α-Pinene is an organic compound of the terpene class, one of two isomers of pinene. It is an alkene and it contains a reactive four-membered ring. It is found in the oils of many species of many coniferous trees, notably the pine. It 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.