74 THE CHEMISTEY OF ESSENTIAL OILS
Seychelles Islands, Uganda, Bermuda, and various other places. There
is, however, nothing particularly characteristic about these oils, the fore-
going figures illustrating the general characters of a number of them.
Lemon-grass oil has the following characters if distilled in the East.
Indies; it may occasionally fall outside these limits, and, as will be seen
from the above, may vary considerably from them when distilled else-
where :—
Specific gravity 0-895 to 0-908
Optical rotation + 1° 30' to - 5°
Refractive index 1-4825 to 1-4885
Citral (bisulphite method)...... 68 to 85 per cent.
The oil is somti'mes soluble in 3 volumes of 70 per cent, alcohol, some-
times insoluble.
Generally speaking, the citral is determined by absorption with sodium
bisulphite, but this determination will include bodies other than citraL
Some chemists prefer to determine it by neutral sodium sulphite, but as
this process gives lower results the method used ought always to be
stated. The sulphite method gives from 3 to 6 per cent, lower results
than the bisulphite method.
According to most chemists who have worked seriously on this oil,
citral is the only aldehyde present in the oil, with the exception of traces
of an isomeric aldehyde, and of decyl aldehyde, and citronellal.
Upon this point Tiemann, Semmler, and Doebner, the three chief
authorities on this oil, are completely agreed. Stiehl
l
has claimed to have
separated the aldehydic constituent into three different bodies. He urges
that the large amount of citral found in the oil usually is due to the
isomerisation of the other aldehydes by means of the bisulphite of sodium
used in separating it, which is generally too acid. If this salt be perfectly
pure he claims that this isomerisation does not take place. These alde-
hydes he terms citral (the aldehyde usually known as such), allolemonal
and citriodoraldehyde, and he gives the following as their properties :—
Citriodoraldehyde. Allolemonal. Citral.
Boiling-point .... 228° to 229° 233° to 235° 225° to 227°
Specific gravity at 20°... -8883 -9017 -8868
Rotation 0° - 5° 6' 0°
Melting-point of compound with
naphthocinchoninic acid. 204° 235° 197°
Doebner, however, has shown—in the author's opinion conclusively
—that these other aldehydes are merely impure citral. By the term
citral is meant a mixture of the stereoisomeric bodies, a-citral and
/3-citral.
Traces of an isomer of citral, having a specific gravity 0*908, have
been detected, and also traces of decyl aldehyde. Methyl-heptenone is
present, as well as geraniol, linalol, dipentene, and limonene. \
Kafaku^2 has identified myrcene, and a new aldehyde in Formosan
lemon-grass oil.
This oil is largely used for the preparation of citral for the manu-
facture of ionone, so that it is usually sold on its citral value.
OIL OF CYMBOPOGON SENNAAKENSIS.
O. D. Eoberts^3 has examined at the Imperial Institute the distillate
from the above grass, which is known in the British Sudan as " ma-(^1) Jour, prakt. Chem., 1898, 51. (^2) Jour. Chem. Ind. Tokyo (1917), 20, 825.
» Jbwr. Chem. Soc., 1915, 1465.