Unlike synthetic chemicals, essential oil chemicals are diverse in their effects. No two oils are alike. Essential oils are constructed from long chains of carbon and hydrogen atoms. Essential oils are chemically different from fatty oils or fatty acids. Fatty acids have a linear carbon-hydrogen structure whereas essential oils have a more complex ring-like structure.
Alcohols are antiseptic, antibacterial, anti-fungal, and antibiotic. They can stimulate the immune responses of the body and function as tonic to the nervous system.
Aldehydes are calming, sedative, anti-inflammatory, antispasmodic, antibacterial, and antifungal.
Esters & Ethers are calming and balancing, soothing, anti-inflammatory. High doses can be uplifting such as in Bergamot or Lavender.
Ketones, e.g. found in Sage. In small doses, ketones can be relaxing and sedative. In small doses, they may promote healing, are anticoagulants, and mucolytic. In large doses, ketones can be overly stimulating thus having the opposite effect. They may be cause miscarriage, convulsions, and epileptic attacks.
Do not use Rosemary or Spike Lavender when pregnant, have a fever, or are prone to epileptic seizures. Furthermore, if you have a liver problem, use essential oils with low levels of ketones. Ketones are difficult for the liver to metabolize.
Phenols are antiseptic. Large doses can cause skin irritation or be toxic to the nervous system
Terpenes are a tonic and stimulant
ORAC stands for Oxygen Radical Absorbance Capacity. It measures the antioxidant levels of chemical compounds such as found in essential oils. The higher the score, the higher the protection against free radicals. On this site, the ORAC Score is expressed in TE/L (micromole Trolox equivalent per liter).
As a quick comparison and to give you an idea...
Clove = 10,786,875 - it is the highest tested oil
Thyme = 159,590
Frankincense = 6125
Lavender = 3669
Rosemary = 3309
Sandalwood = 1655
TOP NOTE is what you immediately smell when you open up a bottle of essential oil, perfume, or cream. A top note evaporates very quickly. The aroma is light, fresh, often citrusy, airy, sharp and penetrating. Top notes are uplifting and can be found in e.g. Bergamot, Lemon, Grapefruit, Orange.
MIDDLE NOTES are harmonizing and balancing any blend. They provide energetic and physical balance. They give the blend a sense of fullness, are the "heart" of the blend, and soften the sharpness of the top notes. They can be found in Chamomile, Geranium, Helichrysum, Lavender, Lemongrass, Pine, Rosemary.
BASE NOTES have a grounding quality. They are the base, are deep and warm and often have an earthy aroma. The base notes are lingering. They are soothing and calming and can be found in Patchouli, Sandalwood, or Ylang Ylang.
An easy way to remember the different notes is by envisioning a tree: the roots and the trunk are your base notes, the stem and the branches make up the middle notes, and the leaves and fruits are the top notes. In the fall, the leaves are falling only to return in the spring. The top notes are evaporating just as the seasons change the tree. The branches are stabilizing the tree and the roots are grounding.