Other varieties of terpenes form when additional isoprene units attach to a monoterpene. Sesquiterpenes are very similar to monoterpenes except for the one additional isoprene unit added to their structure for a total of 15 carbons. Because of their higher molecular weights, sesquiterpenes are less volatile than monoterpenes, and are therefore less prevalent in essential oils overall; however, they have unique properties that distinguish them from monoterpenes and make them important contributors to the synergistic functionality of essential oils. Common sesquiterpenes found in essential oils are cedrene, zingiberene, himachlene, and caryophyllene.
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