Essential Oil Chemistry

Chemistry is the physical science of matter, describing its reactions, composition, structure and properties.  Chemistry describes atoms and the properties of chemical bonds between atoms at atmospheric pressure and temperature and also at different temperatures and pressures.  

The use of steam distillation in particular is relevant to the chemistry of essential oils limiting their composition to compounds of carbon, hydrogen and oxygen as heavier compounds are left behind.

The size of molecules is also limited to a molecular weight of about 440 by the distillation process.  Some essential oil constituents are really tiny.   Analysis of essential oils means we not only know whats in them - many essential oils have over 200 constituents but also how those constituents change over time as the constituents oxidise.  We know it is of importance to have pure essential oils as close to the date of original distillation as possible and have them analysed before embarking on their research.

These essential oils have particular properties and their physical and psychosocial (emotional) use is described as aromatherapy.

While some 400 essential oils can be derived by steam distillation and other processes from plant matter safety considerations limit the essential oils in daily professional and home use to about 60. 
 Two plant families predominate the lamiaceae and rutaceae which give us many of the 34 oils in most common use and which we study in the Shirley Price Aromatherapy Diploma.

The safe 34 oils.  Essential oils are derived from the wood, roots, flowers, fruit, seeds and leaves of Basil, bergamot, black pepper, cedarwood, chamomile roman, chamomile german, clary sage, clove bud, cypress, eucalyptus, frankincense, geranium, ginger, grapefruit, jasmine, juniperberry, lavender, lemon, mandarin, marjoram, melissa, neroli, orange sweet, palmarosa, patchouli, peppermint, petitgrain, rose otto, rosemary, sandalwood, tea tree, thyme (sweet), vetiver, ylang ylang.  General cautions in use apply to all chemicals.  It is important to read the suppliers label and take notice of these.

While one or two compounds predominate an essential oil can consist of up to 300 compounds and given their use in aromatherapy the minor compounds can be as importance as the major ones both in the essential oils effectiveness and avoiding adverse reactions with the skin. 

Life on earth consists of carbon chemistry and the unique ability of carbon to bond to itself in large chains or rings.  The study of this matter is known as organic chemistry which links physical science with biology or the study of life.  Organic chemistry includes the analysis and preparation of carbon based organic compounds - the hydrocarbons and their derivatives. 

Steam distillation limits the organic chemistry of essential oils to carbon, hydrogen and oxygen.  However other heavier compounds of carbon, hydrogen and oxygen can be found in biology or synthesised which include iron, magnesium, nitrogen, the halogens, phosphorous, silicon and sulphur.

Organic compounds form the basis of all processes of life on earth.  They are diverse in structure and number.  The range of application of organic compounds is enourmous.  The form the basis of or are constituents of many products including plastics, drugs, petrochemicals, food, explosives and paints.

In chemistry polarity refers to a separation of electrical charge. 

Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter.  There exist two types of electrical charges called positive and negative.  Positively charge substances are repelled from other positively charged substances are repelled from other positively-charged substances, but attracted to negatively-charged substances; negatively-charged substances are repelled from negative and attracted to positive. The proton has a charge of e, and the electron has a charge of −e. The study of charged particles, and how their interactions are mediated by photons is called quantum electrodynamics.

Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure. For example, a molecule of water  is polar because of the unequal sharing of its electrons between oxygen and hydrogen in which the former has larger electronegativity than the latter, resulting in a "bent" structure, whereas methane is considered nonpolar because the carbon shares the electrons with the hydrogen atoms almost uniformly. Polarity underlies a number of physical properties including surface tension, solubility, and melting- and boiling-points.


Ian Brealey