The Limonene Group of Terrenes. 
315 
In 1864 [Gladstone attempted to introduce order into the then already 
chaotic condition of the terpene literature. Based upon physical and 
chemical properties he classified the terpenes into three great groups. 
It is of special interest to note that he would regard the terpenes from 
sweet orange oil, lemon oil and the oil of citrus medica as identical 
rather than isomeric. 
In 1871 Wright & Piese 11 oxidized hesperidene from orange oil (4- lim¬ 
onene) with chromic acid mixture, and obtained acetic acid. From this 
CH. CH 3 
they conclude that the formula || expresses in part “the group¬ 
's H 13 ) 
ing of the constituent carbon atoms.” Nitric acid oxidizes hesperidene 
to oxalic and carbonic acids. 
Two years later Wright 12 obtained from hesperidene besides acetic and 
formic acid a substance C 10 H 16 O when oxidized with chromic acid 
mixtures. When oxidized with nitric acid he obtained oxalic acid and 
hesperidinic acid, C 30 H 36 0 17 , but no terephthalic acid. 
When a molecule of bromine is added a liquid non-characteristic 
dibromide is formed which when heated splits off hydrobromic acid and 
cymene results which is identical with the cymene obt. from pinene and 
“ citrene ” (-j- limonene from oil of lemon). From this Wright concludes 
“that these three terpenes may all be regarded as dihydrides of cymene.” 
The yield of cymene from “ hesperidene dibromide ” was 80 per cent, and 
from the cymene he obt. by oxidation 45 per cent of terephthalic acid. 
From the latter experiment he concludes that “the production of tereph¬ 
thalic acid “ from cymene by oxidation shows that two ‘ lateral chains ’ 
are present; and as toluic acid is also obtainable from cymene, one of 
these must be methyl (since toluic acid gives rise to methyl-benzene to¬ 
luene); cymene therefore is either a methyl propyl-benzene or a methyl 
isopropyl benzene.” 
As to some of these views, however, Oppenheim 13 has claim of prior¬ 
ity. He had obtained “ citrene cymol ” by addition of bromine to “ cit¬ 
rene” and then splitting off hydrobromic acid by means of anilin; also 
according to Kekules method by adding iodine and splitting off hydro- 
iodic acid with the aid of heat. In his oxidation experiments with dilute 
nitric acid he obtained not only terephthalic acid but also para-toluic 
acid and acetic acid. Neither “terpene cymol” nor “citrene cymol” 
therefore contain an etlyl group, but both have in para-position methyl 
and propyl. The formation of acetic acid makes it probable that the 
latter is isopropyl. 
Already a year previous Oppenheim 14 had remarked that the terpenes 
were hydrocymols. Remarkable are, however his words which he wrote 
shortly after. “ The fact, however, that until now para cymol has only been 
obtained from terpenes may well create the suspicion that even the mild¬ 
est reaction according to which hydrogen may be abstracted, the action 
