322 Wisconsin Academy of Sciences , Arts and Letters. 
among terpenes or their derivatives Wallach removed all doubts and 
thus laid a foundation for a systematic investigation of the terpenes. 
Wallach showed that the composition of the hydrogenchloride addi¬ 
tion product of cineol C 10 H 18 O, the chief constituent of wormseed oil, 
is expressed by the formula (C 10 H 18 0) 8 H Cl. When heated by itself 
this compound splits off water and hydrochloric acid as expressed by 
the following equation: 
(Cio H 18 0) 3 H Cl = 2 H 3 O + H Cl + 2 C 10 H 16 whereby cinene 
(dipentene) is formed. Cineol adds two bromineatoms to form a 
bibromide which is deliquescent. Water and hydrobromic acid split off 
and a tetrabromide C 10 H 16 Br 4 is formed which melts at 125.5°. This 
tetrabromide is identical with the one obtained by the addition of 
bromine to the hydrocarbon cynene (dipentene). At the same time 
Wallach showed that cajeputol 37 is identical with cineol, also cajeputene 
with cynene. The limonene odor of cinene (dipentene) lead Wallach to 
surmise the relations existing between this hydrocarbon and hesperidene 
(+ limonene). The preparation of limonene tetrabromide 38 melting at 
104° proved these suppositions to be true. 
The importance of these tetrabromides soon became apparent. Already 
in his second contribution Wallach showed conclusively by means of 
the tetrabromide reaction that dipentene was formed in a large number 
of reactions. Wallach, at that time, characterized dipentene as follows: 
“ It boils at 180-182 039 . The odor resembles that of limonene. It com¬ 
bines with bromine to form a tetrabromide which crystallizes in the rhom¬ 
bic system and melts at 125-126°, also with two molecules of hydrogen 
chloride, without being changed thereby, to dipentene dihydrochloride 
which melts at 49°. At high temperatures it polymerizes without being 
previously modified.” 40 
Dipentene is classified here as the special hydrocarbon of the dipentene 
group, whereas the closely related “ hesperiden ” (-f limonene) is placed 
in the limonene group. 40 The reason for such a separation is * quite 
apparent. The inactive modifications of pinene and camphene are 
not essentially different from their optically active modifications. 
Laevogyrate limonene was not yet known as such. It was only with the 
discovery of this terpene on the part of Wallach 41 that the true relations 
existing between dipentene and the optically active limonenes could be¬ 
come wholly apparant. Whereas the relations existing between o-pinene 
and ± pinene are those of o-tartaric and to ± tartaric acids, the re¬ 
lations existing between o-dipentine and ± limonene are explained 
by those existing between o-racemic acid and the optically active tar¬ 
taric acids. 
Dipentene, therefore, is not optically inactive limonene in the same 
sense in which o-pinene and o-camphene are the optically inactive mod¬ 
ifications of the respective groups. That dipentene derivatives, pre- 
