332 
Wisconsin Academy of Sciences , Arts and Letters. 
Methods of formation: 
I. Addition of hydrogen chloride: 
1. To the monhydrochlorides of the limonene group. 32 
2 To the hydrocarbons of this group. 33 
The optically active hydrocarbons are hereby inactivated. The indi¬ 
rect formation from cineol 34 is to be classed with 2, since the hydrogen 
chloride acts first as a dehydrating agent forming k ‘ cinene ” (dipentene). 
II. Esterification of the diatomic alcohol terpin and its hydrate: 
1. By means of hydrochloric acid. 35 
2. By means of the tri- and pentachlorides of phosphorus. 36 
Ad I and II. The formation from the terpineols 37 no doubt consists in 
an esterification of the monatomic alcohol and subsequent addition of a 
second molecule of hydrogen chloride. 
III. Inversion and addition: 
e. g. From pinene, 38 phellandrene, 39 and probably from terpino- 
lene. 40 
Preparation. 
Larger quantities of dipentene dihydrochloride are rationally pre¬ 
pared by passing hydrogenchloride over (not into) a solution of limonene 
in one-half its volume of glacial acetic acid. A rise in temperature of 
the solution must be avoided to prevent the formation of oily products. 
After the solution has congealed to a crystalline mass it is poured into 
water and the drained chloride is purified by dissolving it in alcohol and 
precipitating with water. 41 
Small quantities can be readily purified by dissolving in chloroform 
and precipitating the solution with methyl alcohol. 
Dipentene-Dihydrobromide. 
Dibromhydrate de terpiline, Berthelot, 42 1861. 
Zweifach bromwasserstoffsaures terpilen, Oppenheim, 14 1864. 
Cynen-dihydrobromid, Hell and Ritter, 16 1884. 
Dipenten-dihydrobromid, Wallach, 17 1887. 
History. 
Dipentene-dihydrobromide was first prepared by Berthelot 42 from ter- 
pinhydrate analogous to the chloride. In the following year (1862) 
Oppenheim 14 prepared it from terpinhydrate with tri- and penta bromide 
of phosphorus. In 1884 Hell and Ritter 16 prepared it from cineol 
analogous to the chloride. Wallach has repeatedly prepared the dihy¬ 
drobromide from dextrogyrate limonene, dipentene, terpinhydrate, 43 also 
from terpinolene. 44 
