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Wisconsin Academy of Sciences , Arts and Letters . 
of iodine on the terpenes, distnrbes the molecular arrangement of the 
same and causes a rearrangement of the atoms which prevents us from 
recognizing the original structure of the terpenes.” 
In the seventies Tilden introduced a new class of terpene derivatives 
which proved of exceeding interest and importance. His nitrosylchlor- 
ide addition products and the nitro terpenes obtained therefrom afforded 
a better means to distinguish the hydrocarbons of the pinene group 
from those of the limonene group. He showed also that the terpenes 
from orange oil, bergamot oil and caraway oil afforded the same nitro- 
sylchloride addition product. 
With the year 1884 there began a new era in the literature of terpenes 
and volatile oils. Wallach had succeeded in preparing a tetrabromide 17 
of “cynene” C 10 H 16 Br 4 (now Dipentene tetrabromide), a handsomely 
crystallizable body which melted at 124°. Soon after he obtained a 
tetrabromide from “ hesperidene ” (+ limonene), which crystallized in a 
very similar form, but melted at 104°. Wallach wrote at the time, “ There 
seem to exist very close relations between “cynene” and “hesperidene,” 
but no complete identity.” In these tetrabromides we have, as ifc were, 
the key to our present knowledge of the terpenes. Th^se two terpenes 
which for decades had been a stumbling block to chemists, were now 
characterized. The importance of these substances already becomes 
evident from the second contribution 20 of Wallach four months later. 
By means of the tetrabromide Wallach proved the presence of limonene in 
the fraction ITS 1 of the following oils: lemon oil (so-called “ citrene ”), 
bergamot oil, caraway oil (so called “ carvene ”), dill oil, erigeron oil and 
in the oil of the leaves of pinus sylvestris. 
Wallach showed, furthermore, that these fractions could be inverted 
into dipentene by means of heat (temp, of 250-270°) as was shown by 
means of the dipentene tetrabromide. Also by adding two molecules 
of hydrogen chloride and splitting off the same by heating with aniline 
and converting the resulting hydrocarbon into its tetrabromide. 
Wallach then characterizes limonene in the following manner: “ It 
boils between 175-177°, and possesses a lemon like odor. It produces a 
nitroso derivative which melts at 71°, and a tetrabromide which melts at 
104°, and crystalizes in rhombic-hemiedric forms. Hydrogen chloride 
converts it in etherial solution into a dihydrochloride of dipentene 
which melts at 50°. Hesperidene, citrene, carvene, etc., are hereafter to 
be designated as limonene.” 
The true character of limonene, i. e. its position as the dextrogyrate 
member of the limonene group became apparent in 1888 when Wallach 22 
recognized lgevogyrate limonene as such and succeeded in the syntheti¬ 
cal preparation of dipentene derivatives from its destrogyrate and 
lsevogyrate components. The peculiar differences between the limo 
nenes and dipentene as revealed in their derivatives, as also the singular 
