312 
Wisconsin Academy of Sciences , Arts and Letters. 
THE LIMONENE GROUP OF TERPENES. 
By EDWARD KREMERS. 
The following historical study was undertaken with the desire to throw 
some light upon the history of the terpenes. This class of compounds 
has been of interest to chemists ever since organic chemistry may be re¬ 
garded as a science. Year after year the material accumulated until by 
the beginning of the last decade it constituted a special lumber-chamber 
of chemical literature. The reasons for this disorder are of a varied 
character. The thought that every volatile oil contained its peculiar 
terpene, when such was present, for a long time seems to have prevailed 
in the minds of chemical investigators. Another grave fault is to be 
found in the fact that one investigator often gave but little or even no 
attention to the researches of others. This brought about serious con¬ 
fusions in the chemical nomenclature, which in turn gave rise to mis¬ 
understandings everywhere. The reason why, even in later years, so 
little could be done to clear up the subject is to be sought in the fact 
that for the most terpenes, no characteristic reactions were known. 
Since at present, at least some systematic knowledge has been acquired, 
it may not be without interest to look back and see who has identified 
himself with the problems under consideration, who has aided in their 
solution and who has retarded the same. In the course of years the 
amount of material has accumulated to such an extent that a survey of 
the same is a difficult matter, even for the person who has made a special 
study of the subject. In fact an understanding of the Limonene group 
of terpenes became possible only after Prof. Wallach, in 1888, had 
demonstrated the relations existing between the members of this group. 
For the better understanding of the subject the following explanatory 
remarks may serve as a brief introduction. The limonene group of ter¬ 
penes consists of three hydrocarbons: the optically active, dextrogyrate 
and laevogyrate limonene and the optically inactive dipentene. Whether 
the inactive compound resulting from the mixture of equal parts of the 
optically active components is identical with dipentene still remains an 
open question. Suffice it is to say that all crystallizable derivations of 
such an optically inactive mixture are identical with the corresponding 
* These notes are translated from a series of articles published on this subject by the 
writer in the “Pharmaceutische Rundschau” of 1891 and 1892. 
