monoterpene

Monoterpenes are a class of terpenes composed of two isoprene units, yielding the molecular formula C10H16. They occur as hydrocarbons and as oxygenated derivatives known as monoterpenoids. Common hydrocarbon monoterpenes include limonene, pinene, myrcene, and ocimene; oxygenated forms include menthol, linalool, and terpineol. Monoterpenes exhibit acyclic, monocyclic, and bicyclic skeletons.

Biosynthesis begins with the formation of IPP and DMAPP, typically via the ME(P) pathway in plant plastids. Geranyl pyrophosphate (GPP) synthase condenses IPP and DMAPP to form GPP, the precursor to monoterpenes. Monoterpene synthases then convert GPP into specific monoterpene skeletons through cyclization and rearrangement, often followed by oxidation to alcohols, aldehydes, or ketones. The process accounts for much of the chemical diversity observed among monoterpenes.

Monoterpenes are major constituents of essential oils in many plants and contribute to characteristic aromas and flavors. They play ecological roles such as deterring herbivores, protecting against pathogens, and attracting pollinators. Notable examples include limonene in citrus oils; α-pinene and β-pinene in conifers; linalool in lavender; and menthol in mint.

Applications and properties: monoterpenes are highly volatile and lipophilic, typically with low boiling points, making them prominent in flavors and fragrances and in some traditional medicines. They are used in cosmetics, food, and perfumery, and some exhibit biological activities studied for antimicrobial or anti-inflammatory effects. Safety considerations include potential irritation or sensitization, particularly in concentrated essential oils.