YIKES! It is this part of Aromatherapy that is the most difficult to understand (well at least, I think it is). So I thought I would break it down so it much much more easily comprehended.
The following information has been grabbed from the Complete Guide to Aromatherapy by Battaglia - Almost all of the molecules found in essential oils are composed of carbon, hydrogen & oxygen. The chemistry of the essential oil is based on two dominant factors: the extraction process & the biosynthesis of the constituent molecules of the plant itself.
Battaglia explains that the steam distillation process extracts only volatile and water-insoluble constituents of the plant such as terpenes and terpenoid compounds and phenyl propane-derived compounds (confused yet?).
Below are the most commonly found chemical constituents:
Aldehydes: Sedative, antiviral, anti-inflammatory
Ketones: Cell regenerating, mucolytic, neurotoxic (in high doses or prolonged use)
Esters: Antispasmodic, equilibrating, anti-fungal
Sesquiterpenes: Anti-inflammatory, anti-allergenic
Sesquiterpenols: Anti-inflammatory, stimulates glandular secretions & liver function.
Phenylpropanes: Antispasmodic, ANS balancing, antibacterial, stimulant, skin irritant,
Monoterpenols: Antiviral, toners, antiseptic, immuno-stimulants,
Phenols: Strong antibacterial, immuno-stimulant, warming
Monoterpenes: Antiviral, cortisone-like