Limonene is probably the most common monoterpene in plants.
miscible in alcohol, insoluble in water
Limonene is traceable in a variety of plant families, including the oil of citrus fruit peel, in hemp, oregano, aniseed, coriander, and caraway. In celery, limonene is responsible for approx. 60% of the essential oil. There are two enantiomers with different tastes and scents. R-(+)-limonene smells of citrus fruits, l-(-)-limonene of peppermint and pine needles.
Limonene is supposed to enhance the ability to concentrate and to think. Limonene promotes norepinephrine and dopamine production and thus helps against listlessness and depressive moods.
Limonene and its oxidation products may trigger allergies or at least irritate the skin and respiratory tract with extended contact. R-(+)-limonene is currently being investigated for a potential carcinogenic effect.