Twelve families, one entry each — the structure, the behaviour, the oils it leads, and what it asks of you.
~13 min read
A single essential oil can contain two or three hundred distinct compounds. No one memorises that. What makes oil chemistry usable at the shelf is that those hundreds sort into about a dozen families, and a molecule behaves far more like the rest of its family than like anything else.
The sorting is done by the functional group: the arrangement of atoms that decides how a molecule reacts, met already in functional groups. Because reactivity lives in the group rather than the whole molecule, two compounds that look nothing alike on paper — one from a lavender flower, one from a coriander seed — handle alike, age alike and tend to be tolerated alike.
So: open an oil you have never met, read its constituent profile, and you do not need the individual compounds — only which families sit at the top. From there you can predict, before smelling it, how volatile it will be, how it will age, how much dilution it wants and what it will do to a blend. Then you check the label. Prediction is not certainty, but a reasoned guess in thirty seconds beats no framework.
Almost every molecule in an essential oil is built the same way: a carbon skeleton of repeating five-carbon isoprene units, with or without an oxygen-containing group attached. Terpenes covered the skeletons; half of what a family does is decided by size.
Monoterpene — C10
Ten carbons. The most volatile thing in the bottle — these leave first, which is why an oil dominated by them feels bright and thin.
Sesquiterpene — C15
Fifteen carbons. The difference in behaviour is far larger than the difference in size: slow, viscous, still on skin long after the light material has gone.
Diterpene — C20
Twenty carbons. Too heavy to vaporise at steam-distillation temperatures, so they barely appear in distilled oils.
Size decides volatility, viscosity and how long an aroma lasts. It decides almost nothing about reactivity: a fifteen-carbon molecule carrying a hydroxyl is an alcohol first and a large molecule second. Reactivity is the group’s job. Attach nothing and you have a hydrocarbon. Attach a hydroxyl to an open chain and you have an alcohol, mild and well tolerated. Attach the same hydroxyl onto an aromatic ring and you have a phenol, the most reactive family on the shelf.
Diagram to come
Two axes shown as one figure. Along the horizontal, three carbon skeletons drawn to relative scale — a ten-carbon monoterpene, a fifteen-carbon sesquiterpene, a twenty-carbon diterpene — with an arrow beneath for increasing size and decreasing volatility. Along the vertical, the same C10 skeleton repeated four times with a different group attached each time: nothing, a hydroxyl on a chain, a hydroxyl on a ring, a carbonyl. Arrow beside it for increasing reactivity. The point of the drawing is that the two axes are independent.
Each entry has the same shape, so you can jump to a family and find the same things in the same order. Product ranges differ by country, so some oils named here may not be on your shelf — the chemistry is the same wherever a bottle is sold.
Diagram to come
A reference plate: each functional group drawn once as a skeletal structure, arranged in a grid, with the reactive part of each picked out in a second colour so the eye goes straight to it. Draw the group in isolation and then, beneath it, one real named constituent containing it, so the abstract group and a real molecule sit together. Order: monoterpene (limonene), sesquiterpene (beta-caryophyllene), monoterpenol (linalool), sesquiterpenol (patchouli alcohol), ester (linalyl acetate), aldehyde (geranial), ketone (menthone), phenol (carvacrol), phenylpropanoid (cinnamaldehyde), ether (trans-anethole), oxide (1,8-cineole), acid (a carboxyl group), lactone or coumarin (bergapten). This is the plate a member will come back to; legibility on a phone matters more than completeness.
**Monoterpenes.** Ten carbons and hydrogen, nothing else — no oxygen, so strictly no functional group at all. The fastest thing in the bottle to volatilise: bright, fresh, first smelled and first gone.
Example constituents
Limonene, alpha-pinene, gamma-terpinene, myrcene.
Oils led by it
Wild Orange at 80–97% limonene, Grapefruit 90–97%, Lemon 60–75%.
Safety tendency
Gentle while fresh, and the least stable family towards air — an [oxidised monoterpene oil](#how-a-family-changes-with-time) is a different proposition.
**Sesquiterpenes.** Fifteen carbons, still hydrocarbon, still no oxygen — and those extra five change the behaviour out of all proportion to the size. Viscous, slow, woody, arriving late and staying for hours.
Example constituents
Beta-caryophyllene, cedrol, zingiberene.
Oils led by it
Copaiba at 45–65% beta-caryophyllene, Cedarwood 10–47% cedrol, Ginger on zingiberene.
Safety tendency
Among the gentlest on skin, and these oils age better than anything else on the shelf. Beta-caryophyllene is the one constituent here known to dock a human receptor — the endocannabinoid system.
**Monoterpenols.** A hydroxyl group — one oxygen, one hydrogen, written –OH — on a ten-carbon skeleton. That oxygen makes the molecule slightly polar and rounds the aroma into something soft and green-floral.
Example constituents
Linalool, terpinen-4-ol, geraniol, citronellol, menthol.
Oils led by it
Coriander at 60–75% linalool, Basil 40–80%; tea tree leads on terpinen-4-ol, peppermint on menthol at 30–50%.
Safety tendency
The gentle end at ordinary dilution. Menthol is the exception — strongly sensed, which is why peppermint is kept off a young child’s face.
Two names for one family
You will see this family written as monoterpenols in one place and monoterpene alcohols in another, including in this app’s oils library. They are the same thing. Monoterpenals are not a typo but monoterpene aldehydes, two entries down.
**Sesquiterpenols.** The same hydroxyl on a fifteen-carbon skeleton. Heavy, barely volatile, extremely tenacious — deep, creamy, woody, and in a blend the fixatives that slow everything mixed with them.
Example constituents
Alpha- and beta-santalol, patchouli alcohol, khusimol.
Oils led by it
Hawaiian Sandalwood on the santalols, Patchouli on patchouli alcohol, Vetiver on khusimol.
Safety tendency
Among the least likely to irritate. The real caution is a blending one: one drop dominates far longer than intended.
**Esters.** An alcohol joined to an acid with a molecule of water removed, leaving an oxygen bridge. The quietest family chemically, sweet and fruity — and undone by the reverse of their formation: water, warmth and time split them apart again.
Example constituents
Linalyl acetate, bornyl acetate, terpinyl acetate.
Oils led by it
Lavender at 25–45% linalyl acetate over 20–47% linalool, Clary Sage 40–75%, Cardamom 25–50% terpinyl acetate.
Safety tendency
The gentlest family on skin, with one outlier: Wintergreen is 98% or more methyl salicylate — an ester, and nothing like mild.
**Aldehydes.** A carbon at the end of a chain, double-bonded to oxygen and still holding one hydrogen. That exposed terminal position makes them reactive towards proteins and quick to oxidise onward into acids. Intense and lemon-sharp at very low concentrations.
Example constituents
Geranial and neral, together called citral; citronellal.
Oils led by it
Lemongrass at 25–50% neral and 25–50% geranial, Lemon [Eucalyptus](/healer/oils/singles/lemon-eucalyptus) on citronellal.
Safety tendency
The reactive end. Aldehyde oils are dilution-sensitive, and Lemongrass sits on this app’s hot-oil list beside oregano, cassia, cinnamon bark and clove.
**Ketones.** The same carbon–oxygen double bond, within the chain rather than at its end. With no hydrogen to give away a ketone cannot oxidise onward as an aldehyde does, which makes the family unusually stable. Sharp, camphoraceous, and small enough to cross membranes easily.
Example constituents
Menthone, carvone, camphor, and the turmerones.
Oils led by it
Spearmint on carvone, Rosemary on camphor beside its cineole, Peppermint carrying 15–30% menthone behind its menthol.
Safety tendency
The longest caution list, though not the most irritating family. This app advises caution with camphor-rich, sage and rosemary oils during pregnancy.
**Phenols.** A hydroxyl bonded directly onto an aromatic ring rather than an open chain. The ring pulls electron density from the oxygen, the hydroxyl hydrogen becomes far easier to release, and phenols become the most reactive family in essential oils. Hot, pungent, medicinal.
Example constituents
Carvacrol, thymol and eugenol. Carvacrol and thymol are isomers — identical atoms, hydroxyl in a different place on the ring — which is the whole difference between the oregano and thyme notes.
Oils led by it
Oregano is the range’s phenol oil, carvacrol-dominant with thymol behind it; Thyme runs the pair the other way round. Clove is 70–90% eugenol.
Safety tendency
The heaviest dilution of any family — half a per cent or less. Reactive towards skin proteins, and the reason the hot-oil rule exists at all.
**Phenylpropanoids and ethers.** The family that breaks the pattern: built by a separate route in the plant, and recognisable as a six-carbon aromatic ring with a three-carbon tail. An ether is an oxygen bridging two carbons, usually capping what would be a phenol. The aroma is spice.
Example constituents
Cinnamaldehyde, eugenol, trans-anethole, methyl chavicol.
Oils led by it
Cassia at 75–97% cinnamaldehyde, Cinnamon Bark at 45–80% with 3–13% eugenol, Fennel on trans-anethole. On Guard draws heavily on this family.
Safety tendency
The hot end, with the phenols. Cassia and cinnamon bark are the two most irritating oils in the range and both sit on the hot-oil list. Half a per cent or less, never near the face.
**Oxides.** An oxygen atom built into a closed ring as part of the skeleton rather than hanging off it, which makes the group stable and distinctive: highly volatile, strongly penetrating, cool and open. One member dominates so completely that oxide usually means 1,8-cineole.
Example constituents
1,8-Cineole above all; rose oxide and bisabolol oxide in traces.
Oils led by it
Eucalyptus, where cineole is the defining constituent; Rosemary, pairing oxides with ketones; Cardamom at 25–50% 1,8-cineole. Easy Air is built on this family.
Safety tendency
Gentle on adult skin, watched around young children: this app advises against eucalyptus and rosemary under six, and cineole is the reason.
**Acids.** A carboxyl group: a carbonyl and a hydroxyl on the same carbon, written –COOH. The most polar and least volatile of the oxygenated groups, which is why free acids are rare in distilled oils — heavy and water-loving, they stay behind in the still.
Example constituents
Boswellic acids, cinnamic acid, salicylic acid — the last reaching the shelf as its methyl ester.
Oils led by it
None, and that is the point. Boswellic acids are named on frankincense’s constituent list and show the family’s problem exactly: heavy molecules that mostly stay in the resin, which is why that oil’s own families are monoterpenes and sesquiterpenes.
Safety tendency
Rarely a topical consideration at the concentrations found in distilled oils. Acids matter here as a destination — what aldehydes become with age.
**Lactones and coumarins.** A lactone is an ester whose two ends have been joined into a ring; a coumarin is a lactone fused to an aromatic ring, and a furanocoumarin adds a third. Heavy, barely volatile, almost always under one per cent — and per unit weight the most consequential minor constituents in oil chemistry, because furanocoumarins absorb ultraviolet light and turn reactive.
Example constituents
Bergapten, oxypeucedanin, coumarin itself.
Oils led by it
None — but present in every cold-pressed citrus peel oil: Bergamot carries the most, with Lemon, Lime, Grapefruit and Wild Orange.
Safety tendency
This family alone is the reason for the photosensitivity rule: after applying an expressed citrus oil to skin, keep that skin out of direct sunlight and away from UV for at least twelve hours. A distilled citrus does not carry it — these molecules are too heavy to travel over with the steam.
Everything above encourages a habit worth correcting before it hardens: reading the top line of a profile and treating the rest as rounding error. Wild Orange is 80–97% limonene. It is not limonene — the remaining fraction is where most of the character lives.
Start with perception. Molecules become detectable to the nose at wildly different concentrations — the thresholds span several orders of magnitude. A constituent at five hundredths of one per cent can be more perceptible than one at forty per cent. This is why two batches with near-identical major constituents can smell noticeably different: the difference sits in the minor fraction, which is also the fraction that varies most with soil, season and harvest date.
Then stability. Gamma-terpinene, a monoterpene, is a precursor the plant converts onward into thymol and carvacrol. In the bottle it also acts as a sacrificial target, taking up oxygen that would otherwise reach something else. A minor constituent can change how a whole oil ages while contributing nothing to the aroma.
There is also a long-discussed idea called quenching — that one constituent moderates the irritancy of another, so the whole oil is better tolerated than its main constituent alone. It holds in some pairings and not others, and dilution guidance deliberately does not lean on it. An open question, not a reason to dilute less.
The strongest evidence that the mixture matters comes from work at Roseman University College of Pharmacy, covered in the oil effect. Researchers tested whole oils, then tried to reproduce what they measured by recombining purified constituents at matching concentrations. They could not. That was laboratory research, on cells and in research models rather than in people, so read it as a statement about the chemistry rather than the body. As one, it is strong: the arrangement itself is part of what an oil is.
The habit worth keeping
Read the dominant family for the tendency, the second for the texture, and the minor fraction for the reason two bottles of the same oil are not identical. "The whole oil" is a mixture whose behaviour is not the sum of its listed parts.
This is where the families stop being taxonomy and start being handling. Almost every dilution rule in this app follows from the chemistry above, and a rule you can derive is one you remember.
Phenols and aromatic aldehydes need the heaviest dilution because they are the most reactive groups on the shelf; esters and the alcohols are the gentle end because none of them is in a hurry to react.
A phenol releases its hydroxyl hydrogen readily; an aromatic aldehyde carries an exposed carbonyl carbon. Both react with proteins in skin, and skin registers that as irritation — a specific reactivity, not potency in some vague sense, and half a per cent or less is the figure it earns.
Ketones carry their cautions for a different reason. They are not especially irritating; they are small, fat-soluble and stable, so they cross membranes readily and are not quickly broken down afterwards. The pregnancy caution follows from that, not from any surface effect.
Oxides are watched around young children because 1,8-cineole is both highly volatile and strongly sensed in the airway, and a small airway responds more sharply than an adult one. Hence eucalyptus and rosemary away from children under six, and peppermint off a small child’s face and neck.
The expressed-versus-distilled citrus rule is the neatest piece of chemistry in oil safety. Furanocoumarins are heavy and barely volatile: press a citrus peel and they come out with everything else; distil it and they stay behind. Same fruit, two processes, one of them photosensitising.
| Family | Dilution tendency | The usual caution |
|---|---|---|
| Esters | 2% or gentler | Mild. Wintergreen is the outlier — methyl salicylate is not a gentle ester. |
| Monoterpenols and sesquiterpenols | 2% or gentler | Well tolerated. Keep peppermint off a young child’s face. |
| Sesquiterpenes | 2% or gentler | Among the least irritating. Heavy and tenacious in a blend. |
| Monoterpenes | 2% standard, fresh | More likely to irritate once oxidised. Store cool and dark. |
| Oxides | 2% standard for adults | Avoid eucalyptus and rosemary for children under six. |
| Ketones | 1–2%, sparingly | Caution in pregnancy with camphor-rich, sage and rosemary oils. |
| Aldehydes | 0.5–1% | Skin-sensitive. Lemongrass is on the hot-oil list. |
| Phenols and phenylpropanoids | 0.5% or less | Hot oils: oregano, thyme, clove, cassia, cinnamon bark. Never near the face. |
| Lactones and coumarins | Not a dilution rule | Expressed citrus only: no direct sunlight or UV on that skin for twelve hours. |
Those figures come from the safety guide, the authority for dilution here: 2% for general adult use, 1% for sensitive skin and older adults, 0.5–1% for children aged two to ten, 0.5% or less for hot oils at any age. For one family worked through end to end, see oregano.
A family tells you a tendency across dozens of oils; it does not overrule the bottle in your hand, nor your own skin. Wintergreen is the reminder of the first; one diluted patch answers the second.
Keep oils away from eyes and inner ears, and out of reach of children. If you are pregnant, nursing, under medical care or taking medication, speak to your healthcare practitioner before use. Nothing here is intended to diagnose, treat, cure or prevent any disease.
The profile on a batch report describes the oil on the day it was tested. It begins drifting the moment the bottle is opened, along lines the families predict.
Monoterpenes change first, and the change matters most. Their double bonds take up atmospheric oxygen to form hydroperoxides, which are considerably more reactive towards skin than the monoterpene they came from. Limonene is the best-studied case: oxidised limonene is a recognised skin sensitiser, fresh limonene is not. This is why an old citrus oil is more likely to irritate than a new one. It has not become stronger. It has become a different mixture.
Aldehydes are second. That exposed terminal carbonyl oxidises onward to a carboxylic acid — citral becomes the corresponding acids, which is why an ageing lemongrass loses its bright top and takes on a flat, faintly sour edge. The family has become the acid family.
Esters hydrolyse rather than oxidise: given water, warmth and time they split back into the alcohol and acid that formed them. A lavender kept in a hot bathroom for two years holds measurably less linalyl acetate, and more linalool, than its batch report says.
Ketones, sesquiterpenes and sesquiterpenols barely move. A ketone has no hydrogen on its carbonyl carbon to give away, so it cannot take the aldehyde route, and sesquiterpenes are large and comparatively unreactive. This is why a patchouli or a sandalwood improves with age while a lemon deteriorates.
Storage limits three things: oxygen, heat and light. A tight lid limits how much oxygen the oil meets — and the headspace in a half-empty bottle is the neglected part of that, since the air above the oil sits in permanent contact with it. Decanting into a smaller bottle is unglamorous and effective. Cool storage slows every reaction above, which is why citrus oils are best refrigerated, and dark glass blocks the light that starts some of them.
So an oil has no single expiry date; it has a family-dependent one. Expressed citrus and aldehyde oils are the short-lived end, worth buying in sizes you will finish. An oil that smells different from the one you remember has told you something about its chemistry, not about your memory.
Diagram to come
Three short reaction arrows stacked vertically, each showing a before and an after, with a small clock and an oxygen symbol on the arrow. Top: a monoterpene double bond going to a hydroperoxide, marked as the change that raises irritation potential. Middle: an aldehyde carbonyl going to a carboxylic acid. Bottom: an ester splitting into an alcohol and an acid, with a water molecule on the arrow rather than oxygen. Beneath all three, a short bar showing which families sit at the stable end.
The lesson reduced to something you can do at a shelf, with an unfamiliar bottle and a constituent list on the back.
Run that sequence on a dozen bottles and it stops being a sequence: you look at a profile and see the shape of the oil. That is where reading any oil becomes a skill, not a procedure.
Lesson 8 of 18 · Science