Compounds

A compound is a single substance within a mixture. A bottle of essential oil is not one substance — it is a few dozen to a few hundred substances mixed together, and each of them is a compound.

Below they are grouped by chemical family — the second axis of classification. The third axis is the biosynthetic pathway, which answers where a compound comes from. 23 compounds in the library must be declared individually on cosmetic labels. 106 compounds, one page each: how it smells, why it sits where it does on the volatility axis, and where you meet it outside essential oils.

To set a few compounds side by side and see which one really dominates, use the formula reader — it calculates by odour threshold, not by percentage by weight.

Monoterpenes · 17 compounds

α-Thujene	152 °C	Thin wood, slightly pungent, with a dry herbal note. Very light and gone almost instantly.
α-Pinene	155 °C	Pine, dry, sharp. It evaporates so fast it has usually left before you can name it.
Camphene	159 °C	Faint camphor, dry, slightly piney. It is the bridge between the pine group and the camphor group.
Sabinene	163 °C	Woody, slightly peppery, dry. A small presence, but it sharpens the edges of the whole mixture.
β-Pinene	166 °C	Pine, but darker and less sweet than alpha-pinene. The two differ only in the position of one double bond, and the human nose can tell them apart.
Myrcene	167 °C	Green, slightly resinous, with a faint earthy note. Rather coarse on its own, but it links the citrus part to the green part.
δ-3-Carene	170 °C	Pine, sweet, with a clear resinous note. This is the compound that gives turpentine oil its most characteristic part.
β-Phellandrene	172 °C	Mint, slightly citrus, with a light wood note. Close to α-phellandrene but cleaner.
α-Phellandrene	175 °C	Faint mint, wood, with a slight citrus note. Greener than limonene and less sweet.
α-Terpinene	175 °C	Faint lemon, thin wood, with a slight herbal note. Less sharp than limonene.
Limonene	176 °C	Citrus peel, bright, slightly bitter at the edges. This is the most common molecule in the whole fragrance trade, and also one everyone has smelled after it has gone off — the smell of old peel is oxidised limonene. It is not the terpene that spoils fastest; α-terpinene and α-phellandrene are quicker still. But because limonene is everywhere, its trace of decay is the one we know best.
(E)-β-Ocimene	177 °C	Sweet, floral, slightly herbal. Light and much faster to evaporate than the ring terpenes.
cis-β-Ocimene	177 °C	Green, lightly sweet, with a grassy note. Thin and short-lived — it is the coat of paint, not the frame.
p-Cymene	177 °C	Dry, light wood, with a slight solvent note. Fainter than most other monoterpenes and without any freshness.
p-Menth-1-ene	177 °C ~	Faint wood, slightly citrus, with almost no edge. Rarely the main compound anywhere, but worth knowing because it is the simplest form of the skeleton many familiar components share.
γ-Terpinene	183 °C	Dry citrus, slightly woody, not very sweet. It is the backbone of the smell of fruit peel.
Terpinolene	186 °C	Lightly sweet, piney, with a slight floral note. Softer than the other monoterpenes.

Sesquiterpenes · 14 compounds

β-Elemene	252 °C ~	Light wood, slightly herbal, with a waxy note. Faint and hard to pick out on its own.
Longifolene	254 °C	Dry wood, pine resin, slightly dusty. Very lasting and hardly evaporates.
α-Cedrene	262 °C ~	Clear cedarwood, dry, a little like pencil shavings. This is the compound that defines the smell of cedarwood.
α-Copaene	262 °C ~	Wood, slightly spicy, with a faint honey note. Warmer than cadinene and less dusty.
β-Caryophyllene	264 °C	Wood, pepper, dry. It stays a very long time, and almost nobody notices its presence until it is gone.
Zingiberene	269 °C	Dried ginger, warm, slightly earthy. The ginger that remains after the citrusy top has gone.
α-Farnesene	270 °C ~	Green apple, light wood, slightly sweet. Feels lighter than the ring sesquiterpenes.
β-Selinene	270 °C ~	Wood, slightly herbal, with a clear celery note.
Bicyclogermacrene	272 °C ~	Light wood, slightly green, with a herbal note. Very hard to pick out on its own.
Valencene	274 °C ~	Sweet orange, light wood, with a slight grapefruit note. A bridge between the citrus family and the woody-resinous family.
β-Bisabolene	274 °C ~	Warm spice, slightly citrus, with a balsamic note. Softer than most woody sesquiterpenes.
Germacrene D	275 °C ~	Wood, slightly spicy, with a faint earthy note. Hard to pick out because it is rarely alone.
α-Humulene	275 °C ~	Dry wood, slightly pungent, with a hop note. Lighter in impression than caryophyllene although of similar weight.
δ-Cadinene	275 °C ~	Dry wood, slightly medicinal, with a dusty note. Closer to old wood than to fresh.

Alcohols · 20 compounds

Ethanol	78 °C	Sharp, clean, cold, flat. No layer behind it.
cis-3-Hexenol	156 °C	Freshly cut grass, harshly green, cold and wet. The molecule of plant injury.
1-Octen-3-ol	175 °C	Freshly snapped mushroom, damp, slightly metallic. The human nose picks it up at extremely low concentration, so it is both a smell and an alarm bell.
3-Octanol	175 °C	Mushroom, earth, slightly fatty. Softer than 1-octen-3-ol and less fishy.
Linalool	198 °C	Floral, slightly sweet, flat and almost without edges. On its own it is rather faint; its job is to be the ground the other components stand on.
2-Methylisoborneol	207 °C ~	Damp earth, mould, dark cellar. Together with geosmin it builds the smell of first rain.
Terpinen-4-ol	209 °C	Earthy, slightly medicinal, dry. Not pleasant on its own, but without it lavender loses its footing.
Menthol	212 °C	Peppermint, icy in the nasal cavity. It does not lower the temperature; it only convinces the body that the temperature has dropped.
Borneol	213 °C	Camphor but softer, with a damp-wood note. It is the gentle version of camphor.
2-Phenylethanol	219 °C	Rose, honey, with a slight note of bread. Soft and round, with no edge at all.
α-Terpineol	219 °C	Lilac, slightly soapy, soft. One of the molecules that act as connectors and are seldom noticed.
Citronellol	225 °C	Rose, soft, less harsh than geraniol. The same direction of smell, spoken more quietly.
Geraniol	230 °C	Rose, sweet, slightly waxy. More lasting than citral and carries further on the skin.
Geosmin	270 °C	First rain, earth after rain, beetroot. The human nose detects it at a few parts per trillion — one of the lowest thresholds ever measured in humans.
Cedrol	273 °C	Sweet, dry cedarwood with a slight camphor note. Richer than cedrene and far more lasting.
Nerolidol	276 °C	Floral wood, with a note of fresh bark and a hint of apple. A bridge between the floral and the woody-resinous families.
Patchoulol	287 °C ~	Earth, damp wood, deep sweetness. This is the compound that defines the smell of patchouli.
Guaiol	288 °C ~	Damp wood, slightly rosy, with an earthy note. Softer than patchoulol and less sweet.
α-Santalol	301 °C ~	Cream, milk, soft wood. Sweet without sugar, warm without heat.
α-Bisabolol	314 °C ~	Faintly floral, softly sweet, with almost no edge. Very discreet for its weight.

Phenols · 5 compounds

Phenol	182 °C	Antiseptic, asphalt, with a slight note of printing ink. Sharp and dry.
Guaiacol	205 °C	Smoke, tobacco, asphalt. This is the molecule of a fire that has gone out.
Creosol	221 °C	Sweet smoke, tobacco, with a slight antiseptic note. Warmer than guaiacol.
Eugenol	254 °C	Clove, warm, slightly numbing on the tongue. Present in cassia leaf, holy basil and clove — three unrelated plants.
Syringol	261 °C	Wood smoke, smokehouse, black tea. Heavier than guaiacol and stays longer.

Aldehydes · 11 compounds

Furfural	162 °C	Almond, burnt bread, with a slight wood note. Dry sweetness.
5-Methylfurfural	187 °C	Caramel, roasted almond, with a slight spice note. Lightly burnt sweetness.
Citronellal	207 °C	Green, slightly metallic, almost soapy. On its own it builds most of the smell of kaffir lime leaf.
Decanal	208 °C	Orange peel, slightly fatty, waxy. The strange fatty note in Vietnamese coriander is this compound.
Neral	227 °C	A softer lemon than geranial.
Citral	229 °C	Sharp lemon with body, slightly harsh. This is the lemon smell the food industry uses, not the smell of real lemon peel.
Geranial	229 °C	Sharp lemon, edged, slightly metallic. This is the part that gives citral the harshness many people find a little coarse when smelled strongly.
Perillaldehyde	237 °C	Perilla, warm green, with a faint cinnamon note. Very distinctive and very hard to borrow from any other source.
Dodecanal	238 °C	Fatty, waxy, slightly soapy. Heavier than decanal and less fresh.
Cinnamaldehyde	248 °C	Cinnamon bark, hot sweetness, almost a taste. A single molecule responsible for the whole cinnamon tree in everyone’s memory.
Vanillin	285 °C	Vanilla, warm sweetness, with a creamy note and a touch of wood.

Ketones · 9 compounds

Diacetyl	88 °C	Butter, cream, fatty sweetness. At high concentration it turns into an unpleasant artificial-butter note.
3-Octanone	167 °C	Mushroom, slightly fatty, with a faint earthy note. At low concentration it smells like a forest after rain; higher up it smells like a fridge that needs cleaning.
1-Octen-3-one	175 °C	Metallic, fishy, close to blood. Very strong at extremely low concentration.
Fenchone	193 °C	Camphor, slightly sweet, with a faint anise note. Sharper than true camphor and less dusty.
Camphor	204 °C	Dry, sharp, woody, dusty. Heavy and closed; it does not open out like cineole.
Carvone	231 °C	Depending on the isomer it is two unrelated smells: the R form gives spearmint, the S form caraway and dill seed.
cis-Jasmone	248 °C	Warm jasmine, with a hint of celery and a fatty note. This is the part that makes jasmine sound alive rather than merely sweet.
Nootkatone	290 °C ~	Grapefruit peel in concentrated form, bitter and warm. Very lasting, hardly evaporating.
α-Vetivone	300 °C ~	Roots, earth, damp wood. So heavy it almost stands still on a smelling strip.

Esters · 9 compounds

Ethyl butanoate	121 °C	Pineapple, banana, fruit sweets. The molecule of fruit that is just ripe, and of fruit that is overripe.
Isoamyl acetate	142 °C	Ripe banana, sweet, with a candy note. Arrives very fast and spreads far.
Ethyl hexanoate	168 °C	Pineapple, ripe apple, slightly winey. Round sweetness with no edge.
Benzyl acetate	213 °C	Jasmine, sweet, slightly waxy. Clean and pleasant — which is exactly why real jasmine needs indole so as not to be bland.
Linalyl acetate	220 °C	Sweet like pear, soft, rounding off the floral part. This is the compound that makes lavender sound gentle rather than harsh.
Methyl salicylate	222 °C	Medicated balm, sweetly medicinal, warm. The familiar smell of rubbing liniments and the family medicine cabinet.
Lavandulyl acetate	230 °C	Herbal, dry sweetness, very particular. Its presence is a sign of genuine lavender; blended products often leave it out because it is expensive and few people notice.
Methyl anthranilate	256 °C	Orange blossom with grape, sweet and slightly powdery. This is the faint floral note at the end of the breath of ripe kumquat.
Methyl cinnamate	263 °C	Sweet cinnamon, strawberry, slightly resinous. Softer than cinnamaldehyde and less hot.

Oxides · 2 compounds

1,8-Cineole	176 °C	Clean, cold in the nasal cavity, edged. One of very few molecules an untrained nose can name at once.
Caryophyllene oxide	280 °C	Faint wood, slightly musty, almost motionless. This is what is still on your collar the next day.

Lactones · 3 compounds

γ-Nonalactone	243 °C	Toasted coconut, cream, ripe peach. Lactones always give a sense of fat even without a single drop of oil.
δ-Decalactone	281 °C	Cream, coconut, ripe peach. Fatty and sweet without sugar.
Coumarin	301 °C	Hay, faint vanilla, baked goods. Sweet without sugar.

Other compounds · 16 compounds

Trimethylamine	3 °C	Fishy, a light ammonia. In good fish sauce it is just one note; in spoiled fish sauce it is everything.
Dimethyl sulfide	37 °C	Boiled cabbage, sea, slightly fishy. At very low concentration it gives the sweet note of boiled corn.
Acetic acid	118 °C	Vinegar, sharply sour, stinging the nose. No layer behind it.
2,5-Dimethylpyrazine	155 °C	Roasted, nutty, with a dry earthy note. This is the smell of something that has met fire and contains protein.
2-Furfurylthiol	155 °C	Roasted coffee in its purest form. At high concentration it becomes the smell of burnt rubber.
Butyric acid	164 °C	Rancid butter, cheese, divisive. At very low concentration it gives a pleasant sense of richness — dose decides everything.
Dimethyl trisulfide	170 °C	Rotten onion, overcooked cabbage, with a metallic note. Very strong at extremely low concentration.
2-Acetyl-1-pyrroline	181 °C ~	Freshly cooked sticky rice, pandan leaf, popcorn. A single molecule behind a whole region of food memory.
Furaneol	188 °C ~	Caramelised sugar, ripe strawberry, caramel. So sweet it can almost be read as a taste.
Dipropyl disulfide	194 °C	Heat-treated onion, sweet and fatty. Without the pungency of raw onion.
Maltol	200 °C ~	Burnt sugar, baked goods.
Naphthalene	218 °C	Mothballs, asphalt, dry and dusty. Very particular and impossible to confuse.
trans-Anethole	234 °C	Star anise, round sweetness, slightly cool at the end. One of the few molecules almost nobody describes wrongly.
Lenthionine	240 °C ~	Dried shiitake, rich, meaty, with deep umami.
2,4,6-Trichloroanisole	241 °C ~	Mould, damp rag, old paper. This is the smell of corked wine.
Indole	253 °C	Overripe white flowers, faecal at high concentration. The most off-putting and also the most indispensable compound in the floral family.

About the boiling points on this page

Every boiling point on this page is the value at atmospheric pressure. This is not a superfluous detail: the boiling points of heavy molecules are often published at reduced pressure, and a great many sources do not state the pressure used.

A concrete example: many sources give 1-octen-3-one as 56–60 °C. That figure is measured at 16 mmHg. At atmospheric pressure it boils at around 175 °C — more than a hundred degrees apart. Take the wrong figure and its position on the volatility axis is completely wrong.

For heavy sesquiterpenes such as α-santalol and nootkatone, we have not yet cross-checked a value at atmospheric pressure from a reliable source. Those compounds carry the estimate mark, and their pages state why.

A ~ after a figure means the value is still an estimate.