissolved in strong alcohol and insoluble waxes are separated by filtration

Absolutes

Resulting products after small quantities of alcohol and soluble waxes are removed by chilling to 20°C

Destructive distillation

• Heating without access of air

• Means of obtaining oils with “empyreumatic or smoke-like” odor (not used in perfumery)

Destructive distillation — example

Tar

Top note

Most volatile; leave the skin rapidly

Top notes — examples (3)

Lemon, Anise, Lavender

Middle note

Intermediate volatility and tenacity

middle notes — examples (3)

Thyme, Neroli, Rose Oils

Base note

Low volatility and high tenacity; “fixatives” or staying power of perfume

Base notes — examples (4)

Vanillin
Musk (male musk deer)
Civet (civet cats)
Ambergris (spermwhale)

Neroli

Orange flower oil (flower of orange fruit)

Vanillin — distinction

Only fixative from plants

Broad classes of volatile oils (2)

1) Terpenoids

2) Phenylpropanoids / Aromatic compounds

Terpenoids

• Formed via the acetate-mevalonate acid pathway

• Most non-aromatic volatile oils (hydrocarbons, alcohol)

Terpenoids — structure

Natural products whose structure may be divided into isoprene units (C5H8)

Most commonly found terpenoids in volatile oils

Monoterpenes

Largest class of terpenoids

Sesquiterpenes

Phenylpropanoids/aromatic compounds

Formed via the shikimic acid pathway

Precursor of phenylpropanoids/aromatic compounds

Cinnamic acid

Monoterpenes

• Derived from limonene

• Occur in plants as glycosides with the presence of a sugar component

Cyclases

Determine skeletal class of the terpene

Geranyl pyrophosphate

Trans; precursor of acyclic monoterpenes (no ring present in their structure)

Neryl pyrophosphate

Cis; precursor of cyclic monoterpenes

Volatile oil — components (2)

1. Eleoptene

2. Stearoptene

Eleoptene

Hydrocarbon portion of the oil (liquid)

Eleoptene — Examples (3)

Eucalyptol, methyl salicylate, eugenol