ISOMERIC CHANGE 243 



It is, therefore, scarcely possible to carry through a chemical 

 operation with limonene or the related compounds without at 

 some stage producing a symmetrical compound and so destroy- 

 ing the optical activity. The remarkable point is, indeed, not 

 the inactivity of the artificially prepared limonene, but the fact 

 that the natural material is produced in an optically active form. 



In striking contrast with the optical instability of limonene 

 and the mono-cyclic terpenes is the extreme stability of 

 camphor and the related members of the dicyclic group of 

 terpenes. Pinene, for instance, gives an inactive product when 

 converted into a monocyclic ^/hydrochloride (limonene or dipen- 

 tene dihydrochloride, C 10 H 1S C1;,) but retains its activity when 

 converted into a ^/cyclic w/cwohydrochloride (pinene hydro- 

 chloride or t'sobovnyl chloride C 10 H 17 C1) in spite of the fact that 

 the latter operation involves a rearrangement of one of the 

 ring-systems ; or, if these operations be pushed to a further stage 

 by removing the hydrogen chloride and so reproducing a hydro- 

 carbon of the formula Ci H 16 , the active pinene is found to give an 

 inactive monocyclic limonene, but an active dicyclic camphene. 



Camphor itself possesses the quality of optical stability in 

 an extraordinary degree, and retains its activity in a very 

 remarkable way when submitted to the action of the most 

 drastic agents. Indeed, the only action in which racemic 

 compounds are produced to any extent is during the preparation 

 of a 7r- sulphonic acid from camphor itself, 1 even the analogous 

 treatment of a-bromo- and a-chloro-camphor failing to produce 

 any but optically-active derivatives. 



If it be admitted that merely shaking the molecule can never 

 produce a rearrangement of the groups, the optical stability ot 

 camphor follows at once from the fact that its activity is due 

 to the presence of three dissimilar chains joining the two 

 asymmetric carbon atoms, and can therefore only be destroyed 

 by breaking one of these chains and converting the compound 

 into a monocyclic derivative. That this actually takes place 

 during the "Optical Inversion of Camphor" 2 is evidenced (1) by 

 the fact that a monocyclic terpene, carvenone, is actually pro- 

 duced from camphor under very similar conditions, 3 and (2) by 

 the difficulty of accounting otherwise for the ready sulphonation 



1 Kipping and Pope, Trans. Chem. Soc. 1897, 71, 958. 



2 As discussed by Armstrong and Lowry, Trans. Chem. Soc. 1902, 81, 1469. 



3 Bredt, Annalen, 1901, 314, 369. 



