From the theoretical and historical points of view, terpin-hydrate is an important compound. But technically also it is of importance as the material from which terpineol is made. (Comp. p. 375.) It is readily formed from turpentine oil by letting this stand with water containing an acid. It is characterized by a remarkable capacity for crystallization, hence it was observed relatively early. According to older statements, terpin-hydrate is said to occur in the oils of cardamom and basilicum. However, these statements have not been substantiated by recent observations. This much appears certain that the terpinhydrate was not present in the original oil but must have been formed upon prolonged standing.

Terpinhydrate melts at 116 to 117°. When heated, it looses a molecule of water with the formation of anhydrous terpin which melts at 104 to 105° and boils at 258° (corr.).1)

Terpin exists in two stereoisomeric modifications, namely a cis- and a trans-form. The cis-terpin only, however, is capable of yielding a hydrate.2)

1) Wallach, Liebig's Annalen 230 (1885), 248. 2) Baeyer, Berl. Berichte 26 (1893), 2865.