The essential oils contain a variety of molecules often in complicated mixtures. These molecules can have different structures comprising of functional groups. These functional groups classify the molecules as alcohols, ethers, aidehydes, ketones, esters, lactones, castor oil products, terpenes, paraffins, and heterocycles. Of these functional groups, aromatic aldehydes, ketones, heterocycles, and terpenes are considered to be the most important. A group of acyclic molecules that are common to many essential oils are the terpenes.
Terpenes are molecules that are composed of two units of isoprene.
Isoprene is the monomer of natural rubber. Two well known terpenes are citral and menthol. Citral is responsible for the odour of lemons, oranges, limes, and lemon grass whereas menthol is present in the oil of peppermint. Not all of the essential oils are pleasantly fragrant when by themselves. An example is indole which has a foul odour, but is often used to add an interesting character to the perfume as many pleasant smelling oils can make a perfume too sweet.