I. THE CHEMISTRY OF ESSENTIAL OILS
Early in his history, man evinced a great
deal of interest in the preservation of the fragrant exhalation of plants, and
those who were later to be called chemists occupied themselves with separating
the essence of the perishable plants. It was probably observed that heating of
the plant caused the odoriferous principle to evaporate and that upon
condensation and subsequent cooling, droplets united and formed a liquid
consisting of two layers water and oil. While, in such primitive experiments,
the water from the plant is used to carry over the oils, additional water or
steam was later introduced in "stills" to obtain better yields and
quality.
In early work, therefore, we find the term
"essential oil" or "ethereal oil" defined as the volatile
oil obtained by the steam distillation of plants. With such a definition, it is
clearly intended to make a distinction between the fatty oils and the oils
which are easily volatile. Their volatility and plant origin are the
characteristic properties of these oils, and it is for thi reason more
satisfactory to include in our definition volatile plant oils obtained by other
means than by direct steam distillation. 1 - 2 Bitter almond and mustard oil,
obtained by enzymatic action, followed by steam distillation; lemon and orange
oil isolated by simple pressing, and certain volatile oils obtained by
extraction are, therefore, included among the essential oils.
In the early stages of development of organic
chemistry, the chemical investigation of oils was limited to the distillation
of a great number of plants, and the oils which were obtained in this way were
used to compose perfumes according to recipes, some of which are still used at
the present time; e.g., the can de Cologne prepared in 1725 by Johann Maria
Farina in Cologne.
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1
Thomas, "Athcrische
Ole," in Klein, "Handbuch der Pflazenanalyse" Vol. Ill, 1 (1932), 454.
2
Rosenthaler, Pharm.
Ada Helv.
10 (1944),
213.
Gradually with the advance of science came
improvements in the methods of preparing the oils, and parallel with this
development a better knowledge of the constituents of the oils was gained. It
was found that the oils contain chiefly liquid and more or less volatile
compounds of many classes of organic substances. Thus, we find acyclic and isocyclic
hydrocarbons and their oxygenated derivatives. Some of the compounds contain nitrogen
and sulfur. Although a list of all the known oil components would include a variety
of chemically unrelated compounds, it is possible to classify a large number of
these into four main groups, which are characteristic of the majority of the essential
oils, i.e. :
1. Terpenes, related to isoprene or isopentene;
2. Straight-chain compounds, not containing any side branches;
3. Benzene derivatives ;
4. Miscellaneous.
Representatives of this last group are incidental
and often rather specific for a few species (or genera) and they contain compounds
other than those belonging to the three first groups (Fig. 2.1).
3 Mannich and Fresenius, Arch. Pharm. 274 (1936), 461.
For example, the mustard oils, containing allyl
isothiocyanate, are found in the family of the Cruciferae; allyl sulfides in the
oil of garlic. The oil from Ferula asafoetida
L., belonging to the family of the Umbelliferae, gained reputation from its active
component, secondary butyl propenyl disulfide, competitor of the odoriferous principles
of the skunk, primary n-butyl mercaptan and dicrotyl sulfide.4 The more
pleasant smelling orange blossom and jasmine perfume betrays the presence of small
amounts of anthranilates and indole, both compounds related to the amino acid, tryptophane.
Although it is possible to list a considerable
number of such singular cases, the most characteristic group present in many essential
oils contains hydrocarbons, as a rule of the formula C10H16
and a group of oxygen-containing compounds with the empirical formula H10O16
and C10H18O. The classical book of Wallach indicates the names
of these two types of compounds in its title "Terpene und Campher." The
English word "terpene" and the German "Terpen" are derived from
the German word "Terpentin," English "turpentine" and
French "trebenthine." The name "Terpen" is commonly attributed
to Klkule*, who is said to have introduced it as a generic term for hydrocarbons
CioHie to take the place of such words as Terebene, Camphene, etc.5,6
The name "camphor" formerly was used to indicate the crystalline oxygen
compounds, such as thyme camphor (C. Neumann, 1719) and peppermint camphor (Gaubius,
1770) ; these are now known respectively as thymol and menthol. The name "camphor"
is at present limited to a specific compound and its more general meaning, covering
the oxygenated derivatives, has been taken over by the term "terpene."
With an increase in our knowledge, this broadened definition in its turn became
too narrow and had to be modified to cover new and more distantly related compounds.
Not all terpenes are represented by the formula C5H8 ; there
exist compounds which contain less hydrogen, still others which are more saturated.
We also find terpenes, like santene (C9H14), which have only
9 carbon atoms. The close resemblance to and probable connection with the C10
compounds through the terpene acid, santalic acid, make it impractical to omit
such a compound from the terpene literature.
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4Stevens, J. Am. Chem.
Soc. 67 (1945), 407.
5Gildemeister and Hoffmann, "Die Atherischen
Ole," 2cl Ed., Vol. I (1910), 90.
6 Kremers and collaborators, "Phytochemical Terminology," J. Ain. Pharm.
Assocn. 22 (1033),
227.
At the present time, therefore, we use the term
terpene both in its broadest sense to designate all compounds which have a distinct
architectural and chemical relation to the simple C5H8 molecule,
and in a more restricted sense to designate compounds with 10 carbon atoms derived
from C10H16 . When confusion with the general designation
is possible, members of the C10 group are often referred to as
monoterpencs. Compounds having a more distant connection with the terpenes, but
still containing features which link them with terpene structures, are sometimes
called terpenoids or iso-prenoids in analogy with the term steroids, which includes
not only sterols, but many more remotely connected relatives. 7,8,9,10
Characteristic for many of these oil constituents
is their instability an the ease with which intramolecular rearrangements occur.
These propertie have been a great hindrance to the study of these compounds.
Anothe drawback in the analysis of these oils is that most of the compounds ar liquids
so that thorough fractionation is necessary to separate the constituents which boil
within a restricted temperature range. Since in the early stages of research it
was difficult to define sharply the isolated fraction, a great number of terpenes
were named after the plant from which they were obtained.
Order was brought into this chaos by Wallach,
who saw clearly that the first task in the study of the oils was the identification
of the terpenes with the help of crystalline derivatives, this being the only practical
way we possess at present to identify chemical substances with certainty. Based
on Wallach's investigation, about 500 compounds have since been isolated and characterized
in the essential oils. After a general idea was obtained of the great number of
distinct chemical compounds in oils, Wallach started the second part of his working
program, i.e., studies of the relationship between the terpenes and the camphors.
By reason of their fundamental nature and the clear presentation of the problems
they involved, these studies provided great stimulus not only to his contemporaries
Semmler, Harries, Tilden and others but had a pronounced influence on the development
of chemistry as a whole. The establishment of the constitution and the relationship
of the terpenes revealed a certain regularity in their structures. As early as 1869
Berthelot had discovered how the hydrocarbons C10H16, C15H24,
and C20H32 are related to the hydrocarbon isoprene (C6H8)
isolated by Williams11,12 a few years before.
However, it was through the combined work of the aforementioned investigators that
this hypothesis was established on a firm basis.
The compounds which we find in the monoterpene
series can be figuratively divided into 2 isopentene chains; such a hypothetical
combination gives substances of the empirical formula C10H16
. If three of these isopentene units can be recognized in the molecule, the name
sesquiterpene is given. In the course of time there have been added diterpenes
derived from C20H32, triterpenes, C30H48,
and tetraterpenes, C40H64 and finally polyterpenes with an
indefinitely large number of these units (Fig. 2.2).
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7
Kremers, ibid.
8
Gildemeister and
Hoffmann, "Die Xtherischen Ole," 3d Ed., Vol. I, 15.
9
Ruzicka, Ann.
Review Biochem. I
(1932), 581.
10
Fieser and Fieser, "Organic
Chemistry," Heath Co. (1944).
11
Kremers and collaborators,
"Phytochemical Terminology” J. Am. Pharm. Assocn. 22 (1933), 227.
12
Williams, Jahresber. (1860), 495.
A saturated acyclic hydrocarbon with 10 carbon
atoms would have the formula C10H22, possessing 6 II atoms
more than a compound C10H16. This lower hydrogen content may
be caused by the occurrence of double bonds, by ring structure, or by both,
giving rise to acyclic, monocyclic and bicyclic representatives, with 3, 2 and 1
double bond, respectively. We have, therefore, the following possibilities for a
molecule with the formula (monoterpene) :
Acyclic.................... No ring 3 double
bonds
Monocyclic ..............One ring 2 double
bonds
Bicyclic ....................Two rings 1 double
bond
Tricyclic ......................Three rings No double
bonds
All these structural variations of the same empirical
formula are found in the constituents of volatile plant oils. A chemical shorthand,
developed by terpene chemists, has been introduced to show more clearly the principal
structural details. This greatly simplified way of writing formulas consists in
assuming a carbon atom at a place where valency lines end, or form an angle. As
many C's and H's as feasible are omitted and only double bonds and substituents,
such as hydroxyl and amino groups, are written in full (Fig. 2.3). Others prefer
to indicate all end groups such as methyl and methylene groups in full.
As examples of the acyclic terpenes with 3 double
bonds, we find ocimene and myrcene. In the frequently occurring acyclic
alcohols geraniol and linalool, in the aldehydes citronellal and citral, and in
dehydrogeranic acid we see several stages of oxidation and reduction of this type
of terpene hydrocarbons (Fig. 2.4). Many of these compounds can be converted into
each other with great ease. Geraniol, the chief
constituent of rose and geranium oil, is easily converted into the monocyclic alcohol
α-terpineol, the chief constituent of the oil of hyacinth, and into linalool, which
as acetate constitutes the characteristic component of lavender oil.
Geraniols of variant origin have variant
constants and odors, due to the presence of isomers. The double bond between carbon
atoms 2 and 3 makes the existence of cis- and Jrans-isomers possible, and the relative
ease of ring formation permits one to distinguish between these forms, which have
been called nerol and geraniol according to their origin. The double bond near the
terminal carbons is another source of isomerism. Thus geraniol, nerol and other
compounds with similar structure, such as citronellol and rhodinol, and citronellal
and rhodinal, consist of varying quantities of isomers containing the double
bond, between either carbon atoms 7 and 8, or 6 and 7, resulting in a further
source of variation in the constants of the oil constituents (Fig. 2.5).
Most of these compounds easily form cyclic derivatives
under the influence of acids, and the formulas are usually written intentionally
in such a way as to indicate where the ring closure takes place. A saturated monocyclic
terpene has the formula C10H20 and is called menthane. If
the compound has the empirical formula C10H16 , there must
be 2 double bonds, since the ring occurs in the place of one of the 3 double bonds
present in aliphatic terpenes. Such hydrocarbons are called menthadienes, and the
method of indicating the position of the double bond given by Baeyer makes use of
the Greek capital letter A (delta), and an index number indicating the carbon atom
from which the double bond starts. If the double bond is in the side chain, then
it will be necessary to indicate toward which carbon atom the double bond goes.
This number is placed in brackets behind the number of the first carbon atom, as
is indicated in Fig. 2.6. 13
We find many representatives of this class of
menthadienes among the terpene fractions in essential oils. For example, dipentene,
formed by the
When the purified fractions are characterized
by preparation of crystalline derivatives, and when these are compared by melting
point and mixed melting point with the known derivatives, there remain always some
fractions which cannot be characterized in this way. In, such cases, chemical degradations
of the molecules have to be applied. The principle involved in these
degradations consists of breaking up the molecule into smaller parts until the pieces
have become so simple that they can be recognized. For this purpose, oxidation with
ozone, potassium permanganate and chromium trioxide is often used. Sometimes several
of these degradations may be necessary before the pieces obtained are small enough
to be identified. On the basis of these degradations, a possible structure is postulated
and attempts are made to confirm this structure by synthesis. This work has been
carried out on about 500 constituents of essential oils. One-fifth of this number
is made up of monoterpenes, and only a start has been made on the investigations
of sesqui- and higher terpenes. In view of the greatly increased possibility of
structural isomerism, every time 5 carbons are added to a molecule we may look forward
to the addition to our present knowledge of a great number of the higher
terpene homologues when a more extensive survey is made.
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