492  Menthol.  {A*s8jffir,~ 
0°/o°  =  0-8067  and  at  I5°/I5°  =  0  796.  It  does  not  take  up 
bromine,  is  not  attacked  by  strong  sulphuric  or  nitric  acid  at  the 
ordinary  temperature,  gives  when  heated  with  fuming  nitric  acid  a 
liquid  nitro-derivative,  and  with  bromine  and  aluminium  bromide  a 
viscid  bromide.  If  the  heating  in  the  sealed  tube  is  continued  for 
only  1 5-20 hours,  some  menthyl iodide,  C10H19I,  is  also  obtained;  this, 
when  not  quite  pure,  boils  at  140-1430  under  30  mm.  pressure, 
and  has  a  sp.  gr.  at  i'5°/i5°  =  1-357.  When  chlorine  is  allowed 
to  react  with  the  vapors  of  menthonaphthene,  an  inactive  liquid  is 
obtained,  volatile  with  steam,  boiling  at  208-2100,  and  with  a  sp, 
gr.  at  o°/o°  ==  0-9553.  It  would  seem  to  be  identical  with  the 
above-mentioned  menthyl  chloride.  When  menthyl  chloride  (70 
grams)  is  heated  with  potassium  acetate  and  acetic  acid  for  20  hours 
in  a  sealed  tube  at  1500,  some  (20  grams)  is  left  unaltered,  and  some 
menthene,  C10H18  (25  grams),  is  formed.  This  menthene  boils  at 
170-1710,  has  sp.  gr.  at  o°/o°  =0-8287  and  at  20°/'20°  =  0816, 
and  refractive  index  [^]D  =  0-145359  at  20°,  corresponding  with 
a  molecular  refraction  of  45-06  (theory  for  one  double  bond  = 
45-65,  for  two  47-34).  It  oxidizes  readily  in  the  air.  But  whilst  the 
menthene  obtained  from  menthol  is  dextrorotatory  with  a  maximum 
rotation  of  -j-  20°»  this  menthene,  obtained  from  inactive  menthyl 
chloride,  is  laevorotatory  (in  a  thickness  of  200  mm.  [«]D  =  —  560), 
and  the  menthyl  chloride  which,  as  mentioned  above,  was  not  con- 
verted into  menthene,  is  found  to  be  no  longer  inactive,  but  is  now 
laevorotatory  (in  a  thickness  of  200  mm.  [«]D  =  —  620).  The 
inactive  menthyl  chloride  must  thus  have  consisted  of  a  mixture  of 
dextro-  and  laevo-rotatory  varieties,  and  the  former  was  converted 
into  laevorotatory  menthene,  while  the  latter  was  unattacked.  By 
further  heating  with  potassium  acetate  and  acetic  acid,  it  may,  how- 
ever, be  converted  into  a  dextrorotatory  menthene  boiling  at 
167-1690;  in  a  thickness  of  50  mm.  [a]D  =  -f28°. 
Menthone,  C10H18O,  was  prepared  by  the  action  of  chromic  acid  on 
menthol  in  acetic  acid  solution,  and  warmed  with  phosphoric  anhy- 
dride. A  diterpene,  C20H32,  was  obtained,  together  with  a  hydro- 
carbon C10H18  which  was  not  isolated.  The  diterpene  is  a  very 
viscid,  fluorescent,  inactive  liquid  boiling  at  320-325 °;  it  takes  up 
bromine  and  hydrochloric  acid,  and  yields  a  yellowish,  amorphous 
nitro-product  when  treated  with  fuming  nitric  acid.  When  men- 
thone  is  added  in  successive  small  portions  to  a  cooled  mixture  of 
