462 



Mr. Gervaise Le Bas on the 



(b) Owing to peculiarities in the contractions, it is 

 sometimes difficult to say whether a ring is single or 

 double. 



Thus the contractions in 



u Tanacetene 



Thujone 



(CH 3 ) 2 = CH — CH — OH — CO 



CH 2 — CH — CH — OH, 



CH 3 



would be, for a. Tanacetene : 



A 3 + A 5 = 6 + 12 = 18, 



but little different from A 6 = 15 or 16 ; 



for Thujone 2A 4 = 2 x 8*6 = 17*2, 



again but little different from 16*0 for A 6 . 



If, however, the " Bridge " include an atom or group, 

 a very different result is obtained : — 



CH 2 C CH 2 



CH 3 -C-CH 2 



(Camphor.) 



CH: 



- C - 



I 

 CH 3 



CO 



The contraction rises to —31*0. 



This result is found because the central group has a 

 contracting influence on all the other constituents of the 

 rings. 



A very interesting deduction can be drawn from the 

 foregoing. In « Tanacetene, 



