Chemistry of Essential Oils

CEUCIFEE^l 495

CRUCIFER^E,

OIL OF MUSTABD.

The essential oil of mustard is obtained by distilling the seeds of
several species of Brassica with water. Brassica nigra is the principal
species employed, but there is no doubt that the mustard seeds of com-
merce contain a substantial amount of the seeds of Brassica juncea and
Brassica rapa. The seeds contain the glucoside potassium myronate,
which, under the influence of a ferment, myrosin, also present in the
seeds, in the presence of water, is decomposed according to the following
equation :—
C 10 H 16 NS 2 KO 9 + H 2 O = C 6 H 12 O 6 + SO 4 KH + OS. N. C 3 R 5.
Potassium myronate. Glucose. Mustard oil.
The chief product of the reaction is thus allyl iso-thiocyanate (allyl
thiocarbimide), a pungent and disagreeable liquid. A small quantity of
the normal allyl thiocyanate is also formed, together with traces of allyl
cyanide and carbon disulphide. As mustard oil consists almost entirely
of allyl iso-thiocyanate, and the latter body is easily prepared artificially,
there is an artificial oil on the market. This is made by distilling allyl
iodide or bromide with alcoholic solution of potassium thiocyanate—
a molecular rearrangement to the iso-thiocyanic radicle taking place.
Thus—
CN. SK + C 3 H 5 I = OS. N. C 3 H 6 + KI.

Pure allyl iso-thiocyanate is a liquid of specific gravity 1

*

017 at 10°,

boiling at 151°.

Essential oil of mustard is an unpleasant smelling liquid of great

pungency, having the following characters :—

Specific gravity 1-015 to 1*025 (rarely 1*030)

Refractive index 1-5267 to 1-5281

Optical activity 0°

It boils almost entirely between 148° and 155°.

A sample believed to be distilled from the seeds of Brassica juncea

•only has been examined by Schimmel & Co.,^1 and found to have the

following characters :—

Specific gravity ......... 0'995

Optical rotation......... + 0° 12'

Refractive index 1-51849

The greater part of the oil at above 160°.

Seven hundred and fifty grams oil, being fractionated three times,

gave:—

From 40° to 150° 53 grams or about 7 per cent.

150° „

160° „

174° ,

, 160°

, 174°

, 178°

200 „

160 „

290 „,,

30

„ 20

40

The above figures by themselves are sufficient indication that the oil

is not a uniform body. The fact that the thio-urea obtained from it,

which solidified very slowly, had no clearly defined melting-point (67° to

70 °) pointed to a mixture of several substances.

(^1) Report, October, 1910, 81.