Volatile oils obtained from plants possess a great variety of odors,
with no two exactly alike, although many are very closely related. A
classification of these oils based on their odors is not satisfactory,
since many which would not be considered as related if judged only by
the sense of smell have chemical relationships, containing substances
belonging to the same general class of chemical compounds. For our
purpose volatile oils are divided into the following classes, basing
the divisions upon odors and constituents. These groups comprise the
majority of oils, but they are not arranged in the order of their
importance:
(1) _Camphoraceous oils_, possessing a characteristic camphorlike odor,
with camphor or camphor-related compounds predominating, as in the oils
obtained from the camphor tree and from many of the sages. The products
obtained from camphoraceous oils are extensively employed in the arts
and in medicine.
(2) _Terebinthinate oils_, having a characteristic turpentinelike odor.
These oils are obtained largely from the pine family, the turpentines
of commerce being examples. They are composed largely of terpene
hydrocarbons and find extensive application in the paint and varnish
industries.
(3) _Sulphur-containing oils_, a small group characterized by extremely
disagreeable and offensive odors, such as those of mustard, asafetida,
garlic, and onion. These oils contain as their chief constituents
sulphids, sulphocyanates, or nitriles, and are used principally for
medicinal purposes.
(4) _Phenol and phenol-related oils_, containing phenols or phenol
derivatives and characterized by strong, persistent odors, some very
pungent and others pleasant. Owing to their phenolic constituents
the density of these oils is usually very high. Common examples of
this class are the oils of thyme, cloves, cinnamon, sassafras, anise,
fennel, and the monardas. The usefulness of phenol and phenol-related
oils depends largely upon their antiseptic properties, the principal
constituents being thymol, carvacrol, eugenol, anethol, chavicol,
safrol, and their derivatives.
(5) _Oils containing esters or alcohols_, by far the largest group,
consisting of the fragrant oils which are used principally for
perfumery purposes, although some find a use in medicine. The chief
constituents of these oils are usually alcohols and esters, some few
containing aldehydes, ketones, oxids, and lactones. Prominent here are
the alcohols menthol, linalool, geraniol, citronellol, sabinol and
their esters, benzyl alcohol and its esters, and anthranilic acid and
its esters, forming the chief constituents of the oils of peppermint,
lavender, geranium and rose, citronella, savine, ylang-ylang, and
orange flowers, respectively. Other constituents are the aldehydes
citral and citronellal from lemon and lemon-grass oils, and the ketones
thujone, menthone, pulegone, carvone, and methyl heptenone from the
oils of wormwood, peppermint, pennyroyal, caraway, and rue. The oxid
cineol from eucalyptus and many other oils, and the lactone sedanolid
from celery oil are further examples.
All volatile oils capable of being isolated from wild aromatic plants
will fall into one or more of the foregoing divisions although, it must
be understood, the classification is far from being satisfactory. It
will, however, serve to elucidate the fact that although plant odors
are of a very variable character they still possess some relationship.