The Limonene Group of Ter penes. 
313 
‘dipentene compounds. The two physical modifications of limonene can 
add one molecule of hydrogen chloride without loosing their optical ac¬ 
tivity. The additions of a second molecule, however, renders them op¬ 
tically inactive, they are then no longer limonene, but dipentene com¬ 
pounds. Pinene, the most characteristic hydro-carbon of the turpentine 
oils, adds one molecule of hydrogen chloride to form the so-called arti¬ 
ficial camphor, or pinene, monhydrochloride. Under certain conditions 
it also will add two molecules of hydrogen chloride and thus be converted 
into dipentene dihydrochloride. Limonene as well as pinene can be 
“ inverted” into dipentene. This inversion can also take place through 
terpin hydrate. Limonene, and pinene even more readily, will add three 
molecules of water to form terpin hydrate. This optically inactive hydrate 
is no longer either a limonene—or pinene—but a dipentene derivative . 
As will be shown later these inversions for a long time baffled chemical 
investigators, and it is but recently that their character is being under¬ 
stood. This knowledge, the outcome of a series of investigations by 
Prof. Wallach, has at last shed considerable light upon the constitution 
und isomeric relations of the terpenes. 
THE HYDROCARBONS. 
SYNONYMS AND HISTORY. 
Dextrogyrate Limonene. 
Citrene — J. Dumas, 1 1833. 
Essence de citron — J. Dumas 1 & 3 . 
Citronyl — Blanchet & Sell, 2 1833. 
Carven 19 -— Schweizer, 4 1840. 
Hesperidene — Wright Piese, 11 1871. 
Citren 19 \ 
Hesperiden ( ^ allach. 1 - 
Limonen — Wallach, 1884. 
Citrene — Yoshida, 1885. 
Rechts-Limonen — Wallach 22 , 1888. 
Dumas 1 stated (1833) that “ Citrene,” the hydrocarbon from oil of lemon 
is isomeric with “ camphene,” the hydrocarbon from turpentine oil, with 
this difference: the molecule of the former is but half as large as that of 
the latter. This distinction is shown by their different capacity for ab¬ 
sorbing hydrogen chloride. He regarded the natural “ citrene ” (dextro¬ 
gyrate limonene) as being identical with the artificial hydrocarbon which 
he recovered from the hydrogen chloride addition-product (namely dipen¬ 
tene), and which he also designated as “ citrene.” 
Blanchet and Sell 2 (1833) designated as “ citronyl ” that hydrocarbon of 
the lemon oil which is capable of forming a solid compound (“ festes 
