In addition to the terpenes C10H16, the higher boiling fractions of many volatile oils, viz., those between 250 and 280°, contain hydrocarbons corresponding to the formula C15H24 which have been named sesquiterpenes. Of these a considerable number have been isolated, but few have been investigated to any extent.
A few oils consist almost exclusively of sesquiterpenes. In most instances, however, they play but a secondary role. Their occurrence is so general that they must be regarded as belonging to the most common constituents of the volatile oils.
Artificially also sesquiterpenes have been prepared from the alcohols C16H260 which accompany the former in many volatile oils. These alcohols seem to stand in the same relation to the sesquiterpenes as does terpineol, C10H18O to the hydrocarbon C10H16. Nevertheless the addition of the elements of a molecule of water to a hydrocarbon C15H24 by means of the glacial acetic acid - sulphuric acid mixture has been carried out successfully in but one instance. Whether the hydrocarbons C15H24 obtained by the dehydration of the "sesquiterpene alcohols" are identical with the natural sesquiterpenes, has not yet been determined.
In general, the boiling points of the sesquiterpenes lie between 250 and 280°; their density 0,86 and 0,93, as a rule above 0,90. In most instances they are slightly colored. They are more viscid than the terpenes and have a faint odor. In part they resinify as readily as do the terpenes and they are difficultly soluble in alcohol. As unsaturated hydrocarbons they add halogens, hydrohalogens, NO Cl, N2O3 and N2O4. Some of them yield crystalline addition products which can be used for their identification. Their oxidation products, however, remain practically unexamined.
1) Liebig's Annalen 340 (1905), 12.
As to their constitution, nothing definite is known. Hence we are dependent on their physical properties for their classification. A better knowledge of the sesquiterpenes is rendered more difficult from the fact that the only method suitable for their isolation, namely that of fractional distillation, precludes the preparation of pure products. Based on their molecular refraction and relative density, they are classified as aliphatic, monocyclic, dicyclic and tricyclic sesquiterpenes.1) As with the terpenes, these groups are characterized by regularity with reference to boiling point and more particularly to density.
„ 0,875 to 0,89,
bi cyclic „
„ 0,90 „ 0,92,
" 0,93 „ 0,94.
In as much as only a few sesquiterpenes have been obtained in a pure state, it is frequently difficult to ascertain to which of the above groups they belong. The conditions of isomerism are even more complicated than among the terpenes. Nevertheless it is not at all probable that the 60 to 70 known sesquiterpenes are all different. No doubt, many of these will be found identical as soon as it becomes possible to obtain a clear view over this field as over that of the terpenes, since the terpenes have become one of the best known groups of organic compounds.
In the compilation which follows only those representatives of the sesquiterpenes will be discussed in detail the characterization of which has been made possible.
1) Theoretically tetracyclic sesquiterpenes are also possible. In as much as none of these have yet been isolated, this mere reference to the group may suffice.