Monoterpenen

Monoterpenen, or monoterpenes, are a class of terpenes composed of two isoprene units, yielding a C10 hydrocarbon skeleton. They occur as hydrocarbons and as oxygenated derivatives, collectively termed monoterpenoids. The structural variety includes acyclic chains, monocyclic rings, and bicyclic frameworks. Common examples are limonene, alpha-pinene, beta-pinene, myrcene, camphene, terpinene, linalool, and terpineol.

Biogenesis: Monoterpenes are synthesized in plants from geranyl pyrophosphate (GPP), the 10-carbon common precursor. In plants they arise via the mevalonate pathway (in the cytosol) or the non-mevalonate (MEP) pathway (in plastids). Enzymatic cyclizations, hydride shifts, and oxidations transform GPP into a wide array of hydrocarbon and oxygenated products. The resulting compounds contribute to plant aromas and defense signaling.

Occurrence and uses: Monoterpenes are prominent constituents of essential oils from citrus, pine, lavender, mint, and many herbs. They are responsible for characteristic citrus, resinous, or floral notes. In industry they are used as fragrance ingredients, flavorings, and starting materials for aroma chemicals; some derivatives serve as solvents or pharmaceutical intermediates.

Safety and regulation: As volatile, often concentrated components of essential oils, monoterpenes can irritate skin or mucous membranes in sensitive individuals, particularly at high concentrations. They are generally regarded as natural products and are subject to standard regulatory controls for cosmetics, flavorings, and fragrances. Proper dilution and adherence to guidelines are advised.