326 
Wisconsin Academy of Sciences , Arts and Letters. 
Methods of Formation. 
If the “ Isoterpenes” of Flawitzky 9 are impure limonenes, then the 
latter can be obtained by inversion of the optically active pinenes 
through the optically active terpineols (terpene hydrates of Flawitzky). 
Dipentene being more stable than its optically active modifications 
results from a large number of reactions: 
I. Polymerisation of Isoprene, a hemiterpene. 40 
II. Inversion of less stable terpenes, viz.: 
1. By means of heat: direct inversion into dipentene. 
2. By means of acids- 
f a. Hydrohologen acids: into dipen¬ 
tene dihydrochloride, etc. 
b. Sulphuric acid : apparent direct in- 
^ version. 
3. By means of water in presence of acids (e. g. nitric acid) con¬ 
version into terpin hydrate. 
According to these methods pinene, 41 limonene 42 and phellan- 
drene 43 can be converted into dipentene. 
III. Destructive distillation of polymerized terpenes : e. g. Kaoutch- 
ouc (Himly’s “ Kautschin ”). 
IV. Regeneration of dipentene from derivatives : 
1. Removal of hydrohalogen from dipentene dihydrochloride 
and dihydrobromide by means of alcoholic potash, 44 sodium 
acetate 45 or anilin. 46 
2. Dehydration of O-Terpineol or terpin, resp. terpinhydrate 47 
Methods of Preparation. 
Dextrogyrate limonene is obtained by fractional distillation. Oil of 
sweet orange consists almost entirely of + limonene. The “ carvene.” of 
commerce and Erigeron oil are also very rich in + limonene and relative¬ 
ly cheap. 
Levogyrate limonene is thus far obtained from “Fichtennadel 61” only. 
Dipentene is best obtained by decomposition of its dihydrochloride or 
bromide by boiling with anilin. 46 The use of sodium acetate and glacial 
acetic acid 45 cause the formation of byproducts. It is rather difficult to 
remove the last traces of halogen. 5 
HYDROHALOGEN ADDITION PRODUCTS. 
Monohydrochlorides. 
History: 
The question whether a limonene monhydrochloride analogous to 
pinene-monhydrochloride can exist has long remained an open one. 
Considerable speculation has been indulged in concerning the liquid 
byproducts, resulting in the preparation of dipentene-dihydrochloride. 
Some regarded it as a monhydrochloride, others as an isomeric liquid 
