226 THE CHEMISTRY OF ESSENTIAL OILS
Curiously enough, the menthones resulting from the oxidation of these
active neomenthols are optically active in the opposite sense to the
menthols themselves.
The characters, therefore, of two of the optically inactive and four of
the optically active isomeric menthols have been fixed with certainty.
By treating the alcoholic residue of Japanese peppermint oil, from which
the bulk of the ordinary Z-menthol had been removed, with phthalic
anhydride in the usual manner, the authors obtained a mixture of
phthalic acid esters, from which after twenty-eight recrystallisations
from 95 per cent, acetic acid, a fraction was obtained which melted at
141° to 144°, and had a specific rotation of + 5716° in chloroform. This
substance was the phthalic acid ester of ^-neomenthol, containing a little
of the ordinary Z-menthol ester. This was proved (1) by mixing it inti-
mately with artificially prepared d-neomenthol hydrogen phthalate, when
the melting-point was practically unchanged, and (2) by hydrolising the
ester and oxidising the resulting menthol, when almost pure Z-menthone
was obtained, having a specific rotation - 2833°, and yielding a semi-
carbazone melting at 187° to 189°. Japanese peppermint oil therefore
contains Z-menthol, together with a small amount of d-neomenthol.
Walbauml has isolated from the fraction of the oil boiling at 250°
to 310° the acetic ester of 3 hexen-I-ol, of the formula
CH 2 (C 6 H 5 )C0 2. CH 2. CH 2. CH : CH .CH 2. CH 2
which, when pure, has the following characters :—
Boiling-point 135° to 136° at 4 mm.
„ 290° „ 760 mm.
Specific gravity at 15° I'OOO
Refractive index at 20° .... 1 '4981
It is a liquid with an onion-like odour.
Pure, natural Japanese peppermint oil, containing all the natural
menthol, has the following characters :—
Specific gravity at 15° 0-900 to 0'912
Optical rotation - 26° „ - 42°
Befractive index 1-4600 „ 1-4635
Acid value 0'5,, 2'5
Menthol as esters....... 3 to 8 per cent.
Total menthol 70 ,, 90 „
Solidifying-point 16° to 28°
The dementholised oil of commerce has the following characters :—
Specific gravity at 15°
Optical rotation
Refractive index
Acid value
Menthol as esters
Total menthol.0-894 to 0-906- 24° „ 36°
1-4590 „ 1-4650
0-5 „ 3
4 to 17 per cent.
45 to 55 per cent, (rarely higher)
Various colour reactions have been proposed with a view to dis-
tinguishing between Japanese and other peppermint oils. These are of
doubtful value, but there is no question that all peppermint oils other
than Japanese give the following reactions to a marked extent, whilst
Japanese oil either gives no reaction, or at most a very slight one. A
colour reaction, however, would be of great use if it were negative with
the Other peppermint oils, and positive with Japanese oil, which is the
cheapest variety obtainable.
1
Jour, prakt. Chem. (1917), 96, 245.