The Limonene Group of Terpenes. 
327 
dihydrochloride. Those who supported the latter view termed it “ cam- 
phre-liquide de citron ” in opposition to the “ camphre solide de citron ” 
(crystalized dipentene dihydrochloride). From this “camphre liquide 
de citron” Soubeiran and Capitain 1 went so far as to prepare the hypo¬ 
thetical hydrocarbon “citrylene”: spec, gravity 0.88, boiling point 168°, 
vapor density 5.08, optically inactive. More than this, they created a 
group of liquid camphors, in distinction to the solid and artificial cam¬ 
phors. 
Characteristic as well as interesting are Berthelot’s 2 views (1882). He 
had succeeded in preparing dipentene-dihydrochloride from pinene. 
From this he concludes that the difference between pinene and limonene 
(“ essence de citron ”) is not “essentielle.” Consequently limonene must 
yield a mono hydrochloride, analogous to artificial camphor. However 
“ sa preparation est difficile, car elle parait se produire seulement en 
faible proportion et d'une maniere aceidentelle .” 
That under such conditions it is perfectly impossible to interpret all 
older statements in the light of present knowledge is certainly apparent. 
A complete catalogue of all the literature that has reference to this par¬ 
ticular subject would therefore be useless. In most cases it may be 
assumed that these liquid products were mixtures of mono- and dihy¬ 
drochlorides. 
A statement by S. de Luca 8 (1887) appears to be more rational. When 
he passed dry hydrogen chloride into relatively pure limonene from the 
citrus bigaradia no solid compounds icould result. Crystallized dipentene- 
dihydrochloride, however, resulted when the oil was shaken with 
aqueous hydrochloric acid, de Luca, however, makes no statements re¬ 
garding composition and character of this liquid chloride. Ribans 4 
statements (1874) are more important. By passing dry hydrogen chloride 
into limonene a liquid monhydrochloride resulted which could be 
converted into crystallizable dihydrochloride by treatment with moist 
hydrogen chloride. Bouchardat 5 (1875) claims to have obtained a liquid 
monhydrochloride and a solid dihydrochloride. Maissen 6 (1883) also 
obtained a hydrohalogen addition-product which was not saturated. 
From these statements by de Luca, Berthelot and Maissen it would 
seem that the hydrogen chloride must be dry to yield a monhydro¬ 
chloride. Others 7 , however, who according to their statements employed 
dry hydrogen chloride obtained dipentene-dihydrochloride and no 
monhydrochloride. It is therefore not at all surprising that Wallach 8 
(1887) after an unsuccessful attempt to prepare a monhydrochloride de¬ 
nied the existence of such a compound. The study of Maissens com¬ 
pound (dipentene-hydrochlor-nitrosate), however, again brought up this 
question. 
As a result of these investigations it became apparent that a monohy¬ 
drochloride of limonene could exist, and that it was an unsaturated com¬ 
pound. The understanding of Riban’s and Maissan’s experiments had 
