Monoterpenes

Monoterpenes are a class of terpenes that consist of two isoprene units, giving hydrocarbons with the molecular formula C10H16. They are among the most abundant volatile compounds in plants and contribute to the scents of many essential oils, including citrus, pine, and herbs. Common monoterpenes include limonene, pinene, myrcene, ocimene, terpinene, and geraniol.

Chemically, monoterpenes are derived from geranyl pyrophosphate (GPP), formed from two isopentenyl pyrophosphate (IPP) units. Through the action of terpene synthases, GPP can cyclize and rearrange to yield a wide range of carbon skeletons, from acyclic (myrcene) to monocyclic (limonene, pinene) and bicyclic structures (sabinene, camphene).

Monoterpenes are broadly classified as hydrocarbon monoterpenes and oxygenated monoterpenes (monoterpenoids). The latter include alcohols (linalool, menthol), ketones (camphor), aldehydes, and oxides (1,8-cineole). They are produced mainly in plant secretory tissues and released as volatiles, playing ecological roles in defense and pollinator attraction.

Applications include use as fragrance and flavor ingredients, food additives, solvents, and chemical intermediates. Some monoterpenes have been studied for biological activities, including antimicrobial, anti-inflammatory, and antioxidant effects, but results are variable and depend on the specific compound and context.