α-Pinene (CAS NO.2437-95-8) is an organic compound of the terpene class, one of two isomers of pinene. It is an alkenes’ and it contains a reactive four-membered ring. It is found in the oils of many species of many coniferous trees, notably the pine. α-Pinene is also found in the essential oil of rosemary (Rosmarinus officinalis). Both enantiomers are known in nature; 1S, 5S- or (−)-α-pinene is more common in European pines, whereas the 1R, 5R- or (+)-α-isomer is more common in North America. The racemic mixture is present in some oils such as eucalyptus oil.