Mentha-di-enes

HaC CH, Cymene C10H14 I-Methyl 4-iso- -propyl benzene CH3 1 CH H3C CH3 Di-hydro cymene C10H16 Terpa-di-ene Mentha-di-ene CH3 1 CH... 

The tetra-hydro cymene containing one double bond or ethylene group is known as menthene or a terpene, while the di-hydro cymene containing two double bonds is named mentha-di-ene, a terpa-di-ene. The terminology and its significance will be recognized as exactly the same as used in the naming of the unsaturated aliphatic hydrocarbons... 

Isomerism.—From an examination of the above formulas it will be seen that the positions occupied by the added hydrogen atoms in the original cymene molecule or, what is the same thing, the positions occupied by the double bonds, makes isomerism possible both in the tetra-hydro cymenes or menthenes with one double bond and the dihydro cymenes or mentha-di-enes with two double bonds. In the former case six isomers are possible while in the latter there are fourteen. This will be clear if we give the skeleton formulas for the six possible menthenes. 

As each of these menthenes will yield isomeric mentha di-enes the number of isomers possible in this group is still larger. That is, one men-thane yields six menthenes and these a larger number, viz., fourteen, mentha-di-enes. Furthermore, stereo-isomerism with accompanying optical activity due to the presence of asymmetric carbons, increases the number of possible isomers. It will not be necessary to dwell further upon the isomerism of the terpenes it being necessary simply to explain the fact of the existence of structural isomers and of stereo-isomers with optical activity. The system of nomenclature of the isomers will not be considered. Reference to larger books will be necessary to make this plain. 

The most important group of the mono-cyclic terpenes is the dihydro cymene group the members of which are known as terpa-di-enes or mentha-di-enes with the composition CioHie which has been mentioned before as the unit terpene formula. Several of the hydrocarbons of this group are natural products in essential oils. 

Di-hydro Carveol, Di-hydro Carvone.—Di-hydro carveol, the other important menthen-ol, is present in kummel oil, together with the corresponding ketone, di-hydro carvone, from which it may be obtained by reduction. This ketone is the di-hydrogen addition product of a mentha-di-ene ketone known as carvone which we shall presently consider. 

As carvone may be converted into limonene, a mentha-di-ene hydrocarbon, the constitution of the two must agree and the formula for the latter is as above which is the same as previously given to it (p. 820). Limonene is thus the mother terpene of carvone. 

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