324 Wisconsin Academy of Sciences , Arts and Letters. 
The behavior of the limonenes toward water is similar to that toward 
The hydrohalogens. After the addition of one molecule of water the 
-hydration product still appears to be a limonene-derivative. 9 
The addition of a second molecule of water makes it a dipentene 
derivative. Terpin and its hydrate are both optically inactive and upon 
dehydration yield dipentene but no limonene. 
The transformation of limonene to dipentene is termed inversion and 
seems to be brought about easily by the presence of acids. Thus e. g. a 
partial inversion takes place in the preparation of the nitrosochlorides 
and nitrosates, and especially of the hydrochlor nitrosochlorides and ni- 
trosates 10 . Limonene can also be inverted into terpinene whereby di¬ 
pentene results as intermediate product 11 . Indirect inversion into ter- 
pinolene 12 also seems to take place under favorable conditions. If the 
isoterpenes of Fl^witzky 9 are but impure limonenes then the pinenes 
can be inverted into limonene by means of alcoholic sulphuric acid. It 
thus becomes apparent that limonene though a relatively stable terpene 
is by no means the most stable. 
Dipentene closely resembles the limonenes in most of its properties 
Boiling poing 178°, spec, gravity 0.845 at 20°, n c = 1.47308. It is readily 
distinguished from the limonenes by being optically inactive. Whether 
dipentene is physically identical with optically inactive limonene, re¬ 
sulting from a mixture of equal parts of dextro-and Isevogyrate-limo- 
limonene has not yet been decided. Chemically they are identical. 
Since in all characteristic cases the optically inactive limonene deriva¬ 
tives have been shown to be identical with the corresponding derivatives 
obtained from dipentene the term dipentene only will be used here¬ 
after. The dihydrochloride is identical with the one resulting from the 
addition of two molecules of hydrogen chloride to the limonenes. The 
tetrabromide, however, is distinguished from the limonene tetrabrom- 
ides in form, melting point and solubility. What has been mentioned 
of the limonenes with reference to nitrosylchloride also holds true for 
dipentene. It might yet be stated that all dipentene derivatives can 
also be obtained by mixing solutions of equal parts of the corresponding 
detro- and lsevogyrate limonene derivatives. It is to be supposed that 
dipentene like limonene will add water to form terpin hydrate, but this 
has not yet been demonstrated. 
Limonene as well as pinene and phellandrene can be inverted into 
dipentene, which in turn can be inverted into terpinene only. Dipentene 
is therefore one of the most stable terpenes. 
