68 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS, 
of hydrocarbons, called the terpenes, existing- in plants, 
chiefly in conifer® and citrus species. 
I need not here dwell upon the peculiar properties of 
this remarkable group of isomerides; I wish only to 
point out what we know of their chemical constitution, 
on which just lately a little light has been thrown. 
All terpenes possess the property of being easily con¬ 
verted by several chemical agents into new isomeric 
modifications, and these may be changed again into new r 
isomerides, but all yield at the end one and the same 
pioduct, called tcrcbeue, the most characteristic property 
of which is that it forms with hydrocloric acid a semi¬ 
hydrochloride, (C 10 H 16 )HC1. 
Now, according to Rauer and Verson (‘ Ann. Chom. 
Pharm., cli. 52), the hydrocarbon, C 10 H I6 , formed by 
heating rutjlene dibromide with an alcoholic solution of 
potash, exhibits all the characteristic properties of tere- 
bene, and appears to be identical with it. As the con¬ 
stitution of amylene, from which rutylene is derived, is 
know n, w e are in a position to draw some conclusions as 
to the constitution ot terebene, of which I have to say 
more further on. 
On oxidizing terpenes, terephthalic acid is always 
formed, amongst other products. This compound is also 
a product of oxidation of several hydrocarbons of the 
aromatic group. This fact points. out that the terpenes 
must be nearly related to the aromatic hydrocarbons, 
and this has been quite recently fully proved by Oppen- 
heim, who has showm that, by abstracting two molecules 
of bv drobromic acid from the dibromide of turpentine, 
cymene is formed. 
Aromatic Hydrocarbons .—These hydrocarbons, as well 
as their derivatives, have been very fully investigated 
during the last few years. We owe to Ivekule our pre¬ 
sent theory of the aromatic compounds; he first called 
attention to the following points :_ 
1. All aromatic compounds contain a common nucleus 
consisting of six atoms of carbon. 
2. These six atoms are linked together in such a way 
that six combining units remain unsaturated. 
3. All aromatic compounds are formed by saturating 
these free units with other elements or radicals, 
f. The differences observed in certain groups of iso¬ 
meric aromatic compounds are caused bv the dif- 
uient relative position of certain elements or 
radicals m the nucleus. 
c. The carbon-atoms forming the aromatic nucleus 
are united together by one and two combining 
units alternately. ° 
C—C 
AUhough several chemists have proposed cert; 
modifications of this hypothesis, all the facts w: 
w-hic-h we are as yet acquainted prove that Ivekul 
view is the most simple and most probable. 
P 10 ^ sim P le aromatic hydrocarbon is benzei 
^6-^6; tts homologues are derived from it by substil 
tion of monad alcohol-radicals for hydrogen. T 
nimibei of isomerides amongst these hydrocarbons 
very considerable. Thus, the hydrocarbon C S H ]0 exi 
^ tt ci B 5 L different modifications, viz., ethyl-bonzei 
and three different dimethyl-benzend 
I isomerism of the latter being cans 
by the different position of the two methyl-groups. 
. representing benzene as a hexagon, and numbi 
mg the six corners, wdiere the carbon-atoms are sr 
pcaed to be, we can easily see the possibility of t 
existence of three dimethyl-benzenes_ 
[July 27, 1872- 
l 
4 
The positions of the methyl groups being— 
1:2; 1:3; 1:4. 
No further different positions of the methyl groups are- 
possible; for 1 : 5 = 1 : 3 ; and 1 : 6 = 1 : 2. 
As the isomeric aromatic hydrocarbons have gene¬ 
rally a great resemblance in their physical properties, it 
is of the highest importance to have means for distin¬ 
guishing them from each other. This can be done by 
studying their products of oxidation. 
When an aromatic hydrocarbon is heated with very 
dilute nitric acid, one of the alcohol radicals is oxidized 
to carboxyl. Methyl-benzene, ethyl-benzene, amyl- 
benzene, and all other hydrocarbons containing only one 
alcohol-radical yield one and the same product, viz.,, 
benzoic acid, C f) H 5 .CO.OH. The formation of this acid 
proves, therefore, the existence of only one alcohol- 
radical in a hydrocarbon. The three methyl-benzenes, 
as well as methyl-ethyl-benzene, yield isomeric 
methyl-benzoic acids, C 6 II 4 j whilst diethyl¬ 
benzene gives ethyl-benzoic acid. 
l>y acting on the hydrocarbons or their first oxidation 
products with stronger oxidizing agents, such as dilute 
chromic acid, every alcohol-radical is oxidized to car¬ 
boxyl ; the three dimethyl-benzenes, as well as methyl- 
ethyl benzene and diethyl-benzene, yielding bibasic 
acids, having the composition C c H 4 j qq'qjj, etc. 
Rut we are not only able to fix the number of alcohol 
radicals present in an aromatic hydrocarbon, but we can 
also determine their relative positions. Thus w r e know 
that in these dimethyl-benzenes the positions of the 
methyl-groups are— 
Orthoxylene. Isoxvlene. Methyltoluene. 
1:2. 1:3. 1 : 4. 
This has been proved in the following w r ay:—Three 
bibasic acids are known, having the composition. 
r i tt / CO.OH . ,,, r ^ * 1 
L '6 rL 4 i CO.OH Vlz ’’ acid, isophthalic acid, and 
terephthalic acid. In phthalic acid the two carboxyls 
occupy the position 1:2; this we see from the fact 
that this acid is produced by oxidizing naphthalene, 
a hydrocarbon in which, as will be shown further on, 
two atoms of carbon are linked to two adjoining carbon- 
atoms of the aromatic nucleus. Isophthalic acid has 
been obtained by oxidizing isoxylene, and isoxylene has 
been derived from mesitylene, C 6 H 3 (CH 3 ), a trimethyl- 
benzene in which, as Raeyer first pointed out, the three 
methyl-groups have the symmetrical position 1:3:5.. 
This follows from the mode of formation of this hydro¬ 
carbon, which is obtained by heating acetone with sul¬ 
phuric acid. Three molecules of acetone lose three 
molecules of water, and the residues join together as 
follows:— 
CII 3 
/ 
CO ch 3 
/ \ 
ch 3 CO 
CH 3 —CO—ch 3 
\ 
CH 3 
CHj 
/ 
C—CH 
3H.,0= ^ $5; 
C C. 
/\ /\ 
CH 3 —C = CH CH 3 . 
Ry oxidizing mesitylene with weak nitric acid, we 
. ( CH 3 
obtain monobasic mesitvlenic acid, C c H 3 ’ CO OH 
t ch 3 , 
which, when heated with lime, splits up into isoxylene 
and carbon dioxide. From this it follows that in. 
isoxj lene as well as in isophthalic acid, the two radicals^. 
