The Limonene Group of Terpenes. 
331 
avec 1’essence de citron elle-meme, ce qni fournit le moyen de transformer 
l’essence de terebentine en essence de citron.” 
Berthelot 11 obtained (1852) dipentene dihydrochloride directly from 
turpentine oil by shaking the latter with aqueous hydrochloric acid, 
or by passing hydrogen chloride into a solution of turpentine oil in 
ether, alcohol or glacial acetic acid. 
Trying to replace the three molecules of “ water radicles” in terpin hy- 
chlorine, Oppenheim 14 (1862) always obtained the dihydrochloride of 
dipentene. 
These extracts and remarks may afford sufficient insight into the older 
literature of the dihydrochloride of dipentene. To give complete liter¬ 
ary references up to date would be useless. From the above it becomes 
obvious that with a better knowledge of this compound some light was 
shed over the relations of terpenes in general. It must also be obvious, 
however, that for many it was a stumbling block and brought about 
greater confusion. Some chemists indeed recognized the identity of this 
compound from whatever source they obtained it, others denied the 
same. In the imaginations of some there were as many dihydrochlorides 
as there were terpenes, and these were limited only by the number of 
volatile oils containing hydrocarbons of the formula C 10 H 16 
In spite of Berthelot’s ideas Gladstone’s 24 (1864) classification of the 
terpenes gained ground more and more. According to this dipentene 
dihydrochloride was regarded as the characteristic addition product of 
“ citrene ” (+• limonene) and allied terpenes, whereas artificial camphor 
was taken as characteristic derivative of the “terpenes” (pinene and 
allied terpenes.) 
In the following twenty years little was added to a better understanding 
of this compound. A full understanding was possible only after the dis¬ 
covery of lsevogyrate limonene, (1888) i. e., after the completion of the 
limonene group and synthesis of dipentene derivatives. 
Properties. 
Dipentene dihyrochloride crystallizes in pearly laminae which melt 
at 50°. 3 5 It is insoluble in water, soluble in alcohol and ether, readily 
soluble in chloroform, from which solution it can be precipitated with 
methyl alcohol. Its presence effects a decrease of the melting-point of 
other substances. 22 
Chemically, dipentene dihydrochloride behaves like a saturated com¬ 
pound, and as a terpene derivative it is relatively stable. By splitting off 
one or both hydrogen chloride molecules it is converted back into un¬ 
saturated compounds. Heated by itself, 28 or in alcoholic solution. 29 
it decomposes into its components, dipentene and hydrogen chloride. 
Caustic potassa and anilin split of hydrogen chloride, but the last traces 
of chlorine are very difficult to remove. 30 Solution of permanganate of 
potassium attacks it readily. 31 
