— 173 — 



in reality a-terpinene. It is difficult to say, of course, to what degree 

 this body is pure, for no means of testing this are available. 



The optical examination of zl 1}8 -menthadiene and of the lower 

 homologues prepared by Auwers and von der Heyden made it pos- 

 sible to make most interesting comparisons as to the extent to which 

 a hydrocarbon changes its properties by the passing over of a remote 

 double - linking of the ring into a conjugated double-linking The optical 

 examination of the homologous ketones already showed that when a 

 double-linking of the ring assumes a conjugated position to the double- 

 linking of the carbonyl, an increase of b. p., density, refraction, and 

 dispersion may be observed. In the case of the hydrocarbons the 

 same conditions became clearly apparent, affording new and con- 

 clusive proof that conjugated hydrocarbon double - linkings in a hexa- 

 cyclic ring produce the optical effect which is to be expected in ac- 

 cordance with Bruhl's rule. In the case of these bodies the exaltation 

 of the molecular refraction is for M a and M© on the average 0,8 1, 

 which is quite in accord with other observations. The molecular disper- 

 sion is even increased by about 40% above the normal value. 



These observations induced Auwers 1 ) to go deeper into the ter- 

 pinene question itself. There are about 1 8 different methods of preparing 

 terpinene-products. Five of these may finally be derived from terpin 

 and therefore belong together. They differ from the other preparations 

 by their strikingly high sp. gr. and low index of refraction. In every 

 case the molecular refraction falls below the value Md 45,24 which 

 has been calculated for the formula C 10 H 16 /2T Four other preparations 

 form a second correlated group, all of them having had their origin 

 in the splitting -off of hydrohalogen from the terpin ene mono- and di- 

 hydrohaloids. They are differentiated from the first group by a lower 

 sp. gr. and a higher index of refraction, hence the molecular-refraction 

 is considerably greater in the case of this group. Their average 

 exaltation is 0,45. The other methods of preparation in which sabinene, 

 sabina-ketone, dihydrocarveol, carvylamine, carvenone, and the chlorinated 

 ketone are the primary substances, yield terpinenes of more or less especially 

 high exaltation. This clearly shows the connection with the modes of 

 preparation. In order to convert terpine into terpin ene it is necessary 

 to split off water twice, and conditions ensue which make the for- 

 mation of tetra rings not improbable, but which in any case do not 

 favour the formation of conjugated double linkings. The optical examin- 

 ation also agrees with this. In the case of the terpinene hydro- 

 haloids the conditions are more favourable to the formation of a system 

 of conjugated double-linkings. The high exaltation which is shown by 

 the terpinene-preparation from dihydrocarveol warrants the conclusion 



*) Berl. Berichte 42 (1909), 2424. 



