Chemistry for Aromatherapy

Everyday Aromatherapy - background notes

We recommend you acquire the Caddy blending calculator and books Essential oils in Colour and the Essential blending guide.
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Nothing really brings home the relationship of the essential oil constituents to their uses and properties like seeing the oil analysis and the combined blended oils depicted in a pie chart. I give this software and books a 5 star rating for students. The software costs only £16 and really should be on every aromatherapists desktop.  Using our supersense of sight to help us understand our less developed sense of smell makes perfect sense.

The diverse uses accredited to essential oils are largely due to
their highly complex chemical composition.

You may find this download helpful to print off.  The idea is to reduce the
chemistry and the botany to a single piece of A4 for the 62 of the study oils.
Download item 0001 the 62 Study Oil A4Handout

The aim of this Chemistry section is to equip you with enough
information to understand the implications of some of the chemical
compounds found in essential oils and the potential to create new mixtures by blending oils.
We’ll start by looking at some
basic chemistry and then we’ll move on to look at the
compounds themselves. 

Chemistry is the science that is concerned with the composition of
‘matter’ (everything that makes up the universe!) and the changes
that take place in it under certain conditions.
All matter can exist as a solid, liquid or gas, according to changes
in temperature or pressure, and is made up of atoms.
Atoms are the smallest and simplest unit of matter that can take
part in a chemical reaction. They are too small to be seen even
using a high-powered microscope and are in constant motion.

An atom has a nucleus (central part) containing protons
(positively charged particles) and neutrons (particles with no
electrical charge). The nucleus is surrounded by a number of
electrons (negatively charged particles).
There are 109 different types of atom, distinguished by the number
of each type of particle present, each with its own chemical
Atoms can combine with one another to form a molecule.
Molecules that contain atoms of only one kind are called
elements. An example of this is oxygen, chemical formula O.
Oxygen exists in nature as a molecule of two oxygen atoms, hence
the chemical formula of this element is O2.

Molecules that contain atoms of different kinds are called
compounds. Water is an example of a compound. A water
molecule contains two hydrogen atoms (chemical formula H) and
one oxygen atom, hence its chemical formula H2O.
You will often see the word ‘compound’ preceded by either
‘organic’ or ‘inorganic’. Don’t be put off by this terminology.
These terms can be distinguished by two words – life and carbon.
Organic compounds form the chemical basis for life. The main
feature of all organic compounds is the presence of carbon. All
matter that is living or has once lived will consist of carbon.
Charcoal for example (live matter in the form of a tree before
heating) is a simple carbon compound. Humans are no more than
a complex carbon compound! Needless to say, organic
compounds are more complex and abundant than inorganic
compounds (those that do not contain carbon and have never
been ‘alive’).
The chemistry of life is based on the ability of carbon atoms to
bond with other carbon atoms or with other types of atom,
commonly hydrogen, nitrogen, oxygen or sulphur, to produce an
infinite number of organic compounds.
Essential oils contain many thousands of organic compounds. As
an Aromatherapist, it is useful to have an awareness of the
chemical structure of these compounds to give you an insight into
what can be expected from the oil.
Explanations of the ways in which atoms bond and the structures
they produce can get extremely complicated.
For our purposes the main concept to get to grips with is the
difference between aliphatic and aromatic compounds. You
would automatically think that aromatic compounds are odorous
whilst aliphatic are not. Whilst you would be correct to a degree,
the word aromatic in the context of chemistry pertains purely to the
structure of the compound.

We’ll look at aliphatic compounds first.

Aliphatic Compounds
Aliphatic compounds are simpler than aromatic compounds. They
are also generally non-fragranced. Aliphatic compounds are
produced when atoms bond in chains, although these chains can
curl around on themselves to form cyclic chains (circles). Here
are a few diagrams representing simple aliphatic compounds:
Aliphatic open chain:

Aliphatic branching chain:

Aliphatic cyclic chain:

Note: These structures can consist of a varying number of atoms.

More complex aliphatic compounds are produced when structures
containing 5 carbon atoms, arranged as shown below, combine.
These structures are called isoprene units.
Isoprene units are the building blocks for many of the organic
compounds found in essential oils.
They can link up with other isoprene units to form complex
aliphatic chains both acyclic (non-circular) and cyclic. As the
structure gets more complicated the compound increases in weight
and reactivity.
Aromatic Compounds
Aromatic compounds are more complex, more volatile, and more
unstable than aliphatic compounds.
Aromatic compounds are also irritant and although they generally
have a fragrance, some aromatic compounds are odourless.
Structurally, aromatic compounds are characterised by a ring of 6
carbon atoms arranged as shown below.
Notice that alternate bonds are marked by a double line. This
signifies a double covalent bond. To a chemist this indicates the
way in which the electrons of the atoms are shared. Don’t worry
about the complexities of this. Just remember that for a compound
to be aromatic, the 6 carbon atoms are arranged in a ring with
alternate double covalent bonding.
Aromatic rings can bond with either other aromatic rings or
isoprene units to form complex structures. As long as the ring of 6
carbon atoms (characterised by the alternate double covalent
bonding) is present the compound is still aromatic.
Due to the various structural arrangements of atoms, chemical
compounds can have the same molecular composition and
chemical formula, but different physical structures and therefore
different chemical properties. These are called isomers.
For example, butane and methyl propane are isomers. They both
have 4 carbon atoms and 10 hydrogen atoms and so their
chemical formula is C4H10. Their atoms, however, have different
structural arrangements and so these compounds have different
Atom - smallest and simplest matter that can take part in a
chemical reaction. Made up of protons (positively charged),
neutrons (no electrical charge) and electrons (negatively
Molecule – formed when two or more atoms combine.
Element – molecule made up of only one type of atom e.g.
oxygen (O2).
Compound – molecule made up of more than one type of
atom e.g. water (H2O).
Organic compound - forms the basis for life. The main
feature is the presence of carbon, found in all matter that is
living or has once lived. Organic compounds are more
complex and abundant than inorganic compounds.
Inorganic compound - does not contain carbon. Inorganic
compounds are only found in things that are non-living or
have never lived. They are less abundant than organic
Aliphatic compound - simple, generally non-fragranced,
organic compound found in many essential oils. Atoms of
aliphatic compounds bond in chains (as shown below),
although these chains can curl around on themselves to form
cyclic chains. Aliphatic chains may also be branching.
Aromatic compound - organic compound found in many
essential oils. Aromatic compounds are more complex,
volatile and unstable than aliphatic compounds. They are
generally fragranced and irritant. Aromatic compounds are
characterised by 6 carbon atoms arranged in a ring with
alternate double covalent bonding as shown below.

Isoprene unit - a structure containing 5 carbon atoms
arranged as shown below. Isoprene units are the building
blocks for many of the organic compounds found in essential
oils. They can bond with both aliphatic chains and aromatic

Isomers - organic compounds with the same chemical
formula but different structural arrangements. This allows
chemical compounds with the same molecular composition
to have different chemical properties.

Appreciating the chemistry of an oil allows you to develop a deeper
understanding and respect for the oil, instead of simply accepting
it at face value. This has obvious safety benefits, particularly when
it comes to blending.
With this is mind, we’ll move on to look at the chemical compounds
commonly found in essential oils.
There are 9 chemical compounds that you need to be familiar with:
1. Alcohols
2. Aldehydes
3. Esters
4. Ethers
5. Ketones
6. Lactones (coumarins and furocoumarins)
7. Oxides
8. Phenols
9. Terpenes (mono-, sesqui- and di-)
Don’t be put off by these somewhat unfriendly terms. They are,
after all, only words!
For each of these chemical compounds there are literally
hundreds of specific chemicals included in the group. It would be
virtually impossible to learn them all, but you do need sufficient
knowledge to be able to appreciate the general impact of these
compounds within an essential oil.
To satisfy this need we will tackle these chemical compounds
individually. For each, we will give general details about their
basic chemical composition and use, name a couple of chemicals
from the group, state an oil(s) in which each of these chemicals
can be found and give any presently known precautionary


Essential oils contain many different chemicals. These chemicals
may be present in large quantities or just trace amounts. When we
quote a chemical and name an oil(s) which contains it, this does
not mean that this chemical is only found in the stated oil(s). It is
purely an example. It is likely that the chemical will occur in many
oils in varying amounts.

Alcohols are aliphatic and are derived from 2 isoprene units.
They form esters, aldehydes and acids.
Alcohols have many properties but are generally antiseptic,
anti-viral, bactericidal, stimulating and uplifting.
(Memory hint: When did you last get an infection whilst cheering
yourself up with your favourite tipple?)
Chemical Examples:

Linalool, found in lavender and bergamot

Citronellol, found in rose and geranium

Geraniol, found in palmarosa (75-95%) and geranium
(Memory hint: Notice that all of the chemical names end in
‘ol’, the same as alcoh-ol.)
Alcohols are the most beneficial group of chemicals to the
Aromatherapist. They are generally very safe and effective,
although they may cause skin irritation in some people. 

Aldehydes are derived from 2 isoprene units plus a carbonyl
group consisting of carbon, hydrogen and oxygen atoms.
They are formed from alcohols which have been
dehydrogenated (had water removed), hence their name
They are often chemically reactive.
Although aliphatic, they are important in the aroma of the plant.
Aldehydes have a sedative, fungicidal and anti-inflammatory
Chemical Examples:

Citral, found in lemongrass (65-85%), bergamot and

Neral, found in melissa, lemongrass and grapefruit
(Memory hint: The connection between the ‘citrus’ sounding
word and the lemongrass should help. How to tie that in with
the word aldehyde is a tricky one – how about Mr Alde is
hiding in a lemon field?!)
Aldehydes should be used with care as they can irritate the skin
and mucous membranes. As some people may be sensitive,
essential oils containing no aldehydes or just a low proportion
should be selected. Oils containing a high proportion of aldehydes
should only be used in very small doses. 

Esters are formed by the joining of an acid with an alcohol.
They often smell fruity.
Esters have a general fungicidal, anti-spasmodic and sedative

(Memory hint: Think of Ester Rantzen twitching in her sleep with a
mushroom on her head!)
Chemical Examples:

Linalyl acetate, found in clary-sage, bergamot and

Geranyl acetate, found in marjoram (sweet) and
(Memory hint: Esters tend to end with the letters ‘ate’.)
Esters are very user friendly with few precautions or side
effects. However the oils of sweet birch and winter green should
not be used as they contain the ester methyl salycilate which is
highly toxic. 

Ethers are also called phenyl-propane ethers and they rarely
occur naturally in essential oils.
The atoms are structured in aromatic rings.
Ethers are generally anti-depressant, sedative, balance the
nervous system, stimulate mental processes and reduce
Chemical Example:
Anethol, found in bitter fennel (hazardous oil)
Ethers are not user-friendly. Any oil containing even a trace of an
ether must be used with care. 

Ketones are derived from 2 isoprene units plus a carboxyl group
consisting of carbon and oxygen atoms.
This chemical type is aliphatic and chemically reactive.
Some ketones are toxic, others are not.

Chemical Example of a TOXIC ketone:
Thujone, found in sage and wormwood (both hazardous
Chemical Examples of NON-TOXIC ketones:

Jasmone, found in jasmine

Menthone, found in peppermint
(Memory hint: Notice that all the chemical names end in the
letters ‘one’ the same as ket-one.)
Used for short periods in weak dilutions ketones can be
sedative and also decongestant.
They should not be used during pregnancy. The use of high
ketone combinations should be avoided with epileptics. They may
also irritate the skin of some people.

Lactones are made up of 3 isoprene units.
The atoms are structured in aliphatic cyclic chains.
They are often present in expressed citrus oils.
They are immuno-stimulants, expectorants, anti-inflammatory

and febrifugal.
Chemical Examples:

Bergamottin, found in bergamot
Bergapten, found in bergamot and lemon
There are 2 groups of lactones of interest to the Aromatherapist,
coumarins and furocoumarins.
Coumarins are sedative and calming yet uplifting. They are also
Furocoumarins are anti-fungal and anti-viral.
All lactones are known to cause photosensitivity. Therefore care
should be taken when using these oils during periods of bright
sunshine when direct exposure to the skin is possible. Some may
also raise blood pressure and are anti-coagulant.

The atoms in oxides are structured in aliphatic chains.
Oxides are an expectorant.
(Memory hint: You expect oxygen don’t you?)

Chemical Examples:

1,8 cineole, found in eucalyptus and tea-tree, is the
main oxide found in essential oils
another example is cis-linalool oxide, found in hyssop
Oxides may irritate the skin and so combinations of oils high in
1,8 cineole should not be used.
Phenols are biologically very active.
Their atoms are structured in aromatic rings with one or more
hydroxyl groups consisting of hydrogen and oxygen atoms.
They produce a strong, spicy, herbal, medicated aroma.
Phenols have powerful antiseptic and bactericidal effects and
are also stimulants to the nervous and immune systems.
Chemical Examples:
Eugenol, found in clove (bud)
Thymol, found in thyme (red)
Carvacrol, found in black pepper
(Memory hint: Chemicals in this group often end in with the
letters ‘ol’.)
Phenols need to be treated with caution as many can be highly
irritating to the skin and may toxify the liver.
Oils with a high phenol content should be avoided and all others
should be used in low dilutions (1-2%) for short periods only.


Terpenes are the largest group of organic compounds found in
essential oils.
Terpenes are hydrocarbons, i.e. made up from hydrogen and
The carbon atoms are arranged in isoprene units. Two, three or
four isoprene units combine to form the various types of terpenes,
all of which are aliphatic.
They can react with oxygen and therefore oxidise when exposed
to air, which ruins the oil.
Their uses are generally the ‘anti’s’: anti-viral, antiseptic and anti-
Chemical Examples:
Limonene, which is anti-viral, found in 90% of citrus oils
(Memory hint: Think of lemon to tie in with citrus.)
Pinene, which is antiseptic, found in pine
(Memory hint: Remember pine-ne and pine.)
Chamazulene and Farnasene, which are anti-
inflammatory and bactericidal, found in German
(Memory hint: Chemicals in this group usually end with
the letters ‘ene’.)

There are three types of terpene:
1. Monoterpenes
These are made up of 2 isoprene units (10 carbon atoms). They
are highly volatile and evaporate quickly when exposed to air.
They may irritate the skin and so only weak dilutions should be
Both limonene (found in most citrus oils) and pinene (found in
pine) are monoterpenes.
2. Sesquiterpenes
Sesqui means 1½ so, as you would expect, sesquiterpenes
contain 3 isoprene units (monoterpenes have 2) which equals 15
carbon atoms. They are heavier than monoterpenes and are
prone to oxidation.
Chemical examples include caryophyllene found in clove (bud),
and cadinene found in myrrh.
There are no known precautions.
3. Diterpenes
Diterpenes contain 4 isoprene units (20 carbon atoms). They are
heavier than sesquiterpenes and are not commonly found in
essential oils, but sclareol exists in clary-sage.
Like sesquiterpenes, there are no known precautions.


We have looked at the most common chemical compounds found
in essential oils. Before we leave this section here’s a word of
Always remember that all oils comprise of a number of chemical
compounds occurring in varying quantities, from large amounts
to trace amounts.
For example lavender (Lavandula angustifolia) contains a large
quantity of linalool (an alcohol), a trace amount of pinene (a
terpene), plus a number of other chemical compounds.
Therefore careful consideration must be given when deciding if an
oil is suitable. Take into account all you know about the oil as well
as the knowledge you have of the person.
As if chemistry isn’t complicated enough, the situation is made
even more complex by the presence of chemotypes.
Chemotype is the word given to describe plants which, although
they look the same, may produce significantly different
chemicals. This may be caused by variations within a species,
influences such as the environment in which they are grown and
the effects of cross-pollination.
Thymus vulgaris provides a good demonstration of how the same
species of plant can yield quite different oils. It has the following
Thymol (a phenol)
This is strongly antiseptic and aggressive.
Carvacrol (a phenol)
This has the same properties as thymol.
Linalool (an alcohol)
This chemotype also contains the ester linalyl acetate. It is
gentle, bactericidal, fungicidal and is a tonic to the nerves
and urine.
Thujanol 4 (an alcohol)
This chemotype is only found in the wild. It is anti-infective,
bactericidal, an immuno-stimulant and aids concentration.


Terpineol (an ester)
This is gentle enough to be used on children and sensitive
Geraniol (an ester and an alcohol)
This chemotype contains the ester geranyl acetate and the
alcohol geraniol.
Thymus vulgaris is by no means the only plant to look out for!
Chemotypes also occur with lavender, rosemary and lavandin to
name but a few.
Hopefully, the importance of developing your knowledge of the
properties of the chemical compounds and their associated
precautions is now apparent!